Shockable Cardiac Rhythms
The two shockable cardiac arrest rhythms are ventricular fibrillation (VF) and pulseless ventricular tachycardia (VT). 1
Definition and Classification
Shockable rhythms are cardiac arrest rhythms that, when analyzed by a monitor/defibrillator or automated external defibrillator (AED), are deemed treatable by attempted defibrillation. 1 These rhythms are distinguished from nonshockable rhythms, which include asystole and pulseless electrical activity (PEA). 1
The Two Shockable Rhythms:
Ventricular Fibrillation (VF): A chaotic, disorganized electrical rhythm that prevents effective cardiac contraction and requires immediate defibrillation. 1
Pulseless Ventricular Tachycardia (VT): A rapid ventricular rhythm without a palpable pulse that also requires defibrillation. 1
Clinical Significance
Patients presenting with shockable rhythms have significantly better resuscitation outcomes compared to those with nonshockable rhythms. 1, 2 The presence of a shockable rhythm at initial contact is associated with improved survival to hospital discharge. 1
Survival Rates:
- Shockable rhythms (VF/pulseless VT) are associated with better prognosis and neurological outcomes than cardiac arrest due to other rhythms. 3
- Over 80% of successful defibrillations occur within the first three shocks. 1
- Survival decreases as the number of defibrillation attempts required increases. 1
Treatment Approach
For shockable rhythms, immediate defibrillation without delay is the priority intervention. 1 The recommended initial energy doses are 200 J, 200 J, and 360 J for monophasic defibrillators, with subsequent shocks at 360 J or the biphasic equivalent. 1
Medication Considerations:
- Epinephrine administration: For shockable rhythms, epinephrine should be given after initial defibrillation attempts are unsuccessful. 1
- Antiarrhythmic drugs: Amiodarone or lidocaine may be used for shock-refractory or recurrent VF/pulseless VT. 1, 4
Important Caveats
Not all ventricular tachycardias are shockable. 1 The distinction depends on whether the patient has a pulse and the clinical context:
- VT with a pulse: Treated with synchronized cardioversion (not unsynchronized defibrillation). 5
- Pulseless VT: Treated as VF with immediate unsynchronized defibrillation. 5
AED Performance Considerations:
- AEDs demonstrate high specificity (99-100%) for correctly identifying nonshockable rhythms. 1
- Sensitivity for VF is excellent (94-96%), but sensitivity for rapid VT may be lower (60-71%). 1
- Some AEDs may not consistently recommend shocks for monomorphic VT or Torsades de Pointes at certain rates, despite these being potentially lethal rhythms requiring defibrillation. 6
Clinical pearl: After a defibrillation shock, the monitor may show an isoelectric line for several seconds due to electrical or myocardial "stunning"—this does not necessarily indicate conversion to asystole, and a coordinated rhythm or return of VF/VT may subsequently appear. 1