When the heart stops functioning, it enters cardiac arrest. The heart’s ability to pump blood relies on precise electrical signals that coordinate its contractions. Disruptions in this electrical activity can cause the heart to beat erratically or stop altogether, leading to a sudden loss of blood flow to the body’s vital organs. Certain severe electrical disturbances require immediate electrical intervention to restore a normal heart rhythm and improve survival.
The Heart’s Electrical System and Defibrillation
The heart’s rhythmic pumping is regulated by a specialized electrical conduction system. This system originates in the sinoatrial (SA) node, the heart’s natural pacemaker, located in the right upper chamber. The SA node generates electrical impulses that travel through pathways, coordinating the contraction of the heart’s chambers to pump blood efficiently.
When electrical signals become chaotic or extremely fast, the heart’s pumping action fails, leading to a loss of effective blood circulation. Defibrillation delivers a controlled electrical shock to the heart. This shock momentarily stops disorganized electrical activity, allowing the heart’s natural pacemaker to reset and restore a regular rhythm. Automated External Defibrillators (AEDs) are portable devices for public use, delivering these electrical shocks.
Identifying Shockable Rhythms
Two primary heart rhythms respond to defibrillation: Ventricular Fibrillation (VFib) and Pulseless Ventricular Tachycardia (pVT). In Ventricular Fibrillation, the ventricles quiver rapidly and chaotically instead of contracting in a coordinated manner. This disorganized electrical activity prevents the heart from effectively pumping blood, resulting in cardiac arrest. An electrocardiogram (ECG) would show irregular, unformed electrical deflections without clear P waves.
Pulseless Ventricular Tachycardia occurs when the ventricles beat very fast, often exceeding 100 beats per minute. This rapid rate is so inefficient that the heart cannot pump blood effectively, and no pulse can be detected. While electrical activity may appear somewhat organized compared to VFib, the lack of effective pumping makes it life-threatening. AEDs analyze the heart’s electrical rhythm to identify if either shockable rhythm is present, advising a shock only when indicated.
In contrast, other cardiac arrest rhythms, such as asystole and pulseless electrical activity (PEA), do not respond to defibrillation. Asystole, or “flatline,” indicates a complete absence of electrical activity in the heart. With PEA, organized electrical activity is present on the ECG, but the heart muscle does not contract effectively enough to generate a pulse or blood flow. Since there is no chaotic electrical activity to “reset,” defibrillation is not appropriate for these non-shockable rhythms.
The Urgency of Treatment
Immediate action is important when a shockable rhythm is suspected during cardiac arrest. The American Heart Association emphasizes a “chain of survival,” a sequence of actions that improve outcomes when performed quickly. Early recognition of cardiac arrest and activation of emergency medical services are important initial steps.
Rapid defibrillation is the most impactful link in this chain for individuals in VFib or pVT. Every minute without defibrillation significantly decreases survival chances, with a reported decrease of 7% to 10% per minute. While cardiopulmonary resuscitation (CPR) helps maintain blood flow to the brain and other organs, it cannot convert a shockable rhythm back to normal; defibrillation is required. Early CPR combined with rapid defibrillation increases the likelihood of a positive outcome.