Evidence of an intrinsic sinus node abnormality in patients with postural tachycardia syndrome

Objective: To determine whether an intrinsic sinus node abnormality is involved in the pathophysiology of the postural tachycardia syndrome (POTS). Subjects, Patients, and Methods: In this prospective study, we compared the relationship between P-wave axis (PWA) and heart rate (HR) in 11 healthy controls and 14 patients with POTS by obtaining 12-lead electrocardiographic recordings during supine rest and during gradual head-up tilt. The HR of controls was titrated with isoproterenol infusion to match the HR of patients. The PWA was compared at different HR levels, and the relationship between HR and PWA was assessed for patients and controls. Primary end points were the PWA-HR relationship in healthy controls, comparison of these data with data from patients with POTS as a group, and identification of a possible subgroup of patients with POTS with irregular PWA-HR relationship. Results: The PWA increased with increasing HR following a similar logarithmic trendline in both groups. The PWA of patients was significantly lower at the lowest comparable HR level but not different at faster HR levels. Three patients (21%) had a clearly abnormal HR-PWA relationship with substantial shift toward lower PWA. Conclusions: Our data support the hypothesis of a primary sinus node abnormality in a subset of patients with POTS. The ability to identify patients with primary sinus node abnormality may have important therapeutic implications.