Carotid baroreflex regulation of vascular resistance in high-altitude Andean natives with and without chronic mountain sickness.

We investigated carotid baroreflex control of vascular resistance in two groups of high-altitude natives: healthy subjects (HA) and a group with chronic mountain sickness (CMS), a maladaptation condition characterized by high haematocrit values and symptoms attributable to chronic hypoxia. Eleven HA controls and 11 CMS patients underwent baroreflex testing, using the neck collar method in which the pressure distending the carotid baroreceptors was changed by applying pressures of -40 to +60 mmHg to the chamber. Responses of forearm vascular resistance were assessed from changes in the quotient of blood pressure divided by brachial artery blood velocity. Stimulus-response curves were defined at high altitude (4338 m) and within 1 day of descent to sea level. We applied a sigmoid function or third-order polynomial to the curves and determined the maximal slope (equivalent to peak gain) and the corresponding carotid pressure (equivalent to 'set point'). The results showed that the peak gains of the reflex were similar in both groups and at both locations. The 'set point' of the reflex, however, was significantly higher in the CMS patients compared to HA controls, indicating that the reflex operates over higher pressures in the patients (94.4 +/- 3.0 versus 79.6 +/- 4.1 mmHg; P < 0.01). This, however, was seen only when subjects were studied at altitude; after descent to sea level the curve reset to a lower pressure with no significant difference between HA and CMS subjects. These results indicate that carotid baroreceptor control of vascular resistance may be abnormal in CMS patients but that descent to sea level rapidly normalizes it. We speculate that this may be explained by CMS patients having greater vasoconstrictor activity at altitude owing to greater hypoxic stimulation of chemoreceptors.

Water drinking improves orthostatic tolerance in patients with posturally related syncope.

Water drinking improves OT (orthostatic tolerance) in healthy volunteers; however, responses to water in patients with PRS (posturally related syncope) are unknown. Therefore the aim of the present study was to examine whether water would improve OT in patients with PRS. In a randomized controlled cross-over fashion, nine patients with PRS ingested 500 ml and 50 ml (control) of water 15 min before tilting on two separate days. OT was determined using a combined test of head-up tilting and lower body suction and expressed as the time required to induce presyncope. We measured blood pressure and heart rate (using Portapres) and middle cerebral artery velocity (using transcranial Doppler). SV (stroke volume) and TPR (total peripheral resistance) were calculated using the Modelflow method. OT was significantly (P<0.02) greater after drinking 500 ml of water than after 50 ml (25.4+/-1.5 compared with 19.8+/-2.3 min respectively). After ingestion of 500 ml of water, blood pressure during tilting was higher, the tiltinduced reduction in SV was smaller and the increase in TPR was greater (all P<0.05). The correlation coefficient of the relationship between cerebral blood flow velocity and pressure was lower after 500 ml of water (0.43+/-0.1 compared with 0.73+/-0.1; P<0.05), indicating better autoregulation. In conclusion, drinking 500 ml of water increased OT and improved cardiovascular and cerebrovascular control during orthostasis. Patients with PRS should be encouraged to drink water before situations likely to precipitate a syncopal attack.

Water drinking acutely improves orthostatic tolerance in healthy subjects.

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.