Hypoxia

Hypoxia means an abnormally low level of oxygen in the body’s tissues. It is distinct from hypoxaemia, which specifically refers to low oxygen in the blood, though the two terms are often used interchangeably in clinical practice. When cells receive insufficient oxygen, they cannot maintain normal aerobic metabolism and begin to fail, potentially leading to cell death if the deprivation is severe or prolonged.

In the context of cardiac arrest, hypoxia is both a cause and a consequence. Respiratory hypoxia (caused by choking, drowning, asthma, or other airway and breathing problems) can directly trigger cardiac arrest, particularly in children. After cardiac arrest, any delay in restoring spontaneous circulation means the brain and other vital organs are deprived of oxygenated blood. The brain is the organ most sensitive to hypoxia: irreversible neuronal injury begins after approximately four to six minutes without oxygen, underpinning the critical importance of rapid CPR and defibrillation. The resulting acquired brain injury is called hypoxic-ischaemic brain injury (HIBI).

In intensive care following cardiac arrest, maintaining adequate oxygenation while avoiding excessive oxygen levels (hyperoxia) is a key management goal. Both hypoxia and hyperoxia are independently associated with worse neurological outcomes. Target oxygen saturation in most post-arrest protocols is 94 to 98 percent, avoiding both extremes. Arterial blood gas monitoring provides precise measurements of oxygen levels to guide ventilator settings during targeted temperature management.

Hypoxia can also occur on a smaller scale in heart disease. Areas of myocardium supplied by critically narrowed coronary arteries may be chronically hypoxic, making them vulnerable to ventricular arrhythmias and cardiac arrest during periods of increased demand such as exercise. This tissue-level hypoxia is the mechanism underlying myocardial ischaemia and is the target of revascularisation treatments such as angioplasty and coronary artery bypass graft (CABG) surgery.