An Energy Output Study of 14 Implantable Cardiac Pacemakers

Abstract

Cardiac depolarization can be effectively instituted by artificial means if an energy pulse of sufficient amplitude is delivered to a portion of the viable myocardium. The minimum amplitude or &dquo;threshold&dquo; necessary to initiate a contraction has been reported to increase following implantation of the pacing electrodes.'-' In some instances, threshold values have been known to rise so high that they produce exit block. 1, 2, 8 The increase in threshold energy has been attributed to local fibrosis developing around the pacing electrodes, aspecific inflammatory reactions, and generalized depression of myocardial excitability.l° 2 Originally, the increase in pacing threshold was attributed to an increase in electrical impedance of the tissues with the passage of time. However it has now been concluded that the impedance rise, although present, is small, and that the increased threshold should be attributed to the fibrous tissue theory.9, 10 Thus effective stimulation of the myocardium is dependent on the energy of the stimulus, the condition of the myocardium, and the electrical impedance of the electrode-tissue circuit presented to the pacemaker. 1,5,9,11,12 The energy of the stimulus pulse can be determined mathematically and can be measured by utilizing the applied voltage resistance and duration as follows:where E = energy of the stimulus in microjoules, v = voltage of the stimulus, t = stimulus duration in milliseconds, and R = resistance of the electrode/ tissue interface + tissue in kilohms.The equivalent circuit of the tissue-electrode interface and tissue consists of capacitive as well as resistive components. Thus it is correct to speak in terms of impedance when referring to the load against which an implantable cardiac pacemaker must operate. Capacitive elements in the equivalent From the