Neurally mediated hypotension

Neurally mediated hypotension or NMH is an abnormality in regulating blood pressure experienced when the person is upright. NNH causes a drop of 20–25 mm Hg in systolic blood pressure when a person is standing still compared to lying down. NMH is a form of chronic orthostatic intolerance.

Signs and symptoms
Symptoms may include lightheadedness, dizziness, pressure-like left-side chest pain, visual changes, weakness, slower speech or slowed verbal response, pallor, an immediate urge to lie down and syncope.

Blood pressure readings taken lying down and when upright can be compared to assess for a drop in systolic blood pressure. For example, a blood pressure of 130/85 mm Hg may change to 100/85 mm Hg when the person stands, and does not return to normal.

Explanation
According to Blood Pressure Updates, neurally mediated hypotension "is a complication that occurs due to unnatural reflex action between the brain and heart, though they are structurally normal. Neurally mediated hypotension is also called fainting reflex, or vasodepressor syncope or neurocardiogenic syncope or autonomic dysfunction or vaso-vagal reflex. Syncope is the formal medical term used for fainting."