Purpose: Postural orthostatic tachycardia syndrome (POTS) is characterized by excessive tachycardia in the upright position. To test the hypothesis that patients with POTS have impaired arterial-cardiac baroreflex function, while exercise training normalizes the baroreflex function in these patients.
Methods: Seventeen POTS patients aged 27 ± 9 (mean ± SD) years underwent an exercise training program for 3 months. Arterial-cardiac baroreflex function was assessed by spectral and transfer function analysis of beat-to-beat R-R interval (RRI) and systolic blood pressure (SBP) variability in the supine position and at 60° upright tilt during spontaneous breathing before and after training. Data were compared with 17 healthy sedentary controls.
Results: Even though upright heart rate (HR) was greater in patients than controls, indexes of RRI variability did not differ between groups. Transfer function gain (SBP to RRI), used as an index of arterial-cardiac baroreflex sensitivity was similar between patients and controls in both low- (LF, P = 0.470) and high-frequency (HF, P = 0.663) ranges. Short-term exercise training decreased upright HR and increased RRI variability in POTS patients. LF baroreflex gain increased significantly in the supine position and during upright tilt [analysis of variance (ANOVA), P = 0.04 for training], while HF gain increased modestly after training (ANOVA, P = 0.105 for training) in these patients; however, the baroreflex gains remained within the normal ranges when compared with healthy controls.
Conclusions: These data suggest that patients with POTS have normal arterial-cardiac baroreflex function in both supine and upright postures. Short-term exercise training increases the baroreflex sensitivity in these patients, associated with a decrease in upright heart rate.