Abstract Body (Do not enter title and authors here): Background:
Cardiac output reserve and exercise capacity are strong predictors of life expectancy. Chronotropic incompetence (CI) is the inability to reach an age appropriate maximum heart rate with exercise. CI reduces cardiac output reserve and exercise capacity, both of which increase all-cause mortality risk. Sinus node dysfunction may manifest as CI and abnormally low resting heart rate (RHR). The relationship between low RHR and CI has yet to be described.

Purpose:
We hypothesize that resting sinus bradycardia (low RHR) could be a predictor of chronotropic incompetence and reduced exercise capacity.

Methods:
The derivation cohort consists of 201 patients with normal Bruce protocol treadmill stress echocardiogram. Patients with ejection fractions <50%, baseline or stress induced wall motion abnormalities, and valvular diseases were excluded. The confirmation cohort, which was obtained from the National Institute of Health, consisted of 3315 patients with normal Bruce protocol treadmill stress test results in the Prospective Multicenter Imaging Study for Evaluation of Chest Pain (PROMISE) Trial. RHR and peak exercise heart rates (PHR) were tabulated. All patients were categorized based on whether their PHR reached 85% of age-predicted maximum heart rates (APMHR) by the Astrand formula (220 - age in years) and the Tanaka formula (208 - 0.7 x age in years).

Results:
The mean(SD) age in the derivation cohort was 60(±11) years old and 45% were female. 46/201 (23%) did not reach their APMHR. The mean(SD) RHR in CI patients was 65.6(±12) beats per minute (BPM), while it was 69.2 (±12) BPM without CI (p =0.04).

In the confirmation cohort, the mean age was 60(±6.9) years old and 48% were female. 427/3315 (13%) did not reach their APMHR. The mean RHR in CI patients was 68.9(±12) BPM versus 74.0 (±13) BPM in the patients that reached APMHR (p <0.001). The Tanaka formula revealed similar results (p <0.001).

Patients with RHR below or equal to 60 BPM were almost twice as likely to have CI (20%) when compared to patients with RHR over 60 BPM (11%), with a statistical significance of p < 0.001. Patients with CI had a greater cumulative hazard ratio for death and hospitalizations due to myocardial infarction or unstable angina (Image 1).

Conclusion:
These results demonstrate that CI is linked to lower RHR. Below 60 BPM RHR may therefore be an indicator of sinus node dysfunction and CI, both of which predict a reduced exercise capacity and increase all-cause mortality risk.