A comparative study of MRI-induced RF heating in pediatric and adult populations with epicardial and endocardial implantable electronic devices

I. Introduction

Cardiac implantable electronic devices (CIEDs) treat cardiac conduction disorders, replacing sinus and atrioventricular nodal function and providing high voltage defibrillation for abnormal heart rhythms in both adults and children [1, 2]. Two types of CIED leads are commonly used in clinical practice depending on patient's body size and venous anatomy: Adults and older children typically receive endocardial systems, with the leads inserted directly through the subclavian vein and the implantable pulse generator (IPG) placed in the left or right subpectoral pocket [3, 4]. Infants and young children often receive an epicardial system, which requires a sternotomy or thoracotomy to sew the cardiac lead directly to the myocardium. The IPG is then usually placed inferior to the abdominal rectus. Although cases of young children being implanted with the endocardial system are rare, there are reports of young patients (< 8 kg) with a transvenous pacing system [4]. Also, for adult patients with residual right-to-left shunts, or those with chambers that cannot be accessed by the transvenous route, epicardial leads implantation is necessary [5].

At least 75% of patients with CIEDs will require magnetic resonance imaging (MRI) exams during their lifetime [6]. Unfortunately, however, MRI is not readily accessible to patients with electronic implants due to risks associated with the interaction of MRI fields and the implanted device. Since the first report of such harmful effects in 1989 [7] significant improvement has been made in implant design and manufacturing, leading to the first generation of MR-conditional CIEDs that obtained FDA approval in 2011. Such improvements include for example, reduction of ferromagnetic material to reduce the risk of device dislodgement, and enhanced device programming to reduce the risk of unintended tissue stimulation. Tissue heating from radiofrequency (RF) excitation fields, however, remains a major issue. This “antenna effect” happens when the electric field of the MRI transmit coil couples with an elongated conductive lead, causing the specific absorption rate (SAR) of the RF energy to amplify at the lead’s tip [8-10]. Consequently, patients with such leads are either contraindicated to receive MRI, or only approved to undergo MRI under strictly controlled conditions.

The Heart and Rhythm Society expert consensus statement on MRI in patients with CIEDs provides Class I recommendation supported by Level A evidence (i.e., strong recommendation supported by high-quality evidence) that patients with MR-conditional CIEDs should only undergo MRI when the product labeling is adhered to [11]. Because FDA-approved MR-conditional labeling is currently only available for CIEDs with endocardial leads, patients with epicardial devices have been mostly excluded from MRI exams [12]. This restriction primarily occurs in the vulnerable pediatric population, or in adults who have congenital heart disease, as the majority of epicardial CIEDs are implanted in children.

The goal of this study was to compare MRI-induced RF heating at tips of epicardial and endocardial leads with realistic trajectories implanted in adult and pediatric phantoms during MRI at 1.5 T. To our knowledge, such comparative study has not been performed before. Because body size and lead trajectory are two factors that substantially affect RF heating [8, 13-15], we performed experiments in human-shaped phantoms of different sizes implanted with leads that followed patient-derived realistic trajectories. We provide the first preliminary results on RF heating of epicardial leads in clinically relevant populations.

II. Methods

III. Results

Figure 5 gives the magnitude of RF heating recorded at the tip of each lead after 280 s RF exposure. For each group, the temporal profile of RF heating is also given for cases that generated maximum and minimum heating.

The mean ± standard deviation of RF heating was 1.99±2.57 °C and 3.12±2.04 °C in pediatric and adult phantoms with the epicardial leads, respectively, and 0.89±0.61 °C and 1.28±0.43 °C in pediatric and adult phantoms with endocardial leads. The highest RF heating was observed in the pediatric phantom with an epicardial lead. Both groups are normally distributed as calculated by the Anderson-Darling normality test (P>0.05). A one-tail independent t-test at significant level of α = 0.05 showed that epicardial leads generated significantly higher heating compared to endocardial leads (Pvalue = 0.037).

Overall, we observed higher variation in the RF heating of epicardial leads compared to the variation of RF heating of endocardial leads. However, heating is trajectory-dependent as seen in figure 5. This could be explained by the fact that in endocardial CIED implantation, the IPG is consistently placed in the subpectoral pocket and leads are passed through the subclavian vein, which limits the variation in their trajectory. In contrast, there are no rules or guidelines for placing epicardial systems. As a result, surgeons position the IPG and leads based on prior experience and best practices handed from surgeon-to-surgeon, leading to substantial variation in lead trajectories. Because RF heating of an elongated implant highly depends on its trajectory which in turn determines the degree of coupling of the lead with MRI electric fields [19-22], the larger variation in epicardial lead trajectory translates to larger variation in RF heating.

This work did not have enough power to draw inferences as to the effect of body size (i.e., child vs adult) on RF heating. More work is required to establish the safety of MRI in age-specific pediatric populations with epicardiac leads.