Browsing by Author "Hurwitz, Barry E."

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Open Access
Assessment and diagnostic applications of heart rate variability
(1993) Nagel, Joachim H.; Han, Kedu; Hurwitz, Barry E.; Schneiderman, Neil
Though heart rate (HR) is one of the basic parameters in medical diagnostics, precise and reliable assessment of its variability is still an engineering and physiological challenge. Measurement of HR and its changes over time, as in a stress test, is a routine diagnostic procedure to evaluate the cardiovascular condition of a patient. Prenatal monitoring, with its unique problems regarding noninvasive acquisition of reliable fetal data, heavily relies on diagnostic HR analysis.
Open Access
Coherent ensemble averaging techniques for impedance cardiography
(1990) Hurwitz, Barry E.; Shyu, Liang-Yu; Reddy, Sridhar P.; Schneiderman, Neil; Nagel, Joachim H.
EKG synchronized ensemble averaging of the impedance cardiogram tends to blur or suppress signal events due to signal jitter or event latency variability. Although ensemble averaging provides some improvement in the stability of the signal and signal to noise ratio under conditions of nonperiodic influences of respiration and motion, coherent averaging techniques were developed to determine whether further enhancement of the impedance cardiogram could be obtained. Physiological signals were obtained from sixteen male and female subjects during resting conditions, while delivering a speech and while undergoing submaximal bicycle exercise. Results indicated that improved resolution of dZ/dt signal events could be obtained using coherent ensemble averaging. Although some improvement in precision of event location was obtained, most enhancement of the impedance cardiogram occurred in measurement of the amplitude of the dZ/dt maximum (ejection velocity) during speaking and exercise conditions. Validated increases in dZ/dt maximum exceeding 20% were obtained in some subjects with coherent averaging, suggesting that the diagnostic utility of impedance cardiography can be improved by using this technique.
Open Access
The cold pressor test : vascular and myocardial response patterns and their stability
(1993) Saab, Patrice G.; Llabre, Maria M.; Hurwitz, Barry E.; Schneiderman, Neil; Wohlgemut, William; Durel, Lynn A.; Massie, Clifford; Nagel, Joachim H.
The purposes of the present study were to compare the cardiovascular response patterns evoked by three versions of the cold pressor test (either forehead stimulation or hand or foot immersion) and to determine the reproducibility of the responses over a 2-week interval. Blood pressure, heart rate, stroke volume, cardiac output, total peripheral resistance, and systolic time intervals were obtained during rest and during the cold pressor test in 42 young men. Across conditions, the pressor response was supported by peripheral resistance increases with concomitant stroke volume decreases. Although the response patterns were generally similar across sites, exceptions were apparent for heart rate. Forehead stimulation was characterized by no significant change in heart rate, whereas limb (hand or foot) immersion was associated with significant heart rate acceleration. The responses elicited by the three cold pressor test conditions were reliable and showed little evidence of attenuation over the test-retest interval.
Open Access
Decomposition of heart rate variability by adaptive filtering for estimation of cardiac vagal tone
(1991) Han, Kedu; Nagel, Joachim H.; Hurwitz, Barry E.; Schneiderman, Neil
Heart rate fluctuations resulting from respiration and other influences upon the cardiovascular system are encoded into the patterns of heart rate variability (HRV). The fluctuations due to respiration are called respiratory sinus arrhythmia (RSA). Since RSA is primarily mediated through the autonomic nervous system (ANS), it is of interest to separate RSA from other influences to assess the underlying ANS function. On the other hand, the RSA may obscure heart rate responses to external manipulations in psychophysiological tests. A method of partitioning the HRV signal which can provide quantitative estimate of RSA as well as true heart rate responses without respiratory disturbances for psychophysiological studies is developed. The analysis of HRV signal is performed using an adaptive filtering system. With the simultaneously recorded respiration signal as a reference input, the HRV signal can be separated into two components, RSA and fluctuation due to other influences. After the separation, the variance of RSA, an estimate of cardiac vagal tone (ECVT), is readily obtained. The performance of the system was evaluated using artificial test signals as well as real HR V data. As a time domain approach, the method is simple, fast and robust.
Open Access
Differential patterns of dynamic cardiovascular regulation as a function of task
(1993) Hurwitz, Barry E.; Nelesen, Richard A.; Saab, Patrice G.; Nagel, Joachim H.; Spitzer, Susan B.; Gellman, Marc D.; McCabe, Philip M.; Phillips, Donna J.; Schneiderman, Neil
In cardiovascular reactivity studies, interpretations of the processes supporting the blood pressure response may become problematic when systolic blood pressure, diastolic blood pressure, and heart rate all increase in response to a behavioral challenge. Therefore, in addition to evaluating these cardiovascular responses, this study examined cardiac output, total peripheral resistance and systolic time intervals derived from impedance cardiogram, electrocardiogram and phonocardiogram recordings during a speech Stressor, a mirror tracing task, and a foot cold pressor test. All of the behavioral Stressors elicited increases in blood pressure and heart rate, with the largest changes occurring during the overt speech. Based on the examination of the response patterns of the underlying hemodynamic variables it would appear that, in both men and women, the blood pressure increase during the speech preparation period was supported by increased cardiac output; the speech itself resulted in a mixed pattern of increased cardiac output and total peripheral resistance; whereas, the mirror tracing and cold pressor tasks produced increased total peripheral resistance. Although men and women produced similar response patterns to the behavioral challenges, sex differences in the estimates of myocardial contractility were observed during rest. These results provide evidence that different behavioral stressors can produce a distinct yet integrated pattern of responses, whose differences may be revealed, when impedance cardiography is used, to derive sufficient response measures for assessing dynamic cardiovascular processes.
