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Background Postural tachycardia syndrome ( POTS) manifests as marked tachycardia while standing. We noticed two forms of circulatory response to orthostatic stress in POTS. We investigated cardiovascular and autonomic nervous response to orthostatic stress in the two forms. Methods We studied 79 patients with POTS and 38 healthy control subjects ( Ct). Beat-to-beat blood pressure ( BP) and heart rate ( HR) were non-invasively and continuously measured in the supine and standing positions. Autonomic nervous function was evaluated on power spectral analysis of HR variability and diastolic BP variability. We divided the subjects into two groups: standing-induced tachycardia ( SI group; increase in HR ≥35 beats/min) and supine tachycardia ( Su group; standing HR ≥115 beats/min with standing-induced HR increase <35 beats/min). Results The Su group had higher supine BP and HR compared with the other groups, indicating dominant sympathetic control of the heart in the supine position. While rising, the SI group had a higher increase in HR than the Ct group, indicating excessive withdrawal of vagal tone. The Su group had a smaller increase in HR and a greater decrease of systolic BP and cardiac index by standing compared with the SI group. These results suggest that compensatory mechanisms of sympathetic function during standing failed in the Su group, probably because of exhaustion by the nearly maximum effort to generate sympathetic drive even in the supine position with low central blood volume. Conclusion There is a difference between the two types of POTS, in the balance of resting autonomic function and hemodynamic response to standing. [ABSTRACT FROM AUTHOR]