The change in Baroreflex Regulation of Heart Rhythm after “Dry” Immersion Appears during Orthostasis, but not Lower Body Negative Pressure Test

Abstract

The ratio of low-frequency (LF, ~0.1 Hz) waves of RR interval duration (RRI) and systolic blood pressure (SAP) reflects the cardiac baroreflex sensitivity (BRS). Gravitational unloading (GU) may alter BRS during the passive orthostatic test (HUT) and lower body negative pressure (LBNP) test. Both effects cause blood redistribution to the lower body, but HUT is accompanied by greater unloading of sinocarotid baroreceptors than LBNP and activation of the vestibulosympathetic reflex but GU effects on BRS in these tests have not been directly compared previously. In this study we tested the hypothesis that the effect of “dry” immersion (DI, on-ground model of GU) on BRS in the same subjects will be more pronounced during HUT than during LBNP, which causes a comparable decrease in stroke volume. Nine healthy men participated in two test sessions (before and after 7-day DI) consisting of five 3-min HUT (65°) and five 3-min LBNP (–35 mmHg) with averaging the parameters in each test. Wavelet analysis was used to determine the amplitude of the RRI and SAP waves in the range of 0.05–0.13 Hz. The amplitude of LF waves of SAP increased in both tests, after DI - more significantly in HUT. The amplitude of LF RRI waves decreased in the two tests; the degree of decrease did not differ between tests and did not change under the influence of DI. The α-coefficient (the ratio of the amplitudes of RRI and SAP LF waves) decreased equally in the two tests before DI, but after DI, the degree of α-coefficient reduction increased in HUT test but did not change in LBNP test. Thus, the effect of DI on BRS is evident in HUT, but not in LBNP, which may be explained by the more pronounced influence of HUT on the mechanisms of neural control of heart rhythm.