Arrhythmias Arising in Atrioventricular NodeThere is both experimental and clinical evidence11 that extrasystoles and tachycardia originating in the atrioventricular node often cannot be differentiated from corresponding arrhythmias arising elsewhere in the atria. Ectopic rhythms initiating from foci in the midportion of the atrium failed to yield P′ waves in standard leads. Since only QRS complexes were visible the electrocardiogram was indistinguishable from that of nodal tachycardia. In contrast upright P′ waves were recorded when the focus was at the cephalic end of the atrium. Inverted P′ waves occurred when the focus was at the caudal end. Some atrial arrhythmias . . .