Original Articles

AUSCULTATORY MEAN BLOOD PRESSURE

G.J. Davis* and L. A. Geddes1"

Davis GJ, Geddes LA. Auscultatory mean blood pressure. J Clin Monit 1990;6:261-265

ABSTRACT.This study examined the relationship between direct mean arterial blood pressure and cu~ffpressure for the maximum acoustic index calculated from the Korotkoff sounds in the dog. The acoustic index was computed by summing the squares of amplitudes in each Korotkoffsound complex, thereby providing a measure of acoustic energy content. Mean arterial pressure was compared with cuff pressure for the maximum acoustic index. Ten mongrel dogs were fitted with appropriately sized blood-pressure cuffs containing a microphone mounted inside the bladder and positioned over the brachial artery. The Korotkoff sounds, cuff pressure, and direct arterial pressure were recorded over a range of mean arterial pressures (23 to 155 mm Hg), achieved by manipulating the depth of anesthesia with halothane. It was found that cuff pressure at the maximum acoustic index overestimated mean arterial blood pressure by a mean of 14% (range, - 8 to +30%). WORN. Measurement techniques: blood pressure. Monitoring: blood pressure.

Mean arterial pressure is the tissue perfusion pressure and is becoming increasingly useful as a monitoring quantity. The osciUometric m e t h o d is the only indirect method currently available for obtaining mean arterial pressure. This method, however, does not have sharp transitions at systolic and diastolic pressures [1]. Although the auscultatory method provides endpoints for systolic and diastolic pressure, to date it has not been used to indicate mean arterial pressure. If one plots the subjective loudness o f the K o r o t k o f f sounds in the 4 phases (Fig 1), it is seen that the sounds are loudest in the phase III to IV boundary, which is above diastolic pressure [2]. In comparing the auscultatory and oscillometric methods in the dog, Erlanger [3] observed that the point o f m a x i m u m subjective loudness for the K o r o t k o f f sounds was near mean arterial pressure. Rodbard [4] also suggested that the point o f m a x i m u m K o r o t k o f f sound loudness was near mean arterial pressure. H o w e v e r , to date there has been no investigation o f this relationship. The objective o f this preliminary dog study was to determine if it is possible to use the intensity o f the K o r o t k o f f s o u n d s to identify mean arterial pressure, thereby extending use o f the auscultatory method to provide all three pressures: systolic, diastolic, and mean. T o quantify the intensity o f the sounds, we used an acoustic index (AI) algorithm. From the *School of Electrical Engineering, and the J'Hillenbrand Biomedical Engineering Center, Purdue University, West Lafayette, IN 47907. ReceivedSep 21, 1987, and in revised form Oct 11, 1989. Acceptedfor publication Nov 2, 1989. Address correspondence to Dr Geddes.

METHODSANDMATERIALS The AI for each K o r o t k o f f sound complex provides a numeric value. Because each sound contains two or Copyright 9 1990 by Little, Brown and Company 261

262 Journal of Clinical Monitoring Vol 6 No 4 October 1990

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more positive and negative peaks, .the AI was obtained by adding the squares of the amplitudes of the positive and negative peaks in a sound complex. This resulting quantity is a rough measure of acoustic energy. In this study, an AI value and its associated cuff pressure were determined for each Korotkoff sound complex in the systolic-to-diastolic pressure range. Figure 2 presents an example of the relationship between the AI and cuffpressure during cuffdeflation. The value of cuff pressure associated with the point of maximum Korotkoff sound AI was then compared with direct mean blood pressure. A computer program was developed to calculate the AI for each Korotkoff sound complex, thereby allowing easy identification of the maximum AI and the cuff pressure at which it occurred. The Korotkoff sounds have major sine-wave frequency components between 20 and 40 Hz, and lesser amplitude components up to 100 Hz [2]. For the spectrum of the Korotkoff sounds to be recorded with minimal distortion, a bandwidth of 20 to 200 Hz at the - 3 - d B points [2] was used. Because the ratio of cuff width to limb circumference plays a role in the accuracy of the indirect measurement o f blood pressure [5-7], the dogs were fitted with appropriately sized cuffs, resulting in a cuff width/limb circumference ratio of approximately 0.40. A piezoelectric contact microphone was mounted inside the cuff bladder, with the microphone positioned over the brachial artery to detect the Korotkoff sounds [8,9]. Cuff pressure, Korotkoff sounds, and direct arterial pressure were monitored on a Physiograph (Narco BioSystems, Houston, TX). Data from the three analog channels were recorded on magnetic tape at a speed of 15 inches (38 cm)/s. The recorded data were transferred to the local computing facility using the D E C PDP 11/ 03 computer (Digital Equipment Corp, Maynard, MA)

