Cardiopulmonary Resuscitation (CPR) is an emergency technique designed to manually keep a person’s heart and lungs functioning until professional medical help can take over. A common point of confusion is how CPR remains effective when the rescuer’s breath contains carbon dioxide. The process involves chest compressions and rescue breaths, but the key to success is understanding that the immediate goal is not to fill the lungs with pure oxygen. Instead, the goal is to move the oxygen that is already present in the bloodstream. The air exhaled by a rescuer contains enough oxygen to temporarily sustain life, a concept rooted in human gas exchange mechanics.
The Composition of Exhaled Air
Atmospheric air, which a rescuer inhales, consists of approximately 21% oxygen and a trace amount of carbon dioxide (around 0.04%). After this air participates in gas exchange within the lungs, its composition changes significantly. The air a rescuer exhales still contains a substantial amount of oxygen, typically around 16%.
During normal metabolism, the body uses only a fraction of the inhaled oxygen. This remaining oxygen is available to the person receiving the rescue breath. While the oxygen content drops to 16%, the carbon dioxide percentage rises to about 4% to 5% as it is expelled. This concentration is sufficient to achieve a life-sustaining effect in the short term.
The brain is highly sensitive to oxygen deprivation, with permanent damage potentially beginning within four to six minutes. The 16% oxygen delivered by a rescue breath is vital because it is far greater than the zero oxygen the person would receive otherwise. This action provides a minimal supply of oxygen to the bloodstream, which is then circulated to the brain and other vital organs.
The Primary Function of Chest Compressions
The most immediate problem during cardiac arrest is the heart’s inability to pump the oxygenated blood that is already present. The primary purpose of chest compressions is to act as an artificial pump, manually moving this existing supply of oxygenated blood. High-quality compressions, delivered hard and fast at a rate of 100 to 120 per minute, create pressure within the chest cavity.
This rhythmic pressure squeezes the heart between the sternum and the spine, propelling blood to the rest of the body. The most important destination for this blood flow is the brain, which has the lowest tolerance for interrupted circulation. Effective compressions ensure that the oxygenated blood already in circulation reaches the brain to prevent immediate damage.
Compressions typically achieve about 20% to 30% of the normal blood flow, but this small amount is sufficient to delay tissue death. Modern CPR focuses on minimizing interruptions to circulation, as even brief pauses cause the artificially generated blood pressure to drop significantly. Therefore, the priority is the movement of oxygenated blood, which is why compressions are emphasized.
Why Rescue Breaths Remain Essential
While compressions circulate existing oxygen, rescue breaths are necessary to replenish that supply over time and to manage the buildup of waste products. After a few minutes of cardiac arrest, the existing oxygen in the blood is depleted. The breaths introduce new oxygen into the person’s lungs, which can diffuse into the bloodstream and be picked up by the circulating blood.
The breaths also help expel accumulated carbon dioxide from the person’s system. Without proper ventilation, carbon dioxide, a byproduct of cellular metabolism, builds up in the blood. This buildup leads to a dangerous imbalance in the body’s pH level known as respiratory acidosis. Therefore, delivering oxygen and removing waste carbon dioxide supports the person’s respiratory and metabolic functions until advanced life support is available.
The combined action of compressions and breaths provides a minimal, temporary level of support. This bridges the gap until professional responders arrive with high-concentration oxygen and equipment to restart the heart. This teamwork between manual circulation and artificial ventilation makes complete CPR most effective, especially when cardiac arrest was caused by a breathing problem like drowning or overdose.