Figure 2. Physiology of Standing in a Healthy Individual and Patient with POTS.

Panel A: With the assumption of an upright posture in a healthy individual, there is a downward shift of ~500 ml of blood with a decrease in venous return, stroke volume (SV) and eventually blood pressure (BP) ¹. This “unloads” the baroreceptors, and triggers reflex sympathetic nervous system (SNS) activation with a resultant increase in heart rate (HR) and systemic vasoconstriction (countering the initial decline in BP). In a healthy individual, the net effect of assumption of upright posture is an increase in HR of 10-20 bpm, a minimal change in systolic BP, and a ~5 mmHg increase in diastolic BP. Panel B: In patients with POTS, the initial response to upright posture can include a profound drop in SV. With engagement of the baroreflex, there can be a vigorous increase in SNS tone and an exaggerated increase in HR. The BP may remain unchanged or even increase if there is excessive SNS mediated vasoconstriction. Presyncope can result from the excessive HR increase and/or the increased SNS tone. CO – cardiac output; PNS – parasympathetic nervous system; TPR – total peripheral resistance.