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Differences Between Monophasic & Biphasic Shock Delivery for AEDs

Monophasic vs. biphasic waveform defibrillation refers to two different shock techniques that can successfully reset the heart of someone in atrial fibrillation.

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 biphasic defibrillator.

Understanding Waveforms

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, this is referred to as a “sinus rhythm.”

In the early 1900s, the delivery of electricity was changed from direct current (DC) to alternating current (AC), which has a waveform much like that of the human heart.

When linemen suddenly began to die from electrocution at the advent of AC electricity, it was discovered that these waveforms were actually converting their normal sinus heart rhythm into atrial fibrillation, leading to sudden cardiac arrest and death.

Then, in 1956, Dr. Paul Zoll used sinusoidal waveform technology to reset the heart of a 14-year-old girl who had gone into sudden cardiac arrest on the operating table using a rudimentary setup with metal spoons. The girl was revived, and modern defibrillation technology was born.

What Is Monophasic Defibrillation?

The first external defibrillators were monophasic, meaning that only one wave of current passed through the patient’s heart from the right side to the left. 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 atrial 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.[1]

What Is Biphasic Defibrillation?

Noticing that some monophasic defibrillators caused electrical burns at the site of the electrode pads, defibrillator makers experimented with biphasic waveform shocks which use a lower dose of energy: 120 Joules for the first shock and 200 Joules for the second shock.

A biphasic waveform had been used in implantable defibrillators (implantable cardioverter defibrillators or ICDs), and it was fairly straightforward to transfer the same biphasic technology to AEDs.

In biphasic devices, the shock waveform has two phases. The first phase passes from the right side of the patient’s chest to the left side, and the second phase passes back from left to right. This makes the shock more effective at lower doses. The AED pad placement used with biphasic defibrillators is the same as for monophasic defibrillators.

Advantages of Biphasic vs. Monophasic Defibrillation

Biphasic defibrillators have several advantages over monophasic defibrillators:

  1. Biphasic Defibrillators Are More Effective

In the pig study cited above, biphasic defibrillation resuscitated 10 out of 10 pigs (compared to 8 out of 10 pigs for monophasic defibrillation) and caused less impairment of cardiac function post-resuscitation. Another study showed that biphasic waveforms were more effective for converting ventricular fibrillation in out-of-hospital cardiac arrest.[2]

  1. Biphasic Defibrillation Is Less Likely to Burn the Patient

The second advantage of biphasic technology is that the lower levels of delivered energy used in biphasic defibrillation reduce the risk of skin burns to the patient. When a patient requires repeated shocks, this reduced risk of burns is crucial.

  1. Biphasic Waveforms Take a Smaller Toll on Battery Life

Finally, 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.

Examples of Biphasic AEDs

Biphasic waveform defibrillation has been used in automated external defibrillators in the United States for the last 20 or 30 years, so if you buy an AED today, it’s most likely to be biphasic.

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 really needed to reset the heart. The more resistance, the more energy 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

The appropriately named HeartSine Samaritan PAD series (Samaritan PAD 350P, Samaritan PAD 360P, Samaritan PAD 450P) has a slightly different shock delivery system.

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: 150J; Shock 2: 150J; Shock 3: 200J

Children: Shock 1: 50J; Shock 2: 50J; Shock 3: 50J

The charging time is typically 6 seconds for a 150J shock and 8 seconds for a 200J shock with a fresh battery or after six shocks.


Named after the defibrillation pioneer mentioned earlier in this article, the ZOLL AED Plus uses a rectilinear biphasic waveform TM with an impedance range of 0 to 300 Ω. The pre-programmed levels of delivered energy for each biphasic shock are:

Adults: Shock 1: 120J; Shock 2: 150J; Shock 3: 200J

Children: Shock 1: 50J; Shock 2: 75J, Shock 3: 80J

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 200J.

Getting the Most Out of Your Monophasic or Biphasic AED

Both biphasic and monophasic shocks can save lives in the case of ventricular fibrillation. However, if we compare monophasic vs. biphasic defibrillation, biphasic waveforms require less energy and have slightly less risk 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 biphasic 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 and ensure your defibrillator is working and able to deliver the appropriate shock. Whether monophasic or biphasic, that shock coupled with immediate, high-quality CPR might one day save a life.





[1] The effects of biphasic and conventional monophasic defibrillation on postresuscitation myocardial function; Journal of the American College of Cardiology; September 1999

[2] Low-energy biphasic AEDs more effective than high-energy monophasic version; Medscape; October 2000

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