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Multiplane Transesophageal Echocardiographic Doppler Imaging Accurately Determines Cardiac Output Measurements in Critically III Patients
Multiplane Transesophageal Echocardiographic Doppler Imaging Accurately Determines Cardiac Output Measurements in Critically Ill Patients* Micha S. Feinberg, MD; William E. Hopkins, MD; Victor G. Davila-Roman, MD; and Benico Barzilai, MD
Objectives: To compare cardiac output and stroke volume measured by multiplane transesophageal Doppler echocardiography with that measured by the thermodilution technique. Design: Prospective direct comparison of paired measurements by both techniques in each patient. Setting: Cardiac surgery and myocardial infarction intensive care units. Patients: Twenty-nine patients, mean age ( ± SD) 67 ± 8 years. Nineteen had undergone open heart surgery and 10 had suffered acute myocardial infarction. Methods: Cardiac output and stroke volume were measured simultaneously by the thermodilution technique and multiplane transesophageal Doppler echocardiography via the transgastric view (119 ± 8°) with the sample volume positioned at the level of the left ventricular outflow tract.
Results: Stroke volume and cardiac output measurements were obtained in 29 of 33 patients (88%). Mean values were 50± 13 mL and 4.8 ± 1.3 L/min by Doppler and 51 ± 14 mL and 4.9 ± 1.4 L/ min by thermodilution (r=0.90, r=0.91, p
Transesophageal echocardiography (TEE) has been found to have a major impact on the management of patients in the operating room and the intensive care unit. 1-7 The assessment of stroke volume and cardiac output by Doppler TEE has been hampered by the inability to align the pulse Doppler beam with the left ventricular outflow tract with the use of imaging planes available with monoplane or biplane TEE. Alternative, although deficient, methods using the mitral or pulmonic valve have been proposed. 8- 11 Multiplane TEE increases the number of imaging planes and enhances diagnostic capabilities by providing a conical continuum of tomographic two-dimensional images.l 2- 15 The impact of the access to additional planes offered by multiplane TEE on the utility of Doppler interrogations has not been defined. The current study was designed to assess the feasibility and accuracy of determination of stroke volume and cardiac output in critically ill patients by a pulsed-wave Doppler multiplane TEE technique in which the sample volume is positioned
at the level of the left ventricular outflow tract, via a new plane obtained by rotating the transgastric short -axis view.
*From the Cardiovascular Division, Washington University School of Medicine, St. Louis. Manuscript received March 24, 1994; revision accepted July 27. Reprint requests: Dr. Barzilai, Cardiology, Box 8086, 660 South Euclid Avenue, St. Louis, MO 63110
TEE=transesophageal echocardiography
Key words: transesophageal echocardiography; cardiac output; stroke volume; Doppler echocardiography
METHODS
Patients The study group consisted of 33 consecutive adult patients who were being hemodynamically monitored with pulmonary artery catheters in intensive care units (20 after cardiac surgery and 13 after myocardial infarction) and who were referred for TEE evaluation. In four patients, Doppler left ventricular output recordings using the transgastric view in a plane between 100° to 135° could not be obtained; these patients were excluded from the study. The remaining 29 patients comprise the study population (88%). There were 12 women and 21 men with a mean age of 67 ± 8 years (range, 48 to 83 years). Indications for TEE included assessment of the following: left ventricular function in 18 patients, right ventricular function in five patients, mitral valve in three patients, pericardia! effusion in two patients, and ventricular septal defect in one patient. Twenty-seven were receiving mechanical ventilation and 7 had an intra-aortic balloon assist device (Table 1).
Doppler Examination The multi plane TEE probe (Omniplane, No. 21364A, HewlettPackard), equipped with a 5.0/3.7-MHz transducer, was used for two-dimensional and Doppler studies. Doppler measurements were oriented by two-dimensional imaging. The transgastric short-axis view was rotated to a plane between 100° and 135° where the aortic valve and left ventricular outflow tract were CHEST /107 / 3 / MARCH, 1995
769
Table !-Patient Characteristics (N=29)
lnterobserver Variability
Characteristic
No.
