Reentrant arrhythmias generated by a computer-based model of the modulated parasystole in an open-chest dog.

It has been demonstrated that parasystolic activity of a linear Purkinje bundle can be converted into a form of reentry, termed reflections, as a function of changes in electrotonic interactions between two active segments separated by an inexcitable gap. In this study, we attempted to reproduce arrhythmic patterns based upon the reflection mechanism by utilizing a hybrid experimental model of modulated parasystole. The experimental system consisted of the intact heart of a dog connected to a computer, which operated as an ectopic focus. The "ectopic" discharges were shifted by the phase-dependent effects of the input ventricular activation of the heart and were transmitted to the ventricle with a conduction delay in the circuit. A high level of the electrotonic influence was incorporated in the "ectopic" focus by programming a phase response curve with dominant acceleratory phase. The focus was repeatedly captured and reset by the ventricular activation over a wide range of basic heart rates. This preparation produced heart rate-dependent changes in arrhythmias defined by the reflection model. Appropriate conduction delays in the simulated exit pathway allowed the focus to be captured by the activity that it initiated. This process of reciprocation generated a ventricular tachycardia, which could be either induced or terminated by a timed premature beat. Thus, arrhythmias of reentrant type, including reciprocating tachycardia, can be produced by a computer-based model of the modulated parasystole.