Design and Testing of a Percutaneously Implantable Fetal Pacemaker

Abstract

We are developing a cardiac pacemaker with a small, cylindrical shape that permits percutaneous implantation into a fetus to treat complete heart block and consequent hydrops fetalis, which can otherwise be fatal. The device uses off-the-shelf components including a rechargeable lithium cell and a highly efficient relaxation oscillator encapsulated in epoxy and glass. A corkscrew electrode made from activated iridium can be screwed into the myocardium, followed by release of the pacemaker and a short, flexible lead entirely within the chest of the fetus to avoid dislodgement from fetal movement. Acute tests in adult rabbits demonstrated the range of electrical parameters required for successful pacing and the feasibility of successfully implanting the device percutaneously under ultrasonic imaging guidance. The lithium cell can be recharged inductively as needed, as indicated by a small decline in the pulsing rate.

Preclinical testing and optimization of a novel fetal micropacemaker

“…7 A single case report provides a pulse-duration curve for a human fetal pacing 4 and is consistent with epicardial pacing parameters in neonates, children and adults. 14 However, as the literature suggests that fetuses exhibit less fibrosis than children and adults, 15 we did not anticipate the observed degree of myocardial inflammation and subsequent capture threshold elevations after electrode placement.…”

“…As a result, we designed and incorporated a recharging system to allow the pacemaker device to function for the duration of the pregnancy (anticipated 3–5 months). While the technical details of this recharging system are reported separately, 7 it is worth noting that the electronic circuit was redesigned during these experiments such that small fluctuations in the recharging field seen by the micropacemaker would result in relatively large changes in the pacing rate. This modification proved very useful for optimizing the recharging time since changes in fetal and maternal positioning could be quickly recognized (by a decrease in the pacer rate) and the recharging coil re-positioned accordingly.…”

“…The output pulse exponentially decays with a total charge that is fixed by the electronic component values and is independent of electrode impedance. 7 In 6 of 7 experiments, the devices had a 3 µC output pulse (3V peak, 250 µs time constant for fully charged battery); this corresponds approximately to a conventional 2 V over 0.4 ms square pulse. The 3mAh lithium battery cell in the implants sustains pacing for approximately 6 days and can be recharged by inductive coupling of a 6.78 MHz electromagnetic field from a transmitting coil positioned outside the maternal body.…”

A percutaneously implantable fetal pacemaker

“…The threshold strength of the electrical stimulus to pace the heart can be minimized by selecting an appropriately sized electrode, thereby maximizing the very limited battery life. In preliminary experiments, we have observed that strained myocardial tissue had a higher threshold for pacing [13], so it is important to design the electrode so that it penetrates the epicardial and myocardial tissues cleanly. The epicardial connective tissue has a complex structure (Fig.…”

A novel method to estimate safety factor of capture by a fetal micropacemaker

“…After the birth, the infant could then be implanted with a standard pacemaker with epicardial leads. We have developed a fetal micropacemaker to meet this prenatal need (Loeb et al ., 2013) and are currently performing pre-clinical studies in fetal sheep (Bar-Cohen et al ., 2015). This paper describes a novel electrophysiological method that provides useful insights into the quality of the electrode-tissue interface during the course of such chronic experiments.…”