The shockable rhythms are those tachyarrhythmias (high frequency arrhythmias) characterized by hyperactivity, disordered or not, of the 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.
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, which are said to be nonshockable (when defibrillation has no effect).
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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).
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 blood 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 that reverses the electrical alteration. The most appropriate therapy is precisely the defibrillation.
It is also, in this case, an alteration of the rhythm originating in the ventricles and characterized electrocardiographically by the presence of QRS complexes of long duration (wide), 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 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.
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.
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 makes all abnormal arrhythmic activity disappear. The objective is that it makes it possible to reestablish 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. Shocks were applied randomly at any time during the cardiac cycle.
By 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 in which 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 is the application of an electrical shock synchronized with the R wave of ventricular depolarization. It serves to reverse a hemodynamically unstable arrhythmia such as fibrillation or atrial flutter and supraventricular tachycardias, avoiding the risk of VF.
It would be the application of the electric shock without taking into account the moment of the cardiac cycle because, when the condition is feared (ventricular fibrillation or pulseless ventricular tachycardia), it is necessary to act quickly.
From the foregoing, it is understood that under the condition of shockable rhythms, only ventricular fibrillation and pulseless ventricular tachycardia are included where there is no point in worrying about timing. In these cases, defibrillation would be used instead of cardioversion.
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 together with ventricular fibrillation and pulseless ventricular tachycardia among the causes of potentially fatal cardiorespiratory arrest. Both are non-shockable arrhythmias.
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.
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. A defibrillation does not make sense here either if the electrical rhythm is normal.
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