Age, yr, mean± SO (range) Gender Men Women Mechanical ventilation Intra-aortic balloon pump Pacemaker After cardiac surgery After myocardial infarction
67±8 (48-83)
19 10 27 7 2
18 11
optimally seen (Fig 1). Pulsed-Doppler recordings of left ventricular outflow tract were obtained by placing the sample volume in the middle of the outflow tract immediately proximal to the leaflets of the aortic valve (Fig 1), as previously described with transthoracic echocardiography 16 Several cycles were recorded on tape at a speed of 100 mm / s. To obtain the diameter of the left ventricular outflow tract, the transducer was pulled back into the esophagus to obtain an image similar to the long-axis view (0° to 35°). All measurements were made from recorded videotapes by use of the off-line computerized-analysis system available (with the Hewlett-Packard Sonos 1500). Left ventricular stroke volume was calculated as the product of the time-velocity integral and cross-sectional area of the aortic anu lus. The outflow velocity curves were measured following the contour of the brightest portion of the spectral envelope. The cross-sectional area of the aortic anu lus was calculated as 1rXr2, where r represents half of the annular diameter measured immediately proximal to the point of inser tion of the aortic leaflets at the time of maximal separation. The diameter was measured from the inner (trailing) edge to the inner (leading) edge. An average of three of the largest measurements selected from five to eight beats that exhibited a narrow frequency bandwidth was used for the determination of the time-velocity integrals. Overall , left and right ventricular size and function were graded by visual assessment as normal, or as demonstrating mild, moderate, or severe dilatation or dysfunction. The severity of mitral, tricuspid, and aortic regurgitation was semiquantitatively assessed by visual evaluation. Regurgitation was graded as mild, moderate, or severe taking to account the relationship of the size of the regurgitation jet to the size of the atrium in the plane and frame where the jet was maximal for mitral or tricuspid regurgitation and the relationship of the regurgitation jet width to the left ven tricular outflow tract.width in the case of aortic regurgitation.
Each measurement was made by an investigator blinded to the thermodilution data. Intraobserver and interobserver variability were determined by repeating measurements in ten randomized patients. Variability was expressed as mean percent error (difference between two observers divided by the mean of two observed values).
Thermodilution-Derived Cardiac Output and Stroke Volume Cardiac output was calculated from three consecutive measurements (with less than 10% difference) by a thermodilution technique in which 10 mL of iced 5% dextrose was used as an indicator. All measurements were completed within 10 min of the Doppler measurements. Stroke volume was determined by dividing cardiac output with the heart rate for eac h measurement.
Statistical Analysis Data are expressed as the mean± standard deviation. Dopplerderived measurements of stroke volume and cardiac output were compared with thermodilution-derived measurements by use of paired t tests and correlation by first-order linear regression analysis. In addition, the two methods were compared by use of the Bland Altman agreement. 17 RESULTS
Multiplane Doppler-Derived Stroke Volume and Cardiac Output The mean pulsed Doppler time-velocity integral across the left ventricular outflow tract was 16 ± 4em (range, 8 to 26 em) at an angle of 119 ± 8° (range, 101 to 126°). The mean left ventricular outflow tract diameter was 2.0 ± 0.25 em (range, 1.53 to 2.88 em), yielding mean stroke volume by pulsed Doppler of 50± 13 mL (26.7 to 78.0 mL) . The mean Dopplerderived cardiac output was 4.8 ± 1.3 L/min, (2.1 to 7.7 L/ min) . Heart rate was 98 ± 18 bpm.
Stroke Volume and Cardiac Output by Thermodilution The mean thermodilution-derived stroke volume and cardiac output were 51± 14 mL (29.5 to 78.8 mL) and 4.9 ± 1.5 L / min (2.0 to 8.3 L/min), and were not significantly different from the Dopplerderived values.
Comparison Between Doppler and Thermodilution Values The Doppler-derived stroke volume and cardiac output showed good correlation with the thermodilution-derived measurements (r=0.90 and r=0.91) (Fig 2). Mean differences between the stroke volume and cardiac output measurements with Doppler and thermodilution were 1 ± 6 mL (2 ± 12%) and 0.1 ± 0 .7 L /min (2±12%) (Fig 3).
