Importance of ventricular rate after mode switching during low intensity exercise as assessed by clinical symptoms and ventilatory gas exchange

Automatic mode su itching from DDD(R) to DDI(R) or VVI(R) pacing modes has improved dual chamber pacing in patients at high risk for supraventricular tachyarrhythmias, However little is known about the effect of ventricular p acing rate adaptation after mode switching. We conducted a single-blinded, crossover study in 15 patients (58 +/- 21 years) with a DDD pacemaker who h ad AV block and normal sinus node function to investigate the influence of pacing rate adaptation to intrinsic heart rate during low intensity exercis e. Patients performed two tests (A/B) of low intensity treadmill exercise ( 0.5 W/kg) in randomized order. They initially walked for 6 minutes while pa ced in DDD mode. The pacing mode was then switched to VVI with a pacing rat e of either 70 beats/min (test A) or matched to the intrinsic heart rate (9 5 +/- 11 beats/min test B). Respiratory gas exchange variables were determi ned and patients classified the effort before and after mode switching on a Borg scale from 6 to 20. Percentage changes of respiratory gas exchange me asurements were significantly larger (O-2 consumption: -8.2 +/- 5.0% vs. -0 .6 +/- 7.2%; ventilatory equivalent of CO2 exhalation: 5.3 +/- 4.9% vs. 4.5 +/- 4.3%; respiratory exchange ratio: 7.0 +/- 2.2% vs 3.5 +/- 3.0%; end-ti dal CO2: -5.7 +/- 2.9% vs. -1.8 +/- 2.7%; all P < 0.01) and the increase in subjective assessment of the effort tended to be higher (mean increase on Borg scale: 1.6 +/- 1.9 vs. 1.1 +/- 1.8, P = 0.07) after heart rate un adju sted than after adjusted mode switching. Mode switching from DDD to Wi paci ng is better tolerated and gas exchange measurements are less influenced if ventricular pacing rate is adjusted to the level of physical activity. Thu s, pacing rate adjustment should be considered as part of automatic mode sw itch algorithms.