Utility of low energy test shocks for estimation of cardiac and electrode impedance with implantable defibrillators.

We assessed the value and safety of using a low voltage test shock to predict cardiac and electrode impedance during subsequent high voltage, transvenous, sequential pulse shocks. Two transvenous defibrillation catheters were inserted in 15 patients undergoing electrophysiology study for the evaluation of ventricular tachycardia/fibrillation. A sequential pulse test shock at stored voltage of 50 or 100 volts was delivered during sinus rhythm and high voltage therapeutic shocks were delivered during induced ventricular tachycardia/fibrillation. The test shocks were well tolerated and caused only minimal discomfort. Peak delivered voltage for each pulse of the test shock ranged from 34 to 116 volts (mean 62 +/- 27) and for each pulse of the therapeutic shocks from 186 to 778 volts (mean 435 +/- 146). There was a very strong relationship between the impedance of the test and the mean impedance of the therapeutic shocks (R = 0.89, P less than 0.001). The mean impedance of the test shock was slightly greater than the mean impedance of the test shock as slightly greater than the mean impedance of the therapeutic shocks (mean 90 +/- 24 ohms compared to 77 +/- 16 ohms, P less than 0.001), with a maximum difference between test and therapeutic shock impedances of 39 ohms (mean difference 8.4 +/- 10.3 ohms). There was a significant inverse relationship between impedance and peak current (R = -0.49, P = 0.001). Cardiac and electrode impedance during transvenous defibrillation can be predicted reliably and safely by a low voltage test shock. This technique provides a safe and simple method to ensure appropriate connections and electrode function for subsequent countershock therapies.