Shockable Cardiac Rhythms
Definition
The shockable cardiac rhythms are ventricular fibrillation (VF) and pulseless ventricular tachycardia (pulseless VT), which are the only two cardiac arrest rhythms that respond to defibrillation. 1, 2
The Two Shockable Rhythms
Ventricular fibrillation (VF) is characterized by chaotic, disorganized electrical activity with no identifiable QRS complexes on the cardiac monitor 2
Pulseless ventricular tachycardia (pulseless VT) appears as an organized wide-complex tachycardia on the monitor, but the patient has no palpable pulse 2
Both rhythms require immediate unsynchronized defibrillation without delay, as this is the single most critical intervention determining survival 3, 2
Non-Shockable Rhythms (For Contrast)
Asystole (flatline) and pulseless electrical activity (PEA) are classified as non-shockable rhythms 2
These non-shockable rhythms have markedly worse prognosis and require high-quality CPR with epinephrine rather than defibrillation 3, 2
Attempting defibrillation on asystole or PEA is not indicated and wastes critical time 2
Clinical Recognition
Automated external defibrillators (AEDs) demonstrate high specificity (99-100%) for correctly identifying non-shockable rhythms 1
AEDs have excellent sensitivity for VF (94-96%) but lower sensitivity for rapid VT (60-71%), meaning some rapid VT may not be detected 1
If an AED delivers a shock, this is diagnostic evidence that VF or pulseless VT was present 4
Critical Distinction: Pulseless vs. Perfusing VT
Not all ventricular tachycardia is shockable - the critical distinction is whether the patient has a pulse 1
VT with a palpable pulse is treated with synchronized cardioversion (different protocol), while pulseless VT is treated identically to VF with immediate unsynchronized defibrillation 3, 5
This distinction is essential because the treatment pathways diverge completely based on pulse presence 1
Immediate Management Algorithm
When cardiac arrest is confirmed (unresponsive, no pulse, no normal breathing), immediately attach a monitor/defibrillator to assess the rhythm 2
If VF or pulseless VT is present: deliver one shock immediately (200 J for monophasic, 120-200 J for biphasic), then resume CPR for 2 minutes before reassessing 3, 2
If asystole or PEA is present: begin CPR immediately and administer epinephrine; do not attempt defibrillation 2
Common Pitfalls to Avoid
Never delay defibrillation to establish an advanced airway or IV access when VF/pulseless VT is present on the monitor 2
Do not perform pulse checks between shocks if VF/VT persists on the monitor - only check pulse if the waveform changes to one compatible with cardiac output 3, 2
Avoid confusing VT with a pulse (which requires synchronized cardioversion) with pulseless VT (which requires immediate defibrillation like VF) 1, 5