Defibrillation is an electrical therapy used to interrupt life-threatening cardiac rhythms, specifically ventricular fibrillation (VF) and pulseless ventricular tachycardia (pVT). These chaotic rhythms prevent the heart from pumping blood effectively, leading to sudden cardiac arrest. The treatment involves delivering a controlled electrical shock across the chest to momentarily stop all electrical activity. This allows the heart’s natural pacemaker to reset a coordinated, effective rhythm. The energy delivered in this shock is measured in joules.
The Critical Difference Between Monophasic and Biphasic Devices
The energy required for a successful shock depends heavily on the defibrillator technology used. Older monophasic devices deliver the electrical current in a single, unidirectional pulse from one electrode to the other. To ensure enough current reaches the heart through the patient’s chest impedance, these devices must deliver a very high total energy dose. This single-direction flow is less efficient and carries a higher risk of causing damage to the heart muscle or skin burns.
Modern biphasic defibrillators represent a significant technological improvement in this therapy. They deliver the electrical current in two phases: the current initially flows in one direction, then quickly reverses and flows back in the opposite direction. This bidirectional flow is much more effective at depolarizing the heart muscle. Because biphasic technology is more efficient and causes less post-shock myocardial dysfunction, it has become the standard of care in nearly all clinical settings.
Current Guidelines for Initial Adult Defibrillation Energy
For adult patients experiencing ventricular fibrillation or pulseless ventricular tachycardia, the recommended initial energy dose for defibrillation varies based entirely on the device type. If a monophasic defibrillator is used, the standard recommendation is a fixed dose of 360 Joules for the first shock. This maximum energy level is required to maximize the probability of success due to the limitations of the unidirectional waveform.
For biphasic devices, the energy recommendation is a range, typically between 120 and 200 Joules for the first shock. This range accounts for the various types of biphasic waveforms and the specific manufacturer’s testing and recommendations. The American Heart Association (AHA) guidelines suggest using the manufacturer’s recommended dose. If that information is unavailable, the highest available energy setting on the device should be selected for the first shock.
The rationale for using lower energy with biphasic devices is their superior efficiency, which reduces the potential for damaging the heart muscle while maintaining high efficacy. Selecting the appropriate energy for the first shock is a time-sensitive decision. The highest priority is to deliver the shock as quickly as possible to the fibrillating heart. Immediately after the shock, chest compressions must be resumed for two minutes before the heart rhythm is reassessed.
Energy Requirements for Subsequent Shocks
If the initial electrical shock fails to convert the chaotic rhythm, subsequent shocks are required as part of the resuscitation protocol. The energy level for these following attempts should be either equivalent to the first successful dose or escalated. For patients using a monophasic device, all subsequent shocks remain at the fixed maximum of 360 Joules.
With biphasic devices, the guidelines recommend that the second and any following shocks should be at least equivalent to the first dose, and increasing the energy is often considered. This escalation is performed in a stepwise fashion, moving toward the maximum output of the specific defibrillator, which is usually 200 Joules or 360 Joules. The goal of escalating the energy is to overcome the potential for a rising defibrillation threshold, which can occur as cardiac arrest is prolonged. The continuous cycle of two minutes of high-quality cardiopulmonary resuscitation (CPR) followed by a shock is repeated until the heart is successfully restarted.
Pediatric and Specialized Defibrillation Settings
Electrical therapy for children requires a significant deviation from adult protocols, as the energy is based on the patient’s body weight. For pediatric defibrillation in ventricular fibrillation or pulseless ventricular tachycardia, the initial energy recommendation is 2 Joules per kilogram (J/kg) of body weight. If this first shock is unsuccessful, the energy for the second and subsequent shocks is escalated to 4 J/kg.
Higher doses may be considered for subsequent shocks, but should not exceed 10 J/kg or the maximum adult dose (typically 360 Joules). This weight-based dosing ensures sufficient electrical current delivery while avoiding excess energy that could cause myocardial damage. For other conditions, such as unstable supraventricular or ventricular tachycardias where the patient still has a pulse, a synchronized shock called cardioversion is used. Cardioversion typically requires a much lower initial energy, often beginning at 50 to 100 Joules for a biphasic device.