Hypoxaemia is the medical term for abnormally low oxygen levels in the blood, specifically a reduced partial pressure of oxygen in arterial blood (PaO2) or a reduced oxygen saturation of haemoglobin (SpO2 or SaO2). Hypoxaemia and hypoxia are related but distinct: hypoxaemia refers to low oxygen in the blood, while hypoxia refers to insufficient oxygen delivery to the tissues. Severe hypoxaemia will cause hypoxia if prolonged, but the two terms are not interchangeable.
Hypoxaemia can result from several mechanisms: failure of ventilation (hypoventilation), impaired gas exchange in the lungs (as in pneumonia, pulmonary oedema, ARDS, or pulmonary embolism), or right-to-left shunting of blood bypassing the lungs (as in cyanotic congenital heart disease). Symptoms include breathlessness, rapid breathing, tachycardia, cyanosis, confusion, and impaired consciousness.
Severe hypoxaemia causing oxygen deprivation to the heart can trigger arrhythmias and is one of the 4H reversible causes of cardiac arrest in the ALS framework. Restoration of oxygenation is therefore a priority in any resuscitation attempt.
After cardiac arrest, the management of hypoxaemia is a central concern in intensive care. Mechanical ventilation is used to maintain adequate oxygenation while treating the underlying cause. Both under-oxygenation (hypoxaemia) and over-oxygenation (hyperoxia) can harm the injured brain; current guidelines recommend targeting an oxygen saturation of 94 to 98% in the post-arrest period.
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