Role of a Defibrillator in Cardiac Arrest
A defibrillator delivers an electrical shock to terminate life-threatening ventricular fibrillation (VF) or pulseless ventricular tachycardia (VT), which are the only cardiac arrest rhythms that can be reversed by defibrillation—the single greatest determinant of survival is the time from collapse to defibrillation. 1
What Defibrillation Actually Does
A defibrillator delivers controlled electrical energy to the heart to terminate chaotic electrical activity (ventricular fibrillation) or rapid, ineffective ventricular contractions (pulseless ventricular tachycardia). 2, 3 The electrical shock depolarizes the myocardium simultaneously, allowing the heart's natural pacemaker to potentially re-establish an organized rhythm. 4
The device only works for "shockable rhythms"—VF and pulseless VT—and will not help patients in asystole (flatline) or pulseless electrical activity (PEA). 1, 3 Automated external defibrillators (AEDs) have >90% sensitivity for detecting VF and 99-100% specificity for identifying non-shockable rhythms, meaning they are highly accurate at determining when a shock is appropriate. 2
Time-Critical Nature of Defibrillation
Survival rates decline 7-10% with every minute that defibrillation is delayed. 1 When defibrillation occurs within the first minute of collapse, survival rates can reach as high as 90%. 1 However, after 12 minutes without defibrillation, survival drops to only 2-5%. 1
This time-dependency explains why immediate defibrillation is prioritized:
- For witnessed cardiac arrest with immediate defibrillator access, deliver the shock first, then begin CPR. 3
- For unwitnessed arrest or when EMS response time exceeds 4-5 minutes, perform 2 minutes of CPR before the first shock. 1, 3, 5 The rationale is that prolonged VF depletes myocardial oxygen and metabolic substrates, and brief chest compressions can improve the likelihood of successful defibrillation. 1
Energy Delivery Protocols
Use biphasic defibrillators with an initial energy of 120-200 J (biphasic waveforms are preferred over monophasic). 1, 6 If using an older monophasic defibrillator, deliver 360 J for all shocks. 1, 6
For subsequent shocks, use at least the same energy level, and consider escalating to higher energy if available. 1, 6 Either fixed or escalating energy strategies are acceptable. 1, 6
Integration with CPR
Defibrillation must be tightly coordinated with high-quality CPR—minimize the interval between stopping compressions and delivering the shock to just a few seconds. 1, 6 Even small delays reduce shock success. 1
The proper sequence is:
- Perform CPR while the defibrillator charges 1
- Pause compressions briefly to "clear" the patient and deliver the shock 1
- Resume CPR immediately after shock delivery without checking pulse or rhythm, beginning with chest compressions 1, 6, 3
- Continue CPR for 2 minutes before the next rhythm check 1, 6
Critical Pitfalls to Avoid
Do not check the pulse or rhythm immediately after delivering a shock—this wastes precious time. 6, 3 The practice of delivering multiple "stacked shocks" in succession has been abandoned because modern biphasic waveforms have very high first-shock success rates. 6
Do not delay defibrillation for prolonged CPR in witnessed arrests. 3 While some early studies suggested benefit from pre-shock CPR when EMS response exceeded 5 minutes, recent randomized trials failed to confirm routine survival benefit. 1, 5
Never use synchronized cardioversion mode for VF—always use unsynchronized shocks. 3 Synchronized mode attempts to time the shock with the QRS complex, which is impossible in the chaotic rhythm of VF.
Limitations and Context
Defibrillation only addresses shockable rhythms (VF/pulseless VT), which represent approximately 17.8% of in-hospital cardiac arrests. 7 The majority of cardiac arrests present with non-shockable rhythms (asystole and PEA), for which defibrillation provides no benefit. 1, 7
In patients with non-shockable rhythms, AED use is associated with lower survival (10.4% vs 15.4% without AED use), likely reflecting delays in other critical interventions. 7 This underscores that defibrillation is not a universal treatment for all cardiac arrests—it is specifically for VF and pulseless VT.
Post-Defibrillation Management
After successful defibrillation achieves return of spontaneous circulation (ROSC), monitor for recurrent arrhythmias, as VF can recur. 2 Correct electrolyte abnormalities (particularly potassium and magnesium), search for reversible causes (the "Hs and Ts"), and maintain adequate ventilation and oxygenation. 3