CPR rarely restarts a heart; its real job is buying the brain time
Television makes chest compressions look like a reset button. In reality CPR alone seldom gets a heart beating again. Its purpose is to push some oxygen-rich blood to the brain and heart, holding off tissue death until a defibrillator shock or other treatment can restore a working rhythm.
The clock is brutal. Brain damage can begin after about four minutes without blood flow and becomes irreversible at around seven, which is why CPR is generally effective only if started within that window. Cold bodies are an exception: low temperatures, as in some near-drownings, extend how long the brain survives. Compressions work by creating a pressure difference between arteries and veins, keeping a trickle of circulation going.
For adults, guidelines call for compressions 5 to 6 centimetres deep at 100 to 120 a minute, alternating 30 compressions with 2 breaths when breaths are given. Speed matters in both directions: pumping faster than 120 a minute can reduce the blood returning to fill the heart. The most important elements are few pauses, enough depth and pace, full release between pushes, and avoiding over-ventilation. Children and infants get shallower compressions, and babies are compressed with two fingers.
In 2010 the American Heart Association and the International Liaison Committee on Resuscitation reordered the classic sequence from airway, breathing, compressions to compressions first, for everyone except newborns. Untrained bystanders helping adults are now advised that compression-only CPR is acceptable. Children are different, because their arrests usually start with breathing problems rather than heart disease, so 2015 guidance warned that compressions alone may lead to worse outcomes for them. Drowning victims likewise need rescue breaths, since their hearts stop from lack of oxygen.
Shocks fix only certain rhythms. Ventricular fibrillation and pulseless ventricular tachycardia respond to defibrillation, while a flat line or pulseless electrical activity needs the underlying cause treated. Portable automated defibrillators talk users through each step and decide whether to shock. Screen portrayals shape what the public expects: a study of 39 resuscitations in Chinese medical dramas found actors used a correct compression rate only 28.2 percent of the time.
Source: Cardiopulmonary resuscitation
