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

Abstract

We have developed a rechargeable fetal micropacemaker in order to treat severe fetal bradycardia with comorbid hydrops fetalis, a life-threatening condition in pre-term non-viable fetuses for which there are no effective treatment options. The small size and minimally invasive form factor of our design limit the volume available for circuitry and a power source. The device employs a fixed-rate and fixed-amplitude relaxation oscillator and a tiny, rechargeable lithium ion power cell. For both research and clinical applications, it is valuable 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 our simple 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 pacing stimuli that successfully generate a premature ventricular contraction. The method was tested in a pilot study in 4 fetal sheep and the data demonstrate that a relative measure of threshold is obtainable. This method provides valuable real-time information about the electrode-tissue interface.

Design and Testing of a Transcutaneous RF Recharging System for a Fetal Micropacemaker

“…When the pacemaker is recharged, oscillations begin again according to the calibration curve. This pacemaker is effective at treating bradycardia when it is pacing at a frequency of about 60 bpm, and it is delivering charge above the stimulation threshold for the cardiac tissue in which it is implanted [13]. Therefore, a lower operational limit will vary from patient to patient.…”