Heart transplantation-spectral and bispectral analysis

Heart transplantation offers a unique view into an unusual cardiovascular system, with a denervated heart. We investigate noninvasively the compensatory cardiovascular control mechanisms, which develop following transplant. ECG, continuous blood pressure and respiration are recorded in supine position and during transition to standing, and analyzed in time and frequency domains. 25 recordings were obtained from 13 male HT patients at time after transplant (TAT) ranging 0.5-65 months. We observed an interesting evolution with TAT in heart rate response to active standing: from no response, via a slow response, to a fast increase. Another important finding was the existence of very high frequency (VHF) peaks in the power spectra of HR and BP fluctuations, in 8 recordings. Analysis using bicoherence indicates that some of those peaks are harmonics of respiration, while others originate from an unknown source. Our results indicate that with TAT, compensatory cardiovascular mechanisms develop in a biphasic process towards seemingly normal control. We found evidence for the direct effect of the old SA node on the transplanted one, yet no indication of vagal reinnervation. The presence of VHF peaks, unrelated to respiration, suggests the existence of a yet unknown control mechanism, which may be masked in normal subjects.