The idea that chest compressions alone can electrically shock a heart back into a normal rhythm is a frequent misunderstanding, often reinforced by popular media. Cardiopulmonary Resuscitation (CPR) is a time-sensitive emergency procedure involving chest compressions and often rescue breaths. It is performed when a person’s heart has stopped beating effectively, a condition known as cardiac arrest. While CPR is lifesaving, its function is mechanical, not electrical, and it is designed to sustain life by manually moving blood until definitive medical intervention can occur.
The True Purpose of Compressions
Chest compressions do not deliver an electrical impulse to the heart. Instead, the immediate goal of high-quality compressions is to maintain a minimal flow of oxygenated blood to the body’s most sensitive organs. The brain, in particular, begins to suffer irreversible damage within minutes of blood flow stopping. By manually circulating blood, a rescuer is essentially buying time for the patient.
This manual circulation is known as perfusion, and its maintenance prevents the rapid death of tissue, especially in the brain. The objective is to keep the brain and the heart muscle supplied with oxygen until the underlying cause of the cardiac arrest can be addressed. Without compressions, the patient’s chance of survival diminishes significantly with every passing minute. Therefore, the compressions function as a temporary, manual pump to bridge the gap between collapse and advanced medical care.
The Physics of Circulation
The rhythmic pressure applied to the center of the chest during compressions works by two primary physiological mechanisms to create artificial circulation. The first is the cardiac pump theory, where the pressure directly squeezes the heart between the sternum and the spine, forcing blood out of the ventricles and into the arteries. The second mechanism is the thoracic pump theory, which relies on changes in pressure within the chest cavity.
When the sternum is pushed down, the overall pressure inside the chest increases, squeezing the large blood vessels and propelling blood toward the rest of the body. When the chest wall is allowed to completely recoil to its normal position, a negative pressure is created within the thorax. This vacuum effect draws deoxygenated blood back into the heart from the major veins, allowing the heart chambers to refill for the next compression.
Effective compressions must be delivered at a depth of about two to two-and-a-half inches for an average adult, and at a consistent rate of 100 to 120 compressions per minute. This specific depth and rate are necessary to generate adequate coronary perfusion pressure, which pushes blood into the heart muscle itself. If compressions are interrupted or recoil is incomplete, this pressure drops immediately, making it harder to achieve a return of spontaneous circulation. Continuous, uninterrupted compressions are paramount to maintaining blood flow.
What Actually Restarts the Heart
The actual mechanism for restarting an arrested heart is typically an electrical one, delivered through defibrillation. Cardiac arrest is often caused by an electrical malfunction in the heart, such as ventricular fibrillation or pulseless ventricular tachycardia. In these rhythms, the heart’s electrical activity becomes chaotic and disorganized, causing the muscle to merely quiver instead of executing a coordinated contraction.
An Automated External Defibrillator (AED) is designed to deliver a controlled electrical shock to the heart muscle. This shock is not meant to jump-start the heart; rather, it is intended to momentarily stop all electrical activity. The goal is to create a brief, complete pause in the chaotic rhythm, which gives the heart’s natural pacemaker, the sinoatrial node, a chance to reset and re-establish a normal, organized rhythm. Defibrillation is the definitive treatment for these chaotic, shockable rhythms.
Advanced medical personnel also employ medications, such as epinephrine, to support the resuscitation effort. Epinephrine, commonly known as adrenaline, is a potent vasoconstrictor that narrows the blood vessels. This action increases the overall blood pressure, which helps improve the flow of blood to the heart muscle and the brain during compressions. While epinephrine does not electrically restart the heart, it raises the perfusion pressure, making the environment more favorable for a successful defibrillation. If the heart has a “flatline” rhythm, known as asystole, defibrillation is ineffective, and only high-quality compressions and medications can potentially improve the chances of a return to a shockable rhythm.