Reference Values for Chronotropic Index from 1280 Incremental Cycle Ergometry Tests

Purpose Clinical cardiopulmonary exercise testing can determine causes of exercise limitation. The slope of heart rate (f(C)) versus oxygen uptake (VO2), which we call the chronotropic index (CI), can help identify cardiovascular impairment. We aimed to develop a reference equation for CI based on a large number of subjects considered to have normal exercise responses. Methods From a database of 13,728 incremental cycle ergometry exercise tests, we identified 1280 normal tests based on the absence of a clinical diagnosis, normal body mass index, and normal aerobic performance plus absence of cardiovascular disease, medications, or ventilatory limitation. A linear mixed-model approach was used to analyze the relationship between CI and other variables. Results Subjects were age 18-84 yr, and 693 (54.1%) were men. Mean +/- SD CI in men was lower than in women, 41.2 +/- 9.3 beats per liter versus 63.4 +/- 15.7 L-1. Age (in years), sex (0, male; 1, female), height (in centimeters), and weight (in kilograms) were significant predictors for CI:CIi = 106.9 + 0.16 x age(i) + 14.3 x sex(i) - 0.31 x height(i) - 0.24 x weight(i). The SE of estimates ranged from 10.6 to 11.2 L-1 (median of 10.7 L-1). Conclusions We report a reference equation for CI derived from normal subjects. The CI can be used in conjunction with VO2max to interpret maximal cardiopulmonary exercise tests. We consider a high CI to be cardiovascular impairment and a low CI plus low VO2max to be chronotropic insufficiency.