Arterial baroreflex control of heart rate during exercise in postural tachycardia syndrome

Patients with postural tachycardia syndrome (POTS) have excessive tachyeardia without hypotension during orthostasis as well as exercise. We tested the hypothesis that excessive tachycardia during exercise in POTS is not related to abnormal baroreflex control of heart rate (HR). Patients (n = 13) and healthy controls (n = 10) performed graded cycle exercise at 25, 50, and 75 W in both supine and upright positions while arterial pressure (arterial catheter) and HR (ECG) were measured. Baroreflex sensitivity of HR was assessed by bolus intravenous infusion of phenylephrine at each workload. In both positions, HR was higher in the patients than the controls during exercise. Supine baroreflex sensitivity (HR/systolic pressure) in POTS patients was - 1.3 +/- 0.1 beats -min(-1). mmHg(-1) at rest and decreased to -0.6 +/- 0.1 beats - min(-1). mmHg(-1) during 75-W exercise, neither significantly different from the controls (P > 0.6). In the upright position, baroreflex sensitivity in POTS patients at rest (- 1.4 +/- 0.1 beats . min(-1). mmHg(-1)) was higher than the controls (-1.0 +/- 0.1 beats . min(-1). mmHg(-1)) (P < 0.05), and it decreased to -0.1 +/- 0.04 beats - min(-1). mmHg(-1).during 75-W exercise, lower than the controls (-0.3 +/- 0.09 beats . min(-1). mmHg(-1)) (P < 0.05). The reduced arterial baroreflex sensitivity of HR during upright exercise was accompanied by greater fluctuations in systolic and pulse pressure in the patients than in the controls with 56 and 90% higher coefficient of variations, respectively (P < 0.01). However, when baroreflex control of HR was corrected for differences in HR, it was similar between the patients and controls during upright exercise. These results suggest that the tachycardia during exercise in POTS was not due to abnormal baroreflex control of HR.