Cardiopulmonary Resuscitation (CPR) is an emergency procedure combining chest compressions with artificial ventilation, or rescue breathing. The purpose is to manually take over the functions of the heart and lungs when a person’s heart has stopped beating (cardiac arrest). Rescuers aim to circulate oxygenated blood to vital organs, specifically the brain and the heart, until medical professionals arrive. A common concern is that a rescuer’s exhaled breath contains carbon dioxide. The effectiveness of this air is explained by the physiological priorities during an emergency and the actual composition of the air being delivered.
The Critical Role of Chest Compressions
Cardiac arrest signifies the abrupt cessation of the heart’s pumping action, immediately halting blood flow throughout the body. The most urgent priority is to re-establish circulation, not necessarily to immediately provide fresh oxygen. Chest compressions are the mechanical action designed to artificially circulate the blood already within the victim’s system. By pressing forcefully and rapidly on the center of the chest, the rescuer squeezes the heart between the sternum and the spine, generating a minimal but life-sustaining blood pressure.
This manual circulation keeps the oxygen still present in the victim’s blood moving toward the brain and heart tissue. Guidelines recommend performing compressions at a rate between 100 and 120 times per minute, pushing down to a depth of at least two inches for an average adult. Maintaining this rhythm and depth is paramount because blood movement prevents irreversible damage to the brain. The quality of these compressions is considered the single greatest factor influencing survival from cardiac arrest.
Oxygen Content in Exhaled Air
The effectiveness of rescue breaths, even with carbon dioxide present, is explained by the chemical composition of exhaled air. Ambient air, which a rescuer inhales, is approximately 21% oxygen. During normal respiration, the body only consumes a fraction of this oxygen before the air is exhaled. Consequently, the air delivered during rescue breathing is not pure carbon dioxide, but still contains a significant concentration of oxygen.
A rescuer’s exhaled air typically holds an oxygen concentration of about 16% to 17%. While this is lower than the 21% found in the atmosphere, it is adequate for emergency resuscitation. The small amount of carbon dioxide, around 4%, is not enough to negate the benefit of the oxygen being delivered. This 16% oxygen concentration is more than adequate to support the minimal gas exchange occurring in the lungs during the resuscitation effort.
Low Metabolic Demand During Cardiac Arrest
The physiological state of the victim during cardiac arrest further explains why a lower concentration of oxygen is sufficient. When the heart stops, the body’s overall metabolic processes slow down dramatically. This simultaneously reduces the demand for oxygen by the body’s tissues, as the body is no longer performing normal functions that require high levels of oxygen consumption.
The goal of oxygen delivery during CPR is narrowly focused on keeping the most vulnerable organs, the brain and heart, from dying. Because the patient is in a state of suspended circulation, the tissues are not actively burning oxygen at a normal rate. This reduction in metabolic oxygen demand means that the 16% to 17% concentration delivered through rescue breaths can effectively saturate the limited blood flow generated by the chest compressions. The combination of minimal circulation and reduced tissue demand allows the less-than-ideal oxygen percentage to still be life-saving.
Hands-Only CPR vs. Rescue Breathing
The understanding that circulation is the immediate priority has led to a change in modern resuscitation guidelines. For adults who experience cardiac arrest, the current recommendation for untrained bystanders is often Hands-Only CPR, which eliminates rescue breaths entirely. This approach is effective because the victim’s blood typically holds enough stored oxygen at the time of collapse to last for the first few minutes, provided that vigorous chest compressions maintain blood flow. Eliminating mouth-to-mouth ventilation encourages more bystanders to intervene immediately without hesitation.
Traditional CPR, which involves the cycle of 30 compressions followed by two rescue breaths, remains the standard in specific situations. Rescue breaths are necessary when the cause of the cardiac arrest is respiratory in nature, such as in cases of drowning, drug overdose, or in pediatric emergencies. In these scenarios, the victim’s blood is likely to have been depleted of oxygen prior to the heart stopping. Therefore, fresh air must be introduced immediately to be circulated by the compressions.