Korotkoff sound versus oscillometric cuff sphygmomanometers: comparison between auscultatory and DynaPulse blood pressure measurements

Journal of the American Society of Hypertension 5(1) (2011) 12–20

Research Article

Korotkoff sound versus oscillometric cuff sphygmomanometers: comparison between auscultatory and DynaPulse blood pressure measurements Shiu-Shin Chio, PhDa, Elaine M. Urbina, MDb, Jeffery LaPointe, BSa, Jeffrey Tsai, BSa, and Gerald S. Berenson, MDc,* a

Pulse Metric, Incorporated, Vista, CA, USA; Department of Preventive Cardiology, Cincinnati Children Hospital Medical Center, Cincinnati, OH, USA; and c Department of Epidemiology, School of Public Health and Tropical Medicine, Tulane University Health Sciences Center, New Orleans, LA, USA Manuscript received September 8, 2010 and accepted October 31, 2010 b

Abstract Listening to Korotkoff sounds (K-sounds) to determine systolic and diastolic blood pressure (BP) has been the standard for noninvasive BP measurement in medical practices for nearly 100 years. It is the essential tool used for evaluation and assessment of patients with hypertension and risks of cardiovascular diseases (CVD) by physicians and nurses despite limited understanding of the nature of K-sounds. Analyzing cuff oscillometric signals to obtain BP has been the foundation of most digital BP monitors available today. DynaPulse is an oscillometric digital BP monitor that records and analyzes subtle changes of pulse waveforms during the course of a BP measurement while cuff pressure slowly decreases from above systolic to below diastolic. This study compares systolic and diastolic readings obtained by K-sound method following the Bogalusa Heart Study protocol and BP measured by DynaPulse (DP2000A) monitor, in order to better understand the nature and difference between K-sound and oscillometric methods. Analysis of means and differences is applied to BP data collected from 803 subjects examined in the Bogalusa Heart Study. The results indicated: 1) DynaPulse systolic was 9 mm Hg higher (P < .0001) than Phase 1 (K1) systolic, 2) DynaPulse diastolic was 5 mm Hg lower (P < .0001) than Phase 4 (K4), and 3) is less than 1 mm Hg higher than Phase 5 (K5) diastolic (P < .0001), when compared with K-sound auscultatory measurement. Understanding the methods and differences of DynaPulse oscillometric and K-sound BP measurements is important for clinic BP screening and self-BP monitoring, as well as future research to improve hypertension and CVD managements. J Am Soc Hypertens 2011;5(1):12–20. Ó 2011 American Society of Hypertension. All rights reserved. Keywords: Korotkoff sounds; auscultatory; oscillometric; sphygmomanometer; DynaPulse; blood pressure measurement; Bogalusa Heart Study.

Introduction The standard instrument for indirect measurement of systolic and diastolic blood pressures (BP) is the cuff S.-S.C. is an owner and the founder/chair of DynaPulse/Pulse Metric, Inc. This study was supported by Pulse Metric, Inc., and funds from National Heart, Lung, and Blood Institute of the U.S. Public Health Services (USPHS), Early Natural History of Arteriosclerosis HL38844. *Corresponding author: Gerald S. Berenson, MD, Tulane Center for Cardiovascular Health, 1440 Canal Street, Room 2140, New Orleans, LA 70112–7103. Tel: 504-988-7197; fax: 504-988-7194. E-mail: [email protected]

mercury sphygmomanometer. Blood pressure measured on the sphygmomanometer depends on the examiner detecting audible sounds during constricted blood flow.1–3 Another methodology, an automated oscillometric system that analyzes pulse-waveform signals obtained from a cuff applying pressure around the brachial artery at the upper arm eliminating dependence on audible sounds, has become the commonly available digital BP monitor type today.4–10 Understanding the fundamental differences between these two methods and the variations of BP readings between them would better assist individuals using digital BP monitors to apply the recorded BP values in comparison to the established hypertension guidelines and clinical studies to effectively control and manage

1933-1711/$ - see front matter Ó 2011 American Society of Hypertension. All rights reserved. doi:10.1016/j.jash.2010.10.005

