Hypo is a Greek prefix meaning ‘below’, ‘under’, or ‘insufficient’. In medical terminology, it indicates that a measurement or condition falls below the normal range. The opposite prefix is hyper (meaning ‘above’ or ‘excessive’).
In cardiovascular and critical care medicine, hypo- appears in many important clinical terms. Hypotension means abnormally low blood pressure. Hypokalaemia means abnormally low blood potassium levels, which can cause dangerous cardiac arrhythmias and is one of the 4H reversible causes of cardiac arrest. Hypoglycaemia means abnormally low blood glucose. Hypothermia means abnormally low body temperature. Hypovolaemia (another of the 4Hs) means abnormally low circulating blood volume.
The 4H and 4T framework used in advanced cardiac life support identifies Hypokalaemia/electrolyte imbalance, Hypoxia, Hypovolaemia, and Hypo/Hyperthermia as four of the reversible causes of cardiac arrest that must be identified and treated during resuscitation.
In targeted temperature management (therapeutic cooling) used after cardiac arrest, the body is deliberately cooled to a controlled hypothermic range to protect the brain from secondary injury during recovery, demonstrating how the term ‘hypo’ can describe both a dangerous pathological state and a deliberate therapeutic intervention.
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