hypercapnia

If a room's ventilation fails, the stale air and exhaled carbon dioxide build up. Hypercapnia is the bloodstream version of that stuffy room: too much carbon dioxide accumulated in the blood, defined by a raised arterial carbon dioxide pressure (PaCO2) above the normal upper limit of roughly 45 mmHg.

Because the lungs are the body's only meaningful route for getting rid of carbon dioxide, hypercapnia almost always means alveolar ventilation has fallen short of the carbon dioxide being produced. The causes group neatly: the drive to breathe is depressed (sedatives, brainstem injury), the bellows cannot move enough air (severe COPD, exhausted or weak respiratory muscles, a stiff chest), or dead space has grown so large that ordinary effort no longer suffices. Rarely, a large rise in carbon dioxide production outpaces breathing.

Carbon dioxide is also an acid in the blood, so a swift rise pushes the pH down — respiratory acidosis. When it climbs gradually, as in long-standing lung disease, the kidneys compensate by retaining bicarbonate and the pH can stay near normal, so the body tolerates surprisingly high levels chronically. A practical caution: in some people with chronic carbon dioxide retention, giving too much oxygen carelessly can worsen the build-up, so oxygen is titrated rather than poured on.