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high BP.11–14 The DynaPulse BP monitor (Pulse Metric, Inc., Vista, CA), having BP levels previously validated and compared with invasive central aortic pressure,15 applies the principle of pulse dynamics oscillometric methodology. This method digitizes, records, analyzes, and displays cuff pulsation signals during the entire event of a BP measurement. Similar to listening to Korotkoff sound phases, the DynaPulse algorithm investigates each individual pulse waveform, identifying the subtle change of its pattern associated to the vibratory effects of blood flowing through the brachial artery constricted by the cuff, and then determines systolic, diastolic, and mean arterial pressures (MAP).8 DynaPulse records and displays the entire oscillometric signal stream. Each individual pulse waveform allows for visual inspection of a BP measurement, screening for artifacts and/or irregular heartbeat, and detection of the changes in pulse waveform. Similar to the recording of Korotkoff sounds, DynaPulse provides detailed information of blood flowing through the cuffrestricted brachial section and allows for comparisons between these two methods. In this study, we compared DynaPulse systolic, diastolic, and MAP to Korotkoff phase-1 (K1) systolic, phase-4 (K4), and phase-5 (K5) diastolics, and calculated MAP ¼ 1/3*K1 þ 2/3*K4 on 803 subjects at Bogalusa Heart Study. Blood pressures by Korotkoff sounds were measured according to the well-established protocol of Bogalusa Heart Study,16,17 DynaPulse 2000A (Pulse Metric, Inc., Vista, CA) was used to record the oscillometric pulse waveform to measure systolic, diastolic, MAP, and heart rate. Bland-Altman statistical analysis18,19 was applied to estimating precision and agreement between the two BP measurement techniques, Korotkoff sound and DynaPulse oscillometric methods. With better understanding of the principles and differences between oscillometric and Korotkoffsound cuff sphygmomanometer BP readings, the DynaPulse oscillometric BP and waveform monitoring and recording system can be a useful tool in routine BP evaluation.

Methods and Materials Population and Screening Procedure The Bogalusa Heart Study is an epidemiologic survey of cardiovascular disease risk factors from birth through mid-adulthood. Participants (n ¼ 1420, 40% male, 70% white, 18–38 years of age) were previously examined as children in this long-term survey. Details of screening and examination procedures, followed in the Bogalusa Heart Study since its inception, are reported elsewhere.20 All data were collected after obtaining informed consent. Estimation of agreement between measurements from the automated system obtained from the DynaPulse 2000A instrument and the sphygmomanometer are reported for (n ¼ 803) subjects examined on both instruments. A 10%

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random sample of subjects examined was chosen for rescreening daily. The original and rescreened examinations (n ¼ 69) were used to estimate precision of observations obtained from DynaPulse.

Methods of Measurement Procedure Three consecutive observations of BP were recorded by each of two trained observers using mercury sphygmomanometers. Right upper-arm length and circumference determined the appropriate BP cuff for use with the sphygmomanometers.20 Three observations were also recorded on DynaPulse. Each subject was examined in random order at the two mercury sphygmomanometers and then at DynaPulse. Examiners were trained according to written and visual with sound protocols and randomly assigned to instruments.

Blood Pressure BP was measured on the right brachial artery while the subject was seated with the arm placed at heart level. First (systolic), fourth (diastolic), and fifth (diastolic) phase Korotkoff sounds on the sphygmomanometer were measured by auscultation to the nearest 2 mm Hg.16,17 Systolic and diastolic pressures on DynaPulse were determined by the pulse dynamic (DynaPulse) pulse wave analysis method.8,15 Pressures were measured to the nearest 1 mm Hg with a cuff chosen according to upper-arm length and circumference. The DynaPulse system uses waveform analysis of arterial pressure signals obtained from a standard cuff sphygmomanometer to determine the ‘‘end’’ systolic and diastolic points of a calibrated pressure waveform.8,15 The waveform analysis method uses a pulse dynamic pattern recognition algorithm to distinguish changes in the continuous oscillometric cuff signal.8,15 The calibrated pressure waveform, with end-systolic at its maximum and end-diastolic at minimum pressure, mostly resembles the central aortic pressure contour. The waveform is incorporated via a physical model of the cardiovascular system that assumes a straight tube brachial artery and T-tube aortic system. The Appendix describes the DynaPulse method that determines systolic, diastolic, and MAP from the cuff oscillometric signals shown in Figure 1. Three distinguished pulses are shown. Patterns, marked with the three triangles indicated at around 120, 84, and 65 mm Hg of the cuff pressures axis, were determination points of DynaPulse systolic, MAP, and diastolic, respectively. This pattern-recognition method determines BP by identifying the subtle changes of cuff pulses generated from the effects of blood flowing through the brachial artery section restricted by the cuff (the Bernoulli principle). The pressure signals are examined directly instead of the indirect listening to the Korotkoff sound phases by sphygmomanometer measurement. Detailed description and comparison of DynaPulse oscillometric waveforms and Korotkoff sounds is also provided in Appendix.

