The pathophysiology of POTS is complex, and are related to abnormal cardiovascular autonomic adaptation to postural changes. Under normal conditions, the assumption of upright posture does not result in major changes in blood pressure due to the integration of complex autonomic, circulatory and neurohumoral responses. Upright posture-induced a fluid shift of approximately 700 mL of blood from the upper thorax to the splanchnic circulation and lower extremities, which result in decrease in venous return to the heart, ventricular filling, and stroke volume. These changes cause unloading of the arterial baroreceptors and increase in sympathetic activity, vasoconstriction and restoration of stroke volume and cardiac output.