RESUMO
BACKGROUND: Cardiovascular reflexes that regulate blood pressure (BP) adapt during repeated exposure to +Gz acceleration separated by short (< 20 s) breaks, but whether this effect is preserved with longer intervals remains unknown. METHODS: There were 17 subjects who completed 5 repeated gradual onset (0.1 G x s(-1)) runs (GOR1-5) to 60 degrees peripheral light loss (PLL) on a human centrifuge, separated by 2 min. Heart rate (HR) and BP were measured before and during each GOR and noninvasive estimation of cardiac output (CO) was used to calculate total peripheral resistance (TPR). RESULTS: Mean resting (+/- SE) systolic BP (138 +/- 4 vs. 128 +/- 3 mmHg) and TPR (13.9 +/- 1.2 vs. 12.7 +/- 1.0 mmHg x L(-1) x min(-1)) were elevated after GOR1 and remained elevated thereafter. Compared with that before GOR1, resting HR was decreased (5-6 bpm) before GOR2-5. Resting CO decreased from 8.3 +/- 0.6 L x min(-1) before GOR1 to a nadir of 7.2 +/- 0.5 L x min(-1) before GOR4 and GOR5. The change in HR under increased +Gz decreased from +39 +/- 3 bpm during GOR1 to +31 +/- 3 bpm during GOR4 and GOR5, but the decrease in eye level BP under +Gz was unaffected. RGT did not change across the five GORs. CONCLUSIONS: Increased resting BP and TPR following a single +Gz exposure suggest alterations in the cardiovascular system expected to confer improved tolerance during subsequent exposures. However, these changes were insufficient to improve +Gz tolerance measured during repeated GORs separated by 2 min.
