Korotkoff sound filtering for automated three-phase measurement of blood pressure

Masking of Korotkoff sounds used in blood pressure measurement through auscultation

Measurement of blood pressure (BP) through manual auscultation and the observation of Korotkoff sounds (KSs) remains the gold standard in BP methodology. Critical to determining BP levels via auscultation is the determination of KS audibility. While absolute sound level audibility is well researched, the problem has not been approached from the point of view of psychoacoustic masking of the sounds. Here, during manual auscultation of BP, a direct comparison is made between what an observer perceives as audible and the electronic analysis of audibility level determined from masking of sound signal levels. KSs are collected during auscultation with an electronic stethoscope, which allows simultaneously observing sound audibility and recording the sound electronically. By time-segmenting the recorded sound around Korotkoff peaks into a test segment and a masking segment, performing Fourier transforms on the segments, and comparing frequency-band sound energy levels, signal-to-noise ratios of a sound to its masking counterpart can be defined. Comparing these ratios to difference limen in the psychoacoustic masking literature, an approximate threshold for sound audibility is obtained. It is anticipated that this approach could have profound effects on future development of automated auscultation BP measurements.

Characterization of the Korotkoff sounds using joint time-frequency analysis

The sounds associated with the five classical Korotkoff phases are clinically important for measuring systolic and diastolic blood pressures. The frequency ranges of the sounds have already been described simply using the overall peak frequencies within each phase by Fourier methods. However, such analysis may be missing potentially useful clinical information. The aim of this study was to compare features associated with the different phases of the Korotkoff sounds obtained during blood pressure measurement using a joint time-frequency analysis (JTFA) technique. A single operator recorded Korotkoff sounds from 25 healthy subjects using a measurement system comprising cardiology stethoscope, microphone, amplifier and recording system for computer sound digitization, and a MiniDisc system for playback to the cardiologist for Korotkoff phase classification. We have shown that using this system the phase classification by the cardiologist is repeatable, with no significant differences found in the number of sounds allocated to phases on two separate recording assessments. The digitized sounds were processed using a MATLAB-based short-time Fourier transform JTFA technique and differences in time, frequency and amplitude characteristics between the phases compared. It was found that on average, phase III had the largest overall amplitude and high frequency energy. Phase II had the greatest high frequency component and longest murmur, and was visibly the most complex phase in terms of time and frequency content. In contrast, phases IV and V had the lowest amplitude and frequency components. Overall, the statistically significant transitions between phases were: phase I to II with increases in high frequency (224 to 275 Hz) (p < 0.01) and sound duration (49 to 98 ms) (p < 0.0001), II to III with a significant decrease in sound duration (to 37 ms) (p < 0.0001), III to IV with decreases in maximum amplitude (0.95 to 0.25), highest frequency (262 to 95 Hz), and relative high frequency energy of the sounds (0.61 to 0.10) (all p < 0.0001), and IV to V with decreases in the maximum amplitude (0.25 to 0.13) (p < 0.0002) and high frequency energy (0.10 to 0.03) (p < 0.005). This study has demonstrated that joint time-frequency analysis of Korotkoff sounds was able to identify characteristic differences associated with the different phases classified by the expert cardiologist. Ultimately, exploiting the joint time and frequency characteristics of the sounds may improve blood pressure measurement and help to assess the stiffness of the peripheral arteries.

Narrowband auscultatory blood pressure measurement

Auscultatory blood pressure measurement uses the presence and absence of acoustic pulses generated by an artery (i.e., Korotkoff sound), detected with a stethoscope or a sensitive microphone, to noninvasively estimate systolic and diastolic pressures. Unfortunately, in high noise situations, such as ambulatory environments or when the patient moves moderately, the current auscultatory blood pressure method is unreliable, if at all possible. Empirical evidence suggests that the pulse beneath an artery occlusion travels relatively slow compared with the speed of sound. By placing two microphones along the bicep muscle near the brachial artery under the occlusion cuff, a similar blood pressure pulse appears in the two microphones with a relative time delay. The acoustic noise, on the other hand, appears in both microphones simultaneously. The contribution of this paper is to utilize this phenomenon by filtering the microphone waveforms to create spatially narrowband information signals. With a narrowband signal, the microphone signal phasing information is adequate for distinguishing between acoustic noise and the blood pressure pulse. By choosing the microphone spacing correctly, subtraction of the two signals will enhance the information signal and cancel the noise signal. The general spacing problem is also presented.

