Diverse effects of seven-day dry immersion on hemodynamic responses in head-up tilt and lower body negative pressure tests

Head-up tilt (HUT) and lower body negative pressure (LBNP) tests are commonly used to assess orthostatic tolerance after space flight. Due to the mechanistic difference between the effects of these tests, they can provide different patterning of post-flight alterations, but this has not been proven directly, to our knowledge. Dry immersion (DI) is a ground-based experimental model which reproduces the effects of microgravity on the cardiovascular system. The aim of this work was to compare the effects of 7-day DI on hemodynamic responses to HUT and LBNP in 9 young men. HUT (65 degrees, 3 min) and LBNP (Chibis suit, -35 mm Hg, 3 min) tests were carried out 3 days before DI, on the 7th day of DI and on the 3rd day of recovery. ECG (to calculate heart rate, HR), arterial pressure (AP), and stroke volume (SV) were continuously recorded. The tests were repeated 5 times and the data for all repetitions were averaged. Before DI, HUT and LBNP similarly decreased SV, a measure of orthostatic test. On the 7th day of DI, the decrease of SV as well as the increases of HR and mean AP were observed in supine position. DI similarly increased SV and HR responses in HUT and LBNP tests, but the effect on mean AP response was observed in HUT test and absent in LBNB test. All indicators recovered on post-immersion day 3. Therefore, 7-day DI weakens autonomic AP control only in HUT test, presumably, due to inappropriate increase of lower body vascular resistance.