Acute and chronic stimulation thresholds of intramyocardial screw-in pacemaker electrodes

Two screw-in intramyocardial electrodes were inserted in the left ventricle of each of 6 mongrel dogs, and the electrical characteristics were studied over a six-month period. The unipolar lead configuration had consistently better threshold and sensing values than the bipolar configuration during this period. Peaking between implantation and 3 weeks later, mean lead thresholds increased significantly (unipolar, 0.4 to 8.2 mu J; bipolar, 0.6 to 10.2 mu J) while R wave amplitude decreased (unipolar, 16.6 to 8.5 mv; bipolar, 10.6 to 5.8 mv). Mean chronic values for stimulation thresholds were as follows: unipolar, 2.6 mu J, and bipolar, 3.1 mu J. Mean values for the R wave amplitude were as follows: unipolar, 10.6 mv, and bipolar, 7.2 mv. Analysis of the results indicated that with certain exceptions, the optimum system has 2 intramyocardial electrodes, 1 in unipolar configuration and 1 "reserve." In general, adequate values at implantation are a stimulation threshold of 1.4 mu J (e.g., 1.1 v at 500 ohms and 0.6 msec pulse width) and an R wave amplitude of 5 mv.