Monophasic vs Biphasic Defibrillation

Mono vs Biphasic Defibrillation Waveforms

Automated external defibrillators (AEDs) have become the standard for emergency treatment of sudden cardiac arrest. These devices have the potential to save the life of a patient by delivering electrical shocks to the heart that force it from a lethal rhythm back into normal sinus rhythm upon restarting its beat pattern. This process is known as defibrillation. There are two common approaches to defibrillation known as monophasic defibrillation and biphasic defibrillation.

Monophasic Defibrillation

Monophasic defibrillation is administered by a monophasic AED. Monophasic AEDs are devices that send a shock in a single direction from an electrode on one side of the patient’s chest to a second electrode on the other side. This technology is known as monophasic waveform defibrillation and it has been used since the invention of AEDs. The image shows the path a monophasic AED shock travels during defibrillation.


Monophasic defibrillation animation

Biphasic Defibrillation

Over time monophasic AEDs were replaced by biphasic AEDs. Biphasic AEDs utilize a bi-directional current flow and a less powerful shock than monophasic defibrillators. The technology was adopted from automatic implantable cardioverter-defibrillators or ICDs. This bi-directional current flow is known as biphasic waveform defibrillation. There are many advantages to this type of shock delivery system, as can be expected in the majority of technological advances.

Biphasic Waveform Animation

History of Defibrillation

In order to fully understand these AED technologies, it is important to understand the concepts behind them and the history of defibrillation as a whole. Below, we cover a broad range of related defibrillation history topics.

Defibrillation Concepts and Facts

When a person collapses during a sudden cardiac event, specifically sudden cardiac arrest (SCA), the most likely cause of the event will be a cardiac arrhythmia known as ventricular fibrillation. This particular rhythm carries a high risk of sudden cardiac death, and there is no manual treatment method in existence, including CPR, that has been shown to convert this rhythm back to normal sinus rhythm. Based on this fact, it’s easy to understand why SCA events often led to death before AEDs.

Early Defibrillation

In the General Electric laboratories, while the rapid push to electrify the nation was underway, when GE “switched from direct-current (DC) transmission [of electricity] to alternating-current (AC) transmission in the early 1900s, linemen began to die by electrocution.”[1]

This ironically deadly aspect of electricity was previously unknown, so the phenomenon was quickly studied by professors at Johns Hopkins University. Their research eventually caught the interest of a cardiac surgeon named Claude Beck.

In an effort to test his theories on the effects of AC current on the heart, he began to deliberately convert animals into ventricular fibrillation. Then in 1947, during surgery on a 14-year-old patient who suffered sudden cardiac death while on the operating room table, Dr. Beck utilized his rudimentary defibrillator (which used tablespoons to deliver the shock) to revive the child by directly shocking her heart. Seventy years later, we now enjoy modern lunchbox-sized, portable, biphasic AEDs.

As a result of Dr. Beck’s discovery of the correlation between electricity and the beat of the heart, research on this incredible finding began on a worldwide scale. The implication of bringing a clinically dead patient essentially “back to life” was arguably one of the most significant medical advances of this era.

This lead to Dr. Paul M. Zoll successfully demonstrating that the electrical current did not have to be delivered directly to the heart during cardiac arrest, but rather the stimulation could be applied externally and still generate an effective heartbeat.[2] This, coupled with Dr. Zoll’s many other discoveries and inventions in regulating heart rhythm, led to the concept of the external defibrillators that exist today.

Monophasic Defibrillators

The first technology used in these modern devices allowed for the current to be delivered in a single direction as discussed above. This is known as a monophasic defibrillation waveform. In this scenario, the electrical current is delivered in one direction from one electrode to the other, which momentarily stops the heart. This allows for a patient’s heart rhythm to convert back into normal sinus rhythm.

All defibrillation currents have a shape that can be visualized as a “waveform, [which] demonstrates how the flow of the current changes over time during the defibrillation.”[3] As the illustration above shows, the waveform associated with monophasic defibrillation contains a single peak. This peak current is critical in determining successful defibrillation because there must be enough current to reach the heart to terminate the fibrillation (i.e. lethal rhythm), while at the same time avoiding too high of a peak current, which can damage the heart.[4]

This peak current coupled with the body’s resistance to the current, known as impedance, comprise the two components that make up defibrillation. In a monophasic defibrillator, the delivered current is typically very high, at 360 joules, thus requiring large internal components to allow for the generation and storage of the required amount of electrical current to be delivered through the paddles or electrodes. As such, monophasic defibrillators used to be bulky machines not suited for placement in the community or operation by bystander-responders.

Biphasic Defibrillators

Conversely, as technology continued on its ever-evolving journey of intermingling with and augmenting the limits of the human body and mind, advances in the medical field eventually paved the way for a new type of waveform technology, known as biphasic waveform defibrillation. Biphasic defibrillation was derived from the same rationale used in the maintenance of arrhythmias by ICD devices, which maintain normal sinus rhythm in patients with difficult-to-manage chronic arrhythmias.

