hypoxemia

Hypoxemia simply means there is too little oxygen in the arterial blood — the blood that has just left the lungs and should be fully charged for the journey to the tissues. It is a finding measured in the blood, defined by a low oxygen pressure (PaO2) or a low oxygen saturation, and it is one of the most common reasons people come to medical attention with breathlessness.

There are a handful of basic mechanisms, and naming them guides the diagnosis. The everyday culprit is ventilation–perfusion mismatch, seen in asthma, COPD and pneumonia. More severe is shunt, where blood bypasses ventilated lung entirely. Others include hypoventilation (simply not breathing enough, which lowers oxygen and raises carbon dioxide together), impaired diffusion across a thickened membrane, and breathing thin air at high altitude. Clinicians often sort these out partly by checking whether the A–a gradient is widened and whether oxygen corrects the problem.

A precise distinction is worth keeping: hypoxemia is low oxygen in the blood, whereas hypoxia is low oxygen reaching or used by the tissues. The two usually travel together but not always — severe anaemia or poor circulation can starve tissues of oxygen even when the arterial blood is well saturated. Untreated, significant hypoxemia stresses the heart and brain, which is why it is sought out and corrected promptly.