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Comparison of pulmonary artery and central venous pressure waveform measurements via digital and graphic measurement methods
Comparison of pulmonary artery and central venous pressure waveform measurements via digital and graphic measurement methods Thomas S. Ahrens, DNS, RN, CCRN, CS, and Lynn Schallom, MSN, RN, CCRN, CS, St Louis, Missouri
BACKGROUND: Techniques to measure pulmonary artery (PA) pressure waveforms include digital measurement, graphic measurement, and freeze-cursor measurement. Previous studies reported the inaccuracy of digital and freeze-cursor measurements. However, many of the previous studies were small and did not thoroughly examine the circumstances of when digital measurements might be inaccurate. OBJECTIVES: To compare digital measurements and graphic measurements of PA and central venous pressure (CVP) waveforms in patients with a variety of respiratory patterns, and to compare digital measurements and graphic measurements of CVPs in patients with abnormal or right ventricular waveforms. METHODS: A total of 928 patients were enrolled in this study. Waveforms from the PA and CVP were collected from each patient. The monitor pressure value (digital measurement) printed on the recorded waveform was compared with the pressure value obtained by a graphic strip recording and measured by one of the primary investigators (graphic measurement). RESULTS: Digital measurements were found to be inaccurate in measuring waveforms in all respiratory categories and in measuring right ventricular waveforms. PA diastolic values and CVP values were the most inaccurately measured waveforms. Digital errors of more than 4 mm Hg were common. CONCLUSION: There were instances in which the monitor’s digital measurement was substantially different from the graphically measured value. This difference has the potential to mislead interpretation of clinical situations. The monitor’s ability to occasionally give digital measurement values similar to the graphic measurements may lead to a false sense of security in clinicians. Because the accuracy of the monitor is inconsistent, the bedside clinician should interpret waveforms through use of a graphic recording rather than rely on the digital measurement on the monitor. (Heart Lung® 2001;30:26-38.)
C
ritical care clinicians routinely measure pulmonary artery (PA) pressure, central venous pressure (CVP), and pulmonary artery wedge pressure (PAWP) to assess and optimize the hemodynamic status of their patients. Evidence suggests From the critical care units of Barnes-Jewish Hospital, St Louis, Missouri. Supported by the St Louis chapter of AACN, Marquette Electronics, and Mennen Medical. Reprint requests: Thomas S. Ahrens, DNS, RN, CCRN, CS, Nursing Service Barnes-Jewish Hospital, One Barnes Hospital Plaza, St Louis, MO 63110. Copyright © 2001 by Mosby, Inc. 0147-9563/2001/$35.00 + 0 2/1/112504 doi:10.1067/mhl.2001.112504
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that nurses and physicians may not measure and interpret these pressures accurately.1-5 Previous research has indicated that the bedside monitor is inaccurate in its measurement of PA pressure waveforms.6-11 Although these studies provided information on monitor accuracy, they were subject to methodologic limitations. For example, sample sizes in some of these studies were small, ranging from 3 to 44 measurements.7-11 The studies with small sample sizes limit their generalization and power. In addition, delineation of various respiratory pattern effects on digital measurement accuracy has not been well studied. Because of the limitations of the past research, an understanding of bedside monitor accuracy in
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Fig 1 Reading a CVP waveform.
measuring PA and CVP pressure waveforms was incomplete. In the past decade, several studies have questioned the value of the PA catheter.12-15 Although no causal link between these studies and the accuracy of bedside monitors can be made, these studies reflect a problem associated with hemodynamic monitoring. Accuracy of digital measurements may be one part of the problem associated with the uncertain value of hemodynamic monitoring.
CURRENT ISSUES IN MEASURING PRESSURE VALUES The bedside clinician, (eg, nurse) can obtain pressure measurements through 1 of 3 methods: (1) reading waveform values directly from the monitor (digital measurement); (2) measuring the waveform directly from a printed copy of the wave (graphic measurement); and (3) using the freeze-cursor function on the bedside monitor. Based on the research that suggests that digital measurements may be inaccurate, the best options are the second and third methods. Bedside monitor programs use filtering techniques, such as variable weight beat averaging, to improve their accuracy in displaying digital measurements. In variable weight beat averaging, as described by Hewlett Packard,16 the monitor establishes a beat (or wave) value. To update the waveform value, the monitor uses a mathematical calculation designed to minimize variability (eg, artifact). In variable weight beat averaging, determination of the new pressure wave occurs in the following manner: The mean of the wave will be called N. The next wave mean (N + 1) is compared with N. The differ-
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ence between the 2 values (N – [N + 1]) is divided by a variable factor. The larger the difference between N and (N + 1), the greater the dividing factor becomes, to minimize the impact of the change. The change in the dividing factor is what inspired the term “variable weight beat averaging.” The result of this calculation is added to (or subtracted from) N to obtain a new beat. For example, assume the CVP value is 12 mm Hg. The next CVP value is measured by the monitor as 8 mm Hg. The monitor would subtract 8 from 12 and obtain 4 mm Hg. The 4 would be divided by a factor—in this case, 2—to give a value of 2. The 2 would be added to N to yield (N + 1) of 10 mm Hg. The value 10 mm Hg, rather than 8 mm Hg, would be displayed on the monitor. However, in previous studies, this filtering technique has not been shown to improve the monitor accuracy in all situations. In addition, the filtering techniques do not cover specific physiologic measurement concepts. For example, digital measurements are not the result of measuring specific waveforms (eg, a, c, or v waves on CVP or PAWP waveforms).16,17 Rather, engineering of the monitor programs makes possible the measurement of peak and trough values and computation of a geometric mean value. This inability to measure specific waves may limit the accuracy of the monitor’s digital measurements. For example, optimal measurements of a CVP or PAWP must be taken at a specific time in the cardiac cycle (when the tricuspid and mitral valves are open) to estimate ventricular end diastolic pressure. This end-diastolic point coincides with the pre–c-wave point or the mean of the a wave (Fig 1).18 The monitor’s method
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Fig 2 Large v-wave in a wedge waveform.
of averaging may or may not approximate the pre– c wave and mean of the a wave. Therefore, any atrial waveforms with variation caused by any number of reasons (eg, atrial fibrillation with loss of a waves, dysrhythmias with atrial-ventricular dissociation, or mitral/tricuspid valve dysfunction) are at risk for measurement error when the digital measurement technique is used (Fig 2). Another inherent limitation of the engineering of the bedside monitor is the interpretation of respiratory artifact. Because of filtering techniques like variable weight beat averaging, digital display measurements partially discard physiologic waves that are at variance from the “average,” or preceding, artifact value. Although this filtering technique may be helpful at times, in some circumstances it is actually detrimental. For example, the filtering program used by Marquette Medical during their automated wedge processing technique tracks the maximum and minimum 1-second averages.17 Once the wedge is complete, the filtering program computes the mean of the entire wedge sample, which is then compared with the maximum and minimum 1-second averages. If the overall mean is closer to the maximum 1-second average, the maximum 1-second average will be displayed as the PAWP value. If the overall mean is closer to the minimum 1-second average, the minimum 1-second average will be used for the PAWP value. This filtering program assumes an inspiratory:expiratory (I:E) ratio of less than 1:1 to determine end expiration. Therefore, the wedge algorithm may not work correctly when
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the I:E ratio is >1 or the patient is on synchronized intermittent mandatory ventilation and taking spontaneous breaths in addition to the set respiratory rate (personal communication, Marquette Medical Systems, Inc, November 1997).
STUDY QUESTIONS To consider the issue of the accuracy of bedside monitoring, the following questions are addressed. 1. What are the differences between those PA and CVP pressure waveform measurements obtained through digital measurement and those obtained through graphic measurement? 2. Does respiratory rate or the mode of mechanical ventilation affect the accuracy of PA and CVP waveform measurements obtained through digital or graphic measurements? 3. Are there differences between the CVP waveform measurements obtained through digital measurement and those obtained through graphic measurement when abnormal or right ventricular (RV) waveforms are present?
METHODS The study was conducted at 3 community and 2 university hospitals between 1995 and 1998. The Human Subjects Committee at Washington University approved an expedited review of the study.
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Table I Total sample size
Company
Total sample size
SB < 20 breaths/min
SB > 20/min
VB only
VB + SB < 20/min
VB + SB > 20/min
Hewlett Packard SpaceLabs Marquette Mennen Totals
442 253 155 115 965
88 33 27 24 172
61 46 25 24 156
86 76 52 37 251
88 38 17 11 154
41 28 4 4 77
Abnormal and RV waves
78 32 30 15 155
SB, Spontaneous breathing; VB, ventilator breathing.
Data collection involved obtaining a monitor strip from any patient with a PA catheter in place. Neither subjects’ names nor any other identifying characteristics were collected as part of the study.
Patient eligibility and data collection Any patient with a PA catheter in place at one of the study sites was eligible for the study. Selection of patients was made through convenience sampling. There were no exclusion criteria. Each patient had a recording of the pulmonary artery systolic (PAS) pressure, pulmonary artery diastolic (PAD) pressure, and CVP, with the numerical digital values of the pressures printed on the graphic recording. PAWP waveforms were not obtained because CVP and PAWP waveforms are both venous tracings. Digital and manual measurements of CVP and PAWP are identical. Both the CVP and PAWP are measured as means of the a wave or the pre–c-wave point.18 Therefore, additional balloon inflation for PAWP measurement was not necessary. If a patient’s condition changed so that the respiratory pattern changed, data collectors may have recorded additional waveforms with the new respiratory pattern. The principal investigators (PIs) and onsite coordinators recorded all the strips. Training of onsite coordinators included an overview of the study, the technique of graphic recording, and the process of documenting the patient’s respiratory rate and ventilator rate. Graphic measurement was performed by the PIs. Routine practice at 1 site involved the measurement of waveforms with the freeze-cursor function. Therefore, additional data were obtained by a PI or data collector when actual observance of the bedside nurse’s measurement was made. The graphic recording was done simultaneously with the nurse’s cursor measurement. Cursor measurements were
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written on the graphic recording for comparison with the graphic measurements.
Interrater reliability Interrater reliability of the PIs was verified through a preliminary waveform analysis conducted before the study. The 2 PIs, both clinical nurse specialists with more than 15 years of experience in hemodynamic monitoring, interpreted 30 sample waveforms. The waveform sample consisted of a total of 36 graphic recordings, including abnormal (6 waves, 3 giant v waves, and 3 RV waveforms) and waveform recordings from each respiratory variation pattern (30 waves, 6 per category) seen in the study. Independent of each other, each PI measured the waveforms at end expiration with the graphic measurement method. The point of end expiration measurement was identified as the point just before a spontaneous or ventilator breath produced a negative or positive change in the waveform baseline.18 The independent measurements were performed, documented, and then compared. The interrater reliability results indicated 100% agreement on graphic measurements between the PIs. Any unclear waveform identified during the remainder of the graphic measurements by either investigator resulted in an analysis of both investigators’ measurements of that waveform and acceptance of an agreed-upon value.
