SHOCKABLE RHYTHMS: VENTRICULAR FIBRILLATION, TVSP, NON-SHOCKABLE RHYTHMS - MEDICINE - 2025

The shockable rhythms are those tachyarrhythmias (arrhythmias high frequency) characterized by hyperactivity, disordered or not, ventricular myocardial tissue. This causes an effective contraction and adequate blood expulsion is not allowed, which translates into a dangerous reduction in cardiac output.

The term “defibrillation” basically refers to the reversal by electric shocks of the clinical condition known as ventricular fibrillation (VF), but it is also used in pulseless ventricular tachycardia (PVT) which is clinically equivalent to ventricular fibrillation and sometimes precedes.

Photograph of a defibrillator (Source: Rama via Wikimedia Commons)

Ventricular fibrillation and pulseless ventricular tachycardia are two of the basic causes of so-called cardiorespiratory arrest. Also included here are ventricular asystole and pulseless electrical activity, both of which are said to be nonshockable (when defibrillation has no effect).

Ventricular fibrillation

It is an alteration of ventricular electrical activity in which well-defined QRS complexes disappear, being replaced by irregular and rapid undulations of variable amplitudes, contours and frequencies in which systoles and diastoles are not recognized (cardiac contraction and relaxation).

Electrocardiographic record of a patient with ventricular fibrillation (Source: Jer5150 via Wikimedia Commons)

This rapid and disorderly electrical activity does not allow an effective ventricular contraction that manages to expel a sufficient volume of blood (stroke volume) with each beat, and that in turn allows to maintain an adequate cardiac output and arterial pressure to maintain circulation.

The appearance of this type of arrhythmia, with the hemodynamic disorders that characterize it, is quickly followed by loss of consciousness and even life if there is no therapy to reverse the electrical alteration. The most appropriate therapy is precisely defibrillation.

Pulseless ventricular tachycardia (PVT)

It is also, in this case, an alteration of the rhythm originating in the ventricles and characterized electrocardiographically by the presence of long-lasting (wide) QRS complexes, but of a high frequency (above 200 cycles per minute).

Due to this high frequency, the cardiac cycle is greatly shortened and the heart does not have enough time to fill up or to expel an adequate systolic volume, therefore, the pulse wave produced by this volume entering the arterial system is attenuated and there is no palpable pulse.

Electrocardiographic record of a patient with ventricular tachycardia (Source: Matador3020 via Wikimedia Commos)

The hemodynamic consequences are similar to those of ventricular fibrillation and can lead to death. DVT can be caused by premature ventricular systole and it can lead to ventricular fibrillation.

Although it is not properly a ventricular fibrillation, it responds to defibrillation and this prevents it.

Why talk about shockable and non-shockable rhythms?

Therapy using electrical shocks applied to the thoracic surface is intended to suppress certain cardiac tachyarrhythmias, which cause hemodynamic instability to varying degrees and which can lead to suppression of cardiac output, arterial hypotension, and death.

The objective, in these cases, is to produce a complete depolarization of the myocardial tissue and a state of temporary refractoriness that eliminates all abnormal arrhythmic activity. The objective is that it makes it possible to restore a more regular rhythm and with more hemodynamic efficiency.

The procedure was called defibrillation and was used in cases of supraventricular tachycardias (with narrow QRS complexes), atrial fibrillation and flutter, fibrillation, and ventricular tachycardia. The shocks were applied randomly at any time during the cardiac cycle.

In doing so, there was a risk that electrical stimulation would fall into the final repolarization phase of the myocardial action potential, when dangerous depolarizations that trigger ventricular fibrillation are more likely in cases where this lethal arrhythmia is absent.

As myocardial depolarization begins with the QRS complex and its repolarization coincides with the T wave, to prevent the stimulation from coinciding with this wave, it was devised to synchronize the electric shock with the R wave and the procedure was renamed to cardioversion..

Cardioversion

Cardioversion is the application of an electrical shock synchronized with the R wave of ventricular depolarization. It is used to reverse a hemodynamically unstable arrhythmia such as fibrillation or atrial flutter and supraventricular tachycardias, avoiding the risk of VF.

Defibrillation

All cardiac arrhythmias other than the two mentioned above are, in principle, non-shockable. The first thing to consider is that the electrical discharge creates the conditions for a normal rhythm to be restored, but it does not produce that normal rhythm.

Electrical therapy is helpful in certain forms of tachyarrhythmias, but not all. It is ineffective, for example, in bradycardias or tachycardias of sinus origin. In conditions such as atrial fibrillation and flutter or supraventricular tachycardia, cardioversion is used rather than defibrillation.

In turn, ventricular asystole and pulseless electrical activity are grouped along with ventricular fibrillation and pulseless ventricular tachycardia among the causes of potentially fatal cardiorespiratory arrest. Both are non-shockable arrhythmias.

Ventricular asystole

It is the most common form of cardiac arrest in children. From the point of view of the electrocardiogram, this is characterized by a flat recording, without cardiac waves, or with the presence of only P waves. Defibrillation will not restart ventricular systole and it is necessary to resort to another therapy.

Pulseless electrical activity

It shows an apparently normal rhythmic cardiac electrical activity, but no pulse is detected because there is no effective cardiac output, the blood pressure is very low, and it is also undetectable. Again, a defibrillation does not make sense here if the electrical rhythm is normal.

References

1. Goyal A, Sciammarella JC, Chhabra L, et al: Synchronized Electrical Cardioversion. In: Stat Pearls (Internet). Treasure Island (FL): StatPearls Publishing; 2019 Jan-.
2. Gray H: Herzrhythmus. In: von der Kurve EKG zur Diagnose, 1 ^st ed. München, Urban & Fisher, 2001.
3. Josephson ME, Zimetbaum P: The tachyarrhythmias, in Harrison's Principles of Internal Medicine, 16 ^th ed, Kasper DL et to the (eds). New York, McGraw-Hill Companies Inc., 2005.
4. Klinge R: Rhythmusstörungen. In: Das Elektrokardiogramm, 8 ^th ed. Stuttgart, Thieme, 2002.
5. Roden DM: Antiarrhythmic Drugs. In: Goodman & Gilman's The Pharmacological Basis of Therapeutics, 10 ^th ed, Hardman JG, Limbird LE Goodman and Gilman (eds). New York, McGraw-Hill Companies Inc., 2001.