Initial evaluation of the ECG showed sinus rhythm with regular PP intervals and QRS complexes of 2 different morphological appearances and amplitudes that occurred regularly in an alternating fashion, consistent with electrical alternans. However, closer inspection of these alternating QRS complexes revealed that one included a delta wave (Figure 2, arrows) and the other did not. This pattern is consistent with alternating conduction via an accessory pathway and the normal atrioventricular (AV) node His-Purkinje system; therefore, electrical alternans was not present.