Pre-excitation with normal PR intervals. A case secondary to slow Kent bundle conduction.

His bundle electrograms were recorded in a patient with pre-excitation and normal PR intervals to determine whether conduction was via anomalous A-V connection (Kent bundle) or infranodal bypass (Mahaim tract). During sinus rhythm (62/ min) both P-delta and P-H measured 155 msec. With atrial pacing at 70 and 75/min, P-H prolonged while P-delta remained constant. At 80/min, the anomalous pathway failed and the QRS normalized. The effective refractory period (ERP) of the anomalous pathway was 700 msec. In summary, the normal PR interval represented slow Kent bundle conduction. Additional findings of depressed Kent bundle conduction included a prolonged ERP and block at a slow paced rate, implying that tachycardia would be unlikely. In WPW, the functional properties of both normal and anomalous pathways can be determined by His bundle recording and atrial pacing.