Figure 1 .
Typical example of the slope of carbon dioxide versus oxygen uptake during graded treadmill exercise in a 14 year old boy with atrial switch operation for transposition of the great arteries. The data represent average values for 10 second intervals. S1 (= 0.79) is the slope of carbon dioxide versus oxygen uptake below the ventilatory anaerobic threshold, and S2 (= 1.23) is the slope of carbon dioxide versus oxygen uptake above the ventilatory threshold up to a heart rate of 170 beats/min. V̇CO2 = carbon dioxide output, expressed as ml/min/kg; V̇O2 = oxygen uptake, expressed as ml/min/kg.