Renin-Aldosterone Paradox and Perturbed Blood Volume Regulation Underlying Postural Tachycardia Syndrome

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Abstract
Background— Patients with postural tachycardia syndrome (POTS) experience considerable disability, but in most, the pathophysiology remains obscure. Plasma volume disturbances have been implicated in some patients. We prospectively tested the hypothesis that patients with POTS are hypovolemic compared with healthy controls and explored the role of plasma renin activity and aldosterone in the regulation of plasma volume. Methods and Results— Patients with POTS (n=15) and healthy controls (n=14) underwent investigation. Heart rate (HR), blood pressure (BP), plasma renin activity, and aldosterone were measured with patients both supine and upright. Blood volumes were measured with 131I-labeled albumin and hematocrit. Patients with POTS had a higher orthostatic increase in HR than controls (51±18 versus 16±10 bpm, PPP=0.010), and total blood volume (689±270 versus 228±353 mL, P−1 · h−1, P=0.996). There was a paradoxically low level of aldosterone in the patients with POTS (190±140 pmol/L versus 380±230 pmol/L; P=0.017). Conclusions— Patients with POTS have paradoxically unchanged plasma renin activity and low aldosterone given their marked reduction in plasma volume. These patients also have a significant red blood cell volume deficit, which is regulated by the renal hormone erythropoietin. These abnormalities suggest that the kidney may play a key role in the pathophysiology of POTS.