#447: Defibrillation 101

On mobile? Listen with your favorite podcast player here.

If you’ve ever watched a medical drama or spent any amount of time in the clinical setting, you’ve seen a crash cart and have a general idea of what defibrillation is. In this lesson, we’ll dive deeper into defibrillation so you can feel confident when working with patients, practicing a mock code blue, or simply answering questions on an exam. 

What is defibrillation?

Defibrillation is the process of delivering an electrical current to the heart in order to restore a normal cardiac rhythm. Defibrillation can occur through externally placed pads or through an implanted device in patients with advanced cardiac disease. 

Why is defibrillation important?

According to the Centers for Disease Control and Prevention, heart disease is the leading cause of death in the United States. Among adults, the most common cause of sudden cardiac arrest is ventricular fibrillation and its definitive treatment is defibrillation. Survival rates depend on a variety of factors but one of the most compelling is the use of early defibrillation. In fact, the success of defibrillation decreases by about 10% for every minute that passes. This is why prompt recognition and early defibrillation are so vitally important.

Heart rhythms that can be defibrillated

There are two deadly heart rhythms that respond to defibrillation and those are ventricular fibrillation (VF) and pulseless ventricular tachycardia (PVT). 

Ventricular fibrillation

Ventricular fibrillation occurs when the transmission of cardiac impulses through the heart’s electrical conduction system is interrupted. This can occur due to myocardial infarction, ischemia, “R on T” phenomenon, QT prolongation, ventricular tachycardia, a significant blow to the chest, and even electrical shock. 

The interrupted cardiac impulses create the chaotic rhythm that is ventricular fibrillation. On the cardiac monitor it looks like a chaotic wavy line that can present as fine or coarse (view an example here). Without normal ventricular contraction, cardiac output ceases and the patient will quickly deteriorate to asystole if not treated.

Pulseless ventricular tachycardia

In pulseless ventricular tachycardia, the contractions are fast and ineffective, leading to a significant drop in cardiac output to the point that the patient is pulseless. In PVT, the rate is over 100 bpm and the QRS is wide (> 0.12 seconds or 120 milliseconds). Note that pulseless VT looks exactly like VT with a pulse on the monitor, which is why it’s vital to also assess the patient. To see an example of VT, click here. The most common cause of PVT is cardiac ischemia which can be related to things like electrolyte imbalances, structural heart disease (CAD, CHF, aortic stenosis, etc…), and medications that cause QT prolongation (metoclopramide, ciprofloxacin, ondansetron, haloperidol, etc…). Other causes include pulmonary embolism, hypothermia, acidosis, hypoxia, hypovolemia and more.

How does defibrillation work?

Defibrillation works by producing depolarization of much of the myocardium which causes a momentary pause in all cardiac activity. In the best case scenario, a viable site within the heart’s conduction system spontaneously begins firing and this restores the heart’s normal cardiac cycle.

To perform defibrillation with an external device, pads are placed on the adult patient’s chest in one of two positions. These are anterior-lateral and anterior-posterior. Note that the specific pads you are using most likely have illustrations showing where to place them for optimal delivery. Click here to see a visual representation of these two pad placements.

Because children are much smaller than adults, the recommended pad placement is anterior-posterior using the appropriate child-sized pads. Click here to see a visual representation of pad placement in children.

Types of defibrillators

There are several different types of defibrillators:

• Automated external defibrillator (AED) – This is the type of defibrillator you see in public spaces. The device interprets the cardiac rhythm and tells the rescuer what to do and delivers shocks when needed. 
• Advanced life support (ALS) defibrillator – This is the type of defibrillator used by medical professionals and the one you’ll see on your crash cart. Popular brands are Zoll, Philips, and MedTronic. While this is considered a manual defibrillator, most (if not all) brands can be utilized in “analyze” mode which makes them act as an AED. 
• Implanted cardioverter defibrillator (ICD) – Individuals at high risk for sudden cardiac arrest or a life-threatening arrhythmia may have an implanted device that automatically detects when a shock is needed (and delivers it!). Some ICDs also act as pacemakers.
• Wearable cardioverter defibrillator (WCD) – This device is similar to an ICD except it is a vest that is worn externally. Wearable defibrillators are typically only used for short periods of time while the patient is awaiting an implanted device.

Defibrillator settings

The key defibrillator setting is the amount of joules that are delivered with each shock. Older defibrillators are monophasic, while current defibrillators are generally biphasic. With a biphasic defibrillator, lower energy levels are utilized to achieve defibrillation. For example, with a monophasic defibrillator, 360 joules are utilized when resuscitating an adult patient. In contrast, a stepwise approach is utilized when using a biphasic defibrillator. With adults, it is generally recommended to start with 120-200 joules and increase each time defibrillation is needed until the max dose of 360 joules is reached. 

Note that with an AED (and with the ALS defibrillator in “analyze” mode, the joules are automatically selected based on the pads that are used (pediatric vs adult pads).

Tips for successful (and safe) defibrillation

• Remove excessive chest hair using the razor in the crash cart. If no razor is available, you can use a spare pad to remove as much hair as possible.
• Remove metal jewelry that may come into contact with the electrode pads.
• If wet, move the patient away from water and dry the skin as best you can before placing the electrodes.
• Press firmly to ensure good contact and reduce transthoracic impedance.
• Avoid placing electrode pads over large amounts of breast tissue. Instead, lift the breast and apply the pad underneath.
• Avoid placing the pads on top of pacemakers or ICDs (position at least 1 inch away).
• When rescuing a child, ensure pads do not overlap (and do not trim pads to fit).
• Ensure oxygen is off before pressing the shock button.
• Ensure all other rescuers are clear of the patient and bed prior to pressing the shock button (“I’m clear, you’re clear, we’re all clear!”).
• Be absolutely certain that you are defibrillating the appropriate rhythm. For example, defibrillating VT with a pulse could cause something called “R on T phenomenon” where the electrical impulse occurs during the refractory period (represented by the T-wave). When this occurs, the patient can go into serious ventricular arrhythmias including polymorphic ventricular tachycardia (Torsades de Pointes) or ventricular fibrillation.
• Resume compressions immediately after the shock – do not pause to check for a pulse.

Remember, successful defibrillation isn’t just about delivering a shock. It’s about doing it safely, quickly, and effectively so your patient has the best chance at survival. For more cardiac topics, click here!

_____________________________________

The information, including but not limited to, audio, video, text, and graphics contained on this website are for educational purposes only. No content on this website is intended to guide nursing practice and does not supersede any individual healthcare provider’s scope of practice or any nursing school curriculum. Additionally, no content on this website is intended to be a substitute for professional medical advice, diagnosis or treatment.

References:

Foglesong, A., & Pulseless, M. D. (2023). Pulseless Ventricular Tachycardia. In StatPearls [Internet]. StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK554467/

Goyal, A., Chhabra, L., Singh, B., & Cooper, J. S. (2025). Defibrillation. In StatPearls. StatPearls Publishing. http://www.ncbi.nlm.nih.gov/books/NBK499899/

Mahanta, D. S., Barik, R. C., & acharya, D. (2023). R on T phenomenon, a dangerous ECG sign. Visual Journal of Emergency Medicine, 32, 101790. https://doi.org/10.1016/j.visj.2023.101790

NHLBI, NIH. (2023, June 6). Defibrillators – What are Defibrillators? NHLBI, NIH. https://www.nhlbi.nih.gov/health/defibrillators

UC Davis Health. (n.d.). Defibrillation | Heart and Vascular. UC Davis Health. https://health.ucdavis.edu/treatments/defibrillation