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Figure 1. The oscillometric signals of DynaPulse cuff sphygmometer and the five pulse waveform patterns.

Mean Arterial Pressure Using systolic and diastolic measurements from the sphygmomanometer, MAP was calculated as (2/3)  diastolic þ (1/3)  systolic. MAP was also determined by DynaPulse system via mathematical averaging of the calibrated Pulse Dynamic pressure waveform. The method of mathematical averaging applies digital integration to a calibrated pulse waveform over its full cycle (systolic and diastolic cycles). MAP is defined as the point at which a line separates the systolic cycle and the diastolic cycle, such that the integrated area of the systolic cycle equals that of the diastolic.8

Heart Rate Heart rate (HR) was determined both manually and automatically, the latter using principles described previously for DynaPulse system.

Statistical Analysis SAS/STAT system21 was used for all statistical analyses in this study. Mean, standard deviation, and standard error were calculated for all measurements on all 803 subjects in two subgroups: 734 population and 69 randomly selected rescreened subjects. In reproducibility studies, additional analyses of mean difference, t-test for paired data, coefficient of variation in %, correlation coefficient, and variance analysis were also performed on data from the 69 rescreened subjects for intra-examiner measurement error and reliability (precision and reproducibility) analysis of DynaPulse instrument. In an agreement study, BlandAltman analyses18,19 and scatter-plots of mean difference versus mean value, of systolic and diastolic between K-sound (K1 and K4), and DynaPulse BP measurements of all 803 subjects, were obtained.

Results Reproducibility Studies A random sample of 69 rescreened subjects (22 male and 51 white) was examined and the BP mean values were compared with the rest of 734 subjects (313 male and 524 white). In Table 1, analysis of the K-sound sphygmomanometer BP measurements, data from the random sample are compared with data from subjects included in the examined population, but not in the random sample. There is no difference among selected variables between the sample and the population (P > .05). Therefore, estimates of precision and reliability obtained from the sample are applicable to the population. We further analyzed data of DynaPulse BP measurements on the 69 sample subjects for variations of intra-examiner and the reproducibility (reliability) and error (precision) of the BP measurements. Results are summarized in Tables 2 and 3, including also the MAP, HR, and brachial artery distensibility obtained from DynaPulse pulse waveform analysis.15,22–24 Overall estimates of precision, determined from analysis of variance, for several variables measured by DynaPulse are shown in Table 3. Precision and reliability for systolic and diastolic BP are of a magnitude similar to that previously observed for data from this community.25,26 The coefficient of variation (CV), which measures relative variation and allows comparison of variation among variables having different units of measurement, indicates that BP variables and HR cluster near their respective means. Measurements of brachial artery distensibility, on the other hand, are slightly more difficult to measure (CV 14.7%) than either BP or HR. We observed that intra-examiner precision and reliability for DynaPulse system are comparable to that

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Table 1 Comparison of selected variables between the population and a random sample of rescreened subjects Variable*

Population

Males, n (%) Whites, n (%) Age, years Height, cm Weight, kg Triceps skinfold, mm Subscapular skinfold, mm Systolic blood pressure, mm Hg First sphygmomanometer Second sphygmomanometer DynaPulse Diastolic blood pressure, mm Hg Phase 4, first sphygmomanometer Phase 4, second sphygmomanometer Phase 5, first sphygmomanometer Phase 5, second sphygmomanometer DynaPulse DynaPulse Mean arterial pressure, mm Hg Distensibility, mm Hg/L/min Heart rate, beats/min Manual heart rate, beats/min

Sample

n

Mean  SE

n

Mean  SE

313 (42.6) 524 (71.4) 734 734 734 734 733

– –

22 (31.9) 51 (73.9) 69 69 69 69 69

– –

734 734 734

29.7 169.1 78.5 23.6 24.0

    

