Minimally invasive implantable fetal micropacemaker: mechanical testing and technical refinements

Emerging Technologies for the Smallest Patients

Pediatric and congenital heart disease patients may require cardiac implantable electronic device implantation, inclusive of pacemaker, ICD, and implantable cardiac monitor, for a variety of etiologies. While leads, generators, and monitors have decreased in size over the years, they remain less ideal for the smallest patients. The potential for a miniature pacemaker, fetal micropacemaker, improving leadless technology, and rechargeable devices creates hope that the development of pediatric-focused devices will increase. Further, alternative approaches that avoid the need for a transvenous or surgical approach may add more options to the toolbox for the pediatric and congenital electrophysiologist.

Care of the Fetus With Congenital Cardiovascular Disease: From Diagnosis to Delivery

BACKGROUND

The majority of congenital cardiovascular disease including structural cardiac defects, abnormalities in cardiac function, and rhythm disturbances can be identified prenatally using screening obstetrical ultrasound with referral for fetal echocardiogram when indicated.

METHODS

Diagnosis of congenital heart disease in the fetus should prompt assessment for extracardiac abnormalities and associated genetic abnormalities once maternal consent is obtained. Pediatric cardiologists, in conjunction with maternal-fetal medicine, neonatology, and cardiothoracic surgery subspecialists, should counsel families about the details of the congenital heart defect as well as prenatal and postnatal management.

RESULTS

Prenatal diagnosis often leads to increased maternal depression and anxiety; however, it decreases morbidity and mortality for many congenital heart defects by allowing clinicians the opportunity to optimize prenatal care and plan delivery based on the specific lesion. Changes in prenatal care can include more frequent assessments through the remainder of the pregnancy, maternal medication administration, or, in selected cases, in utero cardiac catheter intervention or surgical procedures to optimize postnatal outcomes. Delivery planning may include changing the location, timing or mode of delivery to ensure that the neonate is delivered in the most appropriate hospital setting with the required level of hospital staff for immediate postnatal stabilization.

CONCLUSIONS

Based on the specific congenital heart defect, prenatal echocardiogram assessment in late gestation can often aid in predicting the severity of postnatal instability and guide the medical or interventional level of care needed for immediate postnatal intervention to optimize the transition to postnatal circulation.

Prenatal diagnosis and management of congenital complete heart block

Congenital complete heart block (CCHB) is a life-threatening medical condition in the unborn fetus with insufficiently validated prenatal interventions. Maternal administration of medications aimed at decreasing the immune response in the fetus and beta-agonists intended to increase fetal cardiac output have shown only marginal benefits. Anti-inflammatory therapies cannot reverse CCHB, but may decrease myocarditis and improve heart function. Advances in prenatal diagnosis and use of strict surveillance protocols for delivery timing have demonstrated small improvements in morbidity and mortality. Ambulatory surveillance programs and wearable fetal heart rate monitors may afford early identification of evolving fetal heart block allowing for emergent treatment. There is also preliminary data suggesting a roll for prevention of CCHB with hydroxychloroquine, but the efficacy and safety is still being studied. To date, intrauterine fetal pacing has not been successful due to the high-risk invasive placement techniques and potential problems with lead dislodgement. The development of a fully implantable micropacemaker via a minimally invasive approach has the potential to pace fetal patients with CCHB and thus delay delivery and allow fetal hydrops to resolve. The challenge remains to establish accepted prenatal interventions capable of successfully managing CCHB in utero until postnatal pacemaker placement is successfully achieved.

Prenatal therapy for fetal cardiac disorders

Fetal cardiac abnormalities are some of the commonest congenital disorders seen in prenatal life. They can be anatomical or functional and can develop de novo or as a consequence of either maternal or fetal disease. Untreated, morbidity and mortality rates are high for hypoplastic left heart disorders and for some fetal tachy and bradyarrhythmias. Optimum management strategies are often not clear because of the lack of knowledge about the precise natural history of some of these conditions. Prenatal therapy ranges from invasive fetal cardiac intervention to maternal administration of drugs for transplacental transfer to the fetus. This comprehensive review covers many fetal cardiac disorders and various prenatal therapeutic options that are available.

Accelerated life-test methods and results for implantable electronic devices with adhesive encapsulation

We have developed and applied new methods to estimate the functional life of miniature, implantable, wireless electronic devices that rely on non-hermetic, adhesive encapsulants such as epoxy. A comb pattern board with a high density of interdigitated electrodes (IDE) could be used to detect incipient failure from water vapor condensation. Inductive coupling of an RF magnetic field was used to provide DC bias and to detect deterioration of an encapsulated comb pattern. Diodes in the implant converted part of the received energy into DC bias on the comb pattern. The capacitance of the comb pattern forms a resonant circuit with the inductor by which the implant receives power. Any moisture affects both the resonant frequency and the Q-factor of the resonance of the circuitry, which was detected wirelessly by its effects on the coupling between two orthogonal RF coils placed around the device. Various defects were introduced into the comb pattern devices to demonstrate sensitivity to failures and to correlate these signals with visual inspection of failures. Optimized encapsulation procedures were validated in accelerated life tests of both comb patterns and a functional neuromuscular stimulator under development. Strong adhesive bonding between epoxy and electronic circuitry proved to be necessary and sufficient to predict 1 year packaging reliability of 99.97% for the neuromuscular stimulator.

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

We have developed a rechargeable fetal micropacemaker in order to treat severe fetal bradycardia with comorbid hydrops fetalis. The necessarily small form factor of the device, small patient population, and fetal anatomy put unique constraints on the design of the recharging system. To overcome these constraints, a custom high power field generator was built and the recharging process was controlled by utilizing pacing rate as a measure of battery state, a feature of the relaxation oscillator used to generate stimuli. The design and in vitro and in vivo verification of the recharging system is presented here, showing successful generation of recharging current in a fetal lamb model.