FIGUilE l. Doppler flow velocity across the le ft ventricular outflow tract obtained by TEE using the transgastric view rotated to 118°. 770
Intraobserver and Interobserver Variability The mean percent errors for measurements of left ventricular outflow tract diameter and time-velocity integral across the outflow tract were 3 ± 3% and Multiplane TEE Doppler Imaging for Cardiac Output Determination (Feinberg et alj
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Doppler Stroke Volume (ml)
1 2 3 4 5 6 7 8 9 10 Doppler Cardiac Output (Limin)
FIGURE 2. A, left . Correlation between ventricular stroke volume obtained by thermodilution and Doppler techniques. B, right. Correlation between ventricular stroke volume obtained by thermodilution and Doppler echocardiography.
4 ± 5% by the same observer and 6 ±: 3% and 5 ± 5% between two different observers.
Additional Echocardiographic Findings Additional echocardiography findings are presented in Table 2. No patient in the study was found to have aortic stenosis or severe aortic insufficiency.
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Mean SV difference ± SD = 1.3 ± 6.2 ml
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4 7 2 3 5 8 9 6 Mean Cardiac Output (Doppler & Thermodilution)(Limin)
3. A, upper. Difference between ventricular stroke volume by Doppler and thermodilution vs mean of stroke volume estimated by Doppler and thermodilution. B, lower. Differences between cardiac output by Doppler and thermodilution vs mean cardiac output estimated by Doppler and thermodilution. FIGURE
Table 2-Ancillary Echocardiographic Findings in the Study Population (N=29) Finding*
I:
60 iii 50 .§ 40 ]! 30
~
;a
No.
LV dysfunction Mild Moderate Severe RV dysfunction Mild Moderate Severe Mitral regurgitation Mild Moderate Tricuspid regurgitation Mild Moderate Aortic regurgitation Mild Moderate Ventricular septal defect Prosthetic MV MV repair
4 lO
5 2
8 3
6 7
6
l l 2
*LV=Ieft ventricular; RV-right ventricular; MV=mitral valve. DISCUSSION
We assessed the feasibility and accuracy of cardiac output measurements by multiplane TEE pulsedwave Doppler of the left ventricular outflow tract using thermodilution-derived cardiac output as a standard. We found that multiplane TEE Dopplerderived cardiac output and stroke volume could be obtained with high accuracy in patients in the ICU with various degrees of left ventricular dysfunction . Previous studies have demonstrated the accuracy of transthoracic Doppler echocardiography for measurement of cardiac output at the level of each of the valves. 16,l 8-26 All of these techniques require measurement of the diameter at a particular cardiac level and Doppler measurements to obtain the timevelocity integrals. Cardiac output measurements at the level of the left ventricular outflow tract have generally been easier to obtain, and they are accurate and reproducible. 16•20•25 ·26 Transesophageal echocardiography has emerged as a technique with major impact on the management of critically ill patients both intraoperatively and postoperatively ,1· 7 because it allows accurate monitoring of wall motion, wall thickening, and valvular function. The orientation of the left ventricle and the esophagus does not permit alignment of the Doppler beam with the left ventricular outflow tract to obtain accurate time-velocity integrals with monoplane or biplane TEE. The time-velocity integral is usually underestimated. Alternative methods have been proposed to measure cardiac output at the level of the mitral or pulmonary valves. 8· 11 Doppler determination of cardiac output with the use of mitral CHEST /107131 MARCH, 1995
771
