Cardiac arrest
Mechanism

The definitive electrical mechanisms of cardiac arrest, which may arise from any of the functional, structural, or physiologic abnormalities mentioned above, are characterized by arrhythmias. Ventricular fibrillation and pulseless or sustained ventricular tachycardia are the most commonly recorded arrhythmias preceding cardiac arrest. These are rapid and erratic arrhythmias that alter the circulatory pathway such that adequate blood flow cannot be sustained and is inadequate to meet the body's needs.

The mechanism responsible for the majority of sudden cardiac deaths is ventricular fibrillation. Ventricular fibrillation is a tachyarrhythmia characterized by turbulent electrical activity in the ventricular myocardium leading to a heart rate too disorganized and rapid to produce any meaningful cardiac output, thus resulting in insufficient perfusion of the brain and essential organs. Some of the electrophysiologic mechanisms underpinning ventricular fibrillations include ectopic automaticity, re-entry, and triggered activity. However, structural changes in the diseased heart as a result of inherited factors (mutations in ion-channel coding genes, for example) cannot explain the sudden onset of cardiac arrest.

In ventricular tachycardia, the heart also beats faster than normal, which may prevent the heart chambers from properly filling with blood. Ventricular tachycardia is characterized by an altered QRS complex and a heart rate greater than 100 beats per minute. When V-tach is sustained (lasts for at least 30 seconds), inadequate blood flow to heart tissue can lead to cardiac arrest.

Bradyarrhythmias occur following dissociation of spontaneous electrical conduction and the mechanical function of the heart resulting in pulseless electrical activity (PEA) or through complete absence of electrical activity of the heart resulting in asystole. Similar to the result of tachyarrhythmias, these conditions lead to an inability to sustain adequate blood flow as well, though in the case of bradyarrhythmias, the underlying cause is an absence of mechanical activity rather than rapid beats leading to disorganization.
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