Categorization of waveforms Waveforms were placed in 5 respiratory categories, plus 1 category for abnormal waveforms. Respiratory groupings were as follows: 1. Spontaneous breathing with a total respiratory rate of less than 20 breaths per minute (SB < 20 breaths/minute) 2. Spontaneous breathing with a total respira-
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Table II t test values comparing digital and graphic measurements in various breathing categories* Respiratory category
SB < 20 PASd PASg PADd PADg CVPd CVPg SB > 20 PASd PASg PADd PADg CVPd CVPg VB only PASd PASg PADd PADg CVPd CVPg VB + SB < 20 PASd PASg PADd PADg CVPd CVPg VB + SB > 20 PASd PASg PADd PADg CVPd CVPg
Combined data
Hewlett Packard
SpaceLabs
Marquette
40.15 39.57 20.51 21.35 11.94 12.44
± ± ± ± ± ±
13.90 14.50 7.68 9.04 4.87 5.48†
5.0 42.25 22.63 24.48 13.03 13.77
± ± ± ± ± ±
15.85 17.15 8.63 10.49† 5.59 6.43†
40.18 38.30 20.70 18.88 11.69 11.75
± ± ± ± ± ±
11.67 10.04† 6.48 5.82† 4.42 4.61
32.18 31.4 18.24 17.88 10.27 11.64
± ± ± ± ± ±
7.70 8.57 3.43 4.86 3.50 3.17†
41.13 41.86 20.00 22.38 11.04 12.31
± ± ± ± ± ±
15.36 16.66 9.77 10.76† 7.25 8.05†
47.15 47.72 22.49 23.85 11.20 12.55
± ± ± ± ± ±
16.19 16.76 9.06 8.96† 7.36 7.13†
36.59 35.04 20.70 16.39 10.87 9.89
± ± ± ± ± ±
14.45 12.84† 7.97 7.79† 5.21 4.96†
32.60 32.20 17.72 18.40 12.37 11.87
± ± ± ± ± ±
7.57 7.45 5.14 5.12 6.32 5.91
36.90 36.87 19.90 19.10 12.41 12.10
± ± ± ± ± ±
13.74 13.62 7.22 6.89† 5.38 5.33
42.05 42.23 22.73 21.34 12.65 12.03
± ± ± ± ± ±
16.33 16.79 7.61 7.60† 5.52 5.06†
36.42 36.00 19.71 19.21 11.48 12.31
± ± ± ± ± ±
13.36 12.29 6.62 6.45 4.55 4.76†
31.46 31.48 17.75 16.85 13.05 14.17
± ± ± ± ± ±
8.23 8.19 6.59 6.07† 7.24 6.81†
39.84 39.81 20.24 20.00 10.93 11.47
± ± ± ± ± ±
14.73 14.79 7.97 8.34 5.95 5.97†
41.92 44.65 19.00 25.77 11.11 14.65
± ± ± ± ± ±
16.94 18.94† 11.37 11.40† 8.28 8.84†
39.23 36.83 21.98 16.05 10.71 9.06
± ± ± ± ± ±
10.85 9.58† 6.96 5.94† 5.58 6.14†
36.72 35.17 19.22 18.67 9.85 10.54
± ± ± ± ± ±
9.32 9.38 5.92 6.33 5.54 5.49
45.03 45.03 22.81 24.51 13.06 14.25
± ± ± ± ± ±
15.42 15.88 9.22 10.63 4.99 7.00†
43.07 43.05 20.90 25.98 13.26 15.35
± ± ± ± ± ±
16.73 17.85 9.11 11.52† 5.87 8.74
47.14 47.18 25.93 22.82 12.96 13.32
± ± ± ± ± ±
11.57 10.95 9.35 9.87† 4.20 5.29
41.75 42.00 19.00 19.75 12.25 12.50
± ± ± ± ± ±
14.08 16.89 6.73 5.43 3.50 3.77
PASd = Pulmonary artery systolic digital; PASg, pulmonary artery systolic graphic; PASc, pulmonary artery systolic cursor; PADd, pulmonary artery diastolic digital; PADg, pulmonary artery diastolic graphic; PADc, pulmonary artery diastolic cursor; CVPd, central venous pressure digital; CVPg, central venous pressure graphic; CVPc, central venous pressure cursor. *Values are means ± SD. †P ≤ .05.
tory rate of more than 20 breaths per minute (SB > 20 breaths/minute) 3. Mechanical ventilation without spontaneous breathing (VB only) 4. Mechanical ventilation with spontaneous breathing with a total respiratory rate of less than 20 breaths per minute (VB + SB < 20 breaths/minute)
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5. Mechanical ventilation with spontaneous breathing with a total respiratory rate of more than 20 breaths per minute (VB + SB > 20 breaths/minute) Abnormal waveforms (including RV waveforms and abnormal a and v waves) were grouped into a separate category for all respiratory rates and modes.
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Mennen
Comparison of pulmonary artery
Cursor readings
34.06 35.38 15.94 17.48 10.18 11.24
± ± ± ± ± ±
8.30 9.49† 5.12 5.91† 3.74 4.14†
PASc: 38.48 PASg: 38.56 PADc: 7.92 PADg: 19.12 CVPc: 11.42 CVPg: 11.54
± ± ± ± ± ±
10.72 11.57 4.80 5.59 4.23 4.16
35.04 36.79 16.25 18.83 9.96 11.56
± ± ± ± ± ±
8.18 8.74† 4.95 6.63† 3.80 4.27†
PASc: 37.88 PASg: 39.66 PADc: 19.16 PADg: 22.03 CVPc: 9.61 CVPg: 10.54
± ± ± ± ± ±
11.76 11.31† 6.92 7.03† 5.62 5.92†
32.86 33.41 15.78 15.78 11.16 10.46
± ± ± ± ± ±
6.71 6.79 4.81 4.31 3.42 3.46†
PASc: 35.89 PASg: 35.96 PADc: 18.71 PADg: 19.67 CVPc: 12.31 CVPg: 11.60
± ± ± ± ± ±
13.90 14.30 5.73 6.79† 4.42 4.38†
51.87 51.63 21.25 25.75 11.38 11.63
± ± ± ± ± ±
23.99 22.32 10.42 14.92 8.33 8.60
PASc: 34.92 PASg: 35.79 PADc: 17.71 PADg: 20.37 CVPc: 10.03 CVPg: 10.38
± ± ± ± ± ±
13.49 14.26 7.15 8.10† 4.02 4.28
53.50 53.25 24.25 26.00 12.75 14.25
± ± ± ± ± ±
26.19 23.96 7.14 9.80 5.44 5.19†
PASc: 47.29 PASg: 49.71 PADc: 22.38 PADg: 26.86 CVPc: 12.29 CVPg: 13.19
± ± ± ± ± ±
10.31 11.65 4.83 10.52† 4.52 4.59
Establishing technical accuracy of waveform values All readings were obtained after following institutional practice for leveling and zeroing and for dynamic response assessment. Institutional practices were consistent with each other. The stopcock above the transducers was leveled and
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zeroed to the phlebostatic axis. Assessment for dynamic response was then performed. Per institutional policy, if an optimal square waveform was not obtained, the nurse corrected the tubing/catheter system until one was obtained. Recording of the square waveform was not performed. The graphic recording contained the measurement scale that illustrated proper calibration of the printed waveform.
Sample Intended total sample size was 750 patients. The sample was designed to collect patients in adult intensive care units with a variety of clinical diagnoses. The 750-patient sample size was based on a power of 0.8 (assuming an α coefficient of .05 and expected difference between graphic and digital measurements of 10%). The 4 monitoring companies who conducted subanalyses in this study included the following: Hewlett Packard (HP), Andover, Mass; Spacelabs (SL) Redmond, Wash; Marquette, Milwaukee, Wis; and Mennen (MN) Clarence, NY. Because of the number of intensive care unit beds in the participating institutions, HP contributed more of the waveforms (40%) than did SL (30%), Marquette (20%), or MN (10%).
Statistical analysis A 2-tailed t test compared the differences between the monitor’s digital measurement and graphic measurement values. The population was anticipated to have normal distribution (ie, the monitor can read higher or lower than the actual value), with the frequency of large deviations from the actual value assumed to be uncommon. A second analysis with Wilcoxon matched pairs signedrank test determined how often the digital and graphic measurements differed. The use of the t test (parametric assumes normality of data) and the Wilcoxon matched pairs signed-rank test (nonparametric does not assume normality of data) helped to determine the assumption of normality of data. If responses were similar between the 2 testing methods, the concept of normality of distribution was supported. The percentage of times the digital and graphic measurements differed by more than 4 mm Hg was determined. A variation of 4 mm Hg is the amount that previous research has shown to represent clinical change rather than normal fluctuation.19,20 Therefore, a difference between digital and graphic measurements of greater than 4 mm Hg was defined as clinically significant.
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Table III Wilcoxon frequency of differences between digital measurement and graphic measurement* Respiratory category
SB < 20 PAS PAD CVP SB > 20 PAS PAD CVP VB only PAS PAD CVP VB + SB < 20 PAS PAD CVP VB + SB > 20 PAS PAD CVP RV wave PAS PAD CVP Abnormal CVP
Totals: Wilcoxon ranks lower
Totals: Wilcoxon ranks higher
Hewlett Packard lower/higher
SpaceLabs lower/higher
80 75 47†
61 78 61
40/32 36/48 18/32†
20/8† 23/7† 13/11
56 50 43
79† 88† 56†
16/36† 13/43† 8/31†
25/8† 42/4† 26/5†
96 138† 92†
89 66 56
37/30 56/17† 34/14†
26/28 36/25 12/35†
62 64 36†
65 74 60
15/61† 3/75† 9/43†
27/9† 38†/0 24/6†
33 37 21
31 34 33
17/17 12/27† 7/17†
2/1 1/3 0/4
44†/6 45†/11
16†/5 18†/4
5/16
3/5
SB, Spontaneous breathing; VB, ventilator breathing; PAS, pulmonary artery sytolic; PAD, pulmonary artery diastolic; CVP, central venous pressure. *Ranks lower, graphic measurement is lower than digital or cursor measurement; ranks higher, graphic measurement is higher than digital or cursor measurement. †Significant at .05 or less.
RESULTS Sample description During the measurement period, 928 patients were enrolled in the study and 965 measurements were obtained. All patients were from adult critical care units with surgical, cardiac-surgical, cardiacmedical, or general-medical admitting diagnoses. The total sample size and the sample size of each of the respiratory categories for each manufacturer are presented in Table I. The VB-only category comprised the largest group. This group includes early postoperative surgical patients and sedated (or calm) medical and surgical patients. The smallest group was the VB +
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SB > 20 breaths/minute. This group of patients is smaller because they are likely to be the most acutely ill of all ventilator patients.