0.2 0.4 0.7 0.4 0.4

112.0  0.4 111.3  0.4 120.7  0.5

69 69 69

30.1 167.4 75.7 23.4 23.1

    

0.7 1.1 2.8 1.4 1.5

111.4  1.5 110.7  1.6 119.9  1.7

734 734 734 734 734

74.2 73.8 68.3 67.8 68.7

    

0.3 0.3 0.3 0.3 0.3

69 69 69 69 69

73.3 74.2 67.9 68.3 68.8

    

1.2 1.3 1.3 1.3 1.1

734 734 734 732

85.9 0.0654 77.3 71.9

   

0.3 0.0005 0.4 0.3

69 69 69 69

85.6 0.0649 77.4 73.1

   

1.2 0.0018 1.2 1.4

* No difference between population and sample: a) gender: P > .05, race: P > .05 (continuity adjusted chi-square test of association); b) P > .05 (t-test for independent samples, Satterthwaite correction for unequal variances for weight, for measurements of diastolic blood pressure from the sphygmomanometer, and for manually determined heart rate).

observed for the mercury sphygmomanometer in previous studies of the Bogalusa population.25,26 Precision of both systolic and diastolic measurements of BP is approximately 5 mm Hg. Reliability of approximately 0.85 indicates that measurement error is low (precision is high), and that variability in measurements of systolic and diastolic BP is due mainly to variability among rather than within subjects.

Agreement Between the Sphygmomanometer and DynaPulse Each participant was examined in random order on two separate mercury sphygmomanometers and then on DynaPulse. Figures 2a (systolic) and 2b (diastolic) show data for analysis of agreement between the first sphygmomanometer and DynaPulse. The line of ‘‘Ms’’ in Figures 2a and 2b, denoting the mean difference between

Table 2 Intra-examiner measurements for DynaPulse (n ¼ 69)y Variable

Mean  SD (SE) Original

Blood pressure, mm Hg Systolic Diastolic Mean arterial Heart rate, beats/min Distensibility, mm Hg/L/min

119.9 68.8 85.6 77.4 0.06

    

Replicate 14.3 (1.7) 8.9 (1.1) 10.3 (1.2) 9.5 (1.1) 0.02 (0.002)

121.8 70.2 86.6 75.0 0.07

* P < .05. ** P < .025. *** P < .0005 (t-test for paired data). y Difference ¼ original - rescreened; observation is mean of three measurements.

    

13.9 (1.7) 9.8 (1.2) 10.7 (1.3) 9.9 (1.2) 0.01 (0.002)

Differencey 1.9 1.4 1.0 2.4 0.0002

    

7.6 (0.9)* 4.7 (0.6)** 4.3 (0.5)* 5.4 (0.7)*** 0.01 (0.002)

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Table 3 Intra-examiner measurement error (Precision) and reliability (Reproducibility) for DynaPulse

Discussion

Variable*

To understand the differences better between an oscillometric (DynaPulse) and standard Hg BP measurements, reproducibility analysis and estimation of the precision, and quantification of agreement are essential in evaluating suitability of the oscillometric method. We estimated precision of DynaPulse to compare and quantify with brachial artery Korotkoff-sound BP measurement. Reproducibility analysis and elimination of statistical artifacts are needed when comparing readings from two BP measurements. Many approaches to minimize or to eliminate statistical artifacts were suggested and reported by Bland and Altman,18,19 Oldham,27 and Gill.28 Our reproducibility analysis data indicate that measurements from DynaPulse are rather reliable and do not depend on an observer’s hearing ability to detect Korotkoff sounds at low frequencies (18–60 Hz), which is difficult.29 Reliability of DynaPulse BP (systolic, diastolic, and MAP) and HR were good; approximately 5 mm Hg standard division and 0.8–0.95 intraclass correlation coefficient. This is similar to results reported on other automated BP monitors, such as DinaMAP. DynaPulse brachial artery distensibility had lower, but moderate reliability of near 60% of total variability, which is to be expected as distensibility is derived from BP and other waveform parameters. Evaluation of agreement shows DynaPulse measures systolic BP approximately 9 mm Hg higher than K1, whereas diastolic measures 5 mm Hg lower than the K4 and less than 1 mm Hg higher than K5 sphygmomanometer readings. Results at each sphygmomanometer independently confirm the consistency of differences between the two methods. Higher systolic measurements by electronic oscillometric measurement, when compared with those obtained by auscultation, have been documented previously as might be expected.30 DynaPulse diastolic observations in this study

