Differences between remaining ability and loss of capacity in maximum aerobic impairment

In the evaluation of exercise intolerance of patients with respiratory diseases the American Medical Association (AMA) and the American Thoracic Society (ATS) have proposed similar classifications for rating aerobic impairment using maximum oxygen uptake (VO2max) normalized for total body weight (ml min(-1) kg(-1)).However, subjects with the same VO2max weight-corrected values may have considerably different losses of aerobic performance (VO2max expressed as percent predicted). We have proposed a new, specific method for rating loss of aerobic capacity (VO2max, percent predicted) and we have compared the two classifications in a prospective study involving 75 silicotic claimants. Logistic regression analysis showed that the disagreement between rating systems (higher dysfunction by the AMA/ATS classification) was associated with age>50 years (P<0.005) and overweight (P=0.04). Interestingly, clinical (dyspnea score) and spirometric (FEV(1)) normality were only associated with VO2max, percent predicted, normal values (P<0.01); therefore, in older and obese subjects the AMA/ATS classification tended to overestimate the aerobic dysfunction. We conclude that in the evaluation of aerobic impairment in patients with respiratory diseases, the loss of aerobic capacity (VO2max, percent predicted) should be used instead of the traditional method (remaining aerobic ability, VO2max, in ml min(-1) Kg(-1)).