Feasibility of Extravascular Pacing with a Novel Substernal Electrode Configuration: The Substernal Pacing Acute Clinical Evaluation (SPACE) Study: The Substernal Pacing Acute Clinical Evaluation Study.

Subcutaneous ICDs provide an alternative to transvenous defibrillation but require higher shock outputs and offer no anti-tachycardia pacing. The SPACE study evaluated the feasibility of pacing from an extravascular substernal location.The primary purpose of the SPACE study was to characterize pacing from the substernal space. Secondary objectives included evaluating extracardiac stimulation and recording electrograms.The SPACE study prospectively evaluated the feasibility of pacing with a commercially available EP catheter (Medtronic Marinr®, 7 Fr) acutely implanted in the substernal space via minimally invasive subxiphoid access. Pacing data were collected in ≥7 vectors using constant current stimulation up to 20 mA and pulse width (PW) up to 10 msec.Catheter placement was successful in all 26 patients who underwent the procedure, with a mean (± standard deviation) placement time of 11.7 ± 10.1 minutes. Eighteen patients (69%) had successful ventricular capture in ≥ 1 tested vector. Mean pacing threshold at PW 10 msec was 7.3 ± 4.2 mA for all vectors (5.8 ± 4.4 volts). Failed capture was generally associated with sub-optimal catheter placement or presumed air ingression. A low level of extracardiac stimulation was observed in one patient. Average R-wave amplitude ranged from 2.98-4.11 mV unipolar and from 0.83-3.95 mV bipolar.Data from the SPACE study demonstrate that pacing is feasible from the extravascular substernal location. A substernal electrode configuration has the potential to provide pacing in a future extravascular device without need for intracardiac hardware placement.