Optimal spacing of right ventricular bipolar catheter electrodes for detecting cardiac pumping by an automatic implantable defibrillator.

Our defibrillation-detecting system uses both ECG and right ventricular impedance change (delta Z). We studied the effect of catheter electrode spacing on delta Z in 10 dogs with body weights of 10 to ,5 kg and heart weights of 67 to 220 g. Impedance to 20-kHz 100--muA square waves was measured between two 1-cm-long electrodes mounted on a No. 12F catheter and wedged into the right ventricular apex. Catheters with spacings of 5 to 25 mm between the electrodes were tested during sinus rhyhm and ventricular fibrillation. During sinus rhythm the mean beat-to-beat delta Z was 23 +/- 4 omega using the 5-mm spacing. Wider spacing gave smaller, and hence less desirable, delta Z. Some delta Z signals were recorded during ventricular fibrillation, and 5-mm spacing was more sensitive to these than wider spacing. This resulted in some prolongation of the time between onset of fibrillation and application of the defibrillation shock, but should decrease false positive diagnosis of fibrillation. No clear relationship was observed between delta Z and body weight or heart weight. We conclude that the 5-mm spacing is best for detection of pumping by the catheter-impedance method in hearts of this weight range.