Electricity is sent from one electrode to the other in the first phase of this waveform, followed by a return back to the originating electrode in the second phase. Biphasic technology requires a much lower current to achieve successful termination of fibrillation.

Evidence has demonstrated a higher efficacy with regards to successful defibrillation than with monophasic technology. Because of the lower current, the same components that contributed to the bulky, heavy size of monophasic defibrillators were able to be reduced in size tremendously, leading to the advent of the automated external defibrillator. Current biphasic AEDs are about the size of a lunch box, making it realistic for the majority of shopping centers, warehouse stores, super centers, malls, schools, and other social centers of gathering or commerce to be equipped with publicly-accessible devices.

Beyond Monophasic and Biphasic Defibrillation

The constant advance of technology has yielded AEDs which are fully automated, provide clear verbal instruction to the operator, are able to determine if the patient is in a shockable rhythm (Ventricular Fibrillation/Ventricular Tachycardia), and which will not allow the operator to deliver a shock if the patient’s heart is not in a shockable rhythm. Thus, it is impossible for a layperson who may have little-to-no formal cardiovascular training to harm a patient by delivering defibrillation. Today’s AEDs will only “charge,” and allow the rescuer to emit a “shock,” if the patient is in a recognizable lethal rhythm.

Furthermore, by allowing for a lower current delivery, the most common risk associated with defibrillation of electrical skin burns at the electrode sites is dramatically decreased with biphasic technology. The other common risks, including the risk of stroke from a traveling blood clot, are also dramatically lowered proportionately to the amount of trauma to the body. This should not serve to in any way discount the significantly-positive impact on survival rates that occurred in countless cases as a result of the advent of the monophasic external defibrillator, to which an untold number of victims of ventricular fibrillation owe their lives.


Over the past 64 years, from the time that Dr. Zoll demonstrated the efficacy of delivering external currents to achieve defibrillation, technology has progressed in leaps and bounds. To put this into perspective, in less than the average human life expectancy, innovations in defibrillation technology have saved millions of human lives. Many of those lives were saved as a result of the gold-standard defibrillation technology for over 30 years, monophasic technology.

However, the advent of the biphasic automated external defibrillator exponentially increased our exposure to, and the availability of, these life-saving devices by decreasing the size of the machine and simplifying instructions for the operator.

As technology continues to advance and accelerate, one can only imagine where AED technology will be in another generation. Based upon the incredibly short lifespan of the technology as a whole and the rapid advancements we’ve already seen, the possibilities are endless.

S. Joanne Dames - MD, MPH

Updated: 10/30/2018


  1. [1]Kroll, M., et al. (2008) Idiot-proofing the Defibrillator. IEEE Spectrum. Retrieved from
  2. [2]Hatlestad, D. (2004). Biphasic Defibrillation: The Shape of Resuscitation Today. Retrieved from:
  3. [3]Kroll, M., et al. (2008) Idiot-proofing the Defibrillator. IEEE Spectrum. Retrieved from
  4. [4]Zoll Corporation (2015). Milestones in History: Major Events and Corporate Milestones. Retrieved from

8 Responses to “Monophasic vs Biphasic Defibrillation”

June 18, 2018 at 12:17 pm, Ricky Fitzgerald said:

How can you tell if an AED is monophasic or Biphasic? The company that I am employed by has several AED’s and I am interested to know if they are capable of a biphasic cardio-conversion. Thank you.


June 20, 2018 at 8:59 am, AED Superstore said:

Hello Ricky,

Thanks for your question. The simple answer is you can’t really tell by looking at the AED itself. Please feel free to give the manufacturer a call, or give us a call at our customer service line, 1-800-544-0048, and we would be happy to help you find out by looking up the make, model and year of manufacture (usually found on the AED itself on the back). All AEDs currently available for purchase in the US market are biphasic, but this technology was adopted at different times by different manufacturers.


January 07, 2019 at 11:42 am, Baran said:

How big are they?


May 21, 2019 at 1:55 pm, Mike C said:

The size of the device is not a reflection of the device being monophasic or biphasic the way this article leads you to believe. I work with both types of devices, and there is absolutely no difference in the outer size of an older monophasic device when compared to a brand-new biphasic unit. While there are some differences of the internal capacitor, its minimal, and the user would never know the difference.
With that being said, monophasic hasn’t been sold in the U.S. for nearly twenty years. If your device in newer than that, it’s safe to assume that it’s biphasic.


September 12, 2019 at 3:10 am, Pooja M N said:

Beautifully explained mam. Thank you


February 08, 2020 at 9:38 pm, Chris said:

What was the first biphasic external defibrillator?


February 17, 2020 at 8:37 am, AED Superstore said:

A Google search revealed: “Heartstream introduced its first AED, the ForeRunner, in 1996. The Heartstream ForeRunner AED marked the first widespread commercial use of a biphasic waveform in an external defibrillator. ” I hope this answers your question.


September 10, 2020 at 9:43 am, How An AED Works - Blogs said:

[…] To get more details on how automated external defibrillators work, check out this article on bi-phasic vs monophasic defibrillation. → How To Use An […]


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