Alveolar-arterial gas tension differences during progressive exercise in patients after the Fontan operation

We investigated alveolar-arterial gas tension differences as a measurement of ventilatory impairment during exercise in patients who had undergone the Fontan operation. The ventilatory response to exercise in 13 operated patients was compared with that of 11 control subjects. The difference between end-tidal and arterial oxygen tension (P(ET-a)DO2) and between arterial and end-tidal carbon dioxide tension (P(a-ET)DCO2) as well as the physiologic dead space-tidal volume ratio were calculated during progressive treadmill exercise testing. In the Fontan group, P(ET-a)DO2 and P(a-ET)DCO2 were significantly higher than in control subjects and increased in parallel during the study period. The physiologic dead space-tidal volume ratio was higher in the Fontan group than in the control group, and the difference in these ratios between the values obtained using end-tidal carbon dioxide tension and those using arterial carbon dioxide tension correlated well with P(a-ET)DCO2 (r = -0.79 to -0.98, p < 0.001). The physiologic dead space-tidal volume ratio during exercise was significantly higher in patients with low exercise capacities than in those with high exercise capacities (p < 0.05). The alveolar-arterial gas tension differences during exercise were greater in Fontan patients than in control subjects. We conclude that the size of the physiologic dead space can be evaluated from measurements of arterial carbon dioxide tension and is correlated with impaired exercise capacity after the Fontan operation.