Characteristics of Bipolar and Unipolar Electrograms From Substernal Electrodes: Results From the Substernal Pacing Acute Clinical Evaluation (SPACE) Study

Introduction: A totally subcutaneous (SubQ) defibrillator is limited by its inability to provide painless anti-tachycardia pacing and the need for pre-implant screening due to challenging SubQ electrograms (EGMs). As an alternate extravascular approach, the SPACE study revealed that pacing is feasible in humans using widely spaced electrodes positioned in the mediastinal tissue. We assessed the hypothesis that bipolar and unipolar extravascular EGMs are feasible for sensing. Methods: The SPACE prospective feasibility study enrolled adult subjects undergoing midline sternotomy surgery or SubQ ICD implant. A decapolar EP catheter (Marinr, 7F, Medtronic) was temporarily placed into the mediastinal tissues under the sternum at or adjacent to the midline. Using fluoroscopy in a supine position, the middle pair of electrodes was centered over the right ventricle. A skin patch electrode (Fast Patch Plus, Physio Control) was placed on the left thorax approximately at the 5 th intercostal space. Bipolar and unipolar EGMs during normal sinus rhythm were recorded. Results: We analyzed all available EP recordings in 14 subjects (10 males, 4 females; age 63.1 ± 11.5 years) studied at 5 sites in the US and Canada. As shown in Figure 1, bipolar R-wave amplitude was significantly larger with 41 mm electrode spacing (electrodes 1-10, 3.95±2.41 mV) vs. 3.2 mm electrode spacing (electrodes 5-6, 0.83±0.84 mV) or any other electrode pairs (P Conclusions: Sensing cardiac signals with high R-wave amplitudes is attainable from the electrodes in the mediastinal location in both bipolar and unipolar configurations with wide electrode spacing. Further study of electrogram characteristics at different postures is needed to better understand waveform variations.