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Shockable Rhythms

Much of ACLS is about determining the right medication to use at the appropriate time and deciding when to defibrillate. Along with high quality CPR, these are the only two interventions that are likely to restart the arrested heart, apart from determining the underlying cause of the arrest. This makes defibrillation a powerful tool in the hands of the ACLS practitioner. It is important to know when to use defibrillation. Although determining the underlying cause of an arrest is the most important goal in ACLS, defibrillation can reset and restart the heart, buying the practitioner time to explore and treat the H’s and T’s accordingly.

Rhythms that are not amenable to shock include pulseless electrical activity (PEA) and asystole. In these cases, identifying primary causation, performing good CPR, and administering epinephrine are the only tools you have to resuscitate the patient. Shockable rhythms are rhythms that are caused by an aberration in the electrical conduction system of the heart. 

Ventricular Tachycardia

One of the rhythms that responds well to defibrillation is ventricular tachycardia (often shortened to v-tach). This rhythm usually appears on the monitor as a wide, regular, and very rapid rhythm. Ventricular tachycardia is a poorly perfusing rhythm; patients may present with or without a pulse. Most patients are unconscious and pulseless with this rhythm and defibrillation is needed to ‘stun’ the heart so that the primary pacemaker (usually the S-A node) can take over. Multiple shocks may be needed, but good compressions and adequate ventilation are also important.

You should also consider the H’s and T’s that may be causing the rhythm, particularly if defibrillation is not effective in terminating the rhythm. It is important to consider hypoxia as an underlying cause, as this is one of the most common and easily treated problems. The patient’s airway must be firmly secured and ventilations should be optimized. Other underlying causes include acidosis or an increase in potassium in the blood stream. You should consider toxins as well when investigating v-tach, including prescribed medications. Myocardial infarction, cardiac tamponade and tension pneumothorax can also cause v-tach. If defibrillation is not helpful in terminating the rhythm it is necessary to investigate possible causes, as treatment will likely fail unless the underlying cause is identified and treated.

Ventricular Fibrillation

Ventricular fibrillation, or v-fib, is a common cause of out-of-hospital cardiac arrest. In this case, the heart quivers ineffectively and no blood is pumped out of the heart. On the monitor, v-fib will look like a frenetically disorganized wavy line. Ventricular fibrillation may be fine or coarse; coarse ventricular fibrillation is more likely to convert after defibrillation than fine v-fib.

Fine v-fib is sometimes mistaken for asystole. As the treatments for asystole and ventricular fibrillation are different, it is important to differentiate between the two. If in doubt, it is acceptable to deliver a shock. If it is fine v-fib, you may terminate the rhythm; however, if the rhythm is asystole, defibrillation will be ineffective and you can follow the asystole protocol with confidence.

Supraventricular Tachycardia

Supraventricular tachycardia, or SVT, is far different than the rhythms discussed above, which originate in the ventricles. Patients in a supraventricular tachycardia will have a rapid rhythm with a heart rate greater than 150 beats per minute. The patient with an SVT may be relatively stable with few symptoms or profoundly unstable with severe signs and symptoms related to the rapid heart rate.

Patients who are unstable or who do not respond to medication will require electrical therapy. In this case, synchronized cardioversion, rather than defibrillation, is performed. If the patient is conscious sedation should be provided, as cardioversion is painful. The shock that is delivered in cardioversion for SVT is synchronized to occur at a precise time, one that avoids the vulnerable refractory period which could cause ventricular fibrillation. Smaller doses of energy are needed to convert SVTs such as atrial fibrillation and atrial flutter.