Accuracy of oxygen uptake efficiency slope in adults with congenital heart disease

Background: Given its linearity throughout exercise, oxygen uptake efficiency slope (OUES) obtained with a sub-maximal exercise is considered a reliable predictor of exercise capacity. We sought to assess the linearity of OUES across different exercise stages in adults with various forms of congenital heart disease. Methods: Using cardiopulmonary exercise testing, we studied 23 patients after a Fontan operation, and 30 patients after atrial repair for complete transposition of the great arteries, at a mean age of 24 ± 10  years. Thirty-five healthy volunteers were used as controls. OUES was calculated from 100% (OUES), the first 50% (OUES₅₀), and the last 50% (OUES_50-100) of the entire exercise duration. Results: Peak oxygen uptake and OUES were reduced in Fontan patients when compared to atrial repair or control subjects (p < 0.05). However, whereas in atrial repair and in control subjects OUES₅₀, OUES_50-100, and OUES appeared to be similar (p > 0.05), in Fontan patients OUES₅₀ appeared to be lower than OUES_50-100 (1.38 ± 0.46 vs. 1.78 ± 0.51, p = 0.01) and OUES (1.38 ± 0.46 vs. 1.72 ± 0.56, p = 0.032). The difference between OUES₅₀ and OUES_50-100 appeared particularly large in cyanotic Fontan patients (1.40 ± 0.42 vs. 1.93 ± 0.68, p = 0.001), whereas no difference was observed in Fontan patients with normal saturation (1.33 ± 0.59 vs. 1.37 ± 0.67, p = 0.922). Conclusions: In cyanotic Fontan patients, OUES₅₀ differs substantially from OUES_50-100 and OUES. Therefore, OUES₅₀ is unable to predict maximal exercise capacity in this population.