When the heart suddenly stops beating effectively, a person is experiencing cardiac arrest. This cessation of function is often caused by an electrical malfunction, preventing the heart from pumping blood to the brain and other organs. Electrical therapy, known as defibrillation, attempts to restore a normal heart rhythm by delivering a controlled shock. This treatment is only effective for specific types of electrical chaos, which are referred to as the shockable rhythms. Determining which rhythms respond to this intervention is a time-sensitive step in resuscitation protocols.
Ventricular Fibrillation
The most frequently encountered initial rhythm in sudden cardiac arrest is Ventricular Fibrillation (VF). It is characterized by completely disorganized electrical activity in the heart’s lower chambers. Instead of contracting forcefully, the ventricles merely quiver, which looks like a chaotic, wavy line on an electrocardiogram. This disorganized electrical firing means the heart cannot generate any meaningful blood flow or pulse, leading to immediate collapse. Because the underlying problem is a chaotic electrical state, intervention must be prompt, as the chance of survival decreases by 7% to 10% for every minute defibrillation is delayed.
Delivering an electrical shock is the definitive treatment for VF. VF is often categorized as either fine or coarse, depending on the amplitude of the electrical waves seen on a monitor. Coarse VF is generally more responsive to defibrillation than fine VF. Immediate defibrillation attempts to reset the heart’s electrical system, allowing the natural pacemaker to take over with a coordinated rhythm.
Pulseless Ventricular Tachycardia
The second shockable rhythm is Pulseless Ventricular Tachycardia (pVT). This represents a rapid, but somewhat organized, electrical rhythm originating from the ventricles. In pVT, the ventricles beat so quickly that they do not have enough time to properly fill with blood between contractions. This extreme speed results in an ineffective mechanical pump function, meaning that despite organized electrical signals, no pulse is detectable.
The key distinction from VF is that pVT involves rapid, coordinated firing rather than chaotic quivering, yet the outcome is the same: no circulation. Because of this, pVT is treated identically to VF with immediate defibrillation. The goal of the electrical shock is to interrupt the overly fast circuit, allowing the heart’s normal electrical control center to re-establish a slower, effective rhythm.
Non-Shockable Rhythms
For a shock to be effective, there must be disorganized or excessively fast electrical activity present in the heart to disrupt. Two major rhythms that cause cardiac arrest are considered non-shockable because they lack this electrical component. Asystole shows a complete absence of any electrical activity on the cardiac monitor. In this state, there is no electrical signal remaining to be reset by a defibrillator, making a shock ineffective.
The second non-shockable rhythm is Pulseless Electrical Activity (PEA). Here, the cardiac monitor displays an organized electrical rhythm, but the patient has no pulse. In PEA, the electrical system is working, but the heart muscle is not mechanically contracting. Because the electrical issue is not the root cause, delivering a shock will not restore a pulse. Both Asystole and PEA are instead treated with continuous cardiopulmonary resuscitation (CPR) and specific medications, such as epinephrine, while providers search for and reverse the underlying physiological cause.
The Science of Electrical Therapy
The purpose of the electrical therapy used for VF and pVT is to momentarily stop the entire heart’s electrical activity. Delivering a controlled, high-energy electrical current across the chest causes a mass depolarization of the myocardial cells. This forced, simultaneous depolarization extinguishes the chaotic or rapid electrical wavefronts.
The brief period of electrical silence immediately following the shock gives the heart’s natural pacemaker, the sinoatrial (SA) node, an opportunity to resume control. The SA node is typically the first to recover from the shock. This allows it to initiate a coordinated, slower, and more effective rhythm known as a normal sinus rhythm. This specific unsynchronized delivery of electricity for cardiac arrest rhythms is called defibrillation.