Oxygen uptake versus exercise intensity: a new concept in assessing cardiovascular exercise function in patients with congenital heart disease

Objective-To assess the relation between exercise intensity and oxygen upta ke during graded exercise in paediatric patients who underwent surgical rep air of congenital heart disease, and to compare it with conventional measur es of aerobic exercise function. Design-Cross sectional study. Exercise testing was performed on a treadmill and gas exchange was measured on a breath by breath basis. Patients-29 patients who underwent an atrial switch operation fur transposi tion of the great arteries (TGA) (mean (SD) age at testing 10.3 (2.5) years ) and 30 patients who underwent total repair of tetralogy of Fallot (TF) (a ge 12.1 (3.3) years) performed graded exercise testing. Exercise responses were compared with data obtained in 24 normal controls (age 11.4 (2.6) year s). Results-The slope of oxygen uptake versus exercise intensity averaged 1.50 (0.64) ml O-2/min(2)/kg in the patients with TGA and 1.68 (0.75) ml O-2/min (2)/kg after TF repair, both lower (p < 0.005) than in normal controls (2.4 2 (0.68) ml O-2/min(2)/kg). The lower siege of oxygen uptake was correlated with a subnormal value for ventilatory anaerobic threshold, which averaged 78.0 (13.3)% of normal in TGA and 85.1 (10.6)% in TF. This was associated with a steeper slope (p = 0.001) of carbon dioxide output versus oxygen upt ake above the ventilatory anaerobic threshold in TGA (1.26 (0.20)) and TF ( 1.20 (0.18)) compared with the normal controls (1.05 (0.13)), and also a st eeper slope of ventilation versus carbon dioxide in TGA (47.0 (15.4)) and T F (41.5 (13.7)) than in the controls (30.3 (8.5)). Conclusions-Calculation of the steepness of the slope of oxygen uptake vers us exercise intensity is a valid measurement of oxygen flow to the exercisi ng tissues, which may be limited in congenital heart disease.