Emergency Medicine Emergency Medicine · High Yield

4 Hs & 4 Ts — Cardiac Arrest

The 4Hs and 4Ts are the eight reversible causes of cardiac arrest. The ALS algorithm mandates systematic consideration of all eight in any peri-arrest or refractory arrest scenario.

✦ The Mnemonic

"Four Hs, Four Ts — Always Reversible"

Hypoxia · Hypovolaemia · Hypo/Hyperkalaemia · Hypothermia · Tension pneumothorax · Tamponade · Toxins · Thrombosis

H1 Hypoxia Ensure adequate airway and oxygenation — bag-valve-mask, tracheal intubation

H2 Hypovolaemia Haemorrhage, fluid loss — IV fluid bolus, blood transfusion, surgical haemostasis

H3 Hypo/Hyperkalaemia Hyperkalaemia: calcium chloride IV; bicarbonate; dextrose + insulin. Hypokalaemia: IV potassium

H4 Hypothermia Core temperature — 'not dead until warm and dead'; rewarm actively; ECMO for severe

T1 Tension pneumothorax Needle decompression (2nd ICS MCL) immediately, then chest drain

T2 Tamponade Pericardiocentesis (subxiphoid); or EDT (emergency department thoracotomy)

T3 Toxins Drug overdose, anaesthetic agents — antidotes (naloxone, flumazenil, lipid emulsion for LA toxicity)

T4 Thrombosis (PE or MI) Massive PE: IV thrombolysis (alteplase 50 mg) during CPR; STEMI: consider PCI if ROSC achieved

📚 Clinical Breakdown

ALS algorithm: shockable rhythms (VF/pVT) — defibrillate, CPR, adrenaline after 3rd shock, amiodarone after 3rd shock. Non-shockable rhythms (PEA/asystole) — CPR, adrenaline every 3–5 minutes, consider and treat 4Hs and 4Ts. Quality CPR: depth 5–6 cm, rate 100–120/min, full recoil, minimal interruptions.

Thrombolysis in cardiac arrest: if massive PE is confirmed or strongly suspected as the cause of arrest, give IV alteplase 50 mg during CPR. Continue CPR for at least 60–90 minutes after thrombolysis to allow the drug to work. ROSC (return of spontaneous circulation) can occur late after thrombolysis.

Lipid emulsion therapy: for local anaesthetic (LA) systemic toxicity (LAST) — cardiovascular collapse and CNS toxicity after LA injection. Give Intralipid 20% 1.5 mL/kg IV bolus, then infusion. Mechanism: LA extraction from plasma ('lipid sink theory'). Standard protocol is available on most crash trolleys.

Shockable rhythms VF and pulseless VT

Non-shockable rhythms PEA and asystole

Adrenaline dose 1 mg IV every 3–5 minutes

Amiodarone in shockable 300 mg after 3rd shock; 150 mg after 5th

⭐ Clinical Pearl

Post-resuscitation care (ROSC): targeted temperature management (TTM) 32–36°C for 24 hours prevents neurological injury. Percutaneous coronary intervention (PCI) should be performed urgently if STEMI pattern on post-ROSC ECG. 12-lead ECG, chest X-ray, blood glucose control, and CT brain are standard post-ROSC investigations.

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