Blood pressure, mm Hg Systolic Diastolic Mean arterial Heart rate, beats/min Distensibility, mm Hg/L/min

Measurement Error n ¼ 69 SD

CV, %y

5.53 3.41 3.08 4.16 0.0095

4.57 4.91 3.58 5.46 14.67

Intraclass Correlation Coefficient (Reliability) 0.85 0.87 0.91 0.82 0.58

* Observation is mean of three measurements. y Coefficient of variation, % ¼ (SD/x) (100).

instruments, and Table 4, indicate that systolic measurements from DynaPulse are approximately 9 mm Hg higher (P < .0001) and diastolic measurements are 5 mm Hg lower (P < .0001) than measurements obtained for the first (K1) and fourth (K4) phases on the sphygmomanometer, respectively. Table 4 indicates that diastolic measurements from DynaPulse are higher, a difference of less than 1 mm Hg, than the Phase 5 (K5) diastolic from the sphygmomanometer. Systolic measurements, as quantified by the standard error of the mean, are slightly more variable than are diastolic observations on both instruments. Variability of systolic measurements on all instruments is comparable, similar comments apply to variability of diastolic measurements. Random assignment of subjects to the mercury instruments provides independent estimates of differences between sphygmomanometer and DynaPulse. Table 4 also demonstrates the consistency of differences between sphygmomanometer and DynaPulse, not only for systolic and diastolic BP but also for MAP.

BP Measurements

Figure 2. Bland-Altman agreement analysis: (a) systolic, (b) diastolic.

S.-S. Chio et al. / Journal of the American Society of Hypertension 5(1) (2011) 12–20 Table 4 Comparison between mercury sphygmomanometer and DynaPulse measurements of blood pressure Variable, mm Hg

Mean  SD (SE) n ¼ 803

Blood Pressure Systolic First sphygmomanometer Second sphygmomanometer DynaPulse Diastolic Phase 4, first sphygmomanometer Phase 4, second sphygmomanometer Phase 5, first sphygmomanometer Phase 5, second sphygmomanometer DynaPulse Difference* Systolic, first sphygmomanometer Systolic, second sphygmomanometer Phase 4 diastolic, first sphygmomanometer Phase 4 diastolic, second sphygmomanometer Phase 5 diastolic, first sphygmomanometer Phase 5 diastolic, second sphygmomanometer Mean arterial pressure First sphygmomanometer Second sphygmomanometer DynaPulse Difference First sphygmomanometer Second sphygmomanometer

112.0  11.1 (0.4) 111.2  11.3 (0.4) 120.7  12.8 (0.4)

74.1 73.8 68.3 67.8 68.7

    

8.9 8.7 9.3 9.2 8.2

(0.3) (0.3) (0.3) (0.3) (0.3)

8.7  9.4 (0.3)y 9.4  9.1 (0.3)y 5.4  6.9 (0.2)y 5.1  6.4 (0.2)y 0.4  6.8 (0.2) 0.9  6.4 (0.2)y

86.7  8.9 (0.3) 86.3  8.9 (0.3) 85.8  9.2 (0.3) 0.9  6.6 (0.2)y 0.4  6.3 (0.2)

* Difference ¼ mercury sphygmomanometer - automatic instrument. y P < .0001 (t-test for paired data).

were shown to be equivalent to fifth phase measurements from the sphygmomanometer. Understanding such a relationship in a DynaPulse instrument is important for routine clinic BP measurement as well as self-BP monitoring. For example, pulse pressure is greater with the DynaPulse method and pulse pressure is an important hemodynamic variable related to heart diseases.31,32 Differences between auscultatory and oscillometric methods for measuring BP have been observed in other studies. In a sample of 51 subjects of ages 23–54 years (similar to ages observed in this population), Brinton et al33 noted systolic and diastolic measurements that were, respectively, 5.6 mm Hg higher and 2.0 mm Hg lower than observations from the sphygmomanometer. Studies using other noninvasive pulse-waveform analysis devices (Finapres and

