Squat-stand test response following 10 consecutive episodes of head-up tilt

INTRODUCTION - the cardiovascular system is adaptable to multiple exposures to gravity over several days, and to repeated exposures in a single day. This study aimed to investigate if the cardiovascular adaptation observed following 10 +75° head-up tilts (HUT) would improve the responses to the squat-stand test (SST). METHODS - There were 16 subjects who were randomly allocated into either a tilting group that underwent 10 +75° HUTs in 70 min (Tilting) or a control group that underwent 70 min of rest (Control). Before and after HUT or rest, subjects performed a SST (SST1 and SST2, respectively). Heart rate (HR), BP [systolic (SBP), diastolic (DBP), mean (MAP)], total peripheral resistance (TPR), stroke volume (SV), and cardiac output (CO) were determined during both SSTs. The final 30 s of squat and the first 30 s of stand (divided into three 10-s blocks termed Stand10, Stand20, and Stand30) were compared between SST1 and SST2. RESULTS - In the Tilting condition, during the squat phase of SST2, the following were significantly elevated: SBP (131 ± 9 to 140 ± 7.2 mmHg) and MAP (94 ± 8.7 to 105 ± 10.2 mmHg); DBP (76 ± 9.4 to 87 ± 11.9 mmHg); TPR (1197 ± 524.6 to 229 ± 610.5 dyn · s-1 · cm-5) and HR were significantly decreased (78 ± 6.9 to 73 ± 7.5 bpm) compared with SST1. At Stand10, DBP and MAP were significantly increased (59 ± 9.5 to 69 ± 15.7 mmHg and 74 ± 8.9 to 84 ± 14.7 mmHg, respectively); at Stand20, SBP was increased (121 ± 17 to 128 ± 22 mmHg); and at Stand30 the following were increased: DBP (64 ± 8.8 to 75 ± 16.1 mmHg); SBP (127 ± 9.2 to 139 ± 15.8 mmHg); and MAP (79 ± 8 to 90 ± 14.9 mmHg). There were no differences observed between SST1 and SST2 in the Control group. DISCUSSION - Cardiovascular responses to the SST can be improved with 10 consecutive +75° HUTs. This is predominantly due to an increase in DBP, indicative of a change in vascular resistance.