Mechanism of Action: External Chest Defibrillation | Wellbeing

1. What is Emergency External Defibrillation?

Emergency external defibrillation involves the delivery of a high-voltage electrical impulse for a very brief duration (0.03 – 0.10 seconds) through the heart. This process simultaneously depolarizes the entire myocardium (heart muscle) to restore a normal heart rhythm. Its primary goal is to extinguish tachyarrhythmias that have usurped the sinus node's role, thereby creating the conditions necessary for the sinus node to regain control as the dominant pacemaker. Mechanistically, the shock depolarizes all excitable cardiac cells, interrupting re-entry circuits or inactivating ectopic foci by resynchronizing the electrical activity within the heart muscle cells.

There are two distinct classifications of electrical therapy:

• Synchronized Cardioversion: The electrical discharge is timed to coincide specifically with the patient's QRS complex (targeting the R-wave, or the S-wave if no R is present) to strictly avoid the T-wave. This synchronization prevents the shock from inducing ventricular fibrillation.

• Defibrillation (Unsynchronized Shock): The electrical discharge is delivered randomly at any point during the patient's cardiac cycle. This is used when there is no coordinated intrinsic electrical activity to synchronize with.

2. Indications and Contraindications

Indications: Emergency external defibrillation is primarily indicated for:

• Ventricular Fibrillation (VF) or Pulseless Ventricular Tachycardia (pVT).

• Tachyarrhythmias (excluding sinus tachycardia) accompanied by hemodynamic instability leading to cardiac arrest.

• Cases involving severe loss of consciousness or profound hemodynamic collapse due to arrhythmia.

Contraindications: The procedure is generally contraindicated in the following scenarios:

• Multifocal Atrial Tachycardia (MAT).

• Situations where there is direct physical contact between the patient and another person (risk of accidental shock).

• Wet skin surfaces at the electrode placement sites (risk of arcing or reduced efficacy).

• Note on Implanted Devices: While not an absolute contraindication, care must be taken if the patient has an implanted pacemaker or internal defibrillator (ICD). Pads should not be placed directly over the device.