This work is based on the premise that hypertension in the spontaneously hypertensive rat (SHR) is a consequence of an imbalance in autonomic cardiovascular control. Spontaneous, instantaneous fluctuations in heart rate (HR) and arterial blood pressure (ABP) were studied by spectral analysis in age-matched groups of SHR and normotensive Wistar-Kyoto (WKY) rats. Continuous recordings of ABP and HR made in conscious, unrestrained, young 1-, 2-, 3-, and 6-mo-old rats show that power spectra of HR fluctuations were similar in both strains, whereas ABP fluctuations were significantly different in the low, presumably vasomotor frequency range being reduced in SHR as compared with age-matched WKY rats. In 1-mo-old rats, the difference between strains became smaller with urethan anesthesia and disappeared with spinalization, suggesting that it might have originated in some disparity between the strains at the level of the central nervous system. Low doses of phentolamine elicited a fall in low-frequency ABP fluctuations of WKY but a rise in SHR, whereas larger doses produced a fall in the power of both. In SHR, reduction in the power spectrum of ABP fluctuations in the presence of unaltered HR fluctuations may indicate an impaired control of sympathetic drive to resistance vessels compared with WKY. Note that in young SHR, hypertension also cannot be detected by conventional measurement. Fluctuations in ABP in WKY may reflect an optimal level of sympathetic activity; reduction in fluctuations may be associated with an increase or decrease in sympathetic activity at the vascular bed.(ABSTRACT TRUNCATED AT 250 WORDS)

A quantitative, noninvasive method of assessing autonomic control, based on the spectral analysis of beat-to-beat fluctuations in heart rate (HR), was applied to patients with chronic renal failure (RF). Since the power spectrum of HR fluctuations measures the dynamic nervous control of HR, it can be used to quantitate a normal control system as opposed to a disturbed or depressed system. Indeed, in RF patients, a strong reduction in the HR power spectrum was observed in all frequency ranges, both sympathetically and parasympathetically mediated. A similar depression in autonomic control was demonstrated in patients on hemodialysis or peritoneal dialysis. RF patients not yet undergoing dialysis show a lesser degree of depression. Spectral analysis of HR fluctuations in RF patients makes it possible to quantitate autonomic dysfunction and to reliably measure its development as a function of time, and requires only a 10-min standard electrocardiogram recording.

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In the present study, our aim was to evaluate the applicability of the nonlinear technique to the investigation of cardiovascular control. We applied an approach known as the "surrogate data method" to test for nonlinear components in the blood pressure (BP) signal. Our results strongly indicate that there are nonlinear components in the BP time series taken from a Wistar-Kyoto rat (WKY), suggesting that the use of nonlinear methods may provide new information about the BP control system. We developed a procedure appropriate for the stable and reliable calculation of the Grassberger-Procaccia correlation dimension (D) of the arterial BP signal. The saturation value D was 5.48 +/- 0.30 for the WKY group and 5.92 +/- 0.26 for the spontaneously hypertensive rat (SHR) group, with P < 0.001. We also found that in the WKY group D displays a significant response to complete alpha 1 blockade and bleeding, whereas no response is observed in the SHR group. These results imply that differences in the control mechanisms may be detected by the nonlinear dynamics approach both under baseline conditions and when interfering with cardiovascular control.