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A study was conducted comparing the pacing and sensing performance of an endocardial pacing lead (5076, Medtronic, Inc., Minneapolis, MN) in vivo to an ex vivo isolated heart model designed to work all four chambers under simulated in vivo physiologic conditions. The lead was implanted in vivo using fluoroscopic guidance. Standard stimulation and sensing parameters were recorded. Using standard cardiac surgical techniques, 8 porcine hearts were excised and reperfused in a working heart model retaining their original lead placement. Following stabilization, the electrical measurements were repeated. Ex vivo electrical values were significantly decreased relative to the in vivo measurements: R-wave amplitude by 39%, P-wave amplitude by 39%, slew rate by 62%, and pacing impedance by 34%. Pacing threshold values were similar in vivo and ex vivo. Variations in performance were in part attributed to local tissue deformation in the isolated heart preparation leading to differences in lead slack and orientation. This comparative study substantiated the value of utilizing the isolated heart model to visualize and simultaneously evaluate lead pacing and sensing performance in comparison with in vivo experimental procedures.