Tachycardia-induced cardiomyopathy long after a pacemaker implantation for the treatment of unusual 2:1 atrioventricular block: What is the mechanism?

Case summary A 49-year-old man experienced syncope while he crossed a street, and he was transferred to our hospital. An electrocardiogram (ECG) showed a second-degree 2:1 atrioventricular block with narrow QRS complexes (Figure 1A). Curiously, the RR intervals were not constant while the PP intervals remained steady during the 2:1 AV block (Figure 1B). The echocardiogram revealed a normal left ventricular systolic function and no morphologic abnormalities. No significant coronary stenosis was observed on the coronary angiogram. No electrolyte abnormalities were noted. He did not have any relevant medical history and did not take any medications. Because the 2:1 AV block did not improve, a dual-chamber pacemaker (Advisa DR MRI A3DR01, Medtronic, Minneapolis, MN) was implanted. The alternating RR intervals during the 2:1 AV block seen in the present case were uncommon. A quite similar ECG finding was however previously reported. The authors offered the concealed conduction concept as the causal mechanism. The term “concealed conduction” is used when a proximal atrial impulse penetrates the AV node but fails to