The Influence of Opening the Thorax on Defibrillation Threshold in Canines

To determine if intraoperative testing is predictive of implantable defibrillator performance postoperatively, we measured sequential pulse defibrillation thresholds (DFTs) in 16 adult canines (28.0 +/- 3.5 kg, mean +/- SD body weight) at the time of epicardial defibrillation electrode implantation. Three epicardial defibrillation electrodes were sutured directly to the anterior, posterior, and left lateral epicardial surfaces of the heart through a left fifth intercostal thoracotomy. The pericardium was sutured closed over the electrodes and DFT was measured first with the thorax open and again after closing all surgical wounds, evacuating the thorax, and reinflating the lungs. Mean +/- SD DFT voltage, current, and impedance (pulse 1), and total delivered energy (both pulses) for the open chest measurements were 321 +/- 87 volts, 4.2 +/- 1.9 amps, 80 +/- 14 ohms and 5.3 +/- 3.7 joules, respectively. The corresponding DFT values for the closed chest measurements were 321 +/- 92 volts, 5.1 +/- 1.9 amps, 64 +/- 10 ohms and 6.1 +/- 3.9 joules, respectively. Paired Student's t-test comparison of open versus closed chest DFT values indicated that there were no significant differences in voltage (P greater than 0.80) or energy (P greater than 0.20), but there were significant differences in both current (P less than 0.01) and impedance (P less than 0.001). It is concluded that despite alterations in impedance and current flow, voltage and energy DFT are not significantly different between open and closed chest animals. This suggests that intraoperative testing of implantable defibrillators is predictive of postoperative performance.