Ventricular arrhythmias in the subacute myocardial infarction period. High-resolution activation and refractory patterns of reentrant rhythms.

Patterns of activation, functional conduction block, and effective refractory periods during reentrant activation were investigated in a 4-day postinfarction canine model using a 64-channel high-resolution (1 mm) bipolar electrode array. Lower resolution (3-10 mm) isochronal activation maps of the entire epicardial surface were constructed from 126 sites during the initiation and sustenance of reentry and showed reentrant wave fronts that circulated around arcs of functional conduction block. During initiation of reentry by premature stimulation, high-density recordings from these same regions showed that conduction block occurred abruptly, within 1 mm, and without prior decrement of the impulse. Electrograms recorded in proximity to the arc of block were comprised of two deflections: a local activation potential and an electrotonic potential reflecting activation 1 mm away; the reverse order of activation and electrotonus was observed on the opposite side of the arc of block. The occurrence of functional conduction block during premature stimulation in this model was correlated with abrupt increases in effective refractory periods of 10-120 msec (27 +/- 24 msec; mean +/- SD) within 1 mm or less. Neither the abrupt change of refractoriness nor functional conduction block appeared to depend on differences in excitability, the geometrical characteristics of the surviving epicardial layer, or the orientation of the myocardial fibers. During sustained reentrant activation, high-density recordings along the arcs of block showed split electrograms comprised of local activation and electrotonus, which were identical in morphology to those recorded during the initiation of reentry. The interval between the deflections was shorter at the ends of the arc and increased to a maximum value at the center of the arc. The activation potentials corresponded in time with activation of large isochronal regions on either side of the arc of block. There was evidence that at least part of the arc of block during sustained reentry represented thin discrete zones of constant block due to electrotonic influences of impulse penetration from both sides of the arc. Our results strongly suggest that continuous arcs of functional conduction block are a necessary prerequisite for both the initiation and the sustenance of reentrant activation in subacute canine myocardial infarction. Functional conduction block during the initiation of reentry was due to abrupt changes in refractoriness, within a distance of 1 mm or less.