velocity recordings assumes a circular shape of the mitral valve orifice and constant area during diastole; both of these assumptions may cause considerable inaccuracy. 27 In addition, various degrees of mitral regurgitation are frequently seen in patients receiving intensive care who require TEE, as shown in our patient population (Table 2). In these patients, estimation of stroke volume from the mitral Doppler tracings will reflect total stroke volume and not the forward component. The lack of clear visual definition of the pulmonic valve hampers accurate measurement of its diameter and limits its utility in the determination of cardiac output; moreover, the considerable change in pulmonary valve cross-sectional areas seen during systole partly accounts for the inaccuracy of cardiac output measurements at the level of this valve. 20 Multiplane TEE allows access to new imaging planes. 12· 15 The transgastric short -axis view rotated to a plane of 120 ± 10° appears to permit the measurement of pulsed-Doppler recordings of the left ventricular outflow tract with the transducer beam aligned almost parallel to blood flow. Multiplane TEE also permits clear visualization and measurement of the diameter of the aortic anulus by the use of the transesophageal view obtained between 0 and 35° at the basal long-axis level. The combination of the two measurements provides accurate stroke volume and cardiac output measurements, as previously described with transthoracic echocardiography. The major limitation of the proposed technique is that it may not be possible to obtain the transgastric left ventricular outflow tract view in all patients. In our patient population, this could be achieved in 88%. Individual variations in cardiac anatomy and orientation may not permit parallel alignment of the Doppler beam with the outflow tract in the proposed plane, resulting in underestimation of the true cardiac output. In addition, we used thermodilution values as a reference standard to determine cardiac output. Although widely used in previous studies8· 11 J 6•18·20·22·24·25 and in clinical practice, this technique has an inherent variability and limitations.28·30 However, in this study, the Dopplerdetermined cardiac output was within the range of error of the thermodilution standard (Fig 3). No particular group of patients with wide discrepancies between the two methods, namely Doppler TEE and thermodilution, could be defined in this study. Transesophageal echocardiogra phic-deri ved measurement of cardiac output should become part of a complete study of patients in ICUs. Transesophageal echocardiography cannot replace a pulmonary artery catheter when needed because it does not provide repeated measurements and monitoring, but it may serve to verify whether the measurements 772
obtained are accurate. In conclusion, multiplane TEE enhances the ability to estimate accurately cardiac output and stroke volume by providing new access for Doppler interrogation of the left ventricular outflow tract in critically ill patients. REFERENCES
2
3 4 5
6 7
8
9 10
11 12 13
14
15 16
17
Mu~iplane
Matsumoto M, Oka Y, Strom J, et al. Application of transesophageal echocardiography to continuous intraoperative monitoring of left ventricular performance. Am J Cardiol1980; 46:95-105 Smith JS, Cahalan MK, Benefiel DJ, et al. Intraoperative detection of myocardial ischemia in high-risk patients: electrocardiography versus two-dimensional transesophageal echocardiography. Circulation 1985; 72:1015-21 Matsuzaki M, Toma Y, Kusukawa R. Clinical applications of transesophagealechocardiography. Circulation 1990; 82:709-22 Pearson AC, Castello R, Labovitz AJ. Safety and utility of transesophageal echocardiography in the critically ill patient. Am Heart J 1990; 119:1083-89 Oh JK, Seward JB, Khandheria BK, et al. Transesophageal echocardiography in critically ill patients. Am J Cardioll990; 66:1492-95 Foster E, Schiller NB. The role of transesophageal echocardiography in critical care: UCSF experience. J Am Soc Echocardiogr 1992; 5:368-74 Barzilai B, Davila-Roman YG, Eaton MH, et al. Transesophageal echocardiography predicts successful withdrawal of ventricular assist devices. J Thorac Cardiovasc Surg 1992; 104:1410-16 Muhiudeen IA, Kuecherer HF, Lee E, et al. Intraoperative estimation of cardiac output by transesophageal pulsed Doppler echocardiography. Anesthesiology 1991; 74:9-14 Shimamoto H, Kito H , Kawazoe K, et al. Transesophageal Doppler echocardiographic measurement of cardiac output by mitral annulus method. Br Heart J 1992; 68:510-15 Hozumi T, Shakudo M, Applegate R, et al. Accuracy of cardiac output estimation with biplane transesophageal echocardiography. J Am Soc Echocardiogr 1993; 6:62-8 Gorscan J III, Diana P, Ball BA, et al. Intraoperative determination of cardiac output by transesophageal continuous