When a patient arrests, high-quality CPR and early defibrillation are essential—but so is searching for a reversible cause. The classic memory aid is the 4 Hs and 4 Ts. Identifying and treating one of these can be the difference between ROSC and a non-survivable arrest.
The Five Hs
Hypoxia - Inadequate oxygenation rapidly leads to cardiac arrest; ensure the airway is patent and ventilation is effective.
Hypovolaemia - Severe volume loss (haemorrhage, dehydration, anaphylaxis) leaves the heart with nothing to pump; restore circulating volume aggressively.
Hypo-/Hyperkalaemia - Potassium dysregulation destabilises the myocardium; hyperkalaemia is especially common in renal failure patients and requires urgent correction.
Hypothermia — A core temperature below 30°C can cause refractory VF; remember: "not dead until warm and dead."
The Five Ts
Tension pneumothorax - Raised intrathoracic pressure obstructs venous return; requires immediate needle decompression or finger thoracostomy.
Cardiac tamponade - Fluid in the pericardium prevents the heart from filling; suspect after trauma or in known malignancy and treat with pericardiocentesis.
Toxins - Drugs, overdoses, or poisons can cause arrest; contact poison control early and consider specific antidotes where available.
Thrombosis (pulmonary embolism/coronary/MI) - A massive PE obstructs right heart outflow; consider thrombolysis during CPR if strongly suspected. Acute coronary occlusion is one of the most common causes of out-of-hospital arrest; primary PCI remains the definitive treatment post-ROSC.