Digital and graphic differences by respiratory category The first and second questions were concerned with the nature of any differences between PA and CVP values obtained through digital versus graphic measurement and whether respiratory rate or mode of mechanical ventilation affect the accuracy of PA pressure and CVP measurements obtained through digital versus graphic measurement. The 2tailed t test’s results for the combined measurements indicated that the PAD and CVP values mea-
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Cursor readings lower/higher
Marquette lower/higher
Mennen lower/higher
10/3 7/4 0/11†
10/18† 9/19† 5/18†
9/12 8/14 5/7
14/8 5/11 9/2
7/13† 4/16† 0/15†
10/17 5/23† 4/16†
25/21 33/12† 39†/0
11/17 20/14 34†/0
17/21 14/29† 21/7†
13/4 10/7 3/8
3/5 1/6 3/3
11/21† 7/27† 12/15
14/8 5/11 9/2
2/2 1/2 2/2
5/11 2/16† 3/12
8†/0 9†/2
13†/6 21†/7
0/3
2/3
sured by graphic analysis differed significantly from the digital measurements most often (Table II). PAD was statistically different for 2 respiratory categories, SB > 20 breaths/minute (P = .000) and VB only (P = .000). The difference in the CVP measurements was statistically significant in 4 of the 5 respiratory categories: SB < 20 breaths/minute (P = .010), SB > 20 breaths/minute (P = .003), VB + SB < 20 breaths/minute (P = .003), VB + SB > 20 breaths/minute (P = .040). PAS was not significantly different in any respiratory category (Table II). When the different monitoring company measurements were analyzed individually, significant differences between digital and graphic measurements were found in most of the breathing cate-
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gories for 1 of the measurement points—either PAS, PAD, or CVP (Table II). However, some of the respiratory groupings per individual monitoring company had a small sample size that limited statistical power. As demonstrated by the combined data, the digital measurements from the various types of monitors fluctuated around the graphically measured values. The Wilcoxon matched pairs signed-rank test indicated that in the majority of cases the digital measurement value differed from the graphic measurement (Table III). This difference occurred in all respiratory groupings. Lower differences are defined as measurements with a graphic measurement lower than the digital or cursor measurement, and higher differences are defined as measurements with a graphic measurement higher than the digital or cursor measurement. Measurements with the exact same value between digital and graphic methods are not reported in Table III. Statistically significant differences were found in several respiratory categories, and similar to the 2-tailed t test results, the differences were most often found in the PAD and CVP measurements (Table III). Significantly different measurements were found for PAS in SB > 20 breaths/minute (P = .044). For PAD, significant differences were found in SB > 20 breaths/minute (P = .001) and VB only (P = .000). Statistically significant differences were found for the CVP in the following categories: SB < 20 breaths/minute (P = .035), SB > 20 breaths/minute (P = .013), VB only (P = .009), and VB + SB < 20 breaths/minute (P = .003). The frequency with which the digital measurement differed by more than 4 mm Hg from the graphic measurement ranged from 1% to 46% (Table IV). For the combined data, PAD measurements had the highest frequency of clinically significant differences between the 2 measurement techniques. Again, the frequency of difference was greatest with the fast respiratory rate categories, with difference rates of 46% and 34% respectively for graphic measurements being higher than digital measurement for the SB > 20 breaths/minute and VB + SB > 20 breaths/minute categories. All monitoring companies had clinically significant differences in the various categories, most commonly occurring during the rapid breathing events. The clinically and statistically significant differences between the cursor and graphic methods occurred in similar circumstances. Statistically significant differences for the PAD measurements (P = .000–.030) were found in all of the respiratory categories except SB < 20 breaths/minute (Table II).
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Table IV Descriptive statistics on the frequency percentage when digital measurement differed from graphic measurement by >4 mm Hg* Grouping
Combined lower/higher (%)
SB < 20 PAS PAD CVP SB > 20 PAS PAD CVP VB only PAS PAD CVP VB + SB < 20 PAS PAD CVP VB + SB > 20 PAS PAD CVP Abnormal waves RV waves
Hewlett Packard lower/higher (%)
SpaceLabs lower/higher (%)
Marquette lower/higher (%)
2/10 1/20 1/5
4/9 4/9 0/6
8/8 0/40 0/5
4/4 0/4 7/0
7/22 1/46 4/21
8/25 0/53 4/35
0/18 0/55 6/12
20/5 10/10 0/0
3/5 7/4 2/1
7/5 14/5 1/2
1/6 5/7 3/0
2/4 2/0 0/2
12/11 5/19 1/3
16/8 7/22 0/6
0/18 3/21 3/0
6/6 0/6 0/0
12/13 5/34 6/6
15/15 5/41 3/10 5/3 2/21
7/11 7/29 11/0 2/1 3/15
15/15 0/15 15/0 3/2 1/16
SB, Spontaneous breathing; VB, ventilator breathing; PAS, pulmonary artery systolic; PAD, pulmonary artery diastolic; CVP, central venous pressure; RV, right ventricular. *Lower, graphic measurement >4 mm Hg lower than digital measurement; higher, graphic measurement >4 mm Hg higher than digital measurement.
respectively in the VB + SB > 20 breaths/minute category.
Table V Abnormal waveforms Waves
Abnormal a and v waves RV Waves
Sample size
Significance
n = 54
NS
n = 99
PAS = NS PAD = 0.00
NS, Not significant; PAS, pulmonary artery systolic; PAD, pulmonary artery diastolic.
The CVP measurements were significantly different in the SB > 20 breaths/minute (P = .006) and VB only (P = .002). Clinically significant differences occurred most frequently with the faster respiratory rates, with 25% occurrence for both PAS and PAD in the SB > 20 breaths/minute category and 14% and 19%
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Abnormal and RV waveforms The third research question asked whether there are differences between the CVP waveform measurements obtained through digital measurement and those obtained through graphic measurement when abnormal or RV waveforms are present. Waveforms from this category were found incidentally. Data collectors recorded abnormal and RV waveforms any time they were found. Fifty-four abnormal waveforms and 99 RV waveforms were recorded. With these 2 groupings, further breakdown by respiratory category was not done. The study found that digital measurements of abnormal waveforms did not differ significantly from graphically measured waveforms. RV waveforms were significantly different in diastolic values (P = .000) but not systolic values (Table V). However, clinical-
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Cursor readings lower/higher (%)
0/16 0/25 0/12
3/5 3/21 0/0
25/25 0/37 0/0
6/25 0/25 0/4
0/5 0/5 3/0
2/2 0/9 2/0
18/18 0/9 0/0
8/8 4/12 0/0
0/0 0/25 0/0 1/0 0/7
5/14 0/19 0/5
ly significant measurement differences occurred with both the abnormal waveforms and the RV waveforms with all monitors (Table IV).
DISCUSSION The answers to the 3 research questions are consistent with previous research that indicates that digital display measurements have limited accuracy. In several circumstances, the differences between digital and graphic measurement were of clinical importance. For at least 1 value (PAS, PAD, or CVP) in each respiratory category, a statistically significant difference between the digital measurement and graphic measurement values was present (Table II). The Wilcoxon matched pairs signed-rank tests (Table III) indicated that the majority of the monitor’s digital measurements did not display the same value as did the graphic measurement. Clinically and statistically significant differences between digital and graphic measurements were found for most of the respirato-
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ry and abnormal waveform categories for each of the different types of monitors. The similarity of results from both the t test and the Wilcoxon matched pairs signed-rank tests supported the concept of normality of distribution of the monitor readings. Although digital measurements were the same as the graphic measurements on occasion, obtaining exactly the same measurement through both techniques occurred inconsistently. Although the lack of precise agreement was frequently statistically significant, some might argue that the difference was not clinically important. However, as seen in Table IV, the frequency of differences between the digital and graphic measurements that were greater than or equal to 4 mm Hg was disturbingly high. The potential for clinical errors resulting from this difference may be important. For example, if the monitor measured the CVP as 3 mm Hg in conjunction with a low stroke volume, a clinician might treat the patient for hypovolemia. Yet the actual value from a graphic measurement was 8 mm Hg (or higher), which represents possible ventricular dysfunction. Drawing the conclusion of hypovolemia rather than ventricular dysfunction would likely result in different treatment. The lack of agreement between the digital measurements and the graphic measurements ranged from as small as 1 mm Hg to as great as 40 mm Hg. To obtain accurate measurements, it is clear that the clinician must confirm the digital measurement value by waveform analysis of a graphic measurement before assuming that the value can be accepted as accurate. The digital data were erroneous in all respiratory categories; however, PAD and CVP digital measurements in the faster respiratory rate categories had the highest frequency of clinically and statistically significant differences. Our results support the findings of Dobbin et al5 and Cengiz et al6 regarding the influence of spontaneous respiration on PAD pressure measurements. Dobbin et al found a significant difference between PAD and PAWP values obtained through digital monitor values and those obtained through a graphic recording during spontaneous breathing. In our study with the combined data, we found that patients breathing spontaneously with a total respiratory rate exceeding 20 breaths per minute had PAD values with statistically significant differences between digital and graphic measurements. In addition, both HP and SL showed statistically significant PAD differences in SB > 20 and VB with SB > 20, whereas MN showed statistically significant PAD differences in SB > 20.
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Comparison of pulmonary artery
The inaccuracy of digital measurements of PA and CVP pressures with rapid breathing patterns was one of the more important findings of this study. Monitor programs filter potential artifact through mathematical algorithms. As previously explained, these filtering systems are, by nature, not precise, because they allow for inclusion of values that could be artifact. The filters are prone to have more difficulty with substantial, frequent artifact than with infrequent artifact. The results of our study contrast those of Johnson and Schuman7 and Lundstedt,6 who found no significant difference between the PAD values for the spontaneously breathing patient obtained through digital measurement and those obtained through graphic measurement. Variation in systolic readings was found in Lundstedt’s study, and in that of Johnson and Schumann. The smaller sample size and limited respiratory rates may account for the difference in the findings. Johnson and Schumann studied 19 spontaneously breathing patients who had respiratory rates ranging from 10 to 28 breaths per minute with an average of 17 breaths per minute. Lundstedt’s sample size for spontaneously breathing patients was 37, with respiratory rates ranging from 10 to 40 breaths per minute; however, 76% of his patients had respiratory rates between 10 and 20 breaths per minute. Therefore, patients with higher respiratory rates had less representation. Monitors studied may also account for the discrepancy. Johnson and Schumann used HP monitors and Lundstedt used Siemens monitors. Like Johnson and Schumann,7 we found significant differences among the PAD and CVP readings in mechanically ventilated patients. However, Dobbin et al8 found no significant difference between the digital display and the graphic recording in mechanically ventilated patients. A limitation of both of these studies is that discussion of the patient’s respiratory effort while being mechanically ventilated is lacking. Therefore, accurate comparison of the findings is difficult to accomplish. In addition, maximum respiratory rate for both studies was 18 breaths per minute, and this study had a separate category for patients being mechanically ventilated with spontaneous effort breathing at >20 breaths/minute. Cengiz et al6 found significant differences between automated measurements and graphic analysis in spontaneously breathing and mechanically ventilated patients for PAS, PAD, and PAWP. The greatest differences occurred in spontaneously breathing patients. They state that the error was not affected by spontaneous or assisted respirato-
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ry rate. However, in their study discussion, they present an excellent case study to demonstrate the effect of a rapid respiratory rate on digital versus graphic measurements. The case involves an intubated patient breathing spontaneously at a rate of 40 breaths per minute. The patient had a digital PAWP value of 4 to 8 mm Hg and a graphic measurement of 22 mm Hg. They report that pulmonary edema later developed in the patient. Our findings support the impact of high respiratory rates on accuracy of digital measurements as their case study so clearly exemplified at the clinical level. This study found that measurements in all respiratory categories had statistically significant variation as well as clinically important variation. Clinicians cannot assume reliability of their PA digital measurement with any patient. Clinicians must validate the digital measurement with the graphic measurement. This validation is necessary with any change in the patient’s respiratory pattern. For the future, monitoring companies need to upgrade monitors to better identify physiologic principles, such as improved identification of end expiration, avoidance of artifact, and use of the electrocardiogram to identify specific waves. Bedside monitors need to be better able to filter respiratory artifact in patients with rapid spontaneous effort. Filtering respiratory artifact can be accomplished by timing the filtering to respiratory measurements, such as airway pressure or capnography waveforms. Currently, digital filtering programs do not possess the ability to interpret abnormal waveforms such as a giant a or v wave or an RV waveform. The digital measurement has the potential to substantially mislead the clinician if abnormal or RV waveforms are present. For example, the monitor might measure a large v wave of the CVP or PAWP instead of the pre–c wave or a wave mean. This measurement method can result in artificially high readings. In addition, most digital displays have an option called the “cursor” or “wedge” feature. This feature allows the clinician to measure the waveform on the monitor. The monitor wedge algorithm signals to the clinician that “wedging” has occurred when it detects a change in waveform. The monitor, therefore, might not indicate to a clinician that the catheter is wedging when a giant v wave is present. This error occurs because the giant v wave is similar in appearance to a PA waveform (Fig 2). The clinician might then overinflate the balloon (potentially leading to PA rupture), advance the catheter unnecessarily far into the PA, or disregard valuable clinical measurements. Marquette Medical cautions that in patients with valvular disease or respi-
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ratory variation, a wedge may not be detected with the Marquette wedge algorithm and recommends use of the manual method for wedge measurement.17 Although there was not a statistically significant difference between values in the abnormal waveform group, both the Wilcoxon matched pairs signed-rank tests and the incidence of clinical differences of >4 mm Hg support the use of graphic measurement rather than digital measurement. The lack of a statistically significant difference may be a result of the small sample size in this category. Ninety-nine RV waveforms were observed and recorded. The RV waveforms were obtained from right atrial ports inappropriately positioned in the RV or from the PA distal ports pulled back into the RV. Before the data collectors’ observation of RV placement, the bedside nurses were not aware that the measurement was actually an RV waveform. The monitor frequently underrepresented diastole with the RV waveforms (Table V). End diastole on an RV waveform is not the lowest point as the digital display indicates, but rather the point immediately before systole. Perhaps more important than the display of an incorrect value is the possibility of the monitor failing to warn the clinician that an RV waveform might represent malposition of the catheter. In this study, advanced practice clinical nurse specialists interpreted the graphic measurements. It is important to keep in mind that graphic measurements can also be misinterpreted if waveform analysis skills are not well developed. Education for improving the clinician’s ability to measure graphic recordings is needed to ensure accurate waveform analysis. Previous research supports this statement. The findings of Burns et al1 and Iberti et al2 in studies on critical care nurses’ knowledge of the PA catheter regarding waveform analysis warn of the importance of being able to measure waveforms graphically rather than relying on the digital display. Burns et al found that only 61% of nurses in their study were able to correctly measure a PAWP tracing with variations of as much as 10 mm Hg. Iberti et al report similar findings with only 57.7% of nurses in their study correctly measuring a PAWP tracing. In addition, Lundstedt6 found the incidence of difference between cursor measurements and graphic measurements to be PAS at 53% and PAD at 12% in spontaneously breathing patients and PAS at 36% and PAD at 6% in mechanically ventilated patients. Our results also point to the inaccuracies of cursor measurements, especially with patients breathing at fast respiratory rates.
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Comparison of pulmonary artery
Prior research has consistently suggested the potential inaccuracy of PA catheter measurements obtained by some critical care nurses. Because our findings demonstrate the inaccuracies of cursor measurements and the need for accurate analysis of waveforms through the graphic measurement method before reliance on digital measurement values, what can be done about critical care clinicians’ practice of waveform interpretation? Iberti recommended PA catheter credentialing to ensure the safe use of the catheter. As a result of Lundstedt’s study, the study institution now credentials clinicians yearly in the use of cursor and graphic measurements. Our results support the need for further education regarding the interpretation of hemodynamic waveforms and for possible credentialing.
CONCLUSION Use of graphic recordings as the gold standard must continue despite improvements in digital engineering. Clearly, errors occur with all monitors’ digital measurements in all respiratory and abnormal waveform patterns. Monitor program design excludes correct measurement of waveforms such as RV or abnormal venous waves. Some of the digital measurement errors are not clinically important. However, when the errors are clinically important, the only protection for the patient is the bedside clinician. It is the responsibility of the bedside clinician to verify the digital measurement with each patient by use of a graphic measurement before relying on the digital measurement. In addition, these measurements must be reverified with the occurrence of any change in respiratory pattern. Consistently accurate PA waveform measurements are of paramount importance in tending the patient’s hemodynamic status and response to therapy. To optimize the impact on patient outcomes with hemodynamic monitoring, all clinicians caring for patients with invasive monitoring must be competent at measuring hemodynamic waveforms. Proper leveling, zeroing, and dynamic response (square waveform testing) assessment are the first crucial steps. Correct measurement at end expiration with various respiratory patterns is the next step. Finally, these measurements must be integrated with other hemodynamic indexes for treatment decisions. Hemodynamic indexes such as the stroke index mixed venous oxygen saturation, the cardiac index, and clinical assessments are useful supplements to pressure measurements. A combination of these indexes, rather than a single index
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Comparison of pulmonary artery
or pressure measurement, will more likely lead to accurate evaluation of the patient’s response to therapy.
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10. 11. 12.
REFERENCES 1. Burns D, Burns D, Shively M. Critical care nurses’ knowledge of pulmonary artery catheters. Am J Crit Care 1996;5:49-54. 2. Iberti TJ, Daily EK, Leibowitz AB, Schecter CB, Fischer EP, Silverstein JH. Assessment of critical care nurses’ knowledge of the pulmonary artery catheter. The pulmonary artery catheter study group. Crit Care Med 1994;22:1674-8. 3. Iberti TJ, Fischer EP, Leibowitz AB, Panacek EA, Silverstein JH, Albertson TE. A multicenter study of physicians’ knowledge of the pulmonary artery catheter. JAMA 1990;264:2928-32. 4. Gnaegi A, Feihl F, Perret C. Intensive care physicians’ insufficient knowledge of right heart catheterization at the bedside: time to act? Crit Care Med 1997;25:213-20. 5. Trottier SJ, Taylor RW. Physicians’ attitudes toward and knowledge of the pulmonary artery catheter: Society of Critical Care Medicine Membership Survey. New Horiz 1997;5:201-6. 6. Lundstedt JL. Comparison of methods of measuring pulmonary artery pressure. Am J Crit Care 1997;6:324-32. 7. Johnson MK, Schuman L. Comparison of three methods of measurement of pulmonary artery catheter readings in critically ill patient. Am J Crit Care 1995;4:300-7. 8. Dobbin K, Wallace S, Ahlberg J, Chulay M. Pulmonary artery pressure measurement in patients in with elevated pressures: effect of backrest elevation and method of measurement. Am J Crit Care 1992;1:61-9. 9. Cengiz M, Crapo RO, Gardner RM. The effect of ventilation on
13.
14. 15.
16. 17. 18. 19. 20.
the accuracy of pulmonary artery and wedge pressure measurement. Crit Care Med 1983;11:502-7. Gershan JA. Effect of positive end-expiratory pressure on pulmonary capillary wedge pressure. Heart Lung 1983;12:143-5. Maran AG. Variables in pulmonary capillary wedge pressure: variation with intrathoracic pressure, graphic and digital recorders. Crit Care Med 1980;8:102-5. Connors AF Jr, Speroff T, Dawson NV, Thomas C, Harrell FE Jr, Wagner D, et al. The effectiveness of right heart catheterization in the initial care of critically ill patients. JAMA 1996;276:889-97. Gore JM, Goldberg RJ, Spodick DH, Alpert JS, Dalen JE. A community-wide assessment of the use of pulmonary artery catheters in patients with acute myocardial infarction. Chest 1987;92:721-7. Robin ED. Overuse and abuse of Swan-Ganz catheters. Int J Clin Monit Comput 1987;4:5-9. Tuman KJ, McCarthy RJ, Spiess BD, DaValle M, Hompland SJ, Dabir R, et al. Effects of pulmonary artery catheterization on outcome in patients undergoing coronary artery surgery. Anesthesiology 1989;70:199-206. Hewlett Packard. An algorithm for reduction of respiration artifact in pulmonary artery pressure measurements. Andover: Hewlett Packard; 1989. Solar 7000/8000 patient monitor operator’s manual. Milwaukee (WI): Marquette Medical Systems, Inc; 1997. Ahrens TS, Taylor L. Hemodynamic waveform analysis. Philadelphia: WB Saunders; 1992. Eisenberg PR, Jaffe AS, Schuster DP. Clinical evaluation compared to pulmonary artery catherization in the hemodynamic assessment of critically ill patients. Crit Care Med 1984;12:549-53. Nemens EJ, Woods SL. Normal fluctuations in pulmonary artery and pulmonary capillary wedge pressures in acutely ill patients. Heart Lung 1982;11:393-8.
