Pathophysiology
Clinical meaning
Orthostatic vital signs measure the cardiovascular system's ability to maintain adequate blood pressure and cerebral perfusion during the transition from supine to standing — a fundamental hemodynamic stress test. Upon standing, gravitational pooling of 500-1000 mL of blood in the lower extremities and splanchnic circulation decreases venous return, reducing right atrial filling pressure and cardiac output by approximately 20%. Arterial baroreceptors — stretch-sensitive mechanoreceptors in the carotid sinus (CN IX afferent) and aortic arch (CN X afferent) — detect the pressure drop and relay signals to the nucleus tractus solitarius in the medulla. The medullary cardiovascular center responds within 1-2 heartbeats by increasing sympathetic outflow and withdrawing parasympathetic (vagal) tone: heart rate rises by 10-25 bpm, arteriolar vasoconstriction increases systemic vascular resistance, venoconstriction enhances venous return, and the renin-angiotensin-aldosterone system activates for longer-term volume regulation. A positive orthostatic test (SBP drop ≥20 mmHg, DBP drop ≥10 mmHg, or HR increase ≥30 bpm within 3 minutes of standing) indicates that these compensatory mechanisms are insufficient. The heart rate response differentiates etiology: compensatory tachycardia (HR increase >15 bpm) suggests non-neurogenic causes (hypovolemia,...