Early effects of oral salt on plasma volume, orthostatic tolerance, and baroreceptor sensitivity in patients with syncope.

Salt loading of patients with orthostatic-related syncope has been shown to increase plasma volume and orthostatic tolerance and to decrease baroreceptor sensitivity. This study aims to define the time course of these changes and reveal indications as to the likely mechanisms involved. We measured plasma volume by Evans blue dilution, orthostatic tolerance by time to presyncope in a test of combined head-up tilt and lower body suction, and baroreceptor sensitivity as the cardiac interval prolongation to neck suction. Tests were carried out in patients with poor orthostatic tolerance and low initial urinary salt excretion, before and after 7 or 3 days of salt loading (120 mmol/d). In both groups after salt loading plasma volume and orthostatic tolerance increased and baroreceptor sensitivity decreased. These results confirm the beneficial effects of salt in some patients and show that all these changes occur within 3 days.