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, ~0.1 Hz) waves of RR intervalduration (RRI) and systolic blood pressure (SAP) reflects the cardiacbaroreflex sensitivity (BRS). Gravitational unloading (GU) may alter BRSduring head-up tilt test (HUT) and lower body negative pressure(LBNP) test. Both effects cause blood redistribution to the lowerbody, but HUT is accompanied by greater unloading of sinocarotid baroreceptorsthan LBNP and activation of the vestibulosympathetic reflex. However,GU effects on BRS in these tests have not been directly comparedpreviously. In this study we tested the hypothesis that the effectof dry immersion (DI, on-ground model of GU) on BRS in the samesubjects will be more pronounced during HUT than during LBNP, whilecausing a comparable decrease in stroke volume (SV). Nine healthymen participated in two test sessions (before and after 7-day DI)consisting of five 3-min HUT (65°C) and five 3-min LBNP (–35 mmHg)with data averaging in each test. Wavelet analysis was used to determinethe 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, afterDI—more significantly in HUT. The amplitude of LF RRI waves decreasedin two tests; the percentage decrease did not differ between tests anddid not change under the influence of DI. The α-coefficient (theratio of RRI and SAP LF wave amplitudes) decreased equally in twotests before DI. After DI, more pronounced α-coefficient reduction wasobserved in HUT test but not in LBNP test. Thus, the effect of DIon BRS is evident in HUT, but not in LBNP, which may be explainedby the more pronounced influence of HUT on the mechanisms of neuralcontrol of heart rhythm.