New method to calculate the N-2 evolution from mixed venous blood during the N-2 washout

To modelthe normalized phase III slope (Sn) from N-2 expirograms of the mul tibreath N-2 washout is a challenge to researchers. Experimental measuremen ts show that Sn increases with the number of breaths. Previously, we predic ted Sn by setting the concentration (atm) of mixed venous blood (Fbi, N-2) to a constant value of 0.3 after the fifth breath to calculate the amount o f N-2 transferred from the blood to the alveoli. As a consequence, the pred icted curve of the Sn values showed a maximum before the quasi-steady state was reached. In this paper, we present a way of calculating the amount of N2 transferred from the blood to the alveoli by setting Fbi, N-2 in the fol lowing way: In the first six breaths F-bi, N-2 is kept constant at the init ial value of 0.8 because circulation time needs at least 30 s to alter it. Thereafter, a single exponential function with respect to the number of bre aths is used: F-bi = 0.8 exp[0.112(6-n)], in which n is the breath number. The predicted Sn values were compared with experimental data from the liter ature. The assumption of an exponential decay in the N-2 evolved from mixed venous blood is important in determining the shape of the Sn curve but new experimental data are needed to determine the validity of the model. We co ncluded that this new approach to calculate the N-2 evolution from the bloo d is more meaningful physiologically. (C) 2001 Biomedical Engineering Socie ty.