Electrical Activation During Ventricular Fibrillation in the Subacute and Chronic Phases of Healing Canine Myocardial Infarction

Abstract

Background Little information is available regarding the effects of myocardial infarction on the characteristics of ventricular fibrillation (VF). Epicardial activation during VF can be characterized by the cycle length and by the characteristics of activation wave fronts. Methods and Results VF was induced by programmed stimulation in 6 dogs with subacute healing (1 week) myocardial infarction (MI), 5 dogs with chronic (8 week) healing MI, and 6 dogs without MI. Using a plaque electrode array with a 2.5-mm interelectrode distance, 112 electrograms were recorded and 91 vector loops were created for each cycle of VF from either the anterior (infarcted) or lateral (noninfarcted) wall. Direction of maximum epicardial activation was determined at each site for the first 10 cycles of VF (early) and for 10 cycles after 5 seconds of VF (late). Wave front size was determined based on a similarity in epicardial activation directions within a given area and by a statistical analysis that determined the degree of spatial linking at varying distances over the recording plaque. VF cycle length was defined as the mean interval of 10 consecutive local activation times. Differences among groups and differences between the anterior and posterolateral walls were determined by ANOVA. The mean wave front area was significantly larger in the presence of subacute MI (97±4 mm 2 , early; 78±3 mm 2 , late) or chronic MI (94±5 mm 2 , early; 78±5 mm 2 , late) than in noninfarcted animals (73±5 mm 2 , early; 61±3 mm 2 , late). The degree of linking of epicardial activation directions was similar in the three groups at distances of 2.5 and 5.0 mm but was lower at a distance of 7.5 mm among animals without infarction, confirming a smaller wave front size and suggesting less organization of activation. VF cycle length was significantly longer in the presence of infarction (98±5 ms, normal control animals; 121±13 ms, subacute MI; 127±13 ms, chronic MI). VF cycle length was significantly longer over the anterior than the lateral wall in the presence of subacute MI (131±8 ms, anterior; 109±5 ms, lateral) or chronic MI (136±9 ms, anterior; 119±6 ms, lateral) but not in noninfarcted animals (99±5 ms, anterior; 97±5 ms, lateral). The prolongation of VF cycle length among animals with infarction was associated with slower estimated conduction velocities during VF. Conclusions During VF, in animals with subacute or chronic healing MI, (1) the size of activation wave fronts is larger, (2) the cycle length of VF is longer, (3) the conduction velocities are slower, and (4) the degree of organization is greater than in control animals. Thus, the characteristics of VF throughout the heart are altered by the presence of regional myocardial infarction. The implications of these findings for the initiation and maintenance of VF in the presence of different underlying myocardial substrates require further study.