Effect of field stimulation on cellular repolarization in rabbit myocardium.  Implications for reentry induction. 
We have investigated the effects of electric field stimulation on membrane repolarization in rabbit papillary muscles and assessed the consequences of these effects for the dispersion of intracellular potentials and the production of a propagation wave front or unidirectional block in relatively refractory tissue.  The stimuli studied had electric field strength of 0.25-14 V/cm, duration of 2 msec, and field orientation along or across the myocardial fibers.  The field strengths to excite the muscles in diastole were 0.68 or 1.23 V/cm for stimuli oriented along or across the fibers, respectively (p less than 0.01, along versus across).  A 2.5-V/cm stimulus given near the end of the action potential (AP) produced either no response or, after increasing the stimulus delay only 2-3 msec, a full response with almost no AP durations that were intermediate.  For stimulation along and across the fibers, respectively, given at 70% of the AP duration, a 4-V/cm stimulus produced AP prolongation (measured at 90% repolarization) of 20% and 4% (p less than 0.05), an 8-V/cm stimulus produced AP prolongation of 36% and 20% (p less than 0.05), and a 14-V/cm stimulus produced AP prolongation of 36% and 30% (p = NS).  For either orientation, AP prolongation by stimuli of 8 V/cm or 14 V/cm increased gradually as the stimulus delay was increased.  The different effects in relatively refractory tissue of stimuli of 2.5 V/cm compared with 8 V/cm can explain the propagation wave front and block that occur with electrically induced functional reentry in the heart.(ABSTRACT TRUNCATED AT 250 WORDS)