Abstract
Fabiato et al1 discovered the existence of a shock strength above which a defibrillation shock given during the vulnerable period of the cardiac cycle cannot induce ventricular fibrillation (VF), and that this upper limit of vulnerability (ULV) corresponds to the defibrillation threshold. Subsequent studies confirmed this observation both in animal models and in humans.2–5 However, the mechanism of ULV remains unclear. In this issue of the journal, Mazeh and Roth6 studied the importance of myocardial fiber orientation on the mechanisms of ULV. Using computer simulation in an artificially architectured fiber orientation, the authors found that a ULV is present if local heterogeneities are created by a randomly placed fiber angles. If smooth fiber geometry is used in the computer model, the wavefronts induced by break excitation can always propagate and induce reentry regardless of the shock strength. The presence of heterogeneity of fiber angles, therefore, plays an important role in mechanisms underlying ULV and the mechanisms of defibrillation.
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