Epicardial Leadless Pacemaker in a Lamb Model

Objectives: Current pacemaker therapy in newborns and small toddlers requires epicardial lead fixation with the pulse generator, most commonly implanted abdominally. During growth, lead fracture can occur due to tension or mechanical strain. The aim of this study was to investigate the feasibility of epicardial pacing with a commercially available leadless pacemaker in an animal model.

Methods: A total of 16 lambs (median body weight 23.8 [IQR: 22.4–25.9] kg) underwent epicardial implantation of a Micra TPS pacemaker (Medtronic Inc., Minneapolis, Minnesota, United States). After lateral thoracotomy, the Micra was placed through a purse-string suture in the LV pericardium in 10 animals trapping the tines and the electrode between the pericardium and epicardium. In the remaining ix lambs, the device was fixated with its four tines embedded within the myocardium of the left atrial appendage. After confirmation of appropriate pacing thresholds and sensing properties, chest was closed, and pacemaker position and movement during the heart cycle was recorded with biplane fluoroscopy. Pacemakers were programmed to AAI/VVI 30 modus and animals were followed subsequently for 22 weeks.

Result: Following implantation, the median p/r wave amplitude was 4.25/5.5 mV, the median impedance (atrial/ventricular) was 1,155/1,065 ohms, while the median pacing threshold was 1.125/1.9 V at 0.24 ms. After a follow-up of 5.5 months, the median amplitude was 3.3/3.4 mV and median impedance dropped to 540/525. Median atrial pacing threshold was 0.6 V at 0.24 ms. but 8/10 ventricular pacemakers lost capture at standard impulse width of 0.24 ms.

Conclusion: When the tines were embedded in the myocardium, as with the atrial devices, pacing showed excellent results through 5.5 months. However, when the tines were not embedded in the myocardium as in the LV implants, the pacing thresholds were unacceptably high. Therefore, we believe it is important to embed the tines in the myocardium. The next step will be an attempt to implant the Micra through the pericardium with the tines penetrating the LV epicardium via minimal thoracotomy.