Elevated nocturnal blood pressure assessed by ambulatory automatic monitoring during a stay at high altitude

The aim of this study was to explore, in healthy children, the arterial blood pressure response to a 3-week stay at high altitude (4200 m). An auscultatory automatic ambulatory pressuremeter was used to avoid undue environmental influence on the measurement. The blood pressure was monitored three times in a group of ten boys, aged 10.5 (CI 0.9 years): at sea level (control values), at an altitude of 2100 m after at least 24 h of acclimatization and after at least 24 h at 4200 m altitude. Each period of monitoring extended over 24 h with 10-min intervals between successive measurements. Arterial blood pressure was evaluated separately for the night and day periods. Nocturnal recordings revealed an increase with altitude in systolic as well as in the diastolic blood pressure. Because of the technique used to gather data, this is thought to have represented an independent effect of altitude without interference from the medical environment or diurnal activity.

Low-frequency Korotkoff signal analysis and application

The low-frequency components of the Korotkoff signal are recorded, analyzed, and applied in the derivation of blood pressure estimates. The low-frequency components are found to be the dominant feature of the Korotkoff signal throughout the entire occlusive cuff deflation cycle, and a sharp rise in the energy of these components is found to correlate with the occurrence of systolic pressure. This feature is applied in two separate energy thresholding algorithms which produce estimates of systolic blood pressure which correlate well (r = 0.907 and r = 0.938) with those systolic pressure derived via the auscultatory technique.

Noninvasive blood pressure measurement on the temporal artery using the auscultatory method

Blood pressures in the temporal artery of five normotensive subjects were recorded using a modified auscultatory setup. The setup comprised a pediatric cuff to occlude the artery and a piezoelectric contact microphone to record the Korotkoff sounds. Both the cuff and microphone were held in their respective positions with an adjustable head band. The recordings were taken under four different conditions: the subject lying supine, the subject sitting at rest, the subject sitting immediately after exercise and the subject moving the head gently. These recordings were compared with readings from the brachial artery, obtained with a commercially available automatic blood pressure measuring device. Korotkoff sounds were analyzed in the time and frequency domain. Results indicate that Korotkoff sounds in the temporal artery are much smaller in amplitude, and do not exhibit the same distinctive phases as those of the brachial artery. Despite these differences, these sounds can be used to detect blood pressures at head level. The accuracy of the readings was within +/- 10%. Successful readings were also obtained with gentle head motions, demonstrating that this setup has the potential to be developed into an ambulatory blood pressure monitoring system.

Peripheral vascular effects on auscultatory blood pressure measurement

Experiments were conducted to examine the accuracy of the conventional auscultatory method of blood pressure measurement. The influence of the physiologic state of the vascular system in the forearm distal to the site of Korotkoff sound recording and its impact on the precision of the measured blood pressure is discussed. The peripheral resistance in the arm distal to the cuff was changed noninvasively by heating and cooling effects and by induction of reactive hyperemia. All interventions were preceded by an investigation of their effect on central blood pressure to distinguish local effects from changes in central blood pressure. These interventions were sufficiently moderate to make their effect on central blood pressure, recorded in the other arm, statistically insignificant (i.e., changes in systolic [p < 0.3] and diastolic [p < 0.02]). Nevertheless, such alterations were found to modify the amplitude of the Korotkoff sound, which can manifest itself as an apparent change in arterial blood pressure that is readily discerned by the human ear. The increase in diastolic pressure for the cooling experiments was statistically significant (p < 0.001). Moreover, both measured systolic (p < 0.004) and diastolic (p < 0.001) pressure decreases during the reactive hyperemia experiments were statistically significant. The findings demonstrate that alteration in vascular state generates perplexing changes in blood pressure, hence confirming experimental observations by earlier investigators as well as predictions by our model studies.

