Blood pressure and heart rate variability in early pregnancy in rats

Abstract

Changes in the autonomic control of the circulation may contribute to the maternal hemodynamic adaptation to early pregnancy. To evaluate this, we studied mean arterial pressure (MAP) and heart rate (HR) in chronically instrumented, conscious rats in early ( days 4, 6, 8, and 10) and late ( day 18) pregnancy ( n = 8) and in nonpregnant rats ( n = 9). MAP and HR were recorded on a beat-to-beat basis and analyzed by spectral analysis. Spectral density power was calculated in low- (0.047–0.305 Hz), mid- (0.305–0.598 Hz), and high-frequency (0.598–1.494 Hz) bands, which contain oscillations that are among others related to myogenic-, sympathetic/vagal-, and vagal/respiration-related influences, respectively. In addition, baroreceptor reflex sensitivity was determined from spontaneous variations in MAP and HR by a sequential time series method and by calculating the transfer gain between MAP and HR in the midfrequency band. Mean values of HR and MAP did not differ between the two groups on day 4. In the pregnant group, MAP fell gradually over days, whereas HR had significantly increased only on day 18. Overall variability in MAP and HR (expressed as coefficients of variation) did not change during pregnancy. Baroreceptor reflex sensitivity did not differ between the groups and did not change with advancing pregnancy. Spontaneous oscillations of MAP and HR at low, mid, and high frequencies were not different between pregnant and nonpregnant rats on days 4 to 10. On day 18, spectral density power of MAP, but not of HR, in the high-frequency band had significantly increased in pregnant rats only, most likely reflecting the increased impact of breathing on MAP fluctuations. We conclude that, with the methods employed, we could not discern any changes in baroreflex sensitivity and MAP and HR variability in pregnancy. This would imply that changes in autonomic activity do not contribute appreciably to the hemodynamic adaptations in early rat pregnancy.