How the heart responds to defibrillation: Introduction & process

Ahmed Raza

Reviewed by Jessica Munoz DPN, RN, CEN, providing nurse training at Yale New Haven Health-Bridgeport Hospital since 2022. Previously in healthcare and education at Griffin Hospital, St. Vincent's College of Nursing and Sacred Heart University Medical Center.

Introduction

Sudden cardiac arrest (SCA) is a medical emergency often caused by a chaotic heart rhythm known as ventricular fibrillation (VF). In VF, the heart’s chambers contract irregularly and ineffectively, preventing blood flow to vital organs. VF is recognized on cardiac monitors as a rapid, irregular waveform. If untreated, VF is fatal within 8 to 10 minutes.
Defibrillation is the process of delivering an electrical current to the heart to stop VF and restore a normal rhythm. Studies demonstrate that early defibrillation significantly improves survival rates:

• Defibrillation within 1 minute of cardiac arrest results in survival rates as high as 90%.
• After 5 minutes, survival drops to approximately 50%, and beyond 10 minutes, successful resuscitation is rare.

Defibrillation process

Defibrillation is the primary treatment for two life-threatening cardiac rhythms:

1. Ventricular fibrillation (VF).
2. Pulseless ventricular tachycardia (VT).

According to the AHA, VF and pulseless VT require prompt defibrillation to improve survival. VF is the most common initial rhythm in witnessed cardiac arrests, especially in adults.
Heart disease is a leading cause of death globally, responsible for approximately 17.9 million deaths annually. Defibrillation is particularly effective when performed promptly, as survival depends on the rapid return of organized cardiac activity.

Mechanism of defibrillation

Defibrillation works by delivering a controlled electrical shock to the heart, which:

1. Depolarizes the myocardium (heart muscle), stopping disorganized electrical activity.
2. It briefly pauses the heart’s natural pacemaker (the sinoatrial node) to restore normal rhythm.

Factors influencing defibrillation success include:

• Time to defibrillation: Immediate response is critical.
• Electrode size, placement, and contact pressure: These affect the efficiency of current delivery.
• Energy level of the shock: Appropriate energy levels ensure effective defibrillation while minimizing harm.

Modern biphasic defibrillators optimize energy delivery by automatically adjusting for transthoracic impedance—the resistance of the chest wall.

Energy recommendations for defibrillation

Energy levels for defibrillation depend on the type of device and the patient’s age:

Adults

• Biphasic Defibrillators (standard in most hospitals):
  • First shock: 150-200 joules (device-dependent).
  • Subsequent shocks: Same or escalated energy (up to 360 joules).
• Monophasic Defibrillators (less common):
  • Fixed energy: 360 joules for all shocks.

Pediatrics

• The recommended energy is 2 joules/kg for the first shock and 4 joules/kg for subsequent shocks (maximum 10 joules/kg or the adult maximum dose, whichever is lower).

Studies demonstrate that modern biphasic defibrillators are equally effective at lower energy settings compared to higher-energy monophasic shocks, while reducing the risk of tissue damage.

Risks and considerations

While defibrillation is life-saving, certain risks must be considered:

1. Proarrhythmic Effects: Inappropriate or poorly timed shocks can cause arrhythmias.
2. Tissue Damage: High-energy shocks may lead to myocardial injury, though this is less common with modern devices.

Effective defibrillation requires proper electrode placement and adherence to protocols, including prompt response after VF onset.

Public access defibrillation (PAD)

Many countries have adopted public access defibrillation (PAD) programs, recognizing the importance of early intervention. These programs train laypersons to use automated external defibrillators (AEDs), simple portable devices designed to guide users in delivering a shock. Studies show public AED use improves out-of-hospital cardiac arrest (OHCA) survival rates.

Conclusion

Integrating early defibrillation into emergency care systems is critical to improving outcomes in sudden cardiac arrest. Communities must continue expanding PAD programs and public education efforts.
Survival rates are significantly higher when defibrillation is performed within minutes of VF onset. However, after 10 minutes of VF, the odds of survival are nearly zero. Early intervention remains the most effective way to save lives in cardiac emergencies.

References

1. American Heart Association. 2020. “2020 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care.” Circulation. 2020;142(Suppl 2):S366–S468. doi:10.1161/CIR.0000000000000911.
2. World Health Organization (WHO). “Cardiovascular Diseases.” 2025. https://www.who.int/health-topics/cardiovascular-diseases#tab=tab_1
3. American Heart Association. (2010). Part 6: Electrical therapies. Circulation, 122(18_suppl_3), S706–S719. https://www.ahajournals.org/doi/10.1161/CIRCULATIONAHA.110.970954
4. Centers for Disease Control and Prevention. (n.d.). Public access defibrillation (PAD) state law fact sheet. U.S. Department of Health & Human Services. Retrieved, from https://www.cdc.gov/cardiovascular-resources/php/pad-slfs/index.html