AVAILABILITY OF JOURNAL BACK ISSUES As a service to our subscribers, copies of back issues of Heart & Lung: The Journal of Acute and Critical Care for the preceding 5 years are maintained and are available for purchase from Mosby until inventory is depleted. The following quantity discounts are available: 25% off on quantities of 12 to 23, and one-third off on quantities of 24 or more. Please write to Mosby, Subscription Customer Service, 6277 Sea Harbor Dr, Orlando, FL 32887, or call (800)654-2452 or (407)345-4000 for information on availability of particular issues and prices. If unavailable from the publisher, photocopies of complete issues may be purchased from Bell & Howell Information and Learning, 300 N. Zeeb Rd., Ann Arbor, MI 48106, (734)761-4700 or (800)521-0600.
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BACKGROUND: Techniques to measure pulmonary artery (PA) pressure waveforms include digital measurement, graphic measurement, and freeze-cursor measurement. Previous studies reported the inaccuracy of digital and freeze-cursor measurements. However, many of the previous studies were small and did not thoroughly examine the circumstances of when digital measurements might be inaccurate. OBJECTIVES: To compare digital measurements and graphic measurements of PA and central venous pressure (CVP) waveforms in patients with a variety of respiratory patterns, and to compare digital measurements and graphic measurements of CVPs in patients with abnormal or right ventricular waveforms. METHODS: A total of 928 patients were enrolled in this study. Waveforms from the PA and CVP were collected from each patient. The monitor pressure value (digital measurement) printed on the recorded waveform was compared with the pressure value obtained by a graphic strip recording and measured by one of the primary investigators (graphic measurement). RESULTS: Digital measurements were found to be inaccurate in measuring waveforms in all respiratory categories and in measuring right ventricular waveforms. PA diastolic values and CVP values were the most inaccurately measured waveforms. Digital errors of more than 4 mm Hg were common. CONCLUSION: There were instances in which the monitor’s digital measurement was substantially different from the graphically measured value. This difference has the potential to mislead interpretation of clinical situations. The monitor’s ability to occasionally give digital measurement values similar to the graphic measurements may lead to a false sense of security in clinicians. Because the accuracy of the monitor is inconsistent, the bedside clinician should interpret waveforms through use of a graphic recording rather than rely on the digital measurement on the monitor. (Heart Lung® 2001;30:26-38.)
C
ritical care clinicians routinely measure pulmonary artery (PA) pressure, central venous pressure (CVP), and pulmonary artery wedge pressure (PAWP) to assess and optimize the hemodynamic status of their patients. Evidence suggests From the critical care units of Barnes-Jewish Hospital, St Louis, Missouri. Supported by the St Louis chapter of AACN, Marquette Electronics, and Mennen Medical. Reprint requests: Thomas S. Ahrens, DNS, RN, CCRN, CS, Nursing Service Barnes-Jewish Hospital, One Barnes Hospital Plaza, St Louis, MO 63110. Copyright © 2001 by Mosby, Inc. 0147-9563/2001/$35.00 + 0 2/1/112504 doi:10.1067/mhl.2001.112504
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that nurses and physicians may not measure and interpret these pressures accurately.1-5 Previous research has indicated that the bedside monitor is inaccurate in its measurement of PA pressure waveforms.6-11 Although these studies provided information on monitor accuracy, they were subject to methodologic limitations. For example, sample sizes in some of these studies were small, ranging from 3 to 44 measurements.7-11 The studies with small sample sizes limit their generalization and power. In addition, delineation of various respiratory pattern effects on digital measurement accuracy has not been well studied. Because of the limitations of the past research, an understanding of bedside monitor accuracy in
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Fig 1 Reading a CVP waveform.
measuring PA and CVP pressure waveforms was incomplete. In the past decade, several studies have questioned the value of the PA catheter.12-15 Although no causal link between these studies and the accuracy of bedside monitors can be made, these studies reflect a problem associated with hemodynamic monitoring. Accuracy of digital measurements may be one part of the problem associated with the uncertain value of hemodynamic monitoring.
CURRENT ISSUES IN MEASURING PRESSURE VALUES The bedside clinician, (eg, nurse) can obtain pressure measurements through 1 of 3 methods: (1) reading waveform values directly from the monitor (digital measurement); (2) measuring the waveform directly from a printed copy of the wave (graphic measurement); and (3) using the freeze-cursor function on the bedside monitor. Based on the research that suggests that digital measurements may be inaccurate, the best options are the second and third methods. Bedside monitor programs use filtering techniques, such as variable weight beat averaging, to improve their accuracy in displaying digital measurements. In variable weight beat averaging, as described by Hewlett Packard,16 the monitor establishes a beat (or wave) value. To update the waveform value, the monitor uses a mathematical calculation designed to minimize variability (eg, artifact). In variable weight beat averaging, determination of the new pressure wave occurs in the following manner: The mean of the wave will be called N. The next wave mean (N + 1) is compared with N. The differ-
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ence between the 2 values (N – [N + 1]) is divided by a variable factor. The larger the difference between N and (N + 1), the greater the dividing factor becomes, to minimize the impact of the change. The change in the dividing factor is what inspired the term “variable weight beat averaging.” The result of this calculation is added to (or subtracted from) N to obtain a new beat. For example, assume the CVP value is 12 mm Hg. The next CVP value is measured by the monitor as 8 mm Hg. The monitor would subtract 8 from 12 and obtain 4 mm Hg. The 4 would be divided by a factor—in this case, 2—to give a value of 2. The 2 would be added to N to yield (N + 1) of 10 mm Hg. The value 10 mm Hg, rather than 8 mm Hg, would be displayed on the monitor. However, in previous studies, this filtering technique has not been shown to improve the monitor accuracy in all situations. In addition, the filtering techniques do not cover specific physiologic measurement concepts. For example, digital measurements are not the result of measuring specific waveforms (eg, a, c, or v waves on CVP or PAWP waveforms).16,17 Rather, engineering of the monitor programs makes possible the measurement of peak and trough values and computation of a geometric mean value. This inability to measure specific waves may limit the accuracy of the monitor’s digital measurements. For example, optimal measurements of a CVP or PAWP must be taken at a specific time in the cardiac cycle (when the tricuspid and mitral valves are open) to estimate ventricular end diastolic pressure. This end-diastolic point coincides with the pre–c-wave point or the mean of the a wave (Fig 1).18 The monitor’s method
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Fig 2 Large v-wave in a wedge waveform.
of averaging may or may not approximate the pre– c wave and mean of the a wave. Therefore, any atrial waveforms with variation caused by any number of reasons (eg, atrial fibrillation with loss of a waves, dysrhythmias with atrial-ventricular dissociation, or mitral/tricuspid valve dysfunction) are at risk for measurement error when the digital measurement technique is used (Fig 2). Another inherent limitation of the engineering of the bedside monitor is the interpretation of respiratory artifact. Because of filtering techniques like variable weight beat averaging, digital display measurements partially discard physiologic waves that are at variance from the “average,” or preceding, artifact value. Although this filtering technique may be helpful at times, in some circumstances it is actually detrimental. For example, the filtering program used by Marquette Medical during their automated wedge processing technique tracks the maximum and minimum 1-second averages.17 Once the wedge is complete, the filtering program computes the mean of the entire wedge sample, which is then compared with the maximum and minimum 1-second averages. If the overall mean is closer to the maximum 1-second average, the maximum 1-second average will be displayed as the PAWP value. If the overall mean is closer to the minimum 1-second average, the minimum 1-second average will be used for the PAWP value. This filtering program assumes an inspiratory:expiratory (I:E) ratio of less than 1:1 to determine end expiration. Therefore, the wedge algorithm may not work correctly when
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the I:E ratio is >1 or the patient is on synchronized intermittent mandatory ventilation and taking spontaneous breaths in addition to the set respiratory rate (personal communication, Marquette Medical Systems, Inc, November 1997).
STUDY QUESTIONS To consider the issue of the accuracy of bedside monitoring, the following questions are addressed. 1. What are the differences between those PA and CVP pressure waveform measurements obtained through digital measurement and those obtained through graphic measurement? 2. Does respiratory rate or the mode of mechanical ventilation affect the accuracy of PA and CVP waveform measurements obtained through digital or graphic measurements? 3. Are there differences between the CVP waveform measurements obtained through digital measurement and those obtained through graphic measurement when abnormal or right ventricular (RV) waveforms are present?
METHODS The study was conducted at 3 community and 2 university hospitals between 1995 and 1998. The Human Subjects Committee at Washington University approved an expedited review of the study.
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Table I Total sample size
Company
Total sample size
SB < 20 breaths/min
SB > 20/min
VB only
VB + SB < 20/min
VB + SB > 20/min
Hewlett Packard SpaceLabs Marquette Mennen Totals
442 253 155 115 965
88 33 27 24 172
61 46 25 24 156
86 76 52 37 251
88 38 17 11 154
41 28 4 4 77
Abnormal and RV waves
78 32 30 15 155
SB, Spontaneous breathing; VB, ventilator breathing.
Data collection involved obtaining a monitor strip from any patient with a PA catheter in place. Neither subjects’ names nor any other identifying characteristics were collected as part of the study.
Patient eligibility and data collection Any patient with a PA catheter in place at one of the study sites was eligible for the study. Selection of patients was made through convenience sampling. There were no exclusion criteria. Each patient had a recording of the pulmonary artery systolic (PAS) pressure, pulmonary artery diastolic (PAD) pressure, and CVP, with the numerical digital values of the pressures printed on the graphic recording. PAWP waveforms were not obtained because CVP and PAWP waveforms are both venous tracings. Digital and manual measurements of CVP and PAWP are identical. Both the CVP and PAWP are measured as means of the a wave or the pre–c-wave point.18 Therefore, additional balloon inflation for PAWP measurement was not necessary. If a patient’s condition changed so that the respiratory pattern changed, data collectors may have recorded additional waveforms with the new respiratory pattern. The principal investigators (PIs) and onsite coordinators recorded all the strips. Training of onsite coordinators included an overview of the study, the technique of graphic recording, and the process of documenting the patient’s respiratory rate and ventilator rate. Graphic measurement was performed by the PIs. Routine practice at 1 site involved the measurement of waveforms with the freeze-cursor function. Therefore, additional data were obtained by a PI or data collector when actual observance of the bedside nurse’s measurement was made. The graphic recording was done simultaneously with the nurse’s cursor measurement. Cursor measurements were
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written on the graphic recording for comparison with the graphic measurements.
Interrater reliability Interrater reliability of the PIs was verified through a preliminary waveform analysis conducted before the study. The 2 PIs, both clinical nurse specialists with more than 15 years of experience in hemodynamic monitoring, interpreted 30 sample waveforms. The waveform sample consisted of a total of 36 graphic recordings, including abnormal (6 waves, 3 giant v waves, and 3 RV waveforms) and waveform recordings from each respiratory variation pattern (30 waves, 6 per category) seen in the study. Independent of each other, each PI measured the waveforms at end expiration with the graphic measurement method. The point of end expiration measurement was identified as the point just before a spontaneous or ventilator breath produced a negative or positive change in the waveform baseline.18 The independent measurements were performed, documented, and then compared. The interrater reliability results indicated 100% agreement on graphic measurements between the PIs. Any unclear waveform identified during the remainder of the graphic measurements by either investigator resulted in an analysis of both investigators’ measurements of that waveform and acceptance of an agreed-upon value.
