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.