Electrical energy applied to the heart is a specialized medical intervention used to treat life-threatening irregularities in the heart’s rhythm. This procedure, often called “shocking” a patient, is not a universal treatment for all cardiac emergencies. Its function is to deliver a controlled electrical current to the heart muscle, stopping disorganized electrical activity. This momentary pause allows the heart’s natural pacemaker—the sinoatrial node—to attempt to re-establish a normal, effective rhythm. The decision to use this therapy depends entirely on the heart’s underlying electrical pattern and the patient’s physical condition.
Defibrillation Versus Cardioversion
The two primary methods of using electrical therapy are defibrillation and cardioversion. Defibrillation involves delivering an unsynchronized, high-energy electrical shock immediately upon activation. This method is employed during cardiac arrest when there is no pulse and the heart’s electrical system is in complete chaos. The discharge depolarizes a significant mass of the heart muscle simultaneously, aiming to halt the erratic rhythm and allow a stable rhythm to emerge.
Cardioversion, in contrast, is a synchronized procedure where the electrical shock is timed precisely to the peak of the R-wave on an electrocardiogram (ECG). This synchronization prevents the energy from being delivered during the heart’s vulnerable period, known as the T-wave. Hitting the T-wave can inadvertently trigger a much more dangerous, chaotic rhythm, such as ventricular fibrillation. Cardioversion typically uses a lower energy level than defibrillation and is used for patients who still have a pulse but whose heart rhythm is dangerously fast and compromising their stability.
Rhythms That Require Immediate Defibrillation
Immediate, unsynchronized defibrillation is reserved for two specific rhythms representing sudden cardiac arrest: Ventricular Fibrillation (V-Fib) and Pulseless Ventricular Tachycardia (V-Tach). In V-Fib, the ventricles are not contracting effectively but are merely quivering with chaotic electrical signals. This results in the complete loss of the heart’s ability to pump blood, causing immediate collapse and lack of a pulse.
Pulseless V-Tach involves a very rapid, but somewhat organized, electrical pattern that is too fast to allow for effective filling and pumping of blood, also leading to cardiac arrest. Both V-Fib and Pulseless V-Tach require the rapid application of an electrical shock as the definitive treatment. Survival rates decrease by about 10% for every minute defibrillation is delayed, underscoring the urgency. Automated External Defibrillators (AEDs) analyze the heart’s electrical activity and deliver this unsynchronized shock if they detect either rhythm, enabling lay responders to provide this therapy.
Rhythms That Require Timed Cardioversion
The second major scenario involves patients who still have a pulse but are hemodynamically unstable due to a rapid, organized heart rhythm. This situation calls for synchronized cardioversion, used to treat rhythms like unstable Atrial Fibrillation (A-Fib), Atrial Flutter (A-Flutter), and Ventricular Tachycardia (V-Tach) with a pulse. In these cases, the heart beats so quickly that it cannot fill with enough blood between beats, leading to symptoms like low blood pressure, chest pain, or altered mental status.
The synchronization feature is crucial because these rhythms still have an organized electrical pattern, including the vulnerable T-wave. The device must time the electrical delivery to the R-wave to avoid inducing V-Fib. While pulseless V-Tach requires immediate defibrillation, V-Tach with a pulse and instability is treated with lower-energy, precisely timed cardioversion. The goal is to interrupt the abnormal electrical circuit, allowing the heart to resume a slower, more effective rhythm.
When Electrical Therapy is Not Appropriate
Not all causes of cardiac arrest respond to an electrical shock, and attempting to shock the heart in these situations is ineffective. The two primary non-shockable rhythms are Asystole and Pulseless Electrical Activity (PEA). Asystole, commonly referred to as “flatline,” indicates a total absence of electrical activity in the heart. Since there is no electrical signal to reset, delivering a shock serves no purpose.
Pulseless Electrical Activity is a condition where the heart’s electrical system produces an organized rhythm on the monitor, but the heart muscle is not contracting effectively enough to produce a pulse. In PEA, the failure is mechanical rather than electrical, meaning a shock is futile. For both Asystole and PEA, treatment shifts away from electrical intervention to high-quality chest compressions and medications like Epinephrine. Medical teams focus on identifying and reversing the underlying cause of the cardiac arrest.