Evaluation of Four Methods for Estimating Breath-by-Breath Exchange of O2 and CO2

Four methods which have been used to estimate breath-by-breath gas exchange during spontaneous breathing were compared in vivo and in vitro. The first method estimates net gas exchange by taking the difference between the inspired volume and expired volume of the gas. The second and third methods correct the first estimate by accounting for the intracycle difference between the total inspired volume and total expired volume. The fourth method not only corrects the first estimate for inspired and expired volume differences but also corrects for storage of the gas in the lung from one breath to the next. We found no significant difference among the estimates for O2 and CO2 gas exchange. However, the first and fourth estimates had more variability than the second and third.