Additional Study Information: This is a Phase 2 Double-Blind Placebo-Controlled Single-Dose Study of Pharmacodynamics,Pharmacokinetics, Safety, and Tolerability of REGN7544, an NPR1 Antagonist Monoclonal Antibody, in Patients with Postural Orthostatic Tachycardia Syndrome. Participant must be 18 to 55 years of age, body mass index between 18 and 35 kg/m2, increase in HR 30 BPM within 10 minutes of changing from supine to a standing, absence of orthostatic hypotension.