Why they matter
When a patient goes into cardiac arrest, time is everything. While high-quality CPR and early defibrillation are the cornerstones of management, identifying and treating the underlying cause is just as important. That’s where the Four H’s and T’s come in — a simple yet powerful checklist of reversible causes.
These are the conditions we must rule out (or correct) if our patient is going to have any chance of recovery.
The Four H’s
1. Hypoxia
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Pathophysiology: Without oxygen, cardiac muscle becomes irritable, leading to arrhythmias and eventual asystole.
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Recognition: Look for airway obstruction, inadequate ventilation, or oxygen supply issues.
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Management: Ensure airway patency, provide high-flow oxygen, ventilate effectively, and correct reversible airway problems.
2. Hypovolaemia
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Pathophysiology: Severe blood or fluid loss reduces preload, starving the heart of the filling pressure it needs to generate output.
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Recognition: Trauma with haemorrhage, gastrointestinal bleeding, or severe dehydration.
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Management: Control external bleeding, consider pelvic binders, administer fluids or blood products as per local protocols.
3. Hypothermia
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Pathophysiology: Core temperatures below 30°C can cause bradyarrhythmias, reduced myocardial contractility, and refractory VF/VT.
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Recognition: Environmental exposure, drowning, or cold environments.
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Management: Rewarm gradually, limit defibrillation attempts until temperature improves, handle the patient gently to avoid arrhythmias.
4. Hyper-/Hypokalaemia & Metabolic Disorders
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Pathophysiology: Electrolyte imbalances — especially extremes of potassium — can cause lethal arrhythmias. Severe acidosis or hypocalcaemia may also contribute.
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Recognition: History of renal failure, missed dialysis, metabolic disease, toxicological causes. ECG may show peaked T waves (hyperkalaemia) or flattened T waves/arrhythmias (hypokalaemia).
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Management: Treat per local guidance — e.g., calcium chloride/gluconate, insulin and glucose, sodium bicarbonate, or fluid resuscitation.
The Four T’s
1. Tension Pneumothorax
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Pathophysiology: Air trapped in the pleural space collapses the lung and compresses venous return to the heart.
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Recognition: Unilateral absent breath sounds, tracheal deviation, distended neck veins, hypotension.
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Management: Immediate needle decompression followed by chest drain.
2. Tamponade (Cardiac)
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Pathophysiology: Blood or fluid in the pericardial sac restricts ventricular filling, leading to obstructive shock and PEA arrest.
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Recognition: Penetrating trauma to chest, distended neck veins, muffled heart sounds, pulseless electrical activity.
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Management: In hospital, pericardiocentesis. Pre-hospital: rapid recognition and expeditious transport.
3. Toxins
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Pathophysiology: Overdose or poisoning (e.g., tricyclic antidepressants, beta blockers, calcium channel blockers, opioids) can cause cardiac arrest.
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Recognition: History, drug packets, bystander info, specific toxidromes (pupil size, respiratory depression, etc).
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Management: Supportive care, antidotes if available (e.g., naloxone, sodium bicarbonate, intralipid).
4. Thrombosis (Coronary or Pulmonary)
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Pathophysiology:
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Coronary thrombosis: Acute MI blocking coronary blood supply.
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Pulmonary thrombosis: Massive pulmonary embolism obstructing right heart outflow.
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Recognition: History of chest pain, risk factors (immobility, long travel, DVT), sudden collapse.
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Management: Advanced care may include thrombolysis or PCI; pre-hospital focus on high-quality resuscitation and rapid hospital transfer.
Bringing it all together
The Four H’s and T’s give us a structured approach to think beyond CPR and defibrillation. By systematically working through each cause, we improve the chances of identifying and correcting what’s driving the arrest.
For a quick, pocket-sized summary of the Four H’s and T’s — designed specifically for pre-hospital clinicians — check out our reference card
References
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Resuscitation Council UK. Advanced Life Support Guidelines, 2021.
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European Resuscitation Council Guidelines 2021.
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JRCALC Clinical Guidelines, 2024.