wave Doppler. Am Heart J 1992; 123:171-76 Roeland! JRTC, Thomson IR, Vletter WB, et al. Multiplane transesophageal echocardiography: latest in an imaging revolution. J Am Soc Echocardiogr 1992; 5:361-67 Pandian NG, Hsu TL, Schwartz SL, et al. Multiplane transesophageal echocardiography: imaging planes, echocardiographic anatomy, and clinical experience with a prototype phased array omniplane probe. Echocardiography 1992; 9: 649-66 Seward JB, Khandheria BK, Freeman WK, et al. Multiplane transesophageal echocardiography: image orientation, examination technique, anatomic correlations, and clinical applications. Mayo Clin Proc 1993; 68:523-51 Daniel WG, Pearlman AS, Hausmann D, et al. Initial experience and potential applications of multiplane transesophageal echocardiography. Am J Cardiol1993; 71:358-61 Lewis JF, Kuo LC, Nelson JG, et al. Pulsed Doppler echocardiographic determination of stroke volume and cardiac output: clinical validation of two new methods using apical window. Circulation 1984; 70:425-31 Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986; 1:307-10 TEE Doppler Imaging for Cardiac Output Determination (Feinberg eta/)
18 Labovitz AJ, Buckingham T A, Habermehl K, eta!. The effects of sampling site on the two-dimensional echo-Doppler determination of cardiac output. Am Heart J 1985; 109:327-32 19 Dubin J, Wallerson DC, Cody RJ, eta!. Comparative accuracy of Doppler echocardiographic methods for clinical stroke volume determination. Am Heart J 1990; 120:116-23 20 Stewart WJ, Jiang L, Mich R, eta!. Variable effects of changes in flow rate through the aortic, pulmonary and mitral valves on valve area and flow velocity: impact on quantitative Doppler flow calculations. J Am Coli Cardiol1985; 6:653-62 21 Ihlen H, Endresen K, Myreng Y, eta!. Reproducibility of cardiac stroke volume estimated by Doppler echocardiography. Am J Cardiol1987; 59:975-78 22 Bouchard A, Blumlein S, Schiller NB, eta!. Measurement of left ventricular stroke volume using continuous wave Doppler echocardiography of the ascending aorta and M-mode echocardiography of the aortic valve. J Am Coli Cardiol 1987; 9:75-83 23 Zhang Y, Nitter-Hauge S, Ihlen H, eta!. Doppler echocardiographic measurement of cardiac output using the mitral orifice method. Br Heart J 1985; 53:130-36 24 Ihlen H, Amile JP, Dale J, eta!. Determination of cardiac out-
put by Doppler echocardiography. Br Heart J 1984; 51:54-60 25 Dittmann H, Voelker W, Karsch KR, et a!. Influence of sampling site and flow area on cardiac output measurements by Doppler echocardiography. JAm Coli Cardiol1987; 10:818-23 26 Dubin J, Wallerson DC, Cody RJ, et al. Comparative accuracy of Doppler echocardiographic methods for clinical stroke volume determination. Am Heart J 1990; 120:116-23 27 Ormiston JA, Shah PM, Tei C, eta!. Size and motion of the mitral valve annulus in man: I. A two-dimensional echocardiographic method and findings in normal subjects. Circulation 1981; 64:113-20 28 Wessel HU, Paul MH, James GW, eta!. Limitations of thermal dilution curves for cardiac output determinations. J Appl Physiol1971; 30:643-52 29 Fischer AP, Benis AM, Jurado BR, et al. Analysis of errors in measurement of cardiac output by simultaneous dye and thermal dilution in cardiothoracic surgical patients. Cardiovasc Res 1978; 12:190-19 30 Levett JM, Replogle RL. Thermodilution cardiac output: a critical analysis and review of the literature. J Surg Res 1979; 27:392-404
CHEST /107 I 3 1MARCH, 1995
773
Objectives: To compare cardiac output and stroke volume measured by multiplane transesophageal Doppler echocardiography with that measured by the thermodilution technique. Design: Prospective direct comparison of paired measurements by both techniques in each patient. Setting: Cardiac surgery and myocardial infarction intensive care units. Patients: Twenty-nine patients, mean age ( ± SD) 67 ± 8 years. Nineteen had undergone open heart surgery and 10 had suffered acute myocardial infarction. Methods: Cardiac output and stroke volume were measured simultaneously by the thermodilution technique and multiplane transesophageal Doppler echocardiography via the transgastric view (119 ± 8°) with the sample volume positioned at the level of the left ventricular outflow tract.
