An Estimation Model for Cardiorespiratory Fitness in Adults with Rheumatoid Arthritis

Purpose Cardiopulmonary exercise testing of peak oxygen uptake (VO2peak) is the gold standard to measure cardiorespiratory fitness (CRF). For resource-intensive evaluation, equations estimating CRF (eCRF) may be used. The purpose was to investigate if an eCRF equation from a healthy population is useful in persons with rheumatoid arthritis (RA), and if necessary, develop new equations for eCRF in this group. Methods VO2peak results from 93 persons with RA were compared with eCRF calculated by an established equation for healthy individuals including age, sex, physical activity (PA index), resting HR (RHR), and waist circumference. Because of deviation from the observed VO2peak, new equations for eCRF in persons with RA were developed from regression analysis of variables associated with observed VO2peak. Results The established equation overestimated CRF (R-2 = 0.48, root mean square error [RMSE] = 7.07). The new RA equation more accurately estimated CRF (R-2 = 0.81, RMSE = 4.44) (female = 0, male = 1; never smoked = 0, ever smoked = 1): eCRF = 77.961 + (sex x 28.791) - (age x 0.358) - (age-sex interaction x 0.326) - (body mass index [BMI] x 0.700) - (RHR x 0.125) - (smoking x 1.854) + (PA index x 0.211) - (patient global RA assessment x 0.071). Alternative new RA equation (R-2 = 0.79, RMSE = 4.63): eCRF = 77.851 + (sex x 25.460) - (age x 0.381) - (age-sex interaction x 0.254) - (BMI x 0.743) - (RHR x 0.115) - (smoking x 2.154) + (PA index x 0.209). Conclusions The new RA equations better predicted CRF in individuals with RA, preventing overestimation in low-fit persons. The new equation should be preferred when estimating CRF in individuals with RA. The alternative equation, without patient global assessment, is useful for individuals with RA in population-based studies.