Individual time-dependent spectral boundaries for improved accuracy in time-frequency analysis of heart rate variability.

Heart rate variability (HRV) is a major noninvasive technique for evaluating the autonomic nervous system (ANS). Use of time-frequency approach to analyze HRV allows investigating the ANS behavior from the power integrals, as a function of time, in both steady-state and non steady-state. Power integrals are examined mainly in the low-frequency and the high-frequency bands. Traditionally, constant boundaries are chosen to determine the frequency bands of interest. However, these ranges are individual, and can be strongly affected by physiologic conditions (body position, breathing frequency). In order to determine the dynamic boundaries of the frequency bands more accurately, especially during autonomic challenges, we developed an algorithm for the detection of individual time-dependent spectral boundaries (ITSB). The ITSB was tested on recordings from a series of standard autonomic maneuvers with rest periods between them, and the response to stand was compared to the known physiological response. A major advantage of the ITSB is the ability to reliably define the mid-frequency range, which provides the potential to investigate the physiologic importance of this range.

Bicoherence analysis of new cardiovascular spectral components observed in heart-transplant patients: statistical approach for bicoherence thresholding.

Following heart transplant (HT), the patient's heart functions under complete cardiac denervation. As a result, the variability in physiologic signals is extremely reduced. We have previously reported that in addition to the typical spectral components (of very low amplitude), part of the HT patients (above 50%) demonstrated unexpected additional peaks in their heart rate and blood pressure spectra. These peaks may be a result of the development of compensatory mechanism induced by loss of parasympathetic control, or of increased importance of nonlinear control interactions. It is important to quantify these strange, unexpected very-high frequency (VHF) peaks, to understand their origin and their contribution to cardiac control in transplant patients. In this paper, we chose to examine these VHF peaks by applying the bicoherence approach. The reduced signal to noise ratio, occurring in these patients, results, however, in an extremely noisy bicoherence. We, therefore, developed several statistical tools in order to distinguish between "true" bicoherence peaks (reflecting true phase coupling) and spurious peaks. The outcome of these methods was an efficient and sensitive bicoherence thresholding procedure, able to identify most of the spurious peaks. Applying these tools to the bicoherence of cardiovascular signals which display VHF peaks, revealed several significant bicoherence peaks. Interestingly, these peaks consisted of two different types. The first type of VHF peaks simply reflects nonlinear cardiac-respiratory coupling, imposed by nonsinusoidal breathing. The second type, however, is clearly not induced by the respiratory system. We believe that these type-2 VHF peaks reflect the evolution of a new, yet unexplained, compensatory mechanism.

Functional restitution of cardiac control in heart transplant patients.

Cardiovascular control is fundamentally altered after heart transplantation (HT) because of surgical denervation of the heart. The main goal of this work was the noninvasive characterization of cardiac rate control mechanisms after HT and the understanding of their nature. We obtained 25 recordings from 13 male HT patients [age = 28-68 yr, time after transplant (TAT) = 0.5-62.5 mo]. The control group included 14 healthy men (age = 28-59 yr). Electrocardiogram, continuous blood pressure (BP), and respiration were recorded for 45 min in the supine position and then during active change of posture (CP) to standing. The signals were analyzed in the time domain [mean and variance of heart rate (HR) and rise time of HR in response to CP] and the frequency domain [low and high frequency (LF and HF)]. Our principal finding was the consistent pattern of evolution of the HR response to standing: from no response, via a slow response (>40 s, TAT > 6 wk), to a fast increase (<20 s, TAT > 24 mo). HR response correlated with TAT (P < 0.001). LF correlated with HR response to CP (P < 0.0001); HF and HR did not. An important finding was the presence of very-high-frequency peaks in the power spectrum of HR and BP fluctuations. Extensive arrhythmias tended to appear at the TAT that corresponds to the transition from slow to fast HR response to CP. Our results indicate a biphasic evolution in cardiac control mechanisms from lack of control to a first-order control loop followed by partial sympathetic reinnervation and, finally, the direct effect of the old sinoatrial node on the pacemaker cell of the new sinoatrial node. There was no indication of vagal reinnervation.