SPECTRAL-ANALYSIS OF INTERCYCLE HEART FLUCTUATIONS IN THE DIETHYL-ETHER-ANESTHETIZED OR PITHED RAT TREATED WITH L-HYOSCYAMINE

1 Within the context of neural regulation of the activity of sinus nod e pacemaker cells, the study of heart rate variability, as explored in the frequency domain by spectral analysis, was proposed about 15 year s ago as a quantitative tool for the evaluation of short-term autonomi c cardiovascular control. It has since been postulated that the two ma in oscillations observed, one at low and the other at high frequency, may respectively be markers of sympathetic vs. vagal efferent cardiac activity, and that the low- and high-frequency signals may reflect a r eciprocal or 'push-pull' relationship between sympathetic and parasymp athetic control. 2 In our power spectra assessment, ECG R-R intervals were submitted to fast Fourier transformation analysis in order to stu dy the mechanisms underlying the control of heart beats in rats. Data were acquired in conditions of steady arterial blood pressure and card iac and respiratory activity (spontaneous or artificially stimulated) in diethyl-ether-anaesthetized and pithed rats, as well as in a group of control rats, all in the presence and absence of l-hyoscyamine. 3 W ith increasing doses of the parasympathetic antagonist, the fractal di mension of the time-series structure remained stable in most cases. Th e few-frequency spectral component narrowed with increasing drug doses and the high-frequency band underwent either no, or only very slight, changes. 4 In these rodent assays, the low- and high-frequency signal s cannot be interpreted as a push-pull relationship between sympatheti c and parasympathetic control.