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DinaMAP) have reported similar directional differences when automatic devices were compared with random zero34 or mercury30 sphygmomanometers. On the other hand, Graettinger and coworkers35 noted no consistent difference between auscultatory and oscillometric systolic and diastolic measurements. Graettinger35 also observed that both sphygmomanometric and oscillometric techniques underestimated intra-arterial systolic pressures by 5–8 mm Hg, and overestimated diastolic BP by a corresponding amount. Finnegan36 noted a similar mean systolic underestimation of >5 mm Hg, and diastolic overestimation of approximately 8 mm Hg. Stolt37 reported a mean systolic underestimation with respect to intra-arterial pressure of 3 mm Hg and diastolic overestimation of nearly 9 mm Hg. It is likely that the DynaPulse instrument is measuring BP closer to intra-arterial pressures than the mercury sphygmomanometer. Brinton et al15 demonstrated that, in a sample of 36 patients undergoing diagnostic evaluation by left-sided cardiac catheterization, DynaPulse mean measurements differed less than 3 mm Hg from measures of intra-arterial blood pressure (systolic: DynaPulse exceeded intra-arterial (IA) by 1 mm Hg; diastolic: DynaPulse exceeded intra-arterial by 3 mm Hg; MAP: no difference in mean levels; pulse pressure: intra-arterial exceeded Pulse Metric by 2 mm Hg). These differences may be considered clinically insignificant except as measured by mercury sphygmomanometer. Newer algorithms with more clinical information derived from DynaPulse oscillometric measurements, including pulse waveform and hemodynamic profiles, over time should provide better indirect approximations of not only BP values but also the physiologic conditions, such as 24-hour circadian rhythm, left ventricular mass, and arterial structural and functional properties that are important to the effect on cardiac and vascular changes.22–24,38,39

BP Clinical Application Issues The differences between DynaPulse BP measurements (Table 4: higher systolic and lower Phase 4 diastolic) and those obtained from the mercury sphygmomanometer could be a concern and/or viewed as a limitation in some clinical applications. Examples that include routine clinic BP screening and home BP monitoring, in which the Join National Committee (JNC) guidelines are referenced,40 could result in over treatment of hypertension due specifically to the 9 mm Hg difference in the systolic reading. However, once it is understood that the DynaPulse systolic closely represents the central aortic systolic value, the difference may prove advantageous for physicians evaluating hemodynamics for improving hypertension treatments. The Conduit Artery Function Evaluation study,41 which concluded ‘‘BP-lowering drugs can have substantially different effects on central aortic pressures and hemodynamics despite a similar impact on brachial BP.’’ is an example. On the other hand, in instances of BP

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epidemiologic and outcome studies, including the Trial of Preventing Hypertension study,42 in which the treatment of prehypertension was based on Joint National Committee-7 guideline, the higher DynaPulse systolic reading could cause issues if systolic value adjustment was later needed. As with most automated BP monitors, DynaPulse was designed and developed for measuring the BP of adults, age 18 or older, according to American National Standard Institution/Association for the Advancement of Medical Instrumentation protocol,3 which may be a limitation when used with children. The DynaPulse BP validation studied 132 subjects (38 males, ages 11–85, mean ¼ 47).43 DynaPulse diastolic is 5 mm Hg lower than K4, but within 1 mm Hg difference compared with K5, which may be a concern when determining diastolic in children and K4 is recommended.44,45 However, from the Bogalusa Heart Study on K4 and K5 in children,16,46 it suggests that both K4 and K5 be recorded, even though K4 is known to be the more reliable and better predictor of adult hypertension. The ability of DynaPulse to measure K5 reliably may be an advantage to BP measurement in children. To understand children’s BP and the causes of hypertension in adulthood better, further studies on K1, K4, K5, and central BP are needed. Recently, the monitoring of children’s ambulatory BP with full 24-hour BP information to understand the BP loads during the day and night as well as circadian rhythms, was suggested.47 Oscillometric BP measurement can be a useful tool in hypertension management when its differences from traditional auscultatory method are further understood.

Acknowledgment We would like to thank the subjects that made this study possible and the staff members of the Bogalusa Heart Study for assistance in data collection. In addition, we acknowledge the support of Pulse Metric, Inc, which provided DynaPulse2000A devices and support for those at Tulane University.

