Conduction disturbances in the middle internodal tract of the rabbit heart.

Conduction properties between the sinus node and the septal branch of the crista terminalis, the so-called middle internodal tract, were studied in the isolated rabbit right atrium. Recordings were obtained from two simultaneously impaled microelectrodes using the extrastimulation technique after cutting the anterior and posterior internodal tracts. Several types of conduction block were observed associated with steplike prepotentials and distortions of perinodal fibers in the absence of pharmacological intervention. Prolongation of sino-septal conduction time occurred with decreasing coupling intervals. The return cycle of the atria gradually increased with prolongation of the retrograde conduction time of premature impulses to the SN until entrance block was seen. Several observations in this data demonstrate the influence of concealed or manifest retrograde conduction into the perinodal fibers of the sinus node after premature stimulation of the atria, as critical delay within the perinodal region could alter conduction time of the premature depolarization, reset the sinus node, and influence subsequent antegrade conduction alone or in various combinations.