Defibrillation is a medical procedure that uses a targeted electrical shock to stop a life-threatening, irregular heartbeat. When the heart’s electrical system fails and produces a chaotic rhythm, it can no longer pump blood effectively. This intervention halts the erratic electrical activity, providing an opportunity for the heart’s natural pacemaker to re-establish a normal rhythm. The successful application of this technique can be the difference between life and death in a sudden cardiac arrest.
The Biphasic Electrical Current
A biphasic electrical current is delivered in two separate phases. Initially, the current travels from one of the defibrillator paddles to the other, passing through the heart in a single direction. Immediately after this first phase, the device reverses the polarity of the electrodes, causing the current to flow back in the opposite direction for a specified duration.
This method can be compared to bringing a swing to a stop. The first pulse of energy is a firm push in one direction, and the second, reversed pulse acts as a controlled pull back. This dual-phase action allows for a more managed and efficient electrical intervention. Different manufacturers utilize distinct biphasic waveforms, with the most common being the Biphasic Truncated Exponential (BTE) and the Rectilinear Biphasic (RLB) waveforms.
Terminating Lethal Arrhythmias
The primary purpose of a defibrillation shock is to treat lethal heart rhythms, most notably ventricular fibrillation (VF) and pulseless ventricular tachycardia (pVT). In ventricular fibrillation, the heart’s lower chambers, the ventricles, quiver chaotically and are unable to pump blood to the body. Pulseless ventricular tachycardia is a condition where the ventricles beat so rapidly and inefficiently that no pulse can be detected, also leading to a failure of blood circulation.
The biphasic shock works by simultaneously depolarizing a large portion of the heart muscle, known as the myocardium. This widespread electrical stimulation overwhelms the chaotic signals that define VF and pVT, effectively “resetting” the heart’s electrical system. This creates a momentary pause in all electrical activity, observed as asystole on an electrocardiogram. This brief silence gives the heart’s natural pacemaker, the sinoatrial node, a chance to resume its normal function and generate an organized rhythm.
Comparison with Monophasic Technology
The technology preceding biphasic defibrillation involved a monophasic waveform, where the electrical current flows in only one direction. A significant difference between these technologies is the amount of energy required for a successful outcome. Biphasic defibrillators are effective at much lower energy levels, between 120 and 200 joules. In contrast, monophasic defibrillators deliver a fixed, higher dose, often set at 360 joules for all shock attempts.
This reduction in energy is a major advantage of biphasic systems. Studies have demonstrated that biphasic shocks have a higher rate of success in terminating arrhythmias on the first attempt compared to their monophasic counterparts. The ability to achieve defibrillation with less energy is also believed to reduce potential damage to the heart muscle. High-energy shocks can cause myocardial injury, and minimizing this damage may lead to better patient recovery and outcomes after resuscitation.
Application in Modern Defibrillators
Biphasic waveform technology is the current standard and is used in nearly all modern defibrillators. This includes advanced manual defibrillators in hospitals and the Automated External Defibrillators (AEDs) available in public spaces. The development of smaller, lighter, and more efficient devices is partly due to the lower energy requirements of biphasic technology. These devices are more portable and have less demanding battery and maintenance needs.
A feature of these modern devices is their ability to compensate for impedance. Impedance is the body’s natural resistance to the flow of electrical current. Biphasic defibrillators can measure this resistance and automatically adjust the electrical output to ensure a sufficient dose of current reaches the heart. This smart technology helps to ensure that the shock is effective regardless of variations in patient size and body composition.