Clinical experience with a new rate-responsive TX pacemaker.

In this report, we describe our initial experience with a new rate-responsive TX pacemaker in 10 patients. This pacemaker system uses a conventional transvenous ventricular electrode, which senses the evoked QT interval after a ventricular paced beat, as an indicator of physiological metabolic demand. Implantation of the TX pacemaker does not differ from that of conventional VVI units, and two special modes are available after implantation: TX and VVI mode. Bicycle ergometer tests showed that significantly higher rate response and exercise ability were achieved during the TX mode compared with the VVI mode (mean maximal heart rate: 115 +/- 13 bpm vs 96 +/- 21 bpm, p less than 0.01, and mean maximal exercise time; 10.7 +/- 3.9 min vs 8.0 +/- 3.8 min, p less than 0.01, respectively). In one patient, cardiac index was measured by a thermodilution method during the bicycle ergometer test, which confirmed that an adequate increase in cardiac index (rest: 3.38 liters/min/m2, vs exercise; 7.34 liters/min/m2) was obtained by an increase of the pacing rate (rest: 72 bpm vs exercise: 128 bpm). Our results show that physiological rate-responsive pacing using the QT interval provides a simple means of increasing the heart rate and cardiac index.