CLINICAL EXPERIENCE WITH AN IMPLANTABLE PACEMAKER

Previous reports by the have dealt with the development and with the experimental and initial clinical use of a subcutaneously implanted pacemaker. Our accumulated experience with this therapeutic approach to heart block covers now approximately three and one-half years and 60 cases. The early clinical trials were not without the difficulties that beset most new developments, but they have also led to significant improvements in instrumentation and technique. The purpose of this review is to summarize this experience, analyze the failures, and describe the up-to-date equipment. The patients in this series fall into three groups, and the pertinent clinical data on these have been summarized in TABLE 1 . Group 1. In the first 16 cases treated (April 1960 to April 1961) , the major problem arose from the mechanical instability and disruption of the lead wires of the Hunter-Roth electrode then in use. This difficulty was overcome by the development of the helical spring electrode, a detailed description of which is given elsewhere in this monograph. Group 2. Twenty-two patients were operated upon between April 1961 and March 1962, and the significant complications in this group, as well as in the nine cases from the earlier series who secondarily received the platinum coil electrode (TABLE 1, Group 2 and 2a) , developed from certain design features of the pacemaker that were still indirectly related to the electrode problem. These features had been adopted because of the then-prevailing fear of a progressive increase in threshold current requirements and because of the high incidence of mechanical breakdown of the Hunter-Roth electrode wires. Experimental evaluation of the new coil electrode structure had already yielded encouraging results, but sufficient clinical confirmation was then still lacking. To provide an adequate margin in current output to deal with potential late increases of the threshold, a pulse of 10 ma. was selected since it exceeded by a factor of two to three the average threshold observed with the old Hunter-Roth electrode. Ten mercury cells were needed for this, and the timing circuit of the pulse generator had to be supplied from a four-battery level; whereas the output circuit was receiving voltage from all 10 batteries. Because of the greater current drain on the four cells in the timing circuit, it was anticipated that failure of one of these would occur first, lead to a limited increase in the rate, and provide a warning of impending battery depletion. It was realized that with failure of a second battery in the timing circuit, a further increase in the rate could occur, but it was thought that by then the output of the unit probably would have dropped to below threshold levels. Actual experience in this second year revealed a number of instances in which premature battery failure had led to excessive increases in the rate. It became