Electronics design and in vivo evaluation of a wirelessly rechargeable fetal micropacemaker

A fetal pacemaker can dramatically improve the outcome for fetuses that develop complete heart block in utero. We have developed a rechargeable fetal micropacemaker in order to treat severe fetal bradycardia with comorbid hydrops fetalis. Implanting our fetal micropacemaker could reverse the typically fatal outcome of this condition, resulting in the resolution of hydrops within one to two weeks by pacing the heart and restoring adequate blood flow to the fetus. ❧ The main requirements of the device are that it be implanted with a minimally invasive technique and be implanted entirely within the fetal chest. The pacemaker developed meets these requirements by being designed to fit within a standard fetal surgical cannula, putting a hard constraint on the size of the device. The size limitation dictates the volume available for circuitry and a power source inside the implant. A simple fixed-rate and fixed-amplitude relaxation oscillator based on a single transistor provides stimuli while also meeting stringent requirements for low power consumption and low development cost. A commercially available cylindrical, rechargeable 3 mAh lithium ion cell provides power. ❧ In this dissertation, the limits of the simple circuitry and power source are identified and compensated for. A power budget provides an analysis of the battery life with any given combination of components. The main draw of current from the battery is the output pulse, so it is desirable to set the stimulus strength as low as possible to conserve power in order to maximize the recharge interval, but it is also important to include a safety factor to ensure effective ventricular capture for somewhat unpredictable electrode placements and tissue conditions. The unknown conditions of each electrode placement leads to a need to monitor the electrode-myocardium interface in order to determine that adequate pacemaker output is being provided. This is typically accomplished by observing the minimal stimulus strength that achieves threshold for pacing capture. The output of the micropacemaker cannot be programmatically altered to determine this minimal capture threshold, but a safety factor can be inferred by determining the refractory period for ventricular capture at a given stimulus strength. This is done by measuring the minimal timing between naturally occurring QRS complexes and successful stimuli. Upon pilot testing this method in four fetal sheep, data demonstrate that a relative measure of threshold is obtainable, providing valuable real-time information about the electrode-tissue interface. ❧ Limited volume inside the implant also constrains the engineering of the recharging system, which relies on inductive coupling to provide wireless current to the lithium ion cell. To overcome the lack of regulation circuitry within the implant, a method for controlling the recharging process was developed and utilizes pacing rate as a measure of battery state, a feature of the relaxation oscillator used to generate stimuli. The verification of the recharging system shows successful generation of recharging current in a fetal lamb model.