High spatial resolution measurements of specific absorption rate around ICD leads

It has been shown that strong electric shocks can cause local refractoriness in the heart. This is of particular concern if the region of refractoriness is the area sensed by an implant to determine cardiac rhythm, as is the case with many Implantable Cardioverter Defibrillator (ICD) leads which use the same electrodes for shocking and sensing. Failure to sense the true cardiac rhythm can cause application of unnecessary shocks and potential induction of arrhythmias. We developed a system to accurately map the areas where local refractoriness is most probable. We measured the Specific Absorption Rate (SAR) around typical ICD leads. Current density (J), a parameter that determines defibrillation effectiveness, is proportional to the square root of SAR.

SAR measurements were performed in a homogeneous saline media using a variety of ICD leads. Gated 60 Hz shocks were used to produce heating, which was measured by thermistor probes. The temperature-rate-of-change is directly proportional to the SAR. Measurement techniques were developed that produced accurate SAR results at high spatial resolutions. Multiple polarities and configurations of ICD leads were tested.

We confirmed the spatial distribution of the SAR and corresponding current density possessed sharp peaks and were highly localized around the leads' electrodes. Scans with a resolution of 1 mm or less are required in the area of peak SAR in order to capture the peak's value.