Abstract Body (Do not enter title and authors here): Introduction: Accurate assessment of pacing thresholds during permanent pacemaker implantation is critical to ensure device function and longevity. A few reports have described rate-related threshold variability during Micra^TM leadless pacemaker (Medtronic Inc, Minneapolis, MN) procedures. Postulated mechanisms for this phenomenon include variable myocardial contact or micro-dislodgment, and inflammation-induced phase IV block. In this case series, we demonstrate the use of adenosine to induce transient heart block for accurate threshold assessment in patients with tachycardia during leadless pacemaker placement.

Cases: A 69-year-old male with a history of atrial flutter presented with complete atrioventricular (AV) block requiring emergent transvenous pacing. A Micra placement was performed. During the procedure, the patient was tachycardic in rapid atrial flutter despite multiple doses of metoprolol. It was noted that with rapid pacing from the pacemaker at 120 pulses per minute (ppm) the threshold was ≤1V at 0.24 ms. An adequate “tug-test” was performed and at least 2 of 4 tines were noted to be fixated based on fluoroscopic motion. To confirm the capture threshold before final deployment, 12 mg of adenosine was given to induce heart block. However, at a rate of 60 ppm, there was no ventricular capture at 3V at 0.24ms. The device was repositioned and repeat threshold testing with adenosine at 60 ppm was 0.38V at 0.24ms. Pacing parameters were assessed the next day and remained stable.

Similarly, a 75-year-old female with ischemic cardiomyopathy was admitted with new atrial fibrillation (AF). She then developed pauses prompting Micra placement. During initial threshold testing, she was in rapid AF despite receiving multiple doses of metoprolol. Heart block was induced with 12mg adenosine to assess the threshold at a lower pacing rate of 60 ppm. The capture threshold achieved was 0.5V at 0.24ms.

Discussion: Nusbickel et al., Yoshiyama et al., and Sano et al. have reported rate dependent discrepancies in capture thresholds after Micra implantation. Our cases highlight the importance of capture threshold assessment at lower pacing rates during implantation. In patients who are tachycardic at the time of the procedure, adenosine may be useful to more reliably assess device capture threshold and determine the need for repositioning.