Open Access
Improved reliability of impedance cardiography by new signal processing techniques
(1988) Reddy, Sridhar P.; Shyu, Liang-Yu; Hurwitz, Barry E.; Nagel, Joachim H.; Schneiderman, Neil
This study used the impedance cardiogram (ICG), the phonocardiogram (pCG) and the electrocardiogram (ECG) to determine stroke volume and systolic time intervals. Comparisons between a fully automated PC-AT signal processing system and a computer-assisted visual detection processing system revealed comparable means, standard errors, correlations (Spearman r) and reliability (Chronbach's ex) coefficients on all relevant parameters. Other analyses confIrmed the utility of using Q onset to mark the beginning of ventricular systole, the B point of the ICG to denote the aortic opening and end diastole as the reference for determining the amplitude of dZ/dt max.
Open Access
Influence of signal fidelity on impedance cardiographically derived values at resting and accelerated heart rates
(1991) Hurwitz, Barry E.; Shyu, Liang-Yu; Lu, Chih-Cheng; Reddy, Sridhar P.; Schneiderman, Neil; Nagel, Joachim H.
Since the spectrum of the impedance cardiogram (ICG) extends from DC to 50 Hz, any amplifier with an upper band limit less than 50 Hz can be expected to produce attenuation and distortion of the ICG. This signal attenuation may be systematically enhanced under conditions of high heart rates (HR) when a greater proportion of signal energy will be in the upper frequency range of the ICG spectrum. Therefore, the present study was designed to assess the influence of amplifier bandwidth and signal fidelity on dZ/dtmax, stroke volume (SV), and systolic time intervals (LVET, PEP, OZ, HI). The performance of commonly available commercial systems was tested over a broad range of HRs. The results demonstrated that a digitally differentiated dZ/dt signal using a differentiator with a corner frequency of 50 Hz, when compared with the 15 Hz corner frequency used In the commercial impedance cardiograph, systematically enhanced the dZ/dtmax amplitude and SV measurements as HR increased. For SV the increase ranged from 17 to 30% as HR increased from 70 to 150 bpm. Moreover, the digitally filtered signal had greater resolution and produced less prolonged PEP and QZ intervals and greater HI with increasing HR. These findings indicate that impedance cardiographs with insufficient upper band limits will differentially influence ICG-derived measurements as HR varies.
Open Access
New signal processing techniques for improved precision of noninvasive impedance cardiography
(1989) Nagel, Joachim H.; Shyu, Liang-Yu; Reddy, Sridhar P.; Hurwitz, Barry E.; McCabe, Philip M.; Schneiderman, Neil
Impedance cardiographic determination of clinically important cardiac parameters such as systolic time intervals, stroke volume, and related cardiovascular parameters has not yet found adequate application in clinical practice, since its theoretical basis remains controversial, and the precision of beat-to-beat parameter estimation has until recently suffered under severe shortcomings of available signal processing techniques. High levels of noise and motion artifacts deteriorate signal quality and result in poor event detection. To improve the precision of impedance cardiography, new techniques for event detection and parameter estimation have been developed. Specifically, matched filtering and various signal segmentation and decomposition techniques have been tested on impedance signals with various levels of artificially superimposed noise and on actual recordings from subjects in a laboratory study of cardiovascular response to a cognitive challenge. Substantial improvement in the precision of impedance cardiography was obtained using the newly developed signal processing techniques. In addition, some preliminary evidence from comparisons of the impedance cardiogram with invasive aortic electromagnetic flow measurement in anesthetized rabbits is presented to address questions relating to the origin of the impedance signal.
Open Access
Signal fidelity requirements for deriving impedance cardiographic measures of cardiac function over a broad heart rate range
(1993) Hurwitz, Barry E.; Shyu, Liang-Yu; Lu, Chih-Cheng; Reddy, Sridhar P.; Schneiderman, Neil; Nagel, Joachim H.
Our findings indicate that the impedance cardiogram spectrum extends from DC to 50 Hz. Any amplifier with an upper band limit less than 50 Hz can be expected to produce attenuation and distortion of the impedance cardiogram. This signal attenuation may be systematically enhanced under conditions of high heart rate when a greater proportion of signal energy will be in the upper frequency range of the impedance cardiogram spectrum. Therefore, the present study was designed to assess the influence of amplifier bandwidth on dZ/dtmax, stroke volume, and systolic time intervals (LVET, PEP, QZ, QX). Simultaneously measured ΔZ and dZ/dt signals from two impedance cardiographs, with corner frequencies of 120 and 60 Hz for the ΔZ and 50 and 15 Hz for dZ/dt channels, were contrasted over a broad range of heart rate (70–150 bpm). In addition to the analog dZ/dt signals obtained from the instruments, the ΔZ signals were digitally converted to dZ/dt by off-line digital differentiation with a 50 Hz corner frequency. The results demonstrated that the measurements with the 15 Hz corner frequency, when compared with the 50 Hz corner frequency measurements, systematically attenuated the dZ/dtmax amplitude and stroke volume measurements as heart rate increased. The attenuation of dZ/dtmax and stroke volume ranged from about 13% to 26% as heart rate increased from 70 to 150 bpm. When the upper bandlimit was 50 Hz, the dZ/dt signal had greater resolution of waveform events and produced less prolonged systolic time intervals. The 15 Hz amplifier differentially influenced the B point, Z-peak and X minimum, having no apparent effect on the temporal location of the B point, but delaying the Z-peak about 21.7 ms and the X minimum about 7.4 ms. These findings indicate that impedance cardiographs with insufficient upper bandlimits will differentially influence ICG-derived measurements as heart rate varies.