High spatial resolution measurements of specific absorption rate around ICD leads

Introduction: It has been shown that strong electric shocks can cause local refractoriness in the heart. This is of particular concern if the region o f refractoriness is the area sensed by an implant to determine cardiac rhyt hm, as is the case with many Implantable Cardioverter Defibrillator (ICD) l eads which use the same electrodes for shocking and sensing. Failure to sen se 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 Spe cific Absorption Rate (SAR) around typical ICD leads. Current density (J), a parameter that determines defibrillation effectiveness, is proportional t o the square root of SAR. Methods and results: SAR measurements were performed in a homogeneous salin e media using a variety of ICD leads. Gated 60 Hz shocks were used to produ ce heating, which was measured by thermistor probes. The temperature-rate-o f-change is directly proportional to the SAR, Measurement techniques were d eveloped that produced accurate SAR results at high spatial resolutions. Mu ltiple polarities and configurations of ICD leads were tested. Conclusions: We confirmed the spatial distribution of the SAR and correspon ding current density possessed sharp peaks and were highly localized around the leads' electrodes. Scans with a resolution of 1 mm or less are require d in the area of peak SAR in order to capture the peak's value. Published b y Elsevier Science Ltd on behalf of IPEM.