Accuracy of ambulatory blood pressure determination: a comparative study

This study was designed to discriminate, according to their accuracy, between three ambulatory pressurometers (Diasys 200R, Novacor; P IV, Del Mar Avionics; SpaceLab 90202, SpaceLab). The evaluation was performed against invasive arterial reference measurements. Accuracy was assessed by calculating the error on pressure (EOP) as the difference between invasive and non-invasive measurement of arterial blood pressure. For the systolic values, accuracy (mean of EOP differences) and uncertainty (SD of these differences) were -0.9 +/- 9.7, -4.3 +/- 10.1 and -16.7 +/- 10.1 mmHg for, respectively, Diasys, PIV and SpaceLab. For diastolic values, they were, respectively, 5.9 +/- 6.7, 6.8 +/- 8.5 and 9.1 +/- 6.6 mmHg. EOP was then separated in two different types of errors: (i) the error of dispersion appreciated by the index of homogeneity calculated by a Lehmann analysis and leading to a statistical classification (ii) the error due to the drift of EOP with the reference value, this last error being easier to correct. Two different behaviours were observed for the EOP: (i) the drift of EOP of systolic values was significantly larger for the oscillometric (SpaceLab) than for the auscultatory (Diasys and P IV) method, with no difference between Diasys and P IV (ii) the homogeneity index was not statistically different among these three devices. These data suggest that, in case the correction of the drift of EOP is carried out, there is no statistical significant difference in accuracy between these three pressurometers. However, in our experimental conditions, the two ambulatory pressurometers recording the Korotkoff sounds have a better accuracy than the one using the oscillometric approach.

The Korotkoff sound

As the auscultatory method of blood pressure measurement relies fundamentally on the generation of the Korotkoff sound, identification of the responsible mechanisms has been of interest ever since the introduction of the method, around the turn of the century. In this article, a theory is proposed that identifies the cause of sound generation with the nonlinear properties of the pressure-flow relationship in, and of the volume compliance of the collapsible segment of brachial artery under the cuff. The rising portion of a normal incoming brachial pressure pulse is distorted due to these characteristics, and energy contained in the normal pulse is shifted to the audible range. The pressure transient produced is transmitted to the skin surface and stethoscope through deflection of the arterial wall. A mathematical model is formulated to represent the structures involved and to compute the Korotkoff sound. The model is able to predict quantitatively a range of features of the Korotkoff sound reported in the literature. Several earlier theories are summarized and evaluated.

Wideband external pulse recording during cuff deflation: a new technique for evaluation of the arterial pressure pulse and measurement of blood pressure

Analysis of the external brachial pulse recorded during standard blood pressure cuff deflation with use of a transducer with a wide frequency response has revealed a reproducible pattern with three distinct components that we have labeled K1, K2, and K3. K1 is a low-amplitude, low-frequency signal that is present with cuff pressures above systolic pressure. K2 is a triphasic signal appearing at systolic pressure and disappearing at diastolic pressure, which approximately corresponds to the audible Korotkoff sound. K3 appears with cuff pressure between systolic and diastolic pressure and continues to be present below diastolic pressure. Intra-arterial pressure recordings made with a high-fidelity Millar catheter-tip manometer revealed K2 and K3 analogs. K3 resembles the intra-arterial pressure waveform and when calibrated according to the pulse pressure, noninvasive dK3/dt determinations correlated well with intra-arterial dP/dt measurements. The appearance/disappearance property of K2 was designated as the "K2 algorithm" and represents a new, objective noninvasive method for measurement of blood pressure. The K2 algorithm compares favorably with intra-arterial measurements, is more accurate than the auscultatory technique, and may be especially useful in clinical situations in which the auscultatory technique does not work well.

Accurate blood pressure measurement

The most critical requirement for obtaining accurate blood pressure measurements is that the Korotkoff sounds be loud. Loudness can be enhanced by various techniques of cuff inflation and chest piece placement. The type of manometer, cuff size, and cuff placement are also important factors in obtaining accurate blood pressure readings. Correct systolic pressure measurement depends on proper inflation and deflation of the cuff. True diastolic pressure is usually closer to the disappearance point of Korotkoff sounds than to the muffling phase. Blood pressure should be recorded to the nearest 5 mm Hg because measurement to the nearest 2 mm Hg is not meaningful and is too difficult and time-consuming.

Comparison of performance of various sphygmomanometers with intra-arterial blood-pressure readings

Seven types of sphygmomanometer were used in random order on each of nine hypertensive patients and the readings compared with simultaneous intra-arterial blood-pressure recordings. All the devices gave significantly different values for systolic pressure, and only two measured diastolic pressure without significant error. Systolic pressure was consistently underestimated (range 31-7 mm Hg), and all but one instrument overestimated diastolic pressure (range 10-2 mm Hg). The variability of readings was least with the standard mercury sphygmomanometer and the random-zero machine, while with some of the more automated devices single readings were in error up to -68/33 mm Hg. The strong correlations found between intra-arterial and cuff systolic pressures with all devices tested and significant correlations for diastolic pressure with all but one device indicate that, with one possible exception, the sphygmomanometers would give accurate results where a change in blood pressure was the main concern.