Sudden cardiac arrest (SCA) is a life-threatening emergency caused by a malfunction in the heart’s electrical system, which prevents it from pumping blood effectively. This electrical malfunction prevents the heart from pumping blood to the brain and other organs, leading to immediate collapse and loss of consciousness. Intervention often involves defibrillation, a procedure that uses an electrical shock to restore a normal heart rhythm. However, the success of this intervention depends entirely on the specific underlying electrical pattern.
How Defibrillation Works
Defibrillation involves delivering a high-energy, unsynchronized electrical current across the chest to the heart. The purpose of this current is not to “jump-start” the heart, but rather to momentarily stop all disorganized electrical activity. This shock causes nearly all the heart’s muscle cells to depolarize simultaneously, creating an electrical silence. The goal is that, following this brief period of silence, the heart’s natural pacemaker, the sinoatrial (SA) node, will resume control and re-establish a normal, coordinated rhythm.
This process is sometimes compared to “rebooting a computer” because the shock wipes out the chaotic electrical signals so that the heart can restart with its intended operating system. Defibrillation is distinct from synchronized cardioversion, which is a lower-energy shock timed to a specific point in the heart’s electrical cycle. Cardioversion is used for fast but organized rhythms where a pulse is present, whereas defibrillation is reserved for life-threatening, chaotic rhythms in a patient who has no pulse.
The Shockable Rhythms
A heart rhythm is classified as “shockable” when it contains enough disorganized electrical activity that an external shock can reset it. The two primary shockable rhythms are Ventricular Fibrillation and Pulseless Ventricular Tachycardia. These rhythms are the most common cause of sudden cardiac arrest and are the direct target of defibrillation.
Ventricular Fibrillation (VF)
Ventricular Fibrillation (VF) is a state of complete electrical chaos where the heart’s lower chambers, the ventricles, quiver instead of contracting effectively. The heart muscle is still electrically active, but the signals are disorganized and rapid, preventing any meaningful blood pumping. Defibrillation works to terminate the chaotic electrical waves that circulate through the ventricles, allowing the SA node to take over the role of primary pacemaker. Immediate defibrillation is paramount, as the probability of survival decreases significantly for every minute VF goes untreated.
Pulseless Ventricular Tachycardia (pVT)
Pulseless Ventricular Tachycardia (pVT) is the second shockable rhythm, characterized by a rapid, organized electrical signal originating incorrectly in the ventricles. This rapid rate does not allow the heart chambers enough time to fill with blood before contracting. Though the electrical activity is organized on a monitor, the mechanical pumping is so inefficient that no pulse is palpable. This rhythm responds to the high-energy shock because there is an underlying electrical signal that can be interrupted and reset.
The Non-Shockable Rhythms
Non-shockable rhythms are those that lack the electrical activity for defibrillation to be effective. The problem is either a complete absence of electrical function or a mechanical failure despite organized electrical activity. Delivering a shock in these situations is not therapeutic and would only delay the necessary interventions.
Asystole
Asystole, often referred to as “flatline,” represents the complete absence of electrical activity in the heart. On a heart monitor, this appears as a straight line because there are no electrical impulses to record. Since defibrillation works by resetting disorganized electrical activity, a shock is useless when there is nothing electrical to reset. Treatment for asystole focuses on high-quality cardiopulmonary resuscitation (CPR) and administering medications like epinephrine to stimulate the heart.
Pulseless Electrical Activity (PEA)
Pulseless Electrical Activity (PEA) is a situation where the heart monitor shows an organized electrical rhythm, but the heart muscle is not contracting effectively enough to produce a pulse. The heart has electrical function, but the mechanical or pumping function has failed, often due to an underlying cause like trauma, low blood volume, or a collapsed lung. Management for PEA involves CPR and aggressively searching for and treating the underlying cause.
The Role of Automated External Defibrillators
Automated External Defibrillators (AEDs) bridge the gap between technical medical knowledge and public access to life-saving intervention. These devices are designed to be used by laypeople and are programmed to automatically analyze the patient’s heart rhythm. Once the electrode pads are attached to the chest, the AED detects the heart’s electrical activity to determine if a shockable rhythm is present.
The device uses internal algorithms to distinguish between the shockable rhythms (VF and pVT) and the non-shockable rhythms (Asystole and PEA). If the AED detects a shockable rhythm, it will charge up and prompt the user to deliver a shock. If it detects a non-shockable rhythm, it will announce “no shock advised.” This safety mechanism ensures that the device is only used when the electrical intervention has the potential to restore a normal heartbeat.