Monophasic vs. biphasic waveform defibrillation refers to two different shock techniques that can potentially reset the heart of someone in ventricular fibrillation or pulseless ventricular tachycardia.
The main difference between monophasic and biphasic shock delivery is that a monophasic electrical current moves in a single direction while a biphasic current is bidirectional (moving in a straight line and then reversing its direction).
While earlier defibrillators were monophasic, biphasic defibrillation has become the norm in the United States over the past 20-30 years. Expected users of automated external defibrillators (AEDs) can benefit from understanding the differences between a monophasic and a biphasic defibrillator.
If you’ve ever seen a heartbeat displayed on a screen (such as in an electrocardiogram), you will be familiar with the waves and peaks that occur in the human heartbeat. In medicine, a regular cardiac rhythm is referred to as a “sinus rhythm.”
In the late 1800s, alternating current (AC) was introduced alongside direct current (DC). Alternating current has a waveform much like that of the human heart.
When linemen began to die from electrocution, it was discovered that low-voltage shocks were converting their normal heart rhythm into ventricular fibrillation, leading to sudden cardiac arrest and death.
In 1956, Dr. Paul Zoll reported using alternating current to successfully convert ventricular fibrillation to sinus rhythm in humans through the chest (transthoracic defibrillation) rather than shocking the heart directly as had previously been done.
Ongoing work in the Soviet Union as well as in the Western World showed that external defibrillation was possible, using either direct current or alternating current. This work ultimately led to the development of the modern automated external defibrillator.
What Is Monophasic Defibrillation?
The first external defibrillators used monophasic waveform technology, meaning that only one wave of current passed through the patient’s heart. Defibrillation with a monophasic shock typically requires 200 joules of energy for the first shock and 360 joules for the second shock for adults and a smaller dose for children.
Monophasic shocks are very effective for stopping a lethal rhythm (including ventricular fibrillation and pulseless ventricular tachycardia) so that the heart can start up again with a normal sinus rhythm. In a 1999 study of 20 anesthetized, ventilated pigs, monophasic defibrillation resuscitated 8 out of 10 pigs .
What Is Biphasic Defibrillation?
Starting with Gurvich and Yuniev in the 1940s, defibrillator makers started considering biphasic waveform shocks for defibrillation. In biphasic devices, the shock waveform has two phases: The first phase passes from one side of the heart to the other, and the second phase passes back through the heart in the opposite direction. This makes the shock more effective at lower doses—typically 120 joules for the first shock and 200 joules for the second shock. Biphasic waveforms are now used in implantable defibrillators (implantable cardioverter defibrillators or ICDs), external defibrillators, and hospital-based defibrillators .
Advantages of Biphasic vs. Monophasic Defibrillation
Biphasic defibrillators have a few key advantages over monophasic defibrillators:
1. Biphasic Defibrillation Is Less Likely to Burn the Patient
Biphasic defibrillation comes with a reduced risk of skin burns to the patient  thanks to the lower levels of delivered energy. When a patient requires repeated shocks, this reduced risk of burns is especially valuable.
2. Biphasic Waveforms Take a Smaller Toll on Battery Life
Lower energy discharges mean less battery power is needed for each shock. Less battery power means more shocks per battery and smaller, more portable devices. This, in turn, makes it easier for members of the public to rush an AED to a patient’s side.
3. Biphasic Waveform Defibrillation May Be More Effective
In the pig study cited earlier in the article, biphasic shocks resuscitated 10 out of 10 pigs (compared to 8 out of 10 pigs with monophasic waveform shocks) and caused less impairment of cardiac function post-resuscitation. However, the results of a three-year study conducted in Japan between 2005 and 2007 suggest that biphasic waveform defibrillation is not significantly more effective than monophasic waveform defibrillation at improving outcomes one month after cardiac arrest.
While new defibrillators are generally biphasic, there is still some debate about whether or not it’s necessary to replace older monophasic defibrillators with biphasic models. Consult the American Heart Association website for the most up-to-date recommendations.
Examples of Biphasic AEDs
Biphasic waveform defibrillation has been used in automated external defibrillators in the United States since 1996. If you buy an AED today, it’s most likely to be biphasic.
ZOLL AED Plus
Named after the defibrillation pioneer mentioned earlier in this article, the ZOLL AED Plus uses a rectilinear biphasic waveform™ with an impedance range of 0 to 300 Ω. The pre-programmed levels of delivered energy for each biphasic shock are:
Adults: Shock 1: 120 J; Shock 2: 150 J; Shock 3: 200 J
Children: Shock 1: 50 J; Shock 2: 75 J, Shock 3: 80 J
Philips HeartStart FRx
The Philips HeartStart FRx uses a biphasic truncated exponential waveform with energy levels that are adjusted according to the patient impedance so that only as much energy is used as is needed to reset the heart. The more resistance, the more energy is used in the shock.
If the resistance loading is low (25 Ω), 128 joules are delivered for 2.8 milliseconds for the first phase and 2.8 milliseconds for the second phase. However, if the resistance loading is high (175 Ω), 158 joules are delivered for 12.0 milliseconds for the first phase and 8.0 milliseconds for the second phase.
For children, the biphasic shocks are likewise adjusted according to the patient impedance. The minimum dose is 43.4 joules for 2.8 milliseconds and the maximum dose is 52.4 joules for 8.0 milliseconds.
HeartSine Samaritan PAD
Instead of a biphasic truncated exponential waveform, this model uses a SCOPE™ (self-compensating output pulse envelope) biphasic escalating waveform that “compensates energy, slope, and envelope for patient impedance” (HeartSine Samaritan PAD 350P user manual). The HeartSine Samaritan PAD has an impedance range of 20 Ω to 230 Ω and is pre-configured with the following factory levels of delivered energy:
Adults: Shock 1: 150 J; Shock 2: 150 J; Shock 3: 200 J
Children: Shock 1: 50 J; Shock 2: 50 J; Shock 3: 50 J
The charging time is typically 6 seconds for a 150 J shock and 8 seconds for a 200 J shock with a fresh battery or after six shocks.
The Takeaway About Shock Levels Across AED Brands
As you can see, the pre-set shock levels are slightly different between AED brands. However, the actual energy of the shock delivered is adjusted according to the heart analysis and patient impedance. None of the AEDs listed will deliver a shock greater than 200 J.
Getting the Most Out of Your Monophasic or Biphasic AED
Both biphasic and monophasic shocks can save lives in the case of ventricular fibrillation or pulseless ventricular tachycardia. However, if we compare monophasic vs. biphasic defibrillation, biphasic waveforms require less energy than monophasic defibrillation, making this the preferred technology for new AEDs.
If you are the person in charge of the automated external defibrillator at your workplace, school, or place of worship, it’s essential to make sure that you follow all United States AED maintenance requirements and that batteries are replaced before they expire. Successful defibrillation is only possible if the batteries have enough charge to deliver the shock.
For multi-state or multi-site organizations in the United States, AED program management can help you stay on top of your technical and legal obligations to ensure your defibrillator is working and able to deliver the required shock. Whether monophasic or biphasic, that shock coupled with immediate, high-quality CPR can save a life.