orthostatic intolerance
/ or-thoh-STAT-ik /
Stand up too quickly after lying down for a long time and you may feel a wave of dizziness, a greying of vision, or near-fainting. Normally the body corrects this in a heartbeat; but after prolonged bed rest the correction fails, and even sitting up can bring on the swoon. That difficulty tolerating the upright position is orthostatic intolerance.
When you stand, gravity pulls roughly half a litre of blood downward into the legs and belly, momentarily reducing the blood returning to the heart and the brain. A healthy body responds instantly — the nervous system tightens blood vessels and quickens the heart to defend blood pressure. After days of lying flat the body loses this reflex sharpness and also sheds blood volume, so on standing the blood pressure drops, the brain is briefly under-supplied, and the person feels faint, sweaty, or sees stars; in orthostatic hypotension this is defined by a measurable fall in blood pressure within a few minutes of standing.
In rehabilitation this is one of the first and most dramatic hazards of immobility, and a major obstacle to getting patients up. A patient eager to walk may be unable even to sit at the bedside without nearly passing out, which is dangerous and frightening. The remedy is graded re-exposure to the upright position — raising the head of the bed, sitting, then standing with support, sometimes aided by compression stockings or a tilt table — and, above all, avoiding the prolonged flat bed rest that caused it. It is a vivid example of how the body's protective reflexes themselves deteriorate with disuse.
After ten days flat in intensive care, a recovering patient sits up for the first time and immediately goes pale, dizzy, and sweaty, with his blood pressure dropping on the monitor. The team lays him back, then over days reintroduces sitting in small, supported steps.
After long bed rest, even sitting up can overwhelm the body's blood-pressure reflexes.
Orthostatic intolerance after immobility is usually reversible with graded upright re-exposure, but in conditions like spinal cord injury the underlying autonomic control may be permanently impaired, requiring lasting strategies rather than simple reconditioning.