Abstract Body (Do not enter title and authors here): Background
We have engineered the world’s smallest pacemaker, a self-powered, leadless “milli-pacemaker” that is smaller than a single grain of rice. It is 1.8 mm × 3.5 mm × 1.0 mm and able to fit in the lumen of a small-diameter introducer, enabling transcatheter delivery.

Aim
We are developing the means of delivering the smallest pacemaker in the world for resynchronization therapy. In this study, we sought to optimize this delivery system for left bundle branch pacing in beating hearts.

Methods
The prototype system was iteratively developed through ex vivo experiments on perfused, paced, human hearts declined for transplant. The catheter-based system delivers the milli-pacemaker below the aortic valve to the left ventricular septum. A transcatheter mechanical fixation system facilitates milli-pacemaker attachment to the endocardium. Optical stimulation by a light emitting diode triggers milli-pacemaker activity. Pacing threshold testing was performed. Electrocardiogram confirmed left bundle branch capture.

Results
Iterative prototyping was performed through ex vivo testing on perfused, paced, human hearts declined for transplantation (n=3). A satisfactory pacing threshold of 0.6 V with 4 ms 2 Hz pulses was achieved. Electrocardiogram confirmed left bundle branch capture in each heart.

Conclusions
We have tested a transcatheter delivery system for the smallest pacemaker in the world. It delivers the milli-pacemaker to the left ventricular side of the interventricular septum for resynchronization therapy. Pacing the left ventricular endocardial surface limits septum penetration and facilitates left bundle branch capture. It is also more in-line with physiologic cardiac conduction. Our delivery system makes the milli-pacemaker a feasible option for minimally invasive resynchronization therapy.