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to provide analog-to-digital conversion. A sampling rate of 1,000/s with a 12-bit resolution was used. All computer-processed data were checked visually on a graphics terminal. The AI of each Korotkoff sound complex, corresponding cuff pressure, and true mean arterial pressure were obtained from the digitized data. To avoid the inclusion of low-level background noise, we established a dead zone, amounting to 10% of the largest Korotkoff sound amplitude. For each sound complex, the positive and negative amplitudes were squared and summed. The values for all sound complexes were scanned to identify the maximum AI value. The cuff pressure for this AI was then identified. The 10 mongrel dogs (average weight, 25 kg) were sedated with sodium pentobarbital (5 to 10 mg/kg). All animals were then intubated and maintained on a mixture of halothane, nitrous oxide, and oxygen for the duration of each study. The brachial artery of one forelimb was cannulated and connected to a pressure transducer (Cobe Labs, Denver, CO). The transducer was placed at the level of the center of the blood pressure cuffon the opposite forelimb. After a stable arterial pressure was achieved, indirect pressure measurements were made. The cuff was quickly inflated to 250 mm Hg. A linear decay valve (Harvard Apparatus, Cambridge, MA) was used to provide a linear cuff deflation rate of 3 mm Hg/s. The measurement interval ended when the Korotkoff sounds disappeared. After obtaining several measurements at the same arterial pressure, we decreased the arterial pressure incrementally by increasing the depth of anesthesia. Measurements were made in this manner until the depth of anesthesia caused marked respiratory depression. The depth of anesthesia

Davis and Geddes: Auscultatory Mean Blood Pressure 263

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Fig 3. Indirect mean blood pressure, determined j~om the acoustic index, versus direct mean blood pressure for dog 4.

Fig 4. Ratio of indirect to direct mean pressure versus direct mean pressure for dog 4.

Relationship Between Indirect (Auscultatory) and Direct Mean Pressures, with Statistical Analysis, for All 10 Dogs~

Dog No.

Average I/D Ratio

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13.16 8.59 24.00 7.02 11.85 65.56 -5.21 --4.14 -4.07 8.21

0.995 0.996 0.989 0.999 0.993 0.976 0.991 0.984 0.986 0.996

126.4 155.3 159.3 128.2 103.2 117.5 104.0 111.1 105.1 138.0

63.1 73.5 61.2 46.7 30.8 71.1 23.3 56.8 56.8 60.2

~Mean ratio of indirect (I) and direct (D) mean pressures for all 10 dogs was 1.14. was then gradually decreased to increase blood pressure and measurements were takem

RESULTS Figure 3 presents the relationship between indirect (auscultatory) mean and direct (arterial) mean blood pressure for dog 4. Figure 4 presents the ratio o f indirect-todirect mean pressure for the same dog over a direct mean pressure range extending f r o m about 40 to 130 m m H g mean pressure. T h e Table presents the values obtained for the slopes, intercepts, and correlation coefficients for the relationship between indirect (auscultatory) and direct mean arterial pressure for all 10 dogs. Averaged over all 89 data points, the auscultatory mean pressure was 1.14

times the direct mean arterial pressure. In other words, the auscultatory mean pressure was 14% higher than direct mean pressure. T h e average ratios and standard deviations o f indirect-to-direct mean pressure were c o m p u t e d for all dogs o v e r the range o f pressure values examined. These values are s h o w n in Figure 5. For all but 1 dog, the indirect (auscultatory) m e t h o d overestimated direct m e a n arterial pressure. T h e mean indirect-to-direct ratios were plotted versus the ratio o f cuff width to limb circumference; Figure 6 shows this relationship.

