Bispectrum and bicoherence for the investigation of very high frequency peaks in heart rate variability

Recently (2000), we reported the occurrence of very high frequency (VHF) peaks in the heart rate (HR) and blood pressure (BP) spectra in heart transplant (HT) patients. In this study, we apply the bicoherence function, which provides an estimate of correlation between three spectral peaks, thus allowing us to determine the relations between the VHF peaks and other peaks. A statistical approach was used to determine a threshold which discerns true bispectral peaks from spurious ones for any required level of significance. 25 recordings of HR, BP and respiration were obtained from 13 male HT patients. Extensive arrhythmias prevented spectral analysis in eight recordings. VHF peaks were found in the HR and BP in nine recordings. We found two types of VHF peaks: (1) harmonics of respiration, and (2) peaks totally uncorrelated with respiration. Four recordings exhibited type-1 peaks, one recording had only type-2 peaks and four recordings had both types. The nature of the VHF peaks remains to be resolved.