Effects of local ischemia and transient conduction blocks on the induction of cardiac reentries

The present work is aimed at investigating the effects of ischemia and transient blocks on induction of ventricular fibrillation. Electrophysiologic effects of local ischemia induced by minor myocardial infarction (variation of the dispersion of refractory periods as well as conduction velocity) on initiation of reentry mechanisms were studied by means of computer simulations based on a cellular automata model of propagation of activation wave through a ventricular surface element. A local ischemic area was simulated, showing that altered refractoriness can facilitate initiation of reentry phenomena. The probability of reentries is a function of the size of ischemic zone and of the spatial rate of dispersion of refractory periods. Our results show that the dispersion of refractoriness is an important parameter in triggering cardiac reentries. Above a threshold value of dispersion, even very small ischemic zones can induce spiral waves. We also study the effects of transient conduction blocks on reentries. The role of transient blocks in the triggering of reentries is investigated by use of a two-dimensional set of discretely coupled cells described by the Van Capelle and Durrer model. We study the effect of the size and of the time duration of the transient block on the induction of spiral waves. For each block size, we determine a time window in which spiral waves are initiated.