['A high slope of the ventilation vs. carbon dioxide relationship (VE/VCO₂ slope) during incremental exercise has been reported in several congenital heart disease (CHD) types, but it is not clear whether the main cause of high VE/VCO₂ slope is excessive ventilation or reduced perfusion.', 'We studied 169 adolescent and adult patients with repaired, noncyanotic CHD, divided into 2 groups according to VE/VCO₂ slope %predicted values (≤120 and >120), and 15 age- and sex-matched normals. VCO₂/VE max and VO₂/VE max were considered proxies of the perfusion/ventilation relationship, with VCO₂ and VO₂ as indirect descriptors of cardiac output.', 'VCO₂/VE max was significantly and inversely related to VE/VCO₂ slope (r=-0.73, p<0.0001), and higher in normals and ≤120 than in >120 (39.6 ± 7.7, 36.1 ± 5.3 and 28.5 ± 4.1, respectively, p<0.0001). Similarly, VCO₂ at VCO₂/VE max was higher in normals and ≤120 than in >120 (1701 ± 474, 1480 ± 492 and 1169 ± 388 ml/min, respectively, p<0.0001), whereas ventilation at VCO₂/VE max showed no changes (43 ± 8, 41 ± 12, 41 ± 11 and 41 ± 9l/min, respectively, p=0.82) between groups. Thus, differences in VCO₂/VE max and VE/VCO₂ slope between groups were due mostly to changes in VCO₂, i.e. in cardiac output, rather than ventilation. The same behavior was observed for VO₂/VE max.'] Is hemodynamic , not ventilatory , inefficiency associated with high VE/VCO2 slope in repaired , noncyanotic congenital heart disease?