Categorization of waveforms Waveforms were placed in 5 respiratory categories, plus 1 category for abnormal waveforms. Respiratory groupings were as follows: 1. Spontaneous breathing with a total respiratory rate of less than 20 breaths per minute (SB < 20 breaths/minute) 2. Spontaneous breathing with a total respira-
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Table II t test values comparing digital and graphic measurements in various breathing categories* Respiratory category
SB < 20 PASd PASg PADd PADg CVPd CVPg SB > 20 PASd PASg PADd PADg CVPd CVPg VB only PASd PASg PADd PADg CVPd CVPg VB + SB < 20 PASd PASg PADd PADg CVPd CVPg VB + SB > 20 PASd PASg PADd PADg CVPd CVPg
Combined data
Hewlett Packard
SpaceLabs
Marquette
40.15 39.57 20.51 21.35 11.94 12.44
± ± ± ± ± ±
13.90 14.50 7.68 9.04 4.87 5.48†
5.0 42.25 22.63 24.48 13.03 13.77
± ± ± ± ± ±
15.85 17.15 8.63 10.49† 5.59 6.43†
40.18 38.30 20.70 18.88 11.69 11.75
± ± ± ± ± ±
11.67 10.04† 6.48 5.82† 4.42 4.61
32.18 31.4 18.24 17.88 10.27 11.64
± ± ± ± ± ±
7.70 8.57 3.43 4.86 3.50 3.17†
41.13 41.86 20.00 22.38 11.04 12.31
± ± ± ± ± ±
15.36 16.66 9.77 10.76† 7.25 8.05†
47.15 47.72 22.49 23.85 11.20 12.55
± ± ± ± ± ±
16.19 16.76 9.06 8.96† 7.36 7.13†
36.59 35.04 20.70 16.39 10.87 9.89
± ± ± ± ± ±
14.45 12.84† 7.97 7.79† 5.21 4.96†
32.60 32.20 17.72 18.40 12.37 11.87
± ± ± ± ± ±
7.57 7.45 5.14 5.12 6.32 5.91
36.90 36.87 19.90 19.10 12.41 12.10
± ± ± ± ± ±
13.74 13.62 7.22 6.89† 5.38 5.33
42.05 42.23 22.73 21.34 12.65 12.03
± ± ± ± ± ±
16.33 16.79 7.61 7.60† 5.52 5.06†
36.42 36.00 19.71 19.21 11.48 12.31
± ± ± ± ± ±
13.36 12.29 6.62 6.45 4.55 4.76†
31.46 31.48 17.75 16.85 13.05 14.17
± ± ± ± ± ±
8.23 8.19 6.59 6.07† 7.24 6.81†
39.84 39.81 20.24 20.00 10.93 11.47
± ± ± ± ± ±
14.73 14.79 7.97 8.34 5.95 5.97†
41.92 44.65 19.00 25.77 11.11 14.65
± ± ± ± ± ±
16.94 18.94† 11.37 11.40† 8.28 8.84†
39.23 36.83 21.98 16.05 10.71 9.06
± ± ± ± ± ±
10.85 9.58† 6.96 5.94† 5.58 6.14†
36.72 35.17 19.22 18.67 9.85 10.54
± ± ± ± ± ±
9.32 9.38 5.92 6.33 5.54 5.49
45.03 45.03 22.81 24.51 13.06 14.25
± ± ± ± ± ±
15.42 15.88 9.22 10.63 4.99 7.00†
43.07 43.05 20.90 25.98 13.26 15.35
± ± ± ± ± ±
16.73 17.85 9.11 11.52† 5.87 8.74
47.14 47.18 25.93 22.82 12.96 13.32
± ± ± ± ± ±
11.57 10.95 9.35 9.87† 4.20 5.29
41.75 42.00 19.00 19.75 12.25 12.50
± ± ± ± ± ±
14.08 16.89 6.73 5.43 3.50 3.77
PASd = Pulmonary artery systolic digital; PASg, pulmonary artery systolic graphic; PASc, pulmonary artery systolic cursor; PADd, pulmonary artery diastolic digital; PADg, pulmonary artery diastolic graphic; PADc, pulmonary artery diastolic cursor; CVPd, central venous pressure digital; CVPg, central venous pressure graphic; CVPc, central venous pressure cursor. *Values are means ± SD. †P ≤ .05.
tory rate of more than 20 breaths per minute (SB > 20 breaths/minute) 3. Mechanical ventilation without spontaneous breathing (VB only) 4. Mechanical ventilation with spontaneous breathing with a total respiratory rate of less than 20 breaths per minute (VB + SB < 20 breaths/minute)
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5. Mechanical ventilation with spontaneous breathing with a total respiratory rate of more than 20 breaths per minute (VB + SB > 20 breaths/minute) Abnormal waveforms (including RV waveforms and abnormal a and v waves) were grouped into a separate category for all respiratory rates and modes.
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Comparison of pulmonary artery
Cursor readings
34.06 35.38 15.94 17.48 10.18 11.24
± ± ± ± ± ±
8.30 9.49† 5.12 5.91† 3.74 4.14†
PASc: 38.48 PASg: 38.56 PADc: 7.92 PADg: 19.12 CVPc: 11.42 CVPg: 11.54
± ± ± ± ± ±
10.72 11.57 4.80 5.59 4.23 4.16
35.04 36.79 16.25 18.83 9.96 11.56
± ± ± ± ± ±
8.18 8.74† 4.95 6.63† 3.80 4.27†
PASc: 37.88 PASg: 39.66 PADc: 19.16 PADg: 22.03 CVPc: 9.61 CVPg: 10.54
± ± ± ± ± ±
11.76 11.31† 6.92 7.03† 5.62 5.92†
32.86 33.41 15.78 15.78 11.16 10.46
± ± ± ± ± ±
6.71 6.79 4.81 4.31 3.42 3.46†
PASc: 35.89 PASg: 35.96 PADc: 18.71 PADg: 19.67 CVPc: 12.31 CVPg: 11.60
± ± ± ± ± ±
13.90 14.30 5.73 6.79† 4.42 4.38†
51.87 51.63 21.25 25.75 11.38 11.63
± ± ± ± ± ±
23.99 22.32 10.42 14.92 8.33 8.60
PASc: 34.92 PASg: 35.79 PADc: 17.71 PADg: 20.37 CVPc: 10.03 CVPg: 10.38
± ± ± ± ± ±
13.49 14.26 7.15 8.10† 4.02 4.28
53.50 53.25 24.25 26.00 12.75 14.25
± ± ± ± ± ±
26.19 23.96 7.14 9.80 5.44 5.19†
PASc: 47.29 PASg: 49.71 PADc: 22.38 PADg: 26.86 CVPc: 12.29 CVPg: 13.19
± ± ± ± ± ±
10.31 11.65 4.83 10.52† 4.52 4.59
Establishing technical accuracy of waveform values All readings were obtained after following institutional practice for leveling and zeroing and for dynamic response assessment. Institutional practices were consistent with each other. The stopcock above the transducers was leveled and
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zeroed to the phlebostatic axis. Assessment for dynamic response was then performed. Per institutional policy, if an optimal square waveform was not obtained, the nurse corrected the tubing/catheter system until one was obtained. Recording of the square waveform was not performed. The graphic recording contained the measurement scale that illustrated proper calibration of the printed waveform.
Sample Intended total sample size was 750 patients. The sample was designed to collect patients in adult intensive care units with a variety of clinical diagnoses. The 750-patient sample size was based on a power of 0.8 (assuming an α coefficient of .05 and expected difference between graphic and digital measurements of 10%). The 4 monitoring companies who conducted subanalyses in this study included the following: Hewlett Packard (HP), Andover, Mass; Spacelabs (SL) Redmond, Wash; Marquette, Milwaukee, Wis; and Mennen (MN) Clarence, NY. Because of the number of intensive care unit beds in the participating institutions, HP contributed more of the waveforms (40%) than did SL (30%), Marquette (20%), or MN (10%).
Statistical analysis A 2-tailed t test compared the differences between the monitor’s digital measurement and graphic measurement values. The population was anticipated to have normal distribution (ie, the monitor can read higher or lower than the actual value), with the frequency of large deviations from the actual value assumed to be uncommon. A second analysis with Wilcoxon matched pairs signedrank test determined how often the digital and graphic measurements differed. The use of the t test (parametric assumes normality of data) and the Wilcoxon matched pairs signed-rank test (nonparametric does not assume normality of data) helped to determine the assumption of normality of data. If responses were similar between the 2 testing methods, the concept of normality of distribution was supported. The percentage of times the digital and graphic measurements differed by more than 4 mm Hg was determined. A variation of 4 mm Hg is the amount that previous research has shown to represent clinical change rather than normal fluctuation.19,20 Therefore, a difference between digital and graphic measurements of greater than 4 mm Hg was defined as clinically significant.
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Table III Wilcoxon frequency of differences between digital measurement and graphic measurement* Respiratory category
SB < 20 PAS PAD CVP SB > 20 PAS PAD CVP VB only PAS PAD CVP VB + SB < 20 PAS PAD CVP VB + SB > 20 PAS PAD CVP RV wave PAS PAD CVP Abnormal CVP
Totals: Wilcoxon ranks lower
Totals: Wilcoxon ranks higher
Hewlett Packard lower/higher
SpaceLabs lower/higher
80 75 47†
61 78 61
40/32 36/48 18/32†
20/8† 23/7† 13/11
56 50 43
79† 88† 56†
16/36† 13/43† 8/31†
25/8† 42/4† 26/5†
96 138† 92†
89 66 56
37/30 56/17† 34/14†
26/28 36/25 12/35†
62 64 36†
65 74 60
15/61† 3/75† 9/43†
27/9† 38†/0 24/6†
33 37 21
31 34 33
17/17 12/27† 7/17†
2/1 1/3 0/4
44†/6 45†/11
16†/5 18†/4
5/16
3/5
SB, Spontaneous breathing; VB, ventilator breathing; PAS, pulmonary artery sytolic; PAD, pulmonary artery diastolic; CVP, central venous pressure. *Ranks lower, graphic measurement is lower than digital or cursor measurement; ranks higher, graphic measurement is higher than digital or cursor measurement. †Significant at .05 or less.