Results: Stroke volume and cardiac output measurements were obtained in 29 of 33 patients (88%). Mean values were 50± 13 mL and 4.8 ± 1.3 L/min by Doppler and 51 ± 14 mL and 4.9 ± 1.4 L/ min by thermodilution (r=0.90, r=0.91, p
Transesophageal echocardiography (TEE) has been found to have a major impact on the management of patients in the operating room and the intensive care unit. 1-7 The assessment of stroke volume and cardiac output by Doppler TEE has been hampered by the inability to align the pulse Doppler beam with the left ventricular outflow tract with the use of imaging planes available with monoplane or biplane TEE. Alternative, although deficient, methods using the mitral or pulmonic valve have been proposed. 8- 11 Multiplane TEE increases the number of imaging planes and enhances diagnostic capabilities by providing a conical continuum of tomographic two-dimensional images.l 2- 15 The impact of the access to additional planes offered by multiplane TEE on the utility of Doppler interrogations has not been defined. The current study was designed to assess the feasibility and accuracy of determination of stroke volume and cardiac output in critically ill patients by a pulsed-wave Doppler multiplane TEE technique in which the sample volume is positioned
at the level of the left ventricular outflow tract, via a new plane obtained by rotating the transgastric short -axis view.
*From the Cardiovascular Division, Washington University School of Medicine, St. Louis. Manuscript received March 24, 1994; revision accepted July 27. Reprint requests: Dr. Barzilai, Cardiology, Box 8086, 660 South Euclid Avenue, St. Louis, MO 63110
TEE=transesophageal echocardiography
Key words: transesophageal echocardiography; cardiac output; stroke volume; Doppler echocardiography
METHODS
Patients The study group consisted of 33 consecutive adult patients who were being hemodynamically monitored with pulmonary artery catheters in intensive care units (20 after cardiac surgery and 13 after myocardial infarction) and who were referred for TEE evaluation. In four patients, Doppler left ventricular output recordings using the transgastric view in a plane between 100° to 135° could not be obtained; these patients were excluded from the study. The remaining 29 patients comprise the study population (88%). There were 12 women and 21 men with a mean age of 67 ± 8 years (range, 48 to 83 years). Indications for TEE included assessment of the following: left ventricular function in 18 patients, right ventricular function in five patients, mitral valve in three patients, pericardia! effusion in two patients, and ventricular septal defect in one patient. Twenty-seven were receiving mechanical ventilation and 7 had an intra-aortic balloon assist device (Table 1).
Doppler Examination The multi plane TEE probe (Omniplane, No. 21364A, HewlettPackard), equipped with a 5.0/3.7-MHz transducer, was used for two-dimensional and Doppler studies. Doppler measurements were oriented by two-dimensional imaging. The transgastric short-axis view was rotated to a plane between 100° and 135° where the aortic valve and left ventricular outflow tract were CHEST /107 / 3 / MARCH, 1995
769
Table !-Patient Characteristics (N=29)
lnterobserver Variability
Characteristic
No.
Age, yr, mean± SO (range) Gender Men Women Mechanical ventilation Intra-aortic balloon pump Pacemaker After cardiac surgery After myocardial infarction
67±8 (48-83)
19 10 27 7 2
18 11
optimally seen (Fig 1). Pulsed-Doppler recordings of left ventricular outflow tract were obtained by placing the sample volume in the middle of the outflow tract immediately proximal to the leaflets of the aortic valve (Fig 1), as previously described with transthoracic echocardiography 16 Several cycles were recorded on tape at a speed of 100 mm / s. To obtain the diameter of the left ventricular outflow tract, the transducer was pulled back into the esophagus to obtain an image similar to the long-axis view (0° to 35°). All measurements were made from recorded videotapes by use of the off-line computerized-analysis system available (with the Hewlett-Packard Sonos 1500). Left ventricular stroke volume was calculated as the product of the time-velocity integral and cross-sectional area of the aortic anu lus. The outflow velocity curves were measured following the contour of the brightest portion of the spectral envelope. The cross-sectional area of the aortic anu lus was calculated as 1rXr2, where r represents half of the annular diameter measured immediately proximal to the point of inser tion of the aortic leaflets at the time of maximal separation. The diameter was measured from the inner (trailing) edge to the inner (leading) edge. An average of three of the largest measurements selected from five to eight beats that exhibited a narrow frequency bandwidth was used for the determination of the time-velocity integrals. Overall , left and right ventricular size and function were graded by visual assessment as normal, or as demonstrating mild, moderate, or severe dilatation or dysfunction. The severity of mitral, tricuspid, and aortic regurgitation was semiquantitatively assessed by visual evaluation. Regurgitation was graded as mild, moderate, or severe taking to account the relationship of the size of the regurgitation jet to the size of the atrium in the plane and frame where the jet was maximal for mitral or tricuspid regurgitation and the relationship of the regurgitation jet width to the left ven tricular outflow tract.width in the case of aortic regurgitation.
