Myocardial threshold in patients with artificial pacemakers.

T HIS report has a threefold purpose: (1) to describe a simple, clinical method of estimating myocardial threshold in patients with artificial pacemakers, (2) to verify the importance of critically high threshold as a significant cause of “pacemaker failure,” and (3) to suggest for this type of failure a nonsurgical method of restoring myocardial responsiveness by administration of glucocorticoid and sympathomimetic drugs. Although threshold problems have been previously reported in conjunction with at least three commercially available implantable pacemakers,lM3 the biologic consequences of prolonged artificial pacing have not been systematically investigated to date. Several reports have emphasized the technologic causes for pacemaker failure,4-7 and considerable effort has been expended toward the development of improved leads, better pulse generators and longer battery life. These latter considerations are fundamentally engineering problems to which the physician directly contributes very little, except as he serves as an evaluator and advisor. In calling particular attention to the biologic basis for pacemaker failure, referred to subsequently as exit block, we would like to emphasize the need for (1) ways of serially evaluating pacemaker function following implantation and (2) precise diagnosis of the cause of failure when it occurs unexpectedly. We feel the method here described holds great promise as means of meeting both of these needs. MATERIALS AND METHODS In developing a means of quantitating the myocardial threshold level, we took advantage of the features incorporated into the design of our pacemaker (General Electric Co.), which allowed variation in the rate by an externally applied, inductioncoupled, control unit. By a similar method shown in Figure 1, a threshold analyzer has been constructed which prevents the discharge of the output condenser of the implanted unit by introducing a high frequency (2,000 c.p.s.), low energy (1.45 microjoules) signal. The amplitude of each subsequent pacemaker stimulus is then inversely proportional to the number of suppression pulses introduced. By this means the rate of the implanted pacemaker can then be varied from 60 to 120 pulses per minute and the output energy per pulse can be suppressed to 2.5 per cent of normal. The technical details of this instrument will be reported elsewhere and are beyond the scope of this presentation. Operation of the threshold analyzer can be performed by one physician. The electrocardiogram is observed continuously on a separate cardiac monitor. The first step is to place the induction coil directly over the implanted pacemaker. When sufficient inductive coupling is present, as registered by a simple galvanometer incorporated into the instrument, the analyzer is turned on and allowed to capture control of the rate of the implanted unit. The rate is then adjusted as desired, and the output signal of the implanted pacemaker is displayed on a Tektronix 502A oscilloscope with a 10 msec., synchronized sweep. This allows continuous monitoring of exact rate and pulse width.8 A simple electronic counter can also be used to display these two parameters. Finally, through one control dial, the energy delivered by the implanted unit can be slowly reduced. Threshold is