Reciprocal ventricular extrasystoles.

An impulse originating in the A-V node or ventricles may be conducted retrogradely through the A-V node. The retrograde impulse may, under certain circumstances, return anterogradely through the A-V node to the ventricles giving rise to a reciprocal or return extrasystole. One explanation for this re-entry phenomenon is based on the concept of a functional vertical division or longitudinal dissociation of the A-V node into two pathways (Schmitt and Erlanger, 1928)-(A) a nonrefractory pathway that conducts retrograde impulses from the lower centre, and (B) a refractory pathway that blocks retrograde impulses from the lower centre but can conduct impulses anterogradely, i.e. it has the property of unidirectional conduction. The impulse from the lower centre is thus conducted retrogradely through the non-refractory pathway, A, and after reaching the upper regions of the node is conducted anterogradely through the second pathway, B. To enable the retrograde impulse to re-enter the second pathway, deep retrograde penetration of the A-V node to a critical level is necessary. The level at which re-entry can occur will be referred to as the re-entry level. In the case of A-V nodal rhythm this mechanism will result in an inverted "nodal" P wave " sandwiched " between two normal QRS complexes (L in Fig. 1). Should this mechanism occur with