Estimating steady-state Dl_O2 with nonlinear dissociation curves and Va/Q̇ inequality

A Dl_O2 estimate which accounts for the nonlinearity of the oxygen dissociation curve using the Kelman blood gas routines is presented here. The simultaneous differential equations that describe O₂ and CO₂ diffusion between alveolar gas and pulmonary capillary blood in lung compartments with different V̇a/Q̇ ratios were solved numerically with a Runge-Kutta algorithm. These integrated estimates were compared to Dl_O2 estimates the assume the oxygen dissociation curve is linear. In 140 gas exchange data sets from 18 healthy male subjects previously collected at rest and during exercise it was found that Dl_O2 estimates based on linear dissociation curves exceeded integrated Dl_O2 estimates by 14, 31, and 55 percent when the Pi_O2 was 80, 100, and 148 Torr, respectively. We conclude that the linear approximation is accurate when Pi_O2 is less than 100 Torr but that comparisons of Dl_O2 estimates at different levels of inspired oxygen must allow for the difference in curvature of the oxygen dissociation curve as a function of Pi_O2 .