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Background  Postural tachycardia syndrome ( POTS ) is a heterogeneous condition. We stratified patients previously evaluated for  POTS  on the basis of supine resting cardiac output ( CO ) or with the complaint of platypnea or “shortness of breath” during orthostasis. We hypothesize that postural hyperventilation is one cause of  POTS  and that hyperventilation‐associated  POTS  occurs when initial reduction in  CO  is sufficiently large. We also propose that circulatory abnormalities normalize with restoration of  CO  2 .    Methods and Results  Fifty‐eight enrollees with POTS were compared with 16 healthy volunteer controls. Low  CO  in  POTS  was defined by a resting supine  CO  <4 L/min. Patients with shortness of breath had hyperventilation with end tidal  CO  2  <30 Torr during head‐up tilt table testing. There were no differences in height or weight between control patients and patients with POTS or differences between the  POTS  groups. Beat‐to‐beat blood pressure was measured by photoplethysmography, and  CO  was measured by ModelFlow. Systemic vascular resistance was defined as mean arterial blood pressure/ CO . End tidal  CO  2  and cerebral blood flow velocity of the middle cerebral artery were only reduced during head‐up tilt in the hyperventilation group, whereas blood pressure was increased compared with control. We corrected the reduced end tidal  CO  2  in hyperventilation by addition of exogenous  CO  2  into a rebreathing apparatus. With added  CO  2 , heart rate, blood pressure,  CO,  and systemic vascular resistance in hyperventilation became similar to control.    Conclusions  We conclude that all  POTS  is related to decreased  CO , decreased central blood volume, and increased systemic vascular resistance and that a variant of  POTS  is consequent to postural hyperventilation.

journal of the american heart association

Postural Hyperventilation as a Cause of Postural Tachycardia Syndrome: Increased Systemic Vascular Resistance and Decreased Cardiac Output When Upright in All Postural Tachycardia Syndrome Variants