Fainting syncope, commonly known as fainting

Blood pressure[edit]

Orthostatic (postural) hypotensive syncope is caused primarily by an excessive drop in blood pressure when standing up from a previous position of lying or sitting down.^[9][1] When the head is elevated above the feet the pull of gravity causes blood pressure in the head to drop. This is sensed by stretch receptors in the walls of vessels in the carotid sinus and aortic arch.^[9] These receptors then trigger a sympathetic nervous response to compensate and redistribute blood back into the brain. The sympathetic response causes peripheral vasoconstriction and increased heart rate. These together act to raise blood pressure back to baseline.^[9] Apparently healthy individuals may experience minor symptoms ("lightheadedness", "greying-out") as they stand up if blood pressure is slow to respond to the stress of upright posture. If the blood pressure is not adequately maintained during standing, faints may develop.^[3] However, the resulting "transient orthostatic hypotension" does not necessarily signal any serious underlying disease. It is as common or perhaps even more common than vasovagal syncope.
This may be due to medications, dehydration, significant bleeding or infection.^[1] The most susceptible individuals are elderly frail individuals, or persons who are dehydrated from hot environments or inadequate fluid intake.^[3] For example, medical students would be at risk for orthostatic hypotensive syncope while observing long surgeries in the operating room.^[21] There is also evidence that exercise training can help reduce orthostatic intolerance.^[22] More serious orthostatic hypotension is often the result of certain commonly prescribed medications such as diuretics, β-adrenergic blockers, other anti-hypertensives (including vasodilators), and nitroglycerin.
In a small percentage of cases, the cause of orthostatic hypotensive faints is structural damage to the autonomic nervous system due to systemic diseases (e.g., amyloidosis or diabetes) or in neurological diseases (e.g., Parkinson's disease).^[9]
Hyperadrenergic orthostatic hypotension refers to an orthostatic drop in blood pressure despite high levels of sympathetic adrenergic response. This occurs when a person with normal physiology is unable to compensate for >20% loss in intravascular volume.^[23] This may be due to blood loss, dehydration or third-spacing. On standing the person will experience reflex tachycardia (at least 20% increased over supine) and a drop in blood pressure.^[9]
Hypoadrenergic orthostatic hypotension occurs when the person is unable to sustain a normal sympathetic response to blood pressure changes during movement despite adequate intravascular volume. There is little to no compensatory increase in heart rate or blood pressure when standing for up to 10 minutes. This is often due to an underlying disorder or medication use and is accompanied by other hypoadrenergic signs.^[9]