respiratory failure

Think of the lungs as a gas-exchange machine with two jobs: loading oxygen onto the blood and unloading carbon dioxide from it. Respiratory failure is the state where this machine can no longer keep up — either it cannot put enough oxygen into the blood, or it cannot clear enough carbon dioxide, or both. It is not a single disease but a final common pathway that many lung, heart, brain, and muscle problems can reach.

Doctors define respiratory failure by what the blood gases show. The usual thresholds are an arterial oxygen pressure (PaO2) below about 60 millimetres of mercury while breathing room air, or an arterial carbon dioxide pressure (PaCO2) above about 50 millimetres of mercury with a falling blood pH. These numbers turn a vague sense of breathlessness into a measurable line that signals the body's reserves are exhausted.

Respiratory failure can arrive suddenly (acute) — as in pneumonia, a severe asthma attack, or a large blood clot in the lung — or build slowly over years (chronic), as the lungs in advanced COPD gradually lose ground. The distinction matters because the body partly adapts to slow change, so the same blood-gas number can mean a person is in crisis or merely living near their limit. A useful caveat: the label describes a physiological state, not a cause, so finding and treating the underlying problem always comes first.