What are Shockable Rhythms?

Shockable rhythms are heart rhythms that are caused by an aberration in the heart’s electrical conduction system.

One important aspect of ACLS (advanced cardiac life support) is determining the right medication or therapy to use at the appropriate time and this includes deciding when to defibrillate. Along with high-quality CPR, ACLS medications and defibrillation are the only two healthcare interventions that are likely to restart a heart that is in arrest.

Defibrillation is a powerful tool in the hands of an ACLS provider. However, it’s important to know when to use defibrillation to reset the abnormal rhythm and when not to use it.

Finding the underlying causes of a cardiac arrest is one of the most important goals for ACLS providers. With that said, defibrillation can reset and restart the heart, which helps buy the healthcare provider time to explore and treat the Hs and Ts – the underlying causes.

What are the Shockable Rhythms?

There are two shockable rhythms and two non-shockable rhythms. The two shockable rhythms are:

  1. Ventricular Fibrillation, or VFib
  2. Pulseless ventricular tachycardia, or V-tach

The two non-shockable rhythms are:

  1. Asystole, seen as a flat line on an ECG monitor
  2. Pulseless electrical activity, or PEA

Shockable Rhythms and the Adult Cardiac Arrest Algorithm

The adult cardiac arrest algorithm is the most important algorithm for adult resuscitation. This algorithm outlines all assessment and management steps that ACLS providers should take for the pulseless patient who does not initially respond to basic life support interventions, including the first shock from an AED (automated external defibrillator).

The algorithm consists of the two pathways for a cardiac arrest:

  1. A shockable rhythm – displayed on the left side of the algorithm.
  2. A non-shockable rhythm – displayed on the right side of the algorithm.

When ACLS providers conduct a rhythm check, if that rhythm check reveals a shockable rhythm – VFib or pulseless V-tach – they will prepare to deliver a shock, while also ensuring the continuation of high-quality CPR while the unit is charging and in between shocks.

If a healthcare provider’s rhythm check reveals a non-shockable rhythm – asystole or PEA – the course of action is a continuation of CPR, possible medications, and possible advanced airway capnography. However, subsequent rhythm checks may reveal a change in the patient’s rhythm to one that is shockable, at which point a shock would be delivered.

Shockable Rhythm: VFib

Ventricular fibrillation (or VFib) is a common cause of out-of-hospital cardiac arrest. In VFib cases, the heart quivers ineffectively and as a result, no blood is pumped out. On an ECG monitor, VFib will look like a wavy, disorganized line. VFib can either be fine or coarse. Coarse VFib is more likely to convert after defibrillation than fine VFib.

Fine VFib can sometimes be mistaken for asystole. However, the treatments for asystole and VFib are different, therefore, ACLS providers must be able to differentiate between the two. When in doubt, though, it is acceptable to deliver a shock. If the patient is in fine VFib, the healthcare provider may be able to terminate the rhythm. However, if the patient’s rhythm is asystole, defibrillation will be ineffective.

Shockable Rhythm: Pulseless V-tach

Ventricular tachycardia (V-tach) will usually respond well to defibrillation. V-tach typically appears on an ECG monitor as a wide, regular, and very rapid rhythm. V-tach is a poorly perfusing rhythm and patients may present with or without a pulse.

Most patients with this rhythm are pulseless and unconscious and defibrillation is necessary to reset the heart so that the primary pacemaker (usually the SA node) can take over. ACLS providers may have to administer multiple shocks, but high-quality chest compressions and adequate ventilation are also extremely important.

If defibrillation does not help terminate the V-tach rhythm, the healthcare provider should investigate possible underlying causes for the patient’s condition, as treatment will likely fail unless an underlying cause is identified and treated.


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