ZOLL defibrillators are biphasic. Every current ZOLL model uses the company’s proprietary Rectilinear Biphasic waveform, which delivers energy in two phases of opposite polarity. ZOLL did manufacture monophasic models in the past, but those were discontinued around the early 2000s along with the rest of the industry’s shift to biphasic technology.
ZOLL’s Rectilinear Biphasic Waveform
ZOLL doesn’t just use a generic biphasic waveform. The company developed what it calls the Rectilinear Biphasic waveform (RBW), which works differently from the truncated exponential biphasic waveforms used by most other defibrillator manufacturers. The key difference is how it handles variations in patient chest impedance, the electrical resistance that differs from person to person based on body size, chest wall thickness, and other factors.
A standard biphasic defibrillator delivers a set voltage, which means the actual current reaching the heart changes depending on the patient’s impedance. ZOLL’s rectilinear design compensates for those impedance differences by adjusting the voltage to maintain a near-constant current throughout each phase of the shock. The first phase lasts about 6 milliseconds and the second phase about 4 milliseconds, with a tiny pause between them. This current-based approach is the basis for ZOLL’s FDA-cleared claim that its waveform is superior to monophasic waveforms, a distinction no other biphasic waveform manufacturer has received.
Energy Settings on ZOLL Devices
Because biphasic waveforms are more efficient at terminating dangerous heart rhythms, ZOLL devices operate at lower energy levels than the old monophasic machines. The standard defibrillation protocol for adult cardiac arrest follows an escalating sequence: 120 joules for the first shock, 150 joules for the second, and 200 joules for subsequent shocks. That 200J ceiling is the maximum output on current ZOLL defibrillators.
For context, monophasic defibrillators required up to 360 joules. ZOLL’s clinical data showed 99% first-shock success at 120J and 100% success at 150J in a prospective randomized trial, which is why the 200J level exists mainly as a safety margin rather than a routinely needed dose.
For synchronized cardioversion (used for certain abnormal heart rhythms that aren’t cardiac arrest), ZOLL recommends starting even lower, at 70 to 75 joules depending on the model, and escalating through 120J, 150J, and 200J as needed. Pediatric protocols use weight-based dosing at 2 to 4 joules per kilogram.
Which ZOLL Models Are Biphasic
All ZOLL defibrillators currently on the market use the Rectilinear Biphasic waveform. This includes the AED Plus and AED 3 (automated external defibrillators for public access and first responders), the R Series (a hospital and EMS professional defibrillator), and the X Series (ZOLL’s advanced monitor/defibrillator). The X Series can deliver biphasic energy from 1 joule up to 200 joules, and its waveform uses the same phase timing and current control mechanisms as the R Series.
ZOLL’s Monophasic History
If you’ve encountered references to monophasic ZOLL devices, they’re real but outdated. Around the year 2000, ZOLL sold both monophasic and biphasic versions of its M Series defibrillator simultaneously. The monophasic M Series could deliver up to 360 joules, while the biphasic version maxed out at 200 joules. A U.S. Department of Defense evaluation from October 2000 tested both models side by side, documenting this transition period.
Monophasic defibrillators across all manufacturers were last commercially produced in the late 1990s. The 2025 American Heart Association guidelines note that while some monophasic units may still be in service somewhere, biphasic defibrillators have largely replaced them. The AHA states that biphasic waveforms expose patients to lower peak electrical current while achieving equal or greater effectiveness at stopping dangerous heart rhythms, which is why guidelines favor biphasic devices when available.
Why Biphasic Matters
The practical difference comes down to two things: effectiveness and tissue damage. A monophasic shock pushes current in one direction through the heart. A biphasic shock reverses direction partway through, which turns out to be significantly better at resetting the heart’s electrical activity. Because biphasic shocks work at lower energy levels, they also cause less damage to heart muscle tissue. For the person receiving the shock, this means a better chance of successful defibrillation with less collateral injury, particularly when multiple shocks are needed.