Orthostatic symptoms and syncope are common, even in apparently healthy subjects. Drinking water improves orthostatic hypotension in patients with autonomic failure. The aim of this study was to examine the effects of water in healthy volunteers. In addition to measuring its effects upon orthostatic tolerance (OT), we assessed its effects upon the cardiovascular system and cerebral autoregulation.

A randomised, controlled, cross-over study was performed on thirteen healthy volunteers (aged 31 ± 2 years) who drank 500 ml (test) or 50 ml (control) of water on different days after an overnight fast. The Leeds Teaching Hospitals Research Ethics Committee approved the study. We recorded finger arterial pressure (Finapres, calibrated against a sphygmomanometer), heart rate (ECG), stroke volume (impedance plethysmography), middle cerebral artery blood velocity (MCV, transcranial Doppler ultrasonography) and end-tidal CO₂ (nasal catheter). From these data we determined cardiac output, peripheral and cerebral vascular resistance. Fifteen minutes after drinking OT was assessed as time to presyncope during head-upright tilt and combined lower body suction (El-Bedawi & Hainsworth, 1994). The efficiency of autoregulation was quantified as the correlation coefficient (R) of the relationship between MCV and cerebral arterial pressure (CBP), determined as pressure changed during the orthostatic stress. A low value of R indicates good autoregulation. Data are presented as means ± S.E.M. Statistical significance was assessed using Student’s paired t test. Correlations were assessed using the Spearman ranked correlation coefficient.

Drinking 500 ml water significantly improved OT (36 ± 3 compared with 31 ± 3 min, P < 0.001) and increased supine mean blood pressure (P < 0.01) due to increased peripheral resistance (106 ± 1 % of baseline compared with 100 ± 1 % after 50 ml water). Water drinking significantly blunted the increase in heart rate after 10 min of tilting (+10 ± 1 compared with +16 ± 2 b.p.m. in control, P < 0.001) and the decrease in stroke volume (-38 ± 3 compared with -45 ± 2 % in control, P < 0.01). The autoregulation index (R for regression of MCV on CBP) was significantly less after 50 ml water (0.52 ± 0.08 compared with 0.72 ± 0.06, P < 0.05). The autoregulation index (R) was significantly inversely correlated with OT (P < 0.05). Water had no effect on end-tidal CO₂ levels.

Drinking 500 ml water improved the tolerance to orthostatic stress. This was associated with smaller decreases in heart rate and stroke volume and a larger increase in vascular resistance. Cerebral autoregulation was also significantly improved.

All procedures accord with current local guidelines.