A tidal breathing model of the inert gas sinewave technique for inhomogeneous lungs

The tidal breathing model conservation of mass equations for the sinewave technique have been described for a homogeneous alveolar compartment by Gavaghan and Hahn, 1996 [Gavaghan, D.J., Hahn, C.E.W., 1996. A tidal breathing model of the forced inspired gas sinewave technique. Respir. Physiol. 106, 209-221]. We develop these equations first to a multi-discrete alveolar compartment lung model and then to a lung model with a continuous distribution of volume, ventilation and perfusion. The effect on the output parameters of a multi-compartment model is discussed, and the results are compared to those derived from the conventional continuous-ventilation model. Using the barely soluble gas argon as the tracer gas, an empirical index of alveolar inhomogeneity is presented which uses the end-expired and mixed-expired partial pressures on each breath. This index distinguishes between a narrow unimodal distribution of ventilation-volume, a wide unimodal distribution of ventilation-volume and a bimodal distribution of ventilation-volume. By using Monte Carlo simulations, this index is shown to be stable to experimental error of realistic magnitude.