Effects of different postures on the hemodynamics and cardiovascular autonomic control responses to exercise in postural orthostatic tachycardia syndrome.

Purpose: To assess the effects of two different body positions on the cardiovascular autonomic profile during a single bout of exercise in patients with postural orthostatic tachycardia syndrome (POTS).

Methods: Thirteen patients with POTS and thirteen healthy controls (C) participated in the study. ECG, respiration, beat-by-beat arterial pressure and O₂ consumption (VO2) were continuously recorded while on a cycle ergometer in supine and upright positions, before and during exercise (6 min, 50 Watts). Spectral analysis of RR intervals and systolic arterial pressure (SAP) variability provided indexes of cardiac sympathovagal interaction (LF/HF ratio), cardiac vagal modulation (HF_RR, high-frequency component of RR variability, ~ 0.25 Hz), sympathetic vasomotor control (LF_SAP, low-frequency component of SAP variability, 0.1 Hz) and baroreflex sensitivity (BRS, α_LF).

Results: While supine, patients with POTS showed lower HF_RR and α_LF, greater heart rate (HR), LF/HF and LF_SAP, compared with C, suggesting cardiovascular sympathetic over-activity and reduced BRS. While sitting upright, POTS showed greater HR and reduced HF_RR and α_LF compared with C. During supine exercise, SAP, HR, LF/HF increased and HF_RR and α_LF decreased similarly in POTS and C. In POTS, upright sitting exercise was associated with slightly higher $\dot{V}{O}_2$ , a greater increase in HR whereas LF_SAP was lower than in C.

Conclusion: Upright exercise was associated with excessive enhancement of HR and a blunted increase of the sympathetic vasomotor control in POTS. Conversely, supine exercise-induced hemodynamic and autonomic changes similar in POTS and C, thus making supine exercise potentially more suitable for physical rehabilitation in POTS.