No abstract

ABSTRACT. We studied the role of the autonomic nervous system in the regulation of heart rate variation (HRV) in 12 chronically instrumented neonatal lambs. HRV was quantified from ECG tracings by computing periodic HRV distributions at frequencies of 0.02-1.00 Hz, using power spectral analysis of heart rate, and also by HRV indices. Heart rate declined more during the 1st than the 2nd mo after birth. Multiple regression analysis showed that the heart rate responses to vagal and to @-adrenergic blockade had an independent negative association both with age and with the initial mean heart rate, whereas the overall HRV response had a positive association with age. Vagal blockade led to a 70-80% decrease in the beat-to-beat HRV in all lambs ( p < 0.001). The overall HRV indices decreased by 40-65% in lambs (< 30 d old ( p < 0.001) and about 30% in those >30 d old ( p < 0.05). In the power spectrum the greatest decrease was seen in the high-frequency components of HRV. @-Blockade led to a decrease of about 50% in all HRV ( p < 0.001) in the younger lambs, without frequency selection. In the older lambs, it had no effect on the beat-to-beat HRV, but the overall HRV (coefficient of variance) decreased maximally by 40% ( p < 0.01), with a significant reduction in the low-frequency components of HRV. These results suggest that in the regulation of HRV after birth dual control via the autonomic nervous system is most important. In the older lambs, developmental changes result in precise regulation of the fast heart rate fluctuations mainly by the vagal division, whereas the slow fluctuations are partially regulated by the vagal and @-adrenergic divisions. (Pediatv Res 27: 383-391, 1990) Abbreviations HRV, heart rate variation RMSM, root mean square of differences from the mean interval, overall HRV CV, coefficient of variation for RMSM, relative overall HRV RMSR, residual HRV (RMSM) after trend correction CVR, coefficient of variation for RMSR, relative residual HRV RMSSD, root mean square of successive interval differences, beat-to-beat HRV CVS, coefficient of variation for RMSSD, relative beat-tobeat HRV

Heart rate (HR) variability and arterial blood pressure (BP) variability were analysed as functions of foetal breathing movements (FBMs) by means of power spectral analysis in seven foetal lambs during the third trimester of gestation. No evidence of FBM-related changes, either in mean HR, mean systolic or diastolic arterial pressures, were found. Mean arterial pulse pressure, HR variability, and BP variability increased during FBMs. The increase in BP variability occurred at frequencies higher than 0.35 Hz, i.e. those of FBMs. The increase in HR variability occurred at 0.07-1.0 Hz, i.e. at every frequency band except the lowest one. Thus, the increase in HR variability was not frequency-specifically related to FBMs. During FBMs the periodic variability of HR at frequencies > 0.35 Hz was only 10% of total HR variability. We suggest that the FBM-related changes of BP variability may be mediated by direct peripheral, hydraulic mechanisms. HR changes involve autonomic control systems: the vagal component of baroreflex seems to be relatively insensitive, whereas the very slow vasomotor component of HR variability is dominant.

Cardiac and vascular function is mainly under autonomic nervous control within seconds to minutes, although the control is not mature at birth. We studied sympathovagal control of heart rate and blood pressure in chronically catheterized foetal lambs in the last trimester of gestation. Power spectral analysis was used to quantitate the frequency-specific heart rate variability (HRV) and blood pressure variability. We performed 15 experiments in seven foetal lambs. These preliminary studies showed that parasympathetic blockade by atropine (eight experiments) had no significant effect on the distribution of HRV to different frequencies. Beta-sympathetic blockade by propranolol (seven experiments) decreased the ratio of low and mid to high frequency (0.025-0.13 to 0.13-1.00 Hz) HRV (P = 0.02). The increased high frequency HRV in the absence of a similar increase in blood pressure variability and tracheal pressure variability suggests enhanced baroreflex responsiveness after propranolol administration. The frequency-specific sympathetic control of HRV in foetal lambs, the change in ratio of low and mid to high frequency HRV, might have clinical implications in estimating the level of foetal sympathetic activation in the follow-up of high-risk pregnancies.