These would enable a dipole to reach an area that had been refractory and is now able to be depolarised with continuation of the wavefront.In clinical practice, therefore, factors that would lead to the right conditions to favour such re-entry mechanisms include increased heart size through hypertrophy or dilatation, drugs which alter the length of the refractory period and areas of cardiac disease. Therefore, the substrate of ventricular fibrillation is transient or permanent conduction block. Block due either to areas of damaged or refractory tissue leads to areas of myocardium for initiation and perpetuation of fibrillation through the phenomenon of re-entry. Triggered activity
Triggered activity can occur due to the presence of afterdepolarisations. These are depolarising oscillations in the membrane voltage induced by preceding action potentials. These can occur before or after full repolarisation of the fiber and as such are termed either early (EADs) or delayed afterdepolarisations (DADs). All afterdepolarisations may not reach threshold potential, but, if they do, they can trigger another afterdepolarisation, and thus self-perpetuate. Power spectrum
The distribution of frequency and power of a waveform can be expressed as a power spectrum in which the contribution of different waveform frequencies to the waveform under analysis is measured. This can be expressed as either the dominant or peak frequency, i.e., the frequency with the greatest power or the median frequency, which divides the spectrum in two halves.Frequency analysis has many other uses in medicine and in cardiology, including analysis of heart rate variability and assessment of cardiac function, as well as in imaging and acoustics.
The concept of "R on T ectopics" was further brought out by Katz in 1928. This was called the "vulnerable period" by Wiggers and Wegria in 1940, who brought to attention the concept of the danger of premature ventricular beats occurring on a T wave.Another definition of VF was produced by Wiggers in 1940. He described ventricular fibrillation as "an incoordinate type of contraction which, despite a high metabolic rate of the myocardium, produces no useful beats. As a result, the arterial pressure falls abruptly to very low levels, and death results within six to eight minutes from anemia [ischemia] of the brain and spinal cord".Spontaneous conversion of ventricular fibrillation to a more benign rhythm is rare in all but small animals. Defibrillation is the process that converts ventricular fibrillation to a more benign rhythm. This is usually by application of an electric shock to the myocardium and is discussed in detail in the relevant article. See also
References
External links

Interactive models and information on ventricular fibrillation and other arrhythmias