Simulation of expired nitrogen (N2) after inhaling argon (Ar) at 1 atmosphere

Abstract

Expired Ar and N2, after inhaling Ar, were obtained in 1965 and published in 1991 (Cruz. Respir. Physiol. 86:1,1991). However, only Ar data was used to develop a model of inhaled inert gases. Because expired N2 behaved differently than Ar, especially, after holding the breath at 10s and 20s, at the end of inspiration, N2 was ignored. After introducing a new equation where FAr is a function of lung volume and time (unpublished), I was able to understand how Ar was inhaled and distributed in the alveoli. I learned that the expired Ar was generated by uneven flows from parallel regions of the lung. Now, I'm presenting the simulated expirogram of N2 (Figure, heavy line) after inhaling in 2s, 1.9L of 21% O2, 79% Ar from FRC (4L) and exhaling in 3s to RV (1.4L). New concepts were introduced in mixing the inhaled gas with the expiratory reserve volume (ERV), by the mechanism of convection‐diffusion during inspiration (Caucha et al. Am. J. Respir. Crit. Care Med. 183: A5180, 2011). These concepts are here applied. At FRC, the amount of alveolar N2 (3.245L) was distributed from apex‐to‐base of the lung following West lung model. The amount of N2 in the ERV was 2.114L, which was mixed with the amount of Ar that reached the alveoli (1.165L). The simulated expired N2 (thin line) was insufficient to reach experimental N2 (symbols) because of the assumptions of the expired Ar used: VD=425ml and k value (50L−1) of the exponential equation, the latter produced a progressive decline of Ar from VD to ERV. In order to fit the experimental N2 data, it was necessary to reduce the VD to 370ml and k to 40L−1. In addition, it was added 101ml of alveolar N2 present in the RV that diffused to ERV during Ar inhalation. It is concluded that the N2 expirogram is not the mirror image of the Ar expirogram.Supported in part by the Centro de Enseñanza, Investigación y Servicios (CEIS).