Glenn A. Myers

Power Spectral Analysis of Heart Rate Varability in Sudden Cardiac Death: Comparison to Other Methods

Power spectrum analysis of heart rate variability is described and compared to four other reported methods, with respect to their efficacy as predictors of risk of sudden cardiac death (SCD). Approximate frequency domain representations were obtained for each. The underlying physiologic processes which may give rise to spectral components are considered. These methods were employed to analyze 24-h ambulatory ECGs of patient populations at different degrees of risk of SCD. Heart rate variability was found to be reduced in cardiac patients known to be at increased risk of SCD, when compared to those not at increased risk. These differences were greatest in power spectral methods. Thus, power spectrum analysis appears to be more effective than the other methods in segregating these populations, suggesting that this method may be useful in categorizing cardiac patients according to risk of sudden cardiac death.

Heart rate variability and sudden death secondary to coronary artery disease during ambulatory electrocardiographic monitoring

Data are analyzed from 5 patients who died suddenly during ambulatory electrocardiographic monitoring. Three of the patients were also assessed in terms of 2 recently developed indexes of heart rate (HR) variability. One of these, the standard deviation of RR intervals during successive 5-minute segments averaged over 24 hours, has been reported to be a putative index of vagal tone. Comparisons were made with HR variability findings in 20 normal volunteers. Sudden death was due to ventricular tachycardia degenerating into ventricular fibrillation in all cases. Both early (3 patients) and late cycle (2 patients) ventricular premature complexes initiated the terminal dysrhythmia. An increased density of ventricular ectopic activity was noted in the hour before onset of ventricular fibrillation. HR variability as measured by the standard deviation was significantly (p less than 0.01) lower in the patients who died suddenly (30 +/- 10 ms) than in the normal subjects (76 +/- 14 ms). These findings support suggestions that HR variability analysis may be useful in identifying patients at a higher risk of sudden death.

Clinical, hemodynamic and sympathetic neural correlates of heart rate variability in congestive heart failure

Heart rate (HR) variability has long been recognized as a sign of cardiac health. In the presence of heart disease, HR variability decreases, an observation that has been associated with poor prognosis in a number of recent studies. HR variability is particularly altered in congestive heart failure (CHF), a condition associated with a number of typical functional hemodynamic and neurohumoral alterations. The relation of measurements of HR variability to these abnormalities in patients with heart failure has not been carefully examined. Twenty-three patients (19 men, 4 women, mean age 49 years) with New York Heart Association class II to IV CHF were studied prospectively without cardiac medications; radionuclide ventriculography, right-sided heart catheterization, peroneal microneurography, plasma norepinephrine and 24- to 48-hour ambulatory electrocardiography were performed. Average RR interval and its standard deviation, and HR power spectrum (0 to 0.5, 0.05 to 0.15 and 0.2 to 0.5 Hz) were derived from the ambulatory electrocardiographic recordings and compared with left ventricular ejection fraction, thermodilution cardiac output, pulmonary arterial wedge pressure, New York Heart Association class, age, muscle sympathetic nerve activity (peroneal nerve) and norepinephrine level by linear regression. None of the measures of HR variability were significantly related to age, left ventricular ejection fraction, cardiac output or functional classification, whereas the 0.05 to 0.15 and 0.20 to 0.50 Hz components were weakly but significantly related to cardiac output (r = 0.49 and 0.42, p = 0.02 and 0.045, respectively). In contrast, a generally stronger and negative relation was demonstrated between spectral and nonspectral measurements of HR variability, and indicators of sympathoexcitation, muscle sympathetic nerve activity and plasma norepinephrine.(ABSTRACT TRUNCATED AT 250 WORDS)

Eye monitor: microcomputer-based instrument uses an internal mode to track the eye

An eye monitor whose design is inspired by the human visual system is presented. The monitor incorporates an internal representation or model of what the eye looks like to a video camera. The system can measure the position of the eyes and the size of the pupil in the presence of interfering noise and in patients wearing eyeglasses or contact lenses, and it tolerates defocusing due to small movements in depth by the patient. The system makes real-time correction for head and eye movements while measuring pupillary responses to controlled light stimuli. The design and software and hardware components are described, and some applications are noted. Its use for early clinical detection of visual diseases by objectively measuring pupillary responses to carefully controlled light stimuli is examined as an example. Some general observations about using computers in medical measurements are made.<<ETX>>

Unidirectional block in cardiac fibers: effects of discontinuities in coupling resistance and spatial changes in resting membrane potential in a computer simulation study

The mechanisms for conduction and unidirectional block (UDB) in cardiac tissue under spatial changes in cell-to-cell coupling resistivity (R/sub i/) and resting potential (V/sub rest/) were studied. The effects of discontinuities in R/sub i/ as would result from collagenous or fibrotic tissue on propagation characteristics, in the border zone between normal and ischemic tissue, were studied. It was found that conduction block is more likely to occur when an abrupt decrease in R/sub i/ is encountered as compared to an abrupt increase in R/sub i/. Discontinuities in R/sub i/ were found to cause changes in propagation characteristics, changing regions of bidirectional block to UDB or bidirectional propagation. Spatial changes in V/sub rest/ were also studied. It is found that when V/sub rest/ alone was altered, block was not likely to occur, while discontinuities in R/sub i/ superimposed with V/sub rest/ gradients increased the likelihood of block. It is also found that R/sub i/ discontinuities located in the border zone between normal and ischemic tissue can create exit block or propagation of a parasystolic focus.<<ETX>>

Problems in measuring heart rate variability of patients with congestive heart failure

