Emergency Medicine
CPR Mechanism
Cardiopulmonary resuscitation — chest compressions, defibrillation, and the chain of survival
Cardiopulmonary resuscitation manually circulates blood and oxygen to vital organs when the heart stops. Compressions generate ~25% of normal cardiac output via two mechanisms: direct cardiac compression and intrathoracic pressure pump. Current guidelines (AHA 2020): compress to 5-6 cm depth at 100-120 per minute on adults, with full recoil and minimal interruptions. Survival to discharge for out-of-hospital arrest is ~10%; rises 2-3× with bystander CPR plus early defibrillation. Time matters: every minute without CPR drops survival ~10%.
- Compression rate100-120 per minute
- Compression depth5-6 cm (2-2.4 inches)
- Compression-ventilation30:2 (single rescuer)
- AED shock energy150-360 J biphasic
- Survival rate (OHCA)~10% to discharge
- Time to brain damage4-6 min anoxia
Interactive visualization
Press play, or step through manually. The visualization is yours to drive — try it before reading on.
Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
Why CPR knowledge matters
- Bystander response. ~70% of out-of-hospital arrests happen at home; family members are often the only responder.
- Public AED placement. Airports, casinos, gyms, schools — locations where deployment dramatically improves outcomes.
- Resuscitation team. Healthcare workers must perform high-quality compressions and rotate every 2 minutes.
- Pediatric differences. Two-thumb technique for infants; compression depth ~⅓ chest diameter.
- Drowning, electrocution, choking. Specific causes need targeted approach.
- Quality metrics. Compression fraction, rate, depth, recoil tracked with feedback devices.
- Termination decisions. ROSC, futility criteria, ECPR candidacy, family presence.
Common misconceptions
- "Defibrillation restarts the heart." It stops disorganized activity so the SA node can take over.
- Asystole is shockable. Flat line is not VF — shocking wastes time and energy.
- Compression depth doesn't matter much. Each centimeter <5 cm halves coronary perfusion.
- Pause for pulse checks frequently. Limit to ≤10 seconds — interruptions kill perfusion gains.
- Mouth-to-mouth is required. Hands-only is recommended for untrained adults.
- If ribs crack, stop. Continue — broken ribs heal, dead patients don't.
Frequently asked questions
How do compressions move blood?
Two complementary models. Cardiac pump: direct compression of the ventricles between sternum and spine ejects blood. Thoracic pump: rising intrathoracic pressure squeezes all chest vessels; valves direct blood out. Real CPR mixes both. End-tidal CO2 >10 mmHg confirms adequate compressions; <10 mmHg suggests fatigue, malposition, or futility.
Why is depth and rate so specific?
Below 5 cm: insufficient stroke volume. Above 6 cm: rib fractures, organ injury without added benefit. Below 100/min: underperfusion. Above 120/min: insufficient diastolic refill, drops cardiac output. Full recoil is essential — leaning prevents venous return and coronary perfusion. Studies show rescuers consistently push too shallow and lean too much.
What is the chain of survival?
Five links. Recognition and call for help. Early CPR with chest compressions. Rapid defibrillation. Advanced resuscitation. Post-arrest care including targeted temperature management. A weak link breaks the chain. Bystander CPR doubles or triples survival; AED within 3-5 minutes can yield >50% survival in shockable rhythms.
When does defibrillation work?
Only for shockable rhythms: ventricular fibrillation (VF) and pulseless ventricular tachycardia (pVT). Defibrillation depolarizes the entire myocardium simultaneously, allowing the SA node to resume coordinated rhythm. Asystole and PEA are not shocked — they need CPR plus epinephrine and reversible-cause search (Hs and Ts: hypoxia, hypovolemia, hypothermia, tension pneumothorax, tamponade, toxins, thrombosis).
What drugs are used?
Epinephrine 1 mg IV every 3-5 minutes (vasoconstriction raises coronary perfusion pressure). Amiodarone 300 mg IV bolus for refractory VF/pVT. Lidocaine as alternative. Calcium for hyperkalemia or calcium channel blocker overdose. Sodium bicarbonate for severe acidosis or TCA overdose. Magnesium for torsades. Reversal agents (naloxone, flumazenil) if indicated.
What about hands-only CPR?
For untrained bystanders, compressions alone outperform no CPR and are non-inferior to compression+ventilation in adult cardiac arrest of cardiac origin. Continuous compressions maintain coronary and cerebral perfusion that pauses for breaths interrupt. Ventilations matter more in pediatric, drowning, asphyxial arrest where hypoxia precedes arrest.
What is post-arrest care?
ROSC (return of spontaneous circulation) is just the start. Targeted temperature management 32-36°C for 24 hours improves neuro outcomes. Hemodynamic optimization with MAP ≥65 mmHg. Coronary angiography for likely cardiac cause. EEG monitoring for seizures. Glucose, electrolyte management. Neurological prognostication delayed ≥72 hours after rewarming due to medication effects.