The atrial rhythm is sinus tachycardia at a rate of 116 beats per minute. The QRS complexes show a triphasic repeating pattern with increasing duration, initially narrow (88 ms), then intermediate (100 ms), and wide (160 ms). We will refer to these morphologies as A, B, and C, respectively (Fig. 2). There are multiple etiologies which can explain these three morphologies and the differences between them.

Mechanisms to produce widening of a narrow QRS include aberrancy, fusion with a premature