RESULTS Sample description During the measurement period, 928 patients were enrolled in the study and 965 measurements were obtained. All patients were from adult critical care units with surgical, cardiac-surgical, cardiacmedical, or general-medical admitting diagnoses. The total sample size and the sample size of each of the respiratory categories for each manufacturer are presented in Table I. The VB-only category comprised the largest group. This group includes early postoperative surgical patients and sedated (or calm) medical and surgical patients. The smallest group was the VB +
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SB > 20 breaths/minute. This group of patients is smaller because they are likely to be the most acutely ill of all ventilator patients.
Digital and graphic differences by respiratory category The first and second questions were concerned with the nature of any differences between PA and CVP values obtained through digital versus graphic measurement and whether respiratory rate or mode of mechanical ventilation affect the accuracy of PA pressure and CVP measurements obtained through digital versus graphic measurement. The 2tailed t test’s results for the combined measurements indicated that the PAD and CVP values mea-
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Cursor readings lower/higher
Marquette lower/higher
Mennen lower/higher
10/3 7/4 0/11†
10/18† 9/19† 5/18†
9/12 8/14 5/7
14/8 5/11 9/2
7/13† 4/16† 0/15†
10/17 5/23† 4/16†
25/21 33/12† 39†/0
11/17 20/14 34†/0
17/21 14/29† 21/7†
13/4 10/7 3/8
3/5 1/6 3/3
11/21† 7/27† 12/15
14/8 5/11 9/2
2/2 1/2 2/2
5/11 2/16† 3/12
8†/0 9†/2
13†/6 21†/7
0/3
2/3
sured by graphic analysis differed significantly from the digital measurements most often (Table II). PAD was statistically different for 2 respiratory categories, SB > 20 breaths/minute (P = .000) and VB only (P = .000). The difference in the CVP measurements was statistically significant in 4 of the 5 respiratory categories: SB < 20 breaths/minute (P = .010), SB > 20 breaths/minute (P = .003), VB + SB < 20 breaths/minute (P = .003), VB + SB > 20 breaths/minute (P = .040). PAS was not significantly different in any respiratory category (Table II). When the different monitoring company measurements were analyzed individually, significant differences between digital and graphic measurements were found in most of the breathing cate-
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gories for 1 of the measurement points—either PAS, PAD, or CVP (Table II). However, some of the respiratory groupings per individual monitoring company had a small sample size that limited statistical power. As demonstrated by the combined data, the digital measurements from the various types of monitors fluctuated around the graphically measured values. The Wilcoxon matched pairs signed-rank test indicated that in the majority of cases the digital measurement value differed from the graphic measurement (Table III). This difference occurred in all respiratory groupings. Lower differences are defined as measurements with a graphic measurement lower than the digital or cursor measurement, and higher differences are defined as measurements with a graphic measurement higher than the digital or cursor measurement. Measurements with the exact same value between digital and graphic methods are not reported in Table III. Statistically significant differences were found in several respiratory categories, and similar to the 2-tailed t test results, the differences were most often found in the PAD and CVP measurements (Table III). Significantly different measurements were found for PAS in SB > 20 breaths/minute (P = .044). For PAD, significant differences were found in SB > 20 breaths/minute (P = .001) and VB only (P = .000). Statistically significant differences were found for the CVP in the following categories: SB < 20 breaths/minute (P = .035), SB > 20 breaths/minute (P = .013), VB only (P = .009), and VB + SB < 20 breaths/minute (P = .003). The frequency with which the digital measurement differed by more than 4 mm Hg from the graphic measurement ranged from 1% to 46% (Table IV). For the combined data, PAD measurements had the highest frequency of clinically significant differences between the 2 measurement techniques. Again, the frequency of difference was greatest with the fast respiratory rate categories, with difference rates of 46% and 34% respectively for graphic measurements being higher than digital measurement for the SB > 20 breaths/minute and VB + SB > 20 breaths/minute categories. All monitoring companies had clinically significant differences in the various categories, most commonly occurring during the rapid breathing events. The clinically and statistically significant differences between the cursor and graphic methods occurred in similar circumstances. Statistically significant differences for the PAD measurements (P = .000–.030) were found in all of the respiratory categories except SB < 20 breaths/minute (Table II).
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Table IV Descriptive statistics on the frequency percentage when digital measurement differed from graphic measurement by >4 mm Hg* Grouping
Combined lower/higher (%)
SB < 20 PAS PAD CVP SB > 20 PAS PAD CVP VB only PAS PAD CVP VB + SB < 20 PAS PAD CVP VB + SB > 20 PAS PAD CVP Abnormal waves RV waves
Hewlett Packard lower/higher (%)
SpaceLabs lower/higher (%)
Marquette lower/higher (%)
2/10 1/20 1/5
4/9 4/9 0/6
8/8 0/40 0/5
4/4 0/4 7/0
7/22 1/46 4/21
8/25 0/53 4/35
0/18 0/55 6/12
20/5 10/10 0/0
3/5 7/4 2/1
7/5 14/5 1/2
1/6 5/7 3/0
2/4 2/0 0/2
12/11 5/19 1/3
16/8 7/22 0/6
0/18 3/21 3/0
6/6 0/6 0/0
12/13 5/34 6/6
15/15 5/41 3/10 5/3 2/21
7/11 7/29 11/0 2/1 3/15
15/15 0/15 15/0 3/2 1/16
SB, Spontaneous breathing; VB, ventilator breathing; PAS, pulmonary artery systolic; PAD, pulmonary artery diastolic; CVP, central venous pressure; RV, right ventricular. *Lower, graphic measurement >4 mm Hg lower than digital measurement; higher, graphic measurement >4 mm Hg higher than digital measurement.
respectively in the VB + SB > 20 breaths/minute category.
Table V Abnormal waveforms Waves
Abnormal a and v waves RV Waves
Sample size
Significance
n = 54
NS
n = 99
PAS = NS PAD = 0.00
NS, Not significant; PAS, pulmonary artery systolic; PAD, pulmonary artery diastolic.
The CVP measurements were significantly different in the SB > 20 breaths/minute (P = .006) and VB only (P = .002). Clinically significant differences occurred most frequently with the faster respiratory rates, with 25% occurrence for both PAS and PAD in the SB > 20 breaths/minute category and 14% and 19%
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Abnormal and RV waveforms The third research question asked whether there are differences between the CVP waveform measurements obtained through digital measurement and those obtained through graphic measurement when abnormal or RV waveforms are present. Waveforms from this category were found incidentally. Data collectors recorded abnormal and RV waveforms any time they were found. Fifty-four abnormal waveforms and 99 RV waveforms were recorded. With these 2 groupings, further breakdown by respiratory category was not done. The study found that digital measurements of abnormal waveforms did not differ significantly from graphically measured waveforms. RV waveforms were significantly different in diastolic values (P = .000) but not systolic values (Table V). However, clinical-
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Mennen lower/higher (%)
Comparison of pulmonary artery
Cursor readings lower/higher (%)
0/16 0/25 0/12
3/5 3/21 0/0
25/25 0/37 0/0
6/25 0/25 0/4
0/5 0/5 3/0
2/2 0/9 2/0
18/18 0/9 0/0
8/8 4/12 0/0
0/0 0/25 0/0 1/0 0/7
5/14 0/19 0/5
ly significant measurement differences occurred with both the abnormal waveforms and the RV waveforms with all monitors (Table IV).
DISCUSSION The answers to the 3 research questions are consistent with previous research that indicates that digital display measurements have limited accuracy. In several circumstances, the differences between digital and graphic measurement were of clinical importance. For at least 1 value (PAS, PAD, or CVP) in each respiratory category, a statistically significant difference between the digital measurement and graphic measurement values was present (Table II). The Wilcoxon matched pairs signed-rank tests (Table III) indicated that the majority of the monitor’s digital measurements did not display the same value as did the graphic measurement. Clinically and statistically significant differences between digital and graphic measurements were found for most of the respirato-
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ry and abnormal waveform categories for each of the different types of monitors. The similarity of results from both the t test and the Wilcoxon matched pairs signed-rank tests supported the concept of normality of distribution of the monitor readings. Although digital measurements were the same as the graphic measurements on occasion, obtaining exactly the same measurement through both techniques occurred inconsistently. Although the lack of precise agreement was frequently statistically significant, some might argue that the difference was not clinically important. However, as seen in Table IV, the frequency of differences between the digital and graphic measurements that were greater than or equal to 4 mm Hg was disturbingly high. The potential for clinical errors resulting from this difference may be important. For example, if the monitor measured the CVP as 3 mm Hg in conjunction with a low stroke volume, a clinician might treat the patient for hypovolemia. Yet the actual value from a graphic measurement was 8 mm Hg (or higher), which represents possible ventricular dysfunction. Drawing the conclusion of hypovolemia rather than ventricular dysfunction would likely result in different treatment. The lack of agreement between the digital measurements and the graphic measurements ranged from as small as 1 mm Hg to as great as 40 mm Hg. To obtain accurate measurements, it is clear that the clinician must confirm the digital measurement value by waveform analysis of a graphic measurement before assuming that the value can be accepted as accurate. The digital data were erroneous in all respiratory categories; however, PAD and CVP digital measurements in the faster respiratory rate categories had the highest frequency of clinically and statistically significant differences. Our results support the findings of Dobbin et al5 and Cengiz et al6 regarding the influence of spontaneous respiration on PAD pressure measurements. Dobbin et al found a significant difference between PAD and PAWP values obtained through digital monitor values and those obtained through a graphic recording during spontaneous breathing. In our study with the combined data, we found that patients breathing spontaneously with a total respiratory rate exceeding 20 breaths per minute had PAD values with statistically significant differences between digital and graphic measurements. In addition, both HP and SL showed statistically significant PAD differences in SB > 20 and VB with SB > 20, whereas MN showed statistically significant PAD differences in SB > 20.
