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The effects of injected 50 Hz alternating current on the function of cardiac pacemakers has been observed in 18 patients with implanted unipolar VVI units. Current, in the range 0-600 microA was applied via electrodes attached to the patients' upper body and feet and fed from a specially designed current injection unit at the bedside. Most implanted pacemakers reverted to interference mode in the current range 29-250 microA. At current levels just below the reversion current all units developed irregular and inappropriate pacing. This current level was pacemaker dependent and varied in the range 27-246 microA. The total reversion current depended on the location of the injecting electrodes and on the patients' posture. The sensitivity of the units to injected interference was increased by deep inspiration. Temporary pacing catheters fitted to an additional ten patients were used to monitor the interference voltage which would be sensed by an implanted unit. This voltage was similarly dependent on patient posture and on deep respiration. Current injection has proved to be a safe, controllable and reproducible method of testing the sensitivity of implanted pacemakers to 50 Hz external interference.

Electromagnetic inference is a recognized difficulty in concomitant cardiac pacing and nerve stimulation. A case is reported in which a vertebral nerve stimulator was implanted, with a transmitter antenna distal to the right nipple. Six months later, intermittent S-A block with Adams-Stokes attacks urgently necessitated cardiac pacing. The ECG lead II showed nerve stimulator potentials of 0.75 mV. Intracardiac electrogram with unipolar electrode registered potential of 0.25 mV, but with a bipolar system no such potentials were observed. A bipolar pacing electrode was therefore implanted, with an interference from the nerve stiumluator. Various aspects of interference are discussed.

Thirty-five patients fitted with 16 different pacemaker models (from 6 manufacturers) were exposed to 50 Hz electric fields up to a maximum of 20 kV/m. Four different response patterns were encountered: (1) normal sensing and pacing in all Medtronic and some Vitatron units; (2) reversion to the fixed (interference) rate in all Telectronics, all Pacesetter, some Vitatron and CPI units; (3) slow and irregular pacing in one CPI and in all Cordis units; (4) mixed behavior over a critical range of field strengths in which slow and irregular pacing preceded reversion to fixed-rate, in some Telectronics and Pacesetter units. The field strengths required to induce such behavior varied from unit to unit and from model to model, with Telectronics being the most sensitive. In general, the interference threshold depended on the magnitude and distribution of induced body current relative to the pacemaker as well as field strength and thus varied with patient height, build and posture. While only a small proportion of pacemaker patients are likely to encounter electric fields strong enough to interfere with pacemaker behavior, this possible hazard should be recognized.

We attempt here to outline the different sources of interference and the methods of interference detection and protection with their advantages and disadvantages. A proposal is made as to how a better interference suppression could be established. A comparison of 16 pacemakers from ten manufacturers tested with different interference sources revealed a great variety, which does not allow one to generalize the problem of interference. As an example, an electric steel furnace with its magnetic field is described and how its influence on pacemakers can be estimated. Rules are given as to how pacemaker patients may be treated by electrotherapy. A test procedure for characterizing the specific interference behavior is proposed.

The electric current leak through an unsealed connector or insulation break can be a source of problems in cardiac pacing. This study describes a method of leak detection. The underdrive inhibition test--chest wall stimulation with external stimulator (ES)--was used for this purpose. Three cutaneous electrodes (CE) were applied: the first one above the indifferent electrode, the second one above the suspected leak location, and the third one above the active electrode. Two inhibition thresholds (IT) were measured: the first one connecting ES on first CE and second CE, and the second one connecting ES on first CE and third CE. The inhibition threshold is the value of ES output charge at which the inhibition of implanted pacemaker occurs. The relation between two ITs, not the absolute value, is significant for leak estimation. The former should be at least five times higher than the latter, if there is no current leak. During the two years of follow-up of 541 patients, 272 measurements were performed. In 52 cases the connector leak was found-both ITs were approximately equal. Twenty-seven cases were confirmed by surgery for EOL pacemaker replacement; the intrusion of body fluid was evident. Five operations were indicated because of the leak problem: two--muscle twitching, one--loss of capture, one-undersensing, and one--insulation break with lead fracture. The method, although approximate, is a reliable qualitative estimation of leak.

The inhibited pacemaker (VVI or AAI) has become the most popular in recent years because of its ability to combine a physiological advantage with economical current consumption in cases with spontaneous activity. One of its disadvantages is its sensitivity to external electromagnetic interference. Though today's pacemakers possess effective protection against most interference signals there may be instances in which patients are subjected to uncomfortable or even life-threatening situations. This is the case of "amplitude modulated" or "pulsed" fields with modulation frequencies in the physiological range. Fields of that sort have been found in the vicinity of a welder, an electric steel plant, and in medical practice where therapeutic currents were applied. Even touch-actuated switches may influence a demand pacemaker. However, these situations may be overcome by a device within the pacemaker for simple time analysis which can be carried out with few components. If electromagnetic fields of diathermy equipment are applied, today's pacemakers may react with intolerably high or low rates. They should, therefore, be avoided.