Comparison of single breath and steady state methods for the measurement of pulmonary diffusing capacity for carbon monoxide in non-homogeneous lungs.

The effect of non-uniformity in the distribution of ventilation and diffusion on the apparent pulmonary diffusing capacity measured by the single breath and steady state techniques has been assessed on the basis of a two-compartmental lung model. Both the methods give reasonable estimates of the true diffusing capacity provided the fast compartment has a greater KCO. The reverse of this results in gross underestimation of the diffusing capacity by both methods. There is a fairly good correlation between the single breath and the steady state results both in theory as well as in practