HYPOVOLEMIA IN SYNCOPE AND ORTHOSTATIC INTOLERANCE ROLE OF THE RENIN-ANGIOTENSIN SYSTEM

PURPOSE: Orthostatic intolerance is the cause of significant disabilit y in otherwise normal patients. Orthostatic tachycardia is usually the dominant hemodynamic abnormality, but symptoms may include dizziness, visual changes, discomfort in the head or neck, poor concentration, f atigue, palpitations, tremulousness, anxiety and, in some cases, synco pe. It is the most common disorder of blood pressure regulation after essential hypertension. There is a predilection for younger rather tha n older adults and for women more than men. Its cause is unknown; part ial sympathetic denervation or hypovolemia has been proposed. METHODS AND MATERIALS: We tested the hypothesis that reduced plasma renin acti vity, perhaps from defects in sympathetic innervation of the kidney, c ould underlie a hypovolemia, giving rise to these clinical symptoms. S ixteen patients (14 female, 2 male) ranging in age from 16 to 44 years were studied. Patients were enrolled in the study if they had orthost atic intolerance, together with a raised upright plasma norepinephrine (greater than or equal to 600 pg/mL). Patients underwent a battery of autonomic tests and biochemical determinations. RESULTS: There was a strong positive correlation between the blood volume and plasma renin activity (r = 0.84, P = 0.001). The tachycardic response to upright po sture correlated with the severity of the hypovolemia. There was also a correlation between the plasma renin activity measured in these pati ents and their concomitant plasma aldosterone level. CONCLUSIONS: Hypo volemia occurs commonly in orthostatic intolerance. It is accompanied by an inappropriately low level of plasma renin activity. The degree o f abnormality of blood volume correlates closely with the degree of ab normality in plasma renin activity. Taken together, these observations suggest that reduced plasma renin activity may be an important pathop hysiologic component of the syndrome of orthostatic intolerance. (C) b y Excerpta Medica, Inc.