The 4 Hs and 4 Ts are the standard reversible causes of cardiac arrest that should be actively considered during paediatric ALS/APLS, because correcting one of them may restore circulation.

4 Hs

  • Hypoxia: especially important in children, where arrest is often secondary to respiratory failure or asphyxia. Priorities are airway opening, suction if needed, effective bag-mask ventilation with 100% oxygen, and escalation to an advanced airway if ventilation is inadequate.
  • Hypovolaemia: think haemorrhage, severe dehydration, sepsis, burns, or other major fluid loss. Clues include poor perfusion, weak pulses, narrow pulse pressure, and a history suggesting volume loss. Treat with IV/IO access, rapid fluid or blood replacement, and bleeding control when relevant.
  • Hypo-/hyperkalaemia and metabolic disorders: think renal failure, DKA, severe diarrhoea, endocrine disease, or toxicological causes. Use blood gas, glucose, and electrolytes early when available, and treat the specific disturbance.
  • Hypo-/hyperthermia: severe temperature disturbance can precipitate arrest and alter resuscitation physiology. Rewarm hypothermic patients carefully and avoid further heat loss; in hyperthermia, actively cool and treat the trigger.

4 Ts

  • Thrombosis: includes pulmonary or coronary thrombosis. In children, PE is uncommon but possible in high-risk settings such as central lines, malignancy, immobility, thrombophilia, or congenital heart disease.
  • Tension pneumothorax: suspect with sudden deterioration, unilateral absent breath sounds, hyperinflation, rising ventilation pressures, or trauma. Treat with immediate decompression rather than waiting for imaging.
  • Tamponade (cardiac): causes obstructive shock/PEA by impairing ventricular filling. Consider trauma, post-cardiac surgery, malignancy, or pericardial disease; urgent decompression may be required.
  • Toxic agents: overdose, medication error, envenomation, or poisoning can all present as arrest or peri-arrest. Management is supportive resuscitation plus toxin-specific treatment when available.

Practical bedside frame
Ask: are we oxygenating, ventilating, and circulating properly; is there an obstructive cause; is there a metabolic trigger; and could this be toxic?