Reproducibility of cardiopulmonary exercise test variables: getting into an additional strength of the test

Exercise is the primary source of symptom exacerbation, and intolerance to physical activity is one of the most disabling problems experienced by patients with cardiopulmonary disorders. Exercise evaluation combined with gas exchange analysis by cardiopulmonary exercise testing (CPX) can assist the clinician with a wealth of information beyond standard exercise electrocardiography testing. This approach is useful in terms of assessing the mechanism of exercise intolerance, evaluating disability, making activity and training recommendations, quantifying responses to therapy, and predicting outcome. For these distinctive features, CPX has received a particular amount of attention in recent years; numerous studies have adopted CPX variables as endpoints over the last decade and the testing, in all its features, is becoming more common in daily clinician practice. Given the key role of exercise impairment in estimating cardiac and non-cardiac risk, objective and reproducible methods for exercise capacity assessment are needed. Therefore, to be considered as a reliable tool in the follow-up assessment of cardiopulmonary diseases the performance of CPX variables must be proven to be reproducible across different cohorts of patients. In the present issue of the Journal, Barron et al. contribute to our understanding on the repeatability of CPX data by reporting test to test evaluation in a series of patients with cardiac (heart failure, mitral valve disease) and chronic obstructive pulmonary disease (COPD). The study builds on prior evidence of very good test reproducibility confirming that mainstay CPX measurements, such as oxygen consumption (VO2) and ventilation to carbon dioxide production (VE/VCO2), are well-reproducible and extends these findings with some remarkable new evidence. Investigators report on the largest number of patients (93) ever tested who underwent two CPX tests, the first as familiarisation at a fixed ramp protocol (10watt/min) and the second, performed two hours later or sometimes on a separate day, using a protocol to achieve the maximum in 8–10min duration based on the results of the first test. The analysis incorporated a large set, practically all, of gas exchange variables directly measured or derived and the statistical analysis was performed using robust procedures by testing the interclass correlation coefficient (ICC) and the coefficient of variation between tests. Of note, this is the first report assessing test to test repeatability of some variables that are derived and highly prognostic especially in heart failure patients, e.g. oxygen uptake efficiency slope (OUES), circulatory power and exercise oscillatory ventilation (EOV). In addition, the repeatability of ventilation efficiency was determined by testing the three methods proposed in the literature, VE vs VCO2 slope, their nadir ratio and the ratio at the anaerobic threshold (AT). Interestingly, the reproducibility end-point was not limited to variable comparison per se, but extended to ascertain the role of protocol, disease aetiology and inter-test interval. Also, the potential modifier role of other factors, e.g. age, body mass index (BMI) and gender, was taken into account. Peak VO2, O2 pulse and the VE/VCO2 ratio at nadir showed the best repeatability performance, with withinsubject coefficient of variation <0.12. Good repeatability was maintained for most other variables without any significant influence of age, gender, BMI and inter-test interval, with the exception of EOV and VO2/work rate (WR) in older patients. Some specific considerations on these variables seem worthy of note since both are calculated rather than measured and may be operator-dependent and affected by the methodology. EOV was not observed in the COPD patient cohort, whereas it occurred in 23 (33%) cardiac