1 │ CASE PRESENTATION
An asymptomatic 48- year old gentleman presented to our device clinic for a routine interrogation of his pacemaker. He had undergone a coronary artery bypass grafting and mitral valve replacement 5 years ago. Unfortunately, he developed infective endocarditis of his aortic valve within a year which necessitated mechanical aortic valve replacement. This was complicated by persistent post-operative intermittent high grade atrio-ventricular (AV) block. Therefore, we implanted a Medtronic (ADVISA MRI DR) dual chamber transvenous pacemaker with right atrial and right ventricular apical leads. Previous device interrogations had not detected any sensing/pacing abnormalities. The pacemaker was programmed to a DDDR mode with a lower rate of 70 beats per minute (bpm), upper tracking rate of 120 bpm, upper sensor rate of 130 bpm, sensed and paced AV delay of 150 and 180 milliseconds (ms) respectively.
As per our protocol, we performed a 12 lead electrocardiogram (ECG) at baseline (Figure 1A) followed by magnet application (rhythm strip is shown Figure 1B). Figure 1A shows a predominantly wide QRS rhythm (70 bpm) interspersed with shorter coupled relatively narrower QRS complexes (140 ms) of a different morphology. These were initially suspected to be premature ventricular complexes. When magnet is applied, there is transition from the slower wide QRS rhythm (70 bpm) to the relatively narrower QRS rhythm at a slightly faster rate of 85 bpm (Figure 1B). A previous ECG showing intrinsic rhythm without pacing is also compared to the current one obtained after magnet application (Figure 1C). What is the most likely mechanism for the transition from wide to relatively narrower QRS rhythm pursuant to magnet application?