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Comparison of pulmonary artery
The inaccuracy of digital measurements of PA and CVP pressures with rapid breathing patterns was one of the more important findings of this study. Monitor programs filter potential artifact through mathematical algorithms. As previously explained, these filtering systems are, by nature, not precise, because they allow for inclusion of values that could be artifact. The filters are prone to have more difficulty with substantial, frequent artifact than with infrequent artifact. The results of our study contrast those of Johnson and Schuman7 and Lundstedt,6 who found no significant difference between the PAD values for the spontaneously breathing patient obtained through digital measurement and those obtained through graphic measurement. Variation in systolic readings was found in Lundstedt’s study, and in that of Johnson and Schumann. The smaller sample size and limited respiratory rates may account for the difference in the findings. Johnson and Schumann studied 19 spontaneously breathing patients who had respiratory rates ranging from 10 to 28 breaths per minute with an average of 17 breaths per minute. Lundstedt’s sample size for spontaneously breathing patients was 37, with respiratory rates ranging from 10 to 40 breaths per minute; however, 76% of his patients had respiratory rates between 10 and 20 breaths per minute. Therefore, patients with higher respiratory rates had less representation. Monitors studied may also account for the discrepancy. Johnson and Schumann used HP monitors and Lundstedt used Siemens monitors. Like Johnson and Schumann,7 we found significant differences among the PAD and CVP readings in mechanically ventilated patients. However, Dobbin et al8 found no significant difference between the digital display and the graphic recording in mechanically ventilated patients. A limitation of both of these studies is that discussion of the patient’s respiratory effort while being mechanically ventilated is lacking. Therefore, accurate comparison of the findings is difficult to accomplish. In addition, maximum respiratory rate for both studies was 18 breaths per minute, and this study had a separate category for patients being mechanically ventilated with spontaneous effort breathing at >20 breaths/minute. Cengiz et al6 found significant differences between automated measurements and graphic analysis in spontaneously breathing and mechanically ventilated patients for PAS, PAD, and PAWP. The greatest differences occurred in spontaneously breathing patients. They state that the error was not affected by spontaneous or assisted respirato-
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ry rate. However, in their study discussion, they present an excellent case study to demonstrate the effect of a rapid respiratory rate on digital versus graphic measurements. The case involves an intubated patient breathing spontaneously at a rate of 40 breaths per minute. The patient had a digital PAWP value of 4 to 8 mm Hg and a graphic measurement of 22 mm Hg. They report that pulmonary edema later developed in the patient. Our findings support the impact of high respiratory rates on accuracy of digital measurements as their case study so clearly exemplified at the clinical level. This study found that measurements in all respiratory categories had statistically significant variation as well as clinically important variation. Clinicians cannot assume reliability of their PA digital measurement with any patient. Clinicians must validate the digital measurement with the graphic measurement. This validation is necessary with any change in the patient’s respiratory pattern. For the future, monitoring companies need to upgrade monitors to better identify physiologic principles, such as improved identification of end expiration, avoidance of artifact, and use of the electrocardiogram to identify specific waves. Bedside monitors need to be better able to filter respiratory artifact in patients with rapid spontaneous effort. Filtering respiratory artifact can be accomplished by timing the filtering to respiratory measurements, such as airway pressure or capnography waveforms. Currently, digital filtering programs do not possess the ability to interpret abnormal waveforms such as a giant a or v wave or an RV waveform. The digital measurement has the potential to substantially mislead the clinician if abnormal or RV waveforms are present. For example, the monitor might measure a large v wave of the CVP or PAWP instead of the pre–c wave or a wave mean. This measurement method can result in artificially high readings. In addition, most digital displays have an option called the “cursor” or “wedge” feature. This feature allows the clinician to measure the waveform on the monitor. The monitor wedge algorithm signals to the clinician that “wedging” has occurred when it detects a change in waveform. The monitor, therefore, might not indicate to a clinician that the catheter is wedging when a giant v wave is present. This error occurs because the giant v wave is similar in appearance to a PA waveform (Fig 2). The clinician might then overinflate the balloon (potentially leading to PA rupture), advance the catheter unnecessarily far into the PA, or disregard valuable clinical measurements. Marquette Medical cautions that in patients with valvular disease or respi-
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ratory variation, a wedge may not be detected with the Marquette wedge algorithm and recommends use of the manual method for wedge measurement.17 Although there was not a statistically significant difference between values in the abnormal waveform group, both the Wilcoxon matched pairs signed-rank tests and the incidence of clinical differences of >4 mm Hg support the use of graphic measurement rather than digital measurement. The lack of a statistically significant difference may be a result of the small sample size in this category. Ninety-nine RV waveforms were observed and recorded. The RV waveforms were obtained from right atrial ports inappropriately positioned in the RV or from the PA distal ports pulled back into the RV. Before the data collectors’ observation of RV placement, the bedside nurses were not aware that the measurement was actually an RV waveform. The monitor frequently underrepresented diastole with the RV waveforms (Table V). End diastole on an RV waveform is not the lowest point as the digital display indicates, but rather the point immediately before systole. Perhaps more important than the display of an incorrect value is the possibility of the monitor failing to warn the clinician that an RV waveform might represent malposition of the catheter. In this study, advanced practice clinical nurse specialists interpreted the graphic measurements. It is important to keep in mind that graphic measurements can also be misinterpreted if waveform analysis skills are not well developed. Education for improving the clinician’s ability to measure graphic recordings is needed to ensure accurate waveform analysis. Previous research supports this statement. The findings of Burns et al1 and Iberti et al2 in studies on critical care nurses’ knowledge of the PA catheter regarding waveform analysis warn of the importance of being able to measure waveforms graphically rather than relying on the digital display. Burns et al found that only 61% of nurses in their study were able to correctly measure a PAWP tracing with variations of as much as 10 mm Hg. Iberti et al report similar findings with only 57.7% of nurses in their study correctly measuring a PAWP tracing. In addition, Lundstedt6 found the incidence of difference between cursor measurements and graphic measurements to be PAS at 53% and PAD at 12% in spontaneously breathing patients and PAS at 36% and PAD at 6% in mechanically ventilated patients. Our results also point to the inaccuracies of cursor measurements, especially with patients breathing at fast respiratory rates.
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Comparison of pulmonary artery
Prior research has consistently suggested the potential inaccuracy of PA catheter measurements obtained by some critical care nurses. Because our findings demonstrate the inaccuracies of cursor measurements and the need for accurate analysis of waveforms through the graphic measurement method before reliance on digital measurement values, what can be done about critical care clinicians’ practice of waveform interpretation? Iberti recommended PA catheter credentialing to ensure the safe use of the catheter. As a result of Lundstedt’s study, the study institution now credentials clinicians yearly in the use of cursor and graphic measurements. Our results support the need for further education regarding the interpretation of hemodynamic waveforms and for possible credentialing.
CONCLUSION Use of graphic recordings as the gold standard must continue despite improvements in digital engineering. Clearly, errors occur with all monitors’ digital measurements in all respiratory and abnormal waveform patterns. Monitor program design excludes correct measurement of waveforms such as RV or abnormal venous waves. Some of the digital measurement errors are not clinically important. However, when the errors are clinically important, the only protection for the patient is the bedside clinician. It is the responsibility of the bedside clinician to verify the digital measurement with each patient by use of a graphic measurement before relying on the digital measurement. In addition, these measurements must be reverified with the occurrence of any change in respiratory pattern. Consistently accurate PA waveform measurements are of paramount importance in tending the patient’s hemodynamic status and response to therapy. To optimize the impact on patient outcomes with hemodynamic monitoring, all clinicians caring for patients with invasive monitoring must be competent at measuring hemodynamic waveforms. Proper leveling, zeroing, and dynamic response (square waveform testing) assessment are the first crucial steps. Correct measurement at end expiration with various respiratory patterns is the next step. Finally, these measurements must be integrated with other hemodynamic indexes for treatment decisions. Hemodynamic indexes such as the stroke index mixed venous oxygen saturation, the cardiac index, and clinical assessments are useful supplements to pressure measurements. A combination of these indexes, rather than a single index
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or pressure measurement, will more likely lead to accurate evaluation of the patient’s response to therapy.
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10. 11. 12.
REFERENCES 1. Burns D, Burns D, Shively M. Critical care nurses’ knowledge of pulmonary artery catheters. Am J Crit Care 1996;5:49-54. 2. Iberti TJ, Daily EK, Leibowitz AB, Schecter CB, Fischer EP, Silverstein JH. Assessment of critical care nurses’ knowledge of the pulmonary artery catheter. The pulmonary artery catheter study group. Crit Care Med 1994;22:1674-8. 3. Iberti TJ, Fischer EP, Leibowitz AB, Panacek EA, Silverstein JH, Albertson TE. A multicenter study of physicians’ knowledge of the pulmonary artery catheter. JAMA 1990;264:2928-32. 4. Gnaegi A, Feihl F, Perret C. Intensive care physicians’ insufficient knowledge of right heart catheterization at the bedside: time to act? Crit Care Med 1997;25:213-20. 5. Trottier SJ, Taylor RW. Physicians’ attitudes toward and knowledge of the pulmonary artery catheter: Society of Critical Care Medicine Membership Survey. New Horiz 1997;5:201-6. 6. Lundstedt JL. Comparison of methods of measuring pulmonary artery pressure. Am J Crit Care 1997;6:324-32. 7. Johnson MK, Schuman L. Comparison of three methods of measurement of pulmonary artery catheter readings in critically ill patient. Am J Crit Care 1995;4:300-7. 8. Dobbin K, Wallace S, Ahlberg J, Chulay M. Pulmonary artery pressure measurement in patients in with elevated pressures: effect of backrest elevation and method of measurement. Am J Crit Care 1992;1:61-9. 9. Cengiz M, Crapo RO, Gardner RM. The effect of ventilation on
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the accuracy of pulmonary artery and wedge pressure measurement. Crit Care Med 1983;11:502-7. Gershan JA. Effect of positive end-expiratory pressure on pulmonary capillary wedge pressure. Heart Lung 1983;12:143-5. Maran AG. Variables in pulmonary capillary wedge pressure: variation with intrathoracic pressure, graphic and digital recorders. Crit Care Med 1980;8:102-5. Connors AF Jr, Speroff T, Dawson NV, Thomas C, Harrell FE Jr, Wagner D, et al. The effectiveness of right heart catheterization in the initial care of critically ill patients. JAMA 1996;276:889-97. Gore JM, Goldberg RJ, Spodick DH, Alpert JS, Dalen JE. A community-wide assessment of the use of pulmonary artery catheters in patients with acute myocardial infarction. Chest 1987;92:721-7. Robin ED. Overuse and abuse of Swan-Ganz catheters. Int J Clin Monit Comput 1987;4:5-9. Tuman KJ, McCarthy RJ, Spiess BD, DaValle M, Hompland SJ, Dabir R, et al. Effects of pulmonary artery catheterization on outcome in patients undergoing coronary artery surgery. Anesthesiology 1989;70:199-206. Hewlett Packard. An algorithm for reduction of respiration artifact in pulmonary artery pressure measurements. Andover: Hewlett Packard; 1989. Solar 7000/8000 patient monitor operator’s manual. Milwaukee (WI): Marquette Medical Systems, Inc; 1997. Ahrens TS, Taylor L. Hemodynamic waveform analysis. Philadelphia: WB Saunders; 1992. Eisenberg PR, Jaffe AS, Schuster DP. Clinical evaluation compared to pulmonary artery catherization in the hemodynamic assessment of critically ill patients. Crit Care Med 1984;12:549-53. Nemens EJ, Woods SL. Normal fluctuations in pulmonary artery and pulmonary capillary wedge pressures in acutely ill patients. Heart Lung 1982;11:393-8.
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