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Appendix In1988, the first DynaPulse cuff sphygmomanometer was built and recorded the digitized pulse waveform during a blood pressure (BP) measurement. Since then, increasing interest has been directed to both the nature of bell-shaped oscillometric pulse signals and the gradually changing patterns of pulse waveform when cuff pressure decreases from higher than systolic to below diastolic. Figure 1 illustrates this pulse dynamics phenomenon and the five-waveform types of interest, W-1 to W-5, that are observed. Following are descriptions of these five waveforms and their most likely associations to the five phases of Korotkoff sounds.

W-1 The first pulse with its waveform displaying negative pressure at the bottom or beginning of the diastole cycle (the end of systole cycle). This indicates that a small amount of blood is flowing through the cuff-restricted brachial artery area, an effect of the Bernoulli principle. This point (W-1) at the cuff pressure is determined as the systolic of pulse dynamics, or the DynaPulse Systolic BP. It was compared and validated against central aortic systolic BP measured by catheterization method (146  4 vs. 145  5 mm Hg, n ¼ 36, r ¼ 0.94) at University of California San Diego Medical Center with results published elsewhere.15 At point W-1, the amount of blood flowing through the brachial artery under the cuff is small, resulting in small pulse amplitude, and may not have strong enough force to generate Korotkoff phase-1 sound (K1).

W-2 Where cuff pressure further decreases, its restriction to brachial artery becomes less, and significant amount of blood flows through, pulse amplitude increases, which interacts and applies enough force to brachial artery, and K1 can be detected by a stethoscope placed under the cuff and on the down-stream of the brachial artery. As described in the American National Standard Institution/Association for the Advancement of Medical Instrumentation (ANSI/AAMI)

1987 guidelines,3 the K1 was described as ‘‘begins with the sudden appearance of a faint, clear, tapping or thumping sound that gradually increases in intensity.’’

W-3 In this area, DynaPulse oscillometric pulses display peak amplitude, and the following pulses, with its lower portion of pulse waveform, known as the diastolic cycle, showing a balanced ‘‘V’’ shape. The associated cuff pressure at this point is used to determine the mean arterial pressure (MAP). The DynaPulse MAP was compared and validated against invasive catheterization pressure at central aorta with results of (100  3 vs. 100  3 mm Hg, r ¼ 0.95).15 In general clinical practice, MAP may be estimated from Korotkoff K1 systolic and K4 diastolic readings as MAP ¼ 1/3*K1-SBP þ 2/3*K4-DBP. Pulses around this W-3 area is most likely where the Korotkoff sounds phase-2 and phase-3, as described in ANSI/AAMI 1987 Guideline,3 would become detectable. Phase II: Phase I ends and Phase II begins when the sounds change to a loud ‘‘swishing’’ murmur. Phase III: The beginning of Phase III occurs when the sounds assume a loud, distinct, knocking quality. These sounds are less intense than those of Phase I.

W-4 Although cuff pressure continues to drop, the pulse waveform, specifically the diastolic cycle or lower portion, changes shape significantly. This may associate closely to the ‘‘tone’’ change, or the ‘‘muffled’’ or ‘‘murmur-like swishing’’ of the Korotkoff phase-4 (K4) diastolic sound, as described at ANSI/AAMI 1987 Guideline.3 Phase IV: Begins when the sounds suddenly become muffled and have a faint murmur-like or ‘‘swishing’’ quality.

W-5 At this cuff pressure, the DynaPulse oscillometric pulse waveform has its diastolic cycle or lower portion, reach a steady state and remain unchanged for the following several pulses. The beginning of this phase is the determined point of DynaPulse Diastolic pressure (W5). It most likely represents the Korotkoff phase-5 sound detected, according to this study with their difference being less than 1 mm Hg. When compared with invasive catheterization central aortic diastolic measurement, we found DynaPulse W-5 diastolic to be approximately 3 mm Hg higher (80  2 vs. 77  2 mm Hg, n ¼ 36, r ¼ 0.91).15 For reference, the Korotkoff phase-5 sound described in ANSI/ AAMI 1987 Guideline3 was: ‘‘Phase V-begins when silence develops.’’