Functional properties of accessory AV pathways during premature atrial stimulation.

Premature atrial stimulation was performed during His bundle recording in four patients with the type of ventricular pre-excitation in which theforward H deflection appeared after the onset of the delta wave. The AV conduction patterns occurring in these patients seemed to have depended on the physiological differences existing between normal pathway and accessory pathway (AP) at different moments of the cycle. These differences explained the patterns noted in driven beats, namely, exclusively accessory pathway conduction (Case i), combined accessory and normal pathway conduction (Case 2), and exclusive normal pathway conduction (Cases 3 and 4). In Cases i and 3 the effective refractory period of the AP was longer than that of the normal pathway. Moreover, Case 3 had AP conduction only towards mid-cycle, sandwiched in between periods of normal pathway conduction. In Case 2, with very short refractory periods (less than I70 msec), premature atrial impulses reached the ventricles during the peak of the T wave. Case 4 had normal pathway conduction with right bundle-branch block in sinus, driven, and late diastolic testing beats, with AP conduction only at the shorter coupling intervals. The ventricular rates of spontaneous or induced atrial tachyarrhythmias were significantly faster when the accessory pathway's effective refractory period was short (Cases 2 and 4), than in the other two (Cases i and 3) with longer effective refractory periods. Though premature atrial stimulation has increased our knowledge of AV conduction, more studies are necessary to extend their usefulness to other patients with pre-excitation syndrome.