Cardiac arrest is a sudden cessation of effective heart function, leading to loss of blood circulation, unconsciousness, and death if not treated immediately. It is one of the most critical medical emergencies and requires prompt recognition and resuscitation.
Unlike a heart attack (myocardial infarction), which is caused by blocked blood flow to the heart muscle, cardiac arrest results from the heart stopping altogether, often due to electrical disturbances such as ventricular fibrillation or pulseless ventricular tachycardia.
To guide clinicians in identifying reversible causes of cardiac arrest, the mnemonic “4Hs and 4Ts” is used. These represent the most common, treatable factors that should always be checked during resuscitation.
Understanding Cardiac Arrest
What is Cardiac Arrest?
Cardiac arrest occurs when the heart suddenly stops pumping blood, leading to:
- Loss of consciousness within seconds.
- Absence of pulse.
- No breathing or only abnormal gasping (agonal respirations).
If untreated, permanent brain damage occurs within 4–6 minutes, and death follows shortly thereafter.
Difference Between Cardiac Arrest and Heart Attack
Heart Attack (MI): Blood flow to heart muscle is blocked, but the heart usually continues beating.
Causes of Cardiac Arrest – The 4Hs and 4Ts Mnemonic
The mnemonic “4Hs and 4Ts” is an essential memory aid used in Advanced Cardiac Life Support (ACLS) to quickly identify and treat reversible causes during resuscitation.
4Hs – Reversible Metabolic and Systemic Causes
1. Hypoxia
- Low oxygen levels due to respiratory failure, airway obstruction, or drowning.
- Management: Provide 100% oxygen and ensure adequate ventilation.
2. Hypovolemia
- Severe blood or fluid loss (e.g., trauma, dehydration, massive bleeding).
- Management: Rapid IV fluid replacement and control of bleeding.
3. Hypo-/Hyperkalemia & Metabolic Disturbances
Potassium imbalance and acidosis are life-threatening arrhythmia triggers.
Management:
- Hypokalemia – Give potassium supplements.
- Hyperkalemia – Treat with calcium gluconate, insulin + glucose, sodium bicarbonate.
4. Hypothermia
- Core body temperature <35°C (common in drowning, exposure to cold).
- Management: Active rewarming with warm IV fluids, blankets, or extracorporeal rewarming in severe cases.
4Ts – Structural and Toxic Causes
1. Thrombosis (Coronary or Pulmonary)
- Coronary thrombosis → Acute myocardial infarction leading to arrhythmia.
- Pulmonary thrombosis → Pulmonary embolism blocking circulation.
- Management: Immediate thrombolysis, PCI (angioplasty), or surgical embolectomy.
2. Tamponade (Cardiac)
- Fluid accumulation in pericardium compresses the heart, preventing filling.
- Management: Emergency pericardiocentesis to remove fluid.
3. Toxins
- Overdose of drugs (e.g., opioids, tricyclic antidepressants, digitalis, beta-blockers, cocaine).
- Management: Antidotes (e.g., naloxone, digoxin-specific Fab fragments), activated charcoal, supportive care.
- Air trapped in the pleural cavity compresses the heart and lungs.
- Management: Immediate needle decompression followed by chest tube insertion.
Clinical Features of Cardiac Arrest
- Sudden collapse.
- Unresponsiveness.
- Absence of pulse (carotid or femoral).
- No breathing or only gasping.
- Cyanosis (bluish skin).
- Dilated pupils (late sign).
Diagnosis
Cardiac arrest is diagnosed clinically within seconds:
Check unresponsiveness.- Ventricular fibrillation (VF)
- Pulseless ventricular tachycardia (VT)
- Pulseless electrical activity (PEA)
- Asystole (flatline)
Emergency Management of Cardiac Arrest
1. Basic Life Support (BLS)
- Immediate CPR (Cardiopulmonary Resuscitation) – 30 chest compressions : 2 rescue breaths.
- Chest compressions at 100–120/min with depth of 5–6 cm.
- Minimize interruptions in compressions.
2. Defibrillation
- For VF and pulseless VT, early defibrillation is the most effective treatment.
- Use AED (Automated External Defibrillator) if available.
3. Advanced Cardiac Life Support (ACLS)
- Secure airway (endotracheal intubation if possible).
- Provide 100% oxygen.
- Establish IV/IO access.
- Administer Epinephrine 1 mg IV every 3–5 minutes for asystole/PEA.
- Consider Amiodarone for refractory VF/VT.
- Correct reversible causes (4Hs & 4Ts).
4. Post-Resuscitation Care
- Maintain oxygenation and blood pressure.
- Therapeutic hypothermia in comatose patients after ROSC (return of spontaneous circulation).
- Treat underlying cause (e.g., PCI for MI, dialysis for hyperkalemia).
Prognosis
Survival after cardiac arrest depends on:
- Time to CPR and defibrillation (every minute delay decreases survival by 7–10%).
- Quality of CPR.
- Presence of reversible cause.
- Post-resuscitation care quality.
Despite advancements, out-of-hospital cardiac arrest survival rates remain 5–10%, emphasizing the importance of early recognition and community CPR training.
Prevention
- Control risk factors for heart disease (hypertension, diabetes, obesity, smoking).
- Regular cardiac screening for high-risk patients.
- Implantable cardioverter-defibrillator (ICD) in patients with recurrent arrhythmias.
- Public access to AEDs in airports, malls, and public places.
- CPR training for the general public.
Tabular Summary – 4Hs and 4Ts
| Category | Cause | Management |
|---|---|---|
| H | Hypoxia | Oxygenation, airway management |
| H | Hypovolemia | IV fluids, blood transfusion |
| H | Hypo-/Hyperkalemia | Electrolyte correction |
| H | Hypothermia | Active rewarming |
| T | Thrombosis (MI/PE) | PCI, thrombolysis, embolectomy |
| T | Tamponade | Pericardiocentesis |
| T | Toxins | Antidotes, supportive therapy |
| T | Tension Pneumothorax | Needle decompression, chest tube |
Frequently Asked Questions (FAQs)
Q1: What is the first step when you see a person collapse?
Check responsiveness, pulse, and breathing. If absent, start CPR and call for emergency help.
Q2: What is the difference between cardiac arrest and heart attack?
Cardiac arrest is when the heart stops beating; a heart attack is due to blocked blood flow but the heart may still beat.
Q3: Can cardiac arrest be reversed?
Yes, if identified early and treated with CPR, defibrillation, and correction of reversible causes (4Hs & 4Ts).
Q4: What is the most common rhythm in cardiac arrest?
Ventricular fibrillation (VF) is the most common rhythm seen in sudden cardiac arrest.
Q5: Why are the 4Hs and 4Ts important?
They help identify reversible causes of cardiac arrest, ensuring that treatable conditions are not missed during resuscitation.
