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, 31Ϯ2 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°head-up tilt alone, followed by additional increasing steps of lower body negative pressure (Ϫ20, Ϫ40, and Ϫ60 mm Hg for 10 minutes each or until presyncope). Drinking 500 mL of water improved orthostatic tolerance by 5Ϯ1 minute (range, Ϫ1 to ϩ11 minutes, PϽ0.001). After drinking 500 mL of water, supine mean blood pressure increased slightly (PϽ0.01) as the result of increased peripheral resistance (PϽ0.01). 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.

Patients with postural tachycardia syndrome (POTS) are characterized by development of symptoms of orthostatic intolerance during standing that are not because of hypotension but are associated with tachycardia. The goal of this study was to compare the cardiac and vascular responses to orthostatic stress for patients with POTS (n = 8) with those for patients with low orthostatic tolerance (n = 29) and for healthy control subjects (n = 12). Responses of heart rate (ECG), arterial blood pressure, and brachial artery blood velocity (Doppler) were determined during a progressive orthostatic stress test of head-up tilt and lower body suction. Changes in forearm vascular resistance (mean arterial pressure/brachial velocity) were less for patients with POTS than for healthy persons, and also less than for most of the patients with low orthostatic tolerance. However, patients with POTS did not have a low tolerance to orthostatic stress (measured as time to discontinuation of the test). For the patients with POTS, the test was discontinued often because of symptoms associated with tachycardia but not hypotension, whereas for the other two groups, the test was discontinued because of hypotension. This study shows that for patients with POTS, abnormal sympathetic responses exist, with an increased sympathetic drive to the heart but deficient peripheral vascular responses, as measured in the forearm. This supports the hypothesis that patients with POTS may have a selective peripheral neuropathy, with small responses in some regions being compensated by overactivity in other regions.