The ventricular endocardial electrogram and pacemaker sensing.

During cardiac pacemaker implantation and pulse generator replacement, unipolar, right ventricular electrograms were recorded from 133 patients (77 at implantation and 56 at pulse generator replacement) at 200 mm. per second with a band pass of 0.1 to 2,000 Hz on photographic paper. Each signal was analyzed for electrogram structure, peak-to-peak voltage deflection, ST-segment displacement, and maximum voltage deflection/time (slew rate [dv/dt]). The QRS designation of the peripheral electrocardiogram was used for the endocardial electrogram wave forms, which are sufficiently similar to allow QRS terminology to be used. Of the acute electrograms (at implantation) 58 per cent had a small Q followed by an R wave which was 10 per cent or more of the S amplitude; 30 per cent had an R wave less than 10 per cent of the S; all had a straight line segment, within the QRS complex, exhibiting maximum amplitude and slew rate and a large ST-segment elevation. In 12 per cent the Q was followed by an R wave only. The mean voltage was 12.4 mv. +/- 5.5, the slew rate mean 2.9 v. per second +/- 1.5, and the ST displacement 4.0 mv. +/- 2.6. All chronic (over 6 months after implantation) electrograms but one had no ST-segment elevation, and all had an inverted T wave and a straight line segment extending from the highest to the lowest peaks of the QRS complex. The chronic voltage amplitude is 15 per cent lower and the slew rate 41 per cent lower than that of the acute electrogram. The margin of amplitude and slew rate required at implantation to maintain adequate postimplant sensing must accommodate these changes. The possibility of satisfactory pacing threshold associated with poor sensing exists. Knowledge of the amplitude and slew rate allows the determination of the optimal position for electrode placement and the sensitivity required for satisfactory pulse generator selection, design, and operation.