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Ventricular fibrillation most commonly occurs within diseased hearts, and, in the vast majority of cases, is a manifestation of underlying ischemic heart disease. Ventricular fibrillation is also seen in those with cardiomyopathy, myocarditis, and other heart pathologies.
Right after the ventricles contract, the cells enter into a refractory period, where they can’t be stimulated to contract again, and during the upslope of the T wave, some cells are starting to come out of refractory, so those cells might be ready to go again, whereas other cells might still be in a refractory phase.
This period of time is referred to as the refractory period, which is 250ms in duration and helps to protect the heart. In the classical sense, the cardiac refractory period is separated into an absolute refractory period and a relative refractory period. During the absolute refractory period, a new action potential cannot be elicited.
Timing the shock to the R wave prevents the delivery of the shock during the vulnerable period (or relative refractory period) of the cardiac cycle, which could induce ventricular fibrillation. If the patient is conscious, various drugs are often used to help sedate the patient and make the procedure more tolerable.
Effective refractory period in green. In electrocardiography, during a cardiac cycle, once an action potential is initiated, there is a period of time that a new action potential cannot be initiated. This is termed the effective refractory period (ERP) of the tissue.
There are two major classes of cardiac fibrillation: atrial fibrillation and ventricular fibrillation. Atrial fibrillation is an irregular and uncoordinated contraction of the cardiac muscle of atria. It can be a chronic condition, usually treated with anticoagulation and sometimes with conversion to normal sinus rhythm.
Ventricular tachycardia (V-tach or VT) is a cardiovascular disorder in which fast heart rate occurs in the ventricles of the heart. [3] Although a few seconds of VT may not result in permanent problems, longer periods are dangerous; and multiple episodes over a short period of time are referred to as an electrical storm.
Defibrillation is a treatment for life-threatening cardiac arrhythmias, specifically ventricular fibrillation (V-Fib) and non-perfusing ventricular tachycardia (V-Tach). [1] [2] A defibrillator delivers a dose of electric current (often called a counter-shock) to the heart.