Postoperative bigeminy: what is the mechanism?

A 65-year-old man underwent coronary artery bypass graft and mitral valve annuloplasty after presenting with a 3-month history of increasing shortness of breath. His underlying rhythm before surgery was sinus with left bundle branch block (LBBB) and a PR interval of 148 milliseconds. Postoperatively, he had no evidence of sinus node function and required atrial pacing for a junctional escape rhythm with rates of 30 to 70 bpm. Fig. 1 demonstrates his predominant rhythm on postoperative day 5. He also developed the rhythm demonstrated in Fig. 2. What is the mechanism of the bigeminy?