Shockable vs. Non-Shockable Heart Rhythms

Many of our students ask the question "What is the difference between a shockable and non-shockable heart rhythm?" A shockable versus nonshockable initial rhythm can be determined by a shock as opposed to a no-shock message from an automated external defibrillator (AED) or by a review of the electronic recording.
The four rhythms are divided into two groups: two that do not require defibrillation (called “nonshockable”) and two that do require defibrillation (“shockable”). The two nonshockable rhythms are pulseless electrical activity (PEA) and asystole and the two shockable rhythms are pulseless ventricular tachycardia and ventricular fibrilation.
The ECG will distinguish asystole from ventricular fibrillation, ventricular tachycardia and pulseless electrical activity. The ECG appearance of ventricular asystole looks almost like a straight line with the occasional P-wave. Pulseless electrical activity (PEA) is the absence of a palpable pulse or other signs of circulation despite the presence on the ECG monitor of an observable QRS complex which normally produce a pulse. Ventricular fibrillation (v-fib) is a common cause of out-of-hospital cardiac arrests. In v-fib the heart begins to quivers with no blood being pumped out of the heart. On the monitor, v-fib will look like a irregular and disorganized wavy line. Ventricular fibrillation may be fine or coarse; coarse ventricular fibrillation is more likely to convert after defibrillation than fine v-fib. Pulseless ventricular tachycardia is a rhythm that is perfusing poorly with patients may or may not be displaying a pulse. Most patients with this rhythm are unconscious and pulseless and the use of the AED is necessary to “reset” the heart so that the primary pacemaker or the Sinoatrial Node can take over. With shockable heart rhythms, if the patient is being monitored, the rhythm can often be identified before significant deterioration.