Реферат: INTRODUCTION Head-down bed rest (HDBR) and dry immersion (DI) are the two most widely used on-ground models of space flight multisystem deconditioning. While both lead to centralization of blood flow, they differ in mechanisms inducing such a fluid shift (Navasiolava et al., 2011, Watenpaugh 2016). That distinction may lead to different cardiovascular outcomes, including different baroreflex response to orthostasis. However, no comparison was performed previously between two models on their long-term effects (>3 days) on cardiac baroreflex functioning. The analysis of low frequency (LF) waves of systolic arterial pressure (SAP) and RR-interval (RRi) is widely used to assess cardiac baroreflex function. The amplitude relation and phase coupling of these waves may be assessed by α-coefficient (“spontaneous baroreflex sensitivity”) (Pagani M., et al., 1988) and by phase synchronization index (PSI) (Negulyaev V.O., 2019) accordingly. We applied such an analysis to data collected in supine and head-up positions to assess differences in HDBR and DI influence on baroreflex cardiac control. Aim To compare the effects of 19-day HDBR and DI on amplitude and phase coupling of SAP and RRi LF waves during head-up tilt test. METHODS Two groups of healthy men where exposed to 21-day HDBR (“HDBR” group, n=9, age 31±5 yrs., BMI 24.2±2.8 kg/m2) or 21-day DI (“DI” group, n=8, age 29±4 yrs., BMI 22.3±2.9 kg/m2). Head-up tilt test (HUT, 65°, 15 min) was performed before, on 6–7 day, 14 day and 19 day of HDBR and DI. ECG (NVX52, MCS, Russia) and blood pressure (Finometer, Finapres Medical Systems, the Netherlands) were continuously recorded; RRi and SAP were calculated for every cardiac cycle. Mean power of RRi (SRRi-LF) and SAP (SSAP-LF) waves in LF (0.05-0.13 Hz) band were calculated using spectra obtained with fast Fourier transform. LF α-coefficient was calculated as square root of SRRi-LF / SSAP-LF ratio and reported as cardiac baroreflex sensitivity (cBRS); mean PSI for SAP and RRi in LF band (PSILF) was calculated as previously described (Negulyaev V.O., 2019). Percent differences between parameters in 15-min supine position and 15-min HUT position are reported as reaction to test. As two men in HDBR and one in DI skipped tests on day 19 due to medical considerations, instead of 2-way ANOVA we used Mixed-effects model with the Geisser-Greenhouse correction and Sidak's multiple comparisons to examine influence of time (“Time” factor) and different models (“Model” factor) during two gravitational unloading (GU) models on baroreflex reactions to HUT. RESULTS Two groups didn’t differ in any parameter collected during tests before HDBR and DI. While SSAP-LF didn’t change in horizontal position during modelled GU, SRRi-LF tended to decrease (p<0.05 – “Time” factor) with no observable difference between models; thus, cBRS in horizontal position also tended to decrease with GU exposure time (p<0.05 – “Time” factor). PSILF in horizontal position didn’t change with exposure time and didn’t differ between models. Responses to HUT are shown in figure 1. SSAP-LF response increased with the exposure time without observable difference between models (Fig. 1 A), while SRRi-LF response decreased (Fig. 1 B). Notably, in DI the decrease of RRi LF power in HUT was quite dramatic (the decrease closing to -100%) and post-hoc tests revealed significantly higher decrease of SSAP-LF in DI compared to HDBR on day 14. Thus, cBRS reaction to HUT aggravated with GU exposition time and showed greater decrease in DI compared to HDBR (p<0.05 – “Time”x”Model” interaction) (Fig. 1 C). PSILF reaction to HUT didn’t change in HDBR while diminished completely in DI from day 14 (Fig. 1 D). Fig.1 Response of SSAP-LF (A), SRRi-LF (B), cBRS (C) and PSILF (D) to HUT obtained at different stages of DI (red) and HDBR (blue). * - p<0.05 – compared to “Before”; # - p<0.05 – compared to same timepoint in other model (Sidak’s multiple comparisons test). CONCLUSION According to our data, DI impairs cardiac baroreflex control more prominently than HDBR, leading to such dramatic changes in cardiac function during HUT as seen during parasympathetic blockade (Clemson P.T., 2022). Notably, gravitational unloading exerts diverse effects on amplitude relation of LF RRi and SAP waves (cBRS) and their phase relation (PSILF), highlighting the fact that these characteristics reflect different aspects of cardiac baroreflex functioning. FUNDING Cardiovascular studies were performed under the State Program of Basic Research (FMFR-2024-0032). REFERENCES Navasiolava N.M. et al., 2011, European Journal of Applied Physiology, 111(7), 1235-1260. Watenpaugh D. E., 2016, Journal of Applied Physiology, 120(8), 900-914. Negulyaev V.O. et al., 2019, Physiological Measurement, 40(5). Pagani M. et al., 1988, Hypertension, 12(6), 600-610. Clemson P.T. et al., 2022, Frontiers in Network Physiology, 2, 891604.

Библиографическая ссылка: Zhedyaev R.Y. , Tarasova O.S. , Borovik A.S. , Vinogradova O.L.
Direct Comparison of Head-Down Bed Rest and Dry Immersion Effects on Human Cardiac Baroreflex During Orthostatic Stress
43rd Annual the International Society of Gravitational Physiology Meeting 26-31 May 2024