Orthostatic hypotension: new views for an old problem
Measuring blood pressure (BP) and heart rate (HR) in the supine and in the upright position may be the single best test to assess the integrity of human cardiovascular reflex regulation and should be part of routine clinical assessment in the hypertension clinic. Indeed, standing imposes a major burden on the cardiovascular system. Within seconds, 500–1000 ml blood is pooled below the diaphragm. Moreover, hydrostatic pressure forces plasma from the vascular into the interstitial space reaching a maximum of 10–20% after approximately 15-min standing. Despite the marked change in cardiac loading conditions, BP remains surprisingly stable in healthy young persons. BP stabilization is achieved through cardiovascular reflexes, particularly the arterial baroreflex, augmenting sympathetic outflow to vasculature, heart, and kidneys while attenuating cardiac vagal activity. BP decreases with standing when these mechanisms fail or when the hemodynamic stimulus exceeds the counter regulatory capacity. For example, hypovolemia and venous insufficiency may promote orthostatic BP reductions. Orthostatic hypotension is commonly defined as reduction in systolic BP of at least 20 mmHg and/or of diastolic BP by at least 10 mmHg after 3-min standing . Transient reductions in BP in the first minute of standing occur even in perfectly healthy persons but may nevertheless elicit symptoms.