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

The ratio of low-frequency (LF, similar to 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 head-up tilt 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. However, 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, while causing a comparable decrease in stroke volume (SV). Nine healthy men participated in two test sessions (before and after 7-day DI) consisting of five 3-min HUT (65 degrees C) and five 3-min LBNP (- 35 mmHg) with data averaging in each test. Wavelet analysis was used to determine the amplitude of the RRI and SAP waves in the 0.05-0.13 Hz range. 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 two tests; the percentage decrease did not differ between tests and did not change under the influence of DI. The alpha-coefficient (the ratio of RRI and SAP LF wave amplitudes) decreased equally in two tests before DI. After DI, more pronounced a-coefficient reduction was observed in HUT test but not 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.