Of 8 patients with the short PR interval, normal QRS complex syndrome studied recently, 3 reported here displayed gaps in anterograde conduction. Atrial premature beats at decreasing coupling intervals conducted with minimal AH prolongation until a zone within the cardiac cycle was reached where conduction failed at a supra-Hisian level. Conduction resumed at earlier atrial coupling intervals and was associated with a sudden increase in the AH interval and the appearance of atrial echo beats with earliest atrial activation on the proximal coronary sinus electrogram. It is suggested that the failure of anterograde conduction at relatively late atrial coupling intervals was caused by a short AH functional refractoriness produced by the pre-excitation of the lower AV junction by a partial AV nodal bypass. Conduction resumed only when early atrial premature beats found the extranodal pathway refractory and were transmitted with decremental delay through the AV node.
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