Each measurement was made by an investigator blinded to the thermodilution data. Intraobserver and interobserver variability were determined by repeating measurements in ten randomized patients. Variability was expressed as mean percent error (difference between two observers divided by the mean of two observed values).
Thermodilution-Derived Cardiac Output and Stroke Volume Cardiac output was calculated from three consecutive measurements (with less than 10% difference) by a thermodilution technique in which 10 mL of iced 5% dextrose was used as an indicator. All measurements were completed within 10 min of the Doppler measurements. Stroke volume was determined by dividing cardiac output with the heart rate for eac h measurement.
Statistical Analysis Data are expressed as the mean± standard deviation. Dopplerderived measurements of stroke volume and cardiac output were compared with thermodilution-derived measurements by use of paired t tests and correlation by first-order linear regression analysis. In addition, the two methods were compared by use of the Bland Altman agreement. 17 RESULTS
Multiplane Doppler-Derived Stroke Volume and Cardiac Output The mean pulsed Doppler time-velocity integral across the left ventricular outflow tract was 16 ± 4em (range, 8 to 26 em) at an angle of 119 ± 8° (range, 101 to 126°). The mean left ventricular outflow tract diameter was 2.0 ± 0.25 em (range, 1.53 to 2.88 em), yielding mean stroke volume by pulsed Doppler of 50± 13 mL (26.7 to 78.0 mL) . The mean Dopplerderived cardiac output was 4.8 ± 1.3 L/min, (2.1 to 7.7 L/ min) . Heart rate was 98 ± 18 bpm.
Stroke Volume and Cardiac Output by Thermodilution The mean thermodilution-derived stroke volume and cardiac output were 51± 14 mL (29.5 to 78.8 mL) and 4.9 ± 1.5 L / min (2.0 to 8.3 L/min), and were not significantly different from the Dopplerderived values.
Comparison Between Doppler and Thermodilution Values The Doppler-derived stroke volume and cardiac output showed good correlation with the thermodilution-derived measurements (r=0.90 and r=0.91) (Fig 2). Mean differences between the stroke volume and cardiac output measurements with Doppler and thermodilution were 1 ± 6 mL (2 ± 12%) and 0.1 ± 0 .7 L /min (2±12%) (Fig 3).
FIGUilE l. Doppler flow velocity across the le ft ventricular outflow tract obtained by TEE using the transgastric view rotated to 118°. 770
Intraobserver and Interobserver Variability The mean percent errors for measurements of left ventricular outflow tract diameter and time-velocity integral across the outflow tract were 3 ± 3% and Multiplane TEE Doppler Imaging for Cardiac Output Determination (Feinberg et alj
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1 2 3 4 5 6 7 8 9 10 Doppler Cardiac Output (Limin)
FIGURE 2. A, left . Correlation between ventricular stroke volume obtained by thermodilution and Doppler techniques. B, right. Correlation between ventricular stroke volume obtained by thermodilution and Doppler echocardiography.
4 ± 5% by the same observer and 6 ±: 3% and 5 ± 5% between two different observers.
Additional Echocardiographic Findings Additional echocardiography findings are presented in Table 2. No patient in the study was found to have aortic stenosis or severe aortic insufficiency.
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4 7 2 3 5 8 9 6 Mean Cardiac Output (Doppler & Thermodilution)(Limin)
3. A, upper. Difference between ventricular stroke volume by Doppler and thermodilution vs mean of stroke volume estimated by Doppler and thermodilution. B, lower. Differences between cardiac output by Doppler and thermodilution vs mean cardiac output estimated by Doppler and thermodilution. FIGURE
Table 2-Ancillary Echocardiographic Findings in the Study Population (N=29) Finding*
I:
60 iii 50 .§ 40 ]! 30
~
;a
No.
