Reflected reentry, delayed conduction, and electrotonic inhibition in segmentally depressed atrial tissues

Reflection is a subclass of reentrant cardiac arrhythmias in which reexcitation of the heart occurs as a result of to and fro electrotonically mediated transmission of impulses across a narrow zone of impaired conductivity. Although relatively well characterized in ventricular tissues, the reflection mechanism has not been studied in atrial tissues. In this study we examine the possibility of reflected reentry in segmentally depressed atrial tissues and evaluate conduction characteristics in these preparations. Narrow strips of atrial pectinate muscle or crista terminalis (canine and calf) were placed in a three-chambered bath and the central segment was superfused with an isotonic sucrose solution or an "ischemic" Tyrode's solution. Proximal to distal conduction across the 1.0- to 1.2-mm wide ischemic gap showed step delays as long as 210 ms. Reflected reentry was readily demonstrable when prominent step delays occurred during anterograde conduction of the impulse across the gap. Progressive acceleration of the stimulation rate resulted in progressively greater impairment of anterograde conduction until complete block occurred. The incidence and patterns of reflected reentry were therefore a sensitive function of the stimulation rate. Other features exhibited by these preparations include a slow recovery of excitability following the action potential, postrepolarization refractoriness, and electrotonic inhibition and summation. Our data suggest that the characteristics of conduction and reflection in segmentally depressed atrial tissues are qualitatively similar to those in ventricular tissues. The presence of electrotonic inhibition in atrial may also help to explain the functionally inexcitable zone seen in the vortex of the leading circle model of atrial flutter.