CHRONOTROPIC INCOMPETENCE - A COMMON AND PROGRESSIVE FINDING IN PACEMAKER PATIENTS

Thirty-eight patients (ages 40 to 77 years, mean 63) followed in a pacemaker clinic underwent exercise treadmill tests to determine chronotropic incompetence. There were 28 men and 10 women. Twenty-seven patients had atrioventricular (AV) block and 11 patients had sick sinus syndrome. All patients were exercised to fatigue. None of the patients were receiving beta-blockers or other drugs that could reduce heart rate. Maximum heart rate (MHR) and percent predicted maximum heart rate (% PMHR) were used as an index of chronotropic incompetence. Chronotropic incompetence was defined as inability to achieve a % PMHR of > 80%. The overall incidence of chronotropic incompetence was 58% (22 of 38 patients). We examined the relationship between chronotropic incompetence and the time to pacemaker implantation. We found that in patients who had pacemakers for less than 2 years, the mean MHR was 125 +/- 21.6 beats/min compared with 111.9 +/- 23.6 beats/min for patients who had pacemakers implanted for longer than 4 years. Similarly, the mean % PMHR decreased from 76.5 +/- 12.5% to 68.7 +/- 15.4% in patients with pacemakers less than 2 years versus those with pacemakers for more than 4 years. Fifty-three percent of the patients with a pacemaker less than 2 years old were chronotropic incompetent versus 70% of the patients with a pacemaker more than 4 years old. These data suggest that chronotropic incompetence worsens with time after pacemaker implant. To further support this, eight patients with AV block underwent a second stress test an average of 2 years following the first. In every case, both the MHR and the % PMHR decreased on the repeat stress test; mean MHR decreased from 132.6 +/- 12.3 to 105.6 +/- 25.6 beats/min (p < 0.006), and mean % PMHR fell from 82.2 +/- 7.4% to 66.1 +/- 17.1% (p < 0.007). This demonstrates a worsening of chronotropic incompetence over a relatively short period of time (approximately 2 years). Thus we conclude that there is a marked incidence of chronotropic incompetence in this pacemaker population that includes a large percentage (71%) of patients with AV block. Chronotropic incompetence is progressive and worsens over a short time after pacemaker implantation. Therefore chronotropic competent patients requiring pacemakers should be considered for a sensor-driven pacemaker at the initial implant because of the possibility of progressive chronotropic incompetence.