For about ten years we have studied radio-frequency as a means of transmitting pulses for heart stimulation from an external source.l*p The most immediate and simplest solution appeared to be the epicardial application of a passive receiver, leading the stimulus to the heart by means of two electrodes. The electrical pulses, in the form of radio-frequency wave trains, were produced by an external tran~mitter.~*~ With this system, we attempted to resolve the problem of breakage of the wire electrodes used in most pacemakers at that time. A total of 38 patients were treated in this manner.This system was abandoned because of the electrophysiological factors that quickly produced fibrosis formation around the pacing electrodes, thus causing their electrical insulation (FIGURE 1 ) . However, the good results obtained with catheter electrodesSl0 prompted us to connect one of these catheters, of the bipolar type, to a radio-frequency receiver of the type previously used, and to implant this unit subcutaneously in the thoracic wall (FIGURE 2). An external transmitter with an annular antenna (FIGURE 3) was placed upon the receiver but outside the thorax.11-13 The transmitter power supply was furnished by rechargeable nickel-cadmium batteries, charged every five days.Catheter use has the notable advantage that thoracotomy is not necessary, and trauma is minimized. Moreover, the following properties of the first system were retained: (1) the ability to change the stimulus energy and to stop the stimulation whenever necessary; (2) the ability to change the cardiac frequency;(3) the ability to modify or replace the pulse transmitter, without a new surgical operation (FIGURE 4 ) . The characteristics of this radiopacemaker, which we have named model RFlZ/C, compared with those of our other models, are reported in TABLE 1.To date, 46 pacemakers of this model have been implanted, since October, 1966. The first 33 were implanted with the United States Catheter & Instrument Co. catheter 5651, which was the only one available at that time in Italy. Although we were aware that this type of catheter was designed by its manufacturer for temporary pacing, we were encouraged to use it for long-term stimulation after our own successful experience and that of others with it. However, we noticed that after a period of time varying from two to 21 months, the woven Dacron insulator between the coaxial conductors, of which the catheter is formed, tended in several cases to absorb organic fluids. This led to a gradual short circuit of the catheter, making the stimulation ineffective.In the meantime, the Medtronic catheters for long-term stimulation became available, and we substituted them for the old catheters in the patients who showed signs of short circuiting. In addition, we encountered one case of receiver failure; two cases of transmitter failure, one of which was due to the batteries; and one case of catheter displacement that could not be overcome with an increase 846
A cardiac pacemaker capable of responding to blood acidosis by change in its stimulation rate allows adjustment to a patient's metabolic needs. The blood pH is sensed by an iridium oxide electrode in the right atrium. During exercise, the venous pH decreases and the paced ventricular rate increases. If acidosis persists, the paced rate gradually returns to baseline and reaches it after about 70 minutes. A pH-triggered pacemaker has been implanted in a 72-year-old male. The pacemaker remained responsive one year after implant, increasing rate during exercise, cold pressor stress, ischemia of the arms and emotional stress.
Our first clinical experiments with artificial heart stimulation by means of a pacemaker with myocardiac electrodes did not achieve the results we expected. Therefore in 1959, we devoted our efforts to the study of remote heart stimulation by means of a new method1j2 with the following conditions: (1) complete abolition of any direct connection between pacemaker and the heart; and (2) an external pacemaker.With regard to the first condition, it seemed necessary to dispense with the wire electrodes due to the extreme brittleness of the filiform electrode, which may very easily break or get detached.As to the second condition, we thought it necessary to place the pacemaker externally because we were convinced that a device on which the life of a human being is dependent should be easily inspected, perfected, controlled and regulated.With the aim of achieving these conditions, we began to use Radio-Frequency impulses for the transmission of the stimulation from pacemaker to heart.Thanks to the progressive improvement of our devices and to animal experimentation performed for a long time on numerous dogs we were finally able to achieve a stimulating system that gave very satisfactory experimental results. In 1961 we considered that this new method could be safely applied to human patients afflicted with atrioventricular b l o~k s .~,~
Experimental in vitro testing coupled with animal and clinical studies have shown that a pH sensor may sense acute changes in pH occurring in response to exercise. This signal may in turn be used as a determinant for optimal pacing rate. Further investigation into membrane coating of electrodes and algorithm testing is necessary to maximize sensor response and control.
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