Heart rate variability (HRV) has become an important noninvasive measure of the integrity of the autonomic nervous system in various disease states. The power spectrum of HRV is a means to separate the instability oscillations of the various feedback mechanisms that contribute to cardiovascular homeostasis. The reliability of HRV data is largely unexplored. The day-to-day correlations in the low and mid-frequency components of HRV spectra average 91%, and that of the high-frequency component averages 81%. The correlations among spectral and nonspectral measures of HRV (SD) for the same data segment average 50-60%, suggesting that they encode similar information. Heart rate variability spectra exhibit diurnal variation consistent with physiologic expectation: respiratory sinus arrhythmia (thought to be mediated by parasympathetic tone) and to a lesser extent, the low-frequency spectral component (thought to be of mixed sympathetic-parasympathetic origin) are higher at night than in the daytime; the mid-frequency component (associated with the baroreflex, which is more excited when the patient is upright) is slightly higher during the daytime. Increased frequency of ectopic beats, such as occurs in congestive heart failure, reduces the reliability of the power spectrum since the number of usable data segments falls off rapidly with even small increases in rate of ectopy, and the variance of the estimate (in the method of averaged periodograms) is inversely proportional to the square root of the number of data segments. Using shorter data segments increases the number of segments available, but reduces resolution. Interpolation over ectopic beats (by either linear or cubic splint interpolation) increases the apparent power in low frequencies.(ABSTRACT TRUNCATED AT 250 WORDS)

Interpolation over ectopic beats increases low frequency power in heart rate variability spectra

The authors compared spectra of heart rate variability (HRV) computed from 24 and 48 hour Holter tapes on 34 patients with congestive heart failure. Spectra on each patient were computed in two ways: (A) Segments with ectopic beats were included in the analysis. In this method, the intervals corresponding to ectopic beats and three beats following the last ectopic beat were computed by interpolation, and (B) Segments with ectopic beats were discarded. Spectral components of HRV in the low (0-0.05 Hz) and mid (0.05-0.15 Hz) frequency ranges were significantly (p<0.05) higher in method A than in method B. High (0.02-0.5 Hz) frequency components of the spectra were not significantly different between these two types of analysis.<<ETX>>

Boundary conditions in simulations of cardiac propagating action potentials

Novel boundary conditions (BCs) which terminate propagating action potentials (PAPs) in matched impedance are described. Steady-state PAPs are stored and reused, obviating simulation of long cables to allow PAPs to approach steady state. These BCs help avoid misleading results and reduce computer time, allowing detailed representation of cardiac structures.<<ETX>>

Level dependent signal flow in the light pupil reflex

Latency of pupillary responses to light stimuli are smaller for larger steps of light, and larger for smaller steps of light (Alpern 1954; Lowenstein et al. 1964; Lee et al. 1969; Terdiman et al. 1969; Cibis et al. 1977; and many others). Miller and Thompson (1978), however, reported negligible change in pupil cycle time (period of high gain instability oscillations) with increased mean brightness. Sandberg and Stark (1968) reportd a negligible reduction in phase lag of pupillary responses to sinusoidal light stimuli as the modulation coefficient (m) increased. To resolve the inconsistency between the well-documented dependence of latency upon brightness, and the apparent absence of level dependence in the phase characteristics (as reflected directly in the responses to sinusoidal stimuli and indirectly in pupil cycle time experiments) we measured: 1. Latency to step stimuli of light, 2. Phase of responses to sinusoidal light stimuli and 3. Period (pupil cycle time) of high gain instability oscillations. The dependence of pupillary latency upon stimulus level (both light and accommodation) and the interaction between accommodation and light responses were investigated. We show that most of the level dependence of light-pupil latency resides in the afferent path. In the companion papers, we demonstrate that: 1. Phase of pupillary response to sinusoidal light stimuli is reduced by increased mean light level, but is independent of pupil size and accommodative stimulus level; and 2. The period of high gain oscillations is shown to decrease with increased mean light level. Taken together, these results imply the existence of a Level Dependent Signal Flow (LDSF) operator that resides in the light-pupil pathway, but not in the accommodation-pupil pathway. We propose a systems model of this operator in which the neural signals controlling pupil size are treated as waves whose phase velocity increases in response to brighter stimuli, and decreases in response to dimmer stimuli. When parameters of the model are adjusted to fit measured pupillary latency over a range of light levels, the model exhibits reduced phase lag in response to increased mean light level in the sinusoidal paradigm, and it exhibits reduced pupil cycle time in the high-gain oscillation paradigm. The model exhibits saturation of the LDSF effect in all paradigms at high light levels, as do experimental results. It simulates directional asymmetry of pupillary response to positive and negative steps of light, with constriction more rapid than dilatation. Finally, it simulates tonic pupillary constriction in response to modulation of a light simulus without changing average light level (Varju 1964; Troelstra 1968). All of these stimulated results are in accord with experimental observation.

Mechanisms underlying alterations in power spectra of heart rate variability associated with ectopy

The low frequency components in heart rate variability (HRV) spectra are elevated in power spectra of segments with ectopy. The authors considered several hypothetical mechanisms which might give rise to this difference. (1) The phase of oscillations is disturbed by ectopic beats, resulting in lower coherence. (2) Reflex alterations in autonomic activity due to reduced arterial blood pressure may stimulate larger instability oscillations. (3) Underlying changes in autonomic activity give rise to elevated HRV and to increased rate of ectopy. These hypotheses were tested by computing power spectra by the method of average periodograms of segments just before, after, and centered on each ectopic beat. Spectra were not significantly different for segments before and after ectopic beats, or for segments centered on ectopic beats, suggesting that mechanisms 1 and 2 do not contribute significantly to the difference in power.<<ETX>>