RESUMO
BACKGROUND: Orthostatic symptoms and syncope are common, even in apparently healthy subjects. In patients with severe autonomic dysfunction, water drinking elicits an acute pressor response and improves orthostatic hypotension. We tested the hypothesis that water drinking also improves orthostatic tolerance in healthy subjects. METHODS AND RESULTS: In a randomized, controlled, crossover fashion, 13 healthy subjects (9 men, 4 women, 31+/-2 years) ingested 500 mL and 50 mL of mineral water 15 minutes before head-up tilt on two separate days. Finger blood pressure, brachial blood pressure, heart rate, thoracic impedance, and blood flow velocity in the brachial artery and the middle cerebral artery were measured. Orthostatic tolerance was determined as the time to presyncope during a combined protocol of 20 minutes of 60 degrees head-up tilt alone, followed by additional increasing steps of lower body negative pressure (-20, -40, and -60 mm Hg for 10 minutes each or until presyncope). Drinking 500 mL of water improved orthostatic tolerance by 5+/-1 minute (range, -1 to +11 minutes, P<0.001). After drinking 500 mL of water, supine mean blood pressure increased slightly (P<0.01) as the result of increased peripheral resistance (P<0.01). It also blunted both the increase in heart rate and the decrease in stroke volume with head-up tilt. Cerebral blood flow regulation improved after water drinking. CONCLUSIONS: Water drinking elicits an acute hemodynamic response and changes in cerebrovascular regulation in healthy subjects. These effects are associated with a marked improvement in orthostatic tolerance.