A biphasic defibrillator is a device that delivers an electric shock in two phases, reversing the direction of current partway through the pulse. This design stops life-threatening heart rhythms more effectively and with less energy than older single-direction (monophasic) devices. Since roughly 2000, biphasic technology has become the standard in virtually all defibrillators, from hospital crash carts to the automated external defibrillators (AEDs) mounted in airports and office buildings.
How the Two-Phase Shock Works
When a biphasic defibrillator fires, current flows through the heart in one direction for about 5 to 10 milliseconds. The device then reverses polarity so current flows back in the opposite direction for a second phase. This reversal is the defining feature. Monophasic defibrillators, by contrast, push current in a single direction only.
The two-phase approach works better because the first pulse depolarizes heart muscle cells (essentially resetting their electrical activity), while the second pulse in the reverse direction lowers the threshold needed to fully reset any remaining cells. The result is that fewer joules of energy are required to get the heart back into a normal rhythm, which matters both for success rates and for limiting damage to the heart.
Two Main Waveform Types
Not all biphasic shocks look the same on a graph. The two main waveform designs are the biphasic truncated exponential (BTE) and the biphasic rectilinear (BRL). Both deliver current in two directions, but they differ in how voltage changes over the course of each phase. BTE waveforms start at a peak voltage and taper off, while BRL waveforms maintain a more gradual slope. In practice, both perform well for most patients, though they can behave differently at the extremes of chest resistance, such as in very large or very small patients.
Many modern biphasic devices also use impedance compensation. Before delivering the shock, the defibrillator measures the electrical resistance of your chest. It then automatically adjusts the voltage and duration of each phase so that the energy actually reaching the heart stays consistent regardless of body size. This feature is one reason AEDs can be used by bystanders without needing to select a dose.
Biphasic vs. Monophasic: The Evidence
The clinical case for biphasic technology is strong. In a randomized, double-blind trial comparing 200-joule biphasic shocks to 200-joule monophasic shocks for ventricular fibrillation (VF), the biphasic group achieved a 100% first-shock success rate (39 of 39 patients) compared to 90% for monophasic (61 of 68). A separate study of out-of-hospital cardiac arrests found that 200-joule biphasic shocks restored an organized heart rhythm 69% of the time versus 45% for monophasic shocks at the same energy. After adjusting for other variables, a biphasic shock was roughly four times more likely to succeed on the first attempt.
First-shock success matters enormously. Every additional shock takes time, and in cardiac arrest, every minute without circulation reduces the chance of survival. A technology that gets it right on the first try can meaningfully change outcomes.
Less Energy, Less Heart Damage
Because biphasic waveforms work at lower energy levels, they cause less harm to the heart muscle itself. Research comparing the two technologies after prolonged VF found significantly better heart function in the hours following resuscitation when biphasic shocks were used. Monophasic devices typically required escalating doses of 200, 300, and then 360 joules, while biphasic devices commonly operate at 120 to 200 joules. That reduction in delivered energy translates to less post-shock impairment of the heart’s pumping ability, giving patients a better starting point for recovery.
Current Energy Guidelines
For adults, the 2025 American Heart Association guidelines recommend initial shock energies of 200 joules or higher for conditions like atrial fibrillation and atrial flutter. Most biphasic AEDs designed for public use are preset at 150 to 200 joules and do not require the user to select an energy level.
For children, dosing is weight-based. The AHA recommends starting at 2 joules per kilogram of body weight and escalating to 4 joules per kilogram if the first one or two shocks fail. The European Resuscitation Council takes a slightly different approach, recommending 4 joules per kilogram from the start without escalation. Animal research has supported the higher initial dose: biphasic shocks at 2 joules per kilogram succeeded only about 25 to 32% of the time in piglet models, while 4 joules per kilogram pushed success rates above 80%. Pediatric AED pads include a built-in energy reducer that lowers the adult dose to an appropriate range for smaller patients.
How Biphasic Became the Standard
The biphasic concept is not new. A Soviet researcher named Gurvich introduced the first biphasic transthoracic defibrillator in 1970, and it became standard practice in Soviet medicine. Western countries, however, continued using monophasic devices for another two decades. It was not until the late 1990s that clinical trials in Europe and the United States demonstrated clear advantages, and after 2000, manufacturers shifted almost entirely to biphasic designs for both external defibrillators and implantable cardioverter-defibrillators (ICDs). The transition was especially important for ICDs, where device size is limited by battery capacity. Because biphasic waveforms achieve the same result with less stored energy, manufacturers could build smaller, longer-lasting implants.
Today, monophasic defibrillators are essentially obsolete. You may still encounter one in an older facility that has not upgraded its equipment, but new units sold worldwide are biphasic. If you are trained to use a defibrillator or encounter an AED in public, the device in front of you is almost certainly biphasic, and its internal software is handling the waveform adjustments automatically.