LV dysfunction Mild Moderate Severe RV dysfunction Mild Moderate Severe Mitral regurgitation Mild Moderate Tricuspid regurgitation Mild Moderate Aortic regurgitation Mild Moderate Ventricular septal defect Prosthetic MV MV repair
4 lO
5 2
8 3
6 7
6
l l 2
*LV=Ieft ventricular; RV-right ventricular; MV=mitral valve. DISCUSSION
We assessed the feasibility and accuracy of cardiac output measurements by multiplane TEE pulsedwave Doppler of the left ventricular outflow tract using thermodilution-derived cardiac output as a standard. We found that multiplane TEE Dopplerderived cardiac output and stroke volume could be obtained with high accuracy in patients in the ICU with various degrees of left ventricular dysfunction . Previous studies have demonstrated the accuracy of transthoracic Doppler echocardiography for measurement of cardiac output at the level of each of the valves. 16,l 8-26 All of these techniques require measurement of the diameter at a particular cardiac level and Doppler measurements to obtain the timevelocity integrals. Cardiac output measurements at the level of the left ventricular outflow tract have generally been easier to obtain, and they are accurate and reproducible. 16•20•25 ·26 Transesophageal echocardiography has emerged as a technique with major impact on the management of critically ill patients both intraoperatively and postoperatively ,1· 7 because it allows accurate monitoring of wall motion, wall thickening, and valvular function. The orientation of the left ventricle and the esophagus does not permit alignment of the Doppler beam with the left ventricular outflow tract to obtain accurate time-velocity integrals with monoplane or biplane TEE. The time-velocity integral is usually underestimated. Alternative methods have been proposed to measure cardiac output at the level of the mitral or pulmonary valves. 8· 11 Doppler determination of cardiac output with the use of mitral CHEST /107131 MARCH, 1995
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velocity recordings assumes a circular shape of the mitral valve orifice and constant area during diastole; both of these assumptions may cause considerable inaccuracy. 27 In addition, various degrees of mitral regurgitation are frequently seen in patients receiving intensive care who require TEE, as shown in our patient population (Table 2). In these patients, estimation of stroke volume from the mitral Doppler tracings will reflect total stroke volume and not the forward component. The lack of clear visual definition of the pulmonic valve hampers accurate measurement of its diameter and limits its utility in the determination of cardiac output; moreover, the considerable change in pulmonary valve cross-sectional areas seen during systole partly accounts for the inaccuracy of cardiac output measurements at the level of this valve. 20 Multiplane TEE allows access to new imaging planes. 12· 15 The transgastric short -axis view rotated to a plane of 120 ± 10° appears to permit the measurement of pulsed-Doppler recordings of the left ventricular outflow tract with the transducer beam aligned almost parallel to blood flow. Multiplane TEE also permits clear visualization and measurement of the diameter of the aortic anulus by the use of the transesophageal view obtained between 0 and 35° at the basal long-axis level. The combination of the two measurements provides accurate stroke volume and cardiac output measurements, as previously described with transthoracic echocardiography. The major limitation of the proposed technique is that it may not be possible to obtain the transgastric left ventricular outflow tract view in all patients. In our patient population, this could be achieved in 88%. Individual variations in cardiac anatomy and orientation may not permit parallel alignment of the Doppler beam with the outflow tract in the proposed plane, resulting in underestimation of the true cardiac output. In addition, we used thermodilution values as a reference standard to determine cardiac output. Although widely used in previous studies8· 11 J 6•18·20·22·24·25 and in clinical practice, this technique has an inherent variability and limitations.28·30 However, in this study, the Dopplerdetermined cardiac output was within the range of error of the thermodilution standard (Fig 3). No particular group of patients with wide discrepancies between the two methods, namely Doppler TEE and thermodilution, could be defined in this study. Transesophageal echocardiogra phic-deri ved measurement of cardiac output should become part of a complete study of patients in ICUs. Transesophageal echocardiography cannot replace a pulmonary artery catheter when needed because it does not provide repeated measurements and monitoring, but it may serve to verify whether the measurements 772
obtained are accurate. In conclusion, multiplane TEE enhances the ability to estimate accurately cardiac output and stroke volume by providing new access for Doppler interrogation of the left ventricular outflow tract in critically ill patients. REFERENCES
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