mcumoa C o m p a r i s o n o f indirect auscultatory mean blood pressure w i t h direct m e a n a r t e r i a l pressure indicates an aver-

264 Journal of Clinical Monitoring Vol 6 No 4 October 1990

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age overestimation of approxirdately 14% for 89 measurements from 10 dogs over the entire pressure range. The highest overestimation was in the animals with low mean pressure (60 mm Hg and below). The closest agreement was found in the animals with mean pressures around 120 m m Hg. These results suggest that the best correlation occurs with normal to high mean pressure. The cuff width/limb circumference ratios varied between 0.34 and 0.42 for the 10 dogs, yet the average indirect-to-direct pressure ratio values did not vary appreciably among the dogs. This may be because only a narrow range of cuff widths was investigated. However, the optimum ratio for the dog is not known. It is well known from human studies that use of a ratio of less than 0.4 results in an overestimation of indirect blood pressure [6,7]. It is reasonable to expect the maximum Korotkoff sound AI value to occur with a cuff pressure close to mean pressure. At this point the arterial wall is unloaded maximally throughout the cardiac cycle. Consequently, the arterial wall motion will be greatest and the local pulsatile blood flow will also be maximum; both factors would contribute to the genesis of the maximum sound. Determination of systolic and diastolic pressures with the auscultatory method often fails in hypotensive subjects owing to faintness of the Korotkoff sounds. In those dogs with direct mean blood pressures below 50 m m Hg (dogs 4, 5, and 7), mean blood pressure could be estimated by using the maximum AI. We found that the faintest Korotkoff sounds contain enough reliable characteristic peaks to permit identification of the maximum AI, and therefore mean pressure. The average

indirect/direct ratio for these 3 animals was 1.17. Thus, while auscultatory systolic and diastolic pressure measurements may be ambiguous in hypotensive subjects, mean blood pressure can be estimated with our method. The overestimation of mean arterial pressure may be due in part to the algorithm used to compute the AI. A different algorithm might make the agreement closer. Likewise, a different bandpass filter for the Korotkoff sounds could alter the correlation. The use of a larger ratio for cuff width/member circumference may improve the relationship. It is not yet known what the agreement will be in humans with the method described herein. However, the first steps in this direction have been taken. Davis and Geddes [10] compared indirect auscultatory mean and oscillometric mean pressures in humans. It was found that the ratio of auscultatory to oscillometric mean pressure was 1.03 in a 10-subject trial with blood pressures ranging from 65 to 130 mm Hg. Supported by grant HL31089 from the National Heart, Lung, and Blood Institute and sponsored in part by the Kendall Company, Barrington, IL. REFERENCES

1. Geddes, LA, Voelz M, Combs C, Reiner D. Characterization of the oscillometric method for measuring indirect blood pressure. Ann Biomed Eng 1983;10(6):271-280 2. GeddesLA. The direct and indirect measurement of blood pressure. Chicago: Year Book Medical Publishers, 1970 3. Erlanger J. Studies in blood pressure estimation by indirect methods. II. The mechanism of the compression sounds of Korotkoff. Am J Physiol 1916;40:82-125

Davis and Geddes: Auscultatory Mean Blood Pressure 265

4. Rodbard S. The clinical utility of the arterial pulses and sounds. Heart Lung 1972;1(6):776-784 5. Bordley J, Connor C. Hamilton W, et ai. Recommendations for human blood pressure determinations by sphygmomanometers. Circulation 1951;40:503-509 6. Geddes LA, Whistler SJ. The error in indirect blood pressure measurement with the incorrect size of cuff. Am Heart J 1978;96(1):4-8 7. Kirkendall W, Feinlieb M, Fries E, Mark A. Recommendations for human blood pressure determination by sphygmomanometers. Dallas, TX: American Heart Assoc, 1981 8. Geddes L, Moore A. The efficient detection of Korotkoff sounds. Med Biol Eng Comput 1968;6:603-609 9. Wessale JL, Smith LA, Reed ME, et al. Indirect auscultatory systolic and diastolic pressures in the anesthetized dog. Am J Vet Res 1985;46(10):2129-2132 10. Davis GG, Geddes LA. Auscultatory mean blood pressure. J Clin Eng 1989;13:443-446