The simultaneous comparison of acetylene or carbon dioxide flux as a measure of effective pulmonary blood flow in children

Both acetylene (Ac) and carbon dioxide can be used to measure effective pulmonary blood flow (Q'eff) noninvasively. They are safe and reasonably accurate in adults during rest and exercise, but there have been no simultaneous comparisons in children. One hundred and six healthy children (55 males and 51 females, aged 8-17 yrs) were studied using an Innovision quadrupole mass spectrometer. They all underwent five rebreathing manoeuvres at rest, and then single measurements were again taken after 9 min of bicycle exercise. Mixed venous CO2 levels were calculated either by a linear (L) or curvilinear (C) extrapolation method. At rest, the coefficients of variation for Q'eff were Ac 8%, L 20%, and C 16% (p<0.001). The median resting values were: Ac 3.2 (95% confidence interval (95% CI) 3.1-3.4) L 5.1 (95 % CI 4.6-5.4) and C 4.7 (95 % CI 4.3-5.1) L x min(-1) x m(-2), (p<0.001). Compared to Ac, only 14 and 17% of L and C values, respectively, were +/-0.5 L x min(-1) x m(-2), whilst 41 and 29%, respectively were more than +/-2 L x min(-1) x m(-2). During exercise, median values were: Ac 6.7 (95% CI 6.3-7.0); L 8.0 (95% CI 7.3-8.4); and C 7.2 (95% CI 6.5-7.9) L x min(-1) x m(-2). L was significantly greater than C (p<0.001), but C was similar to Ac (p=0.06). More than 50% of L and C values could not be calculated for various reasons, whereas all 106 Ac values could be calculated. Neither carbon dioxide method is sufficiently reliable to be used in children in a clinical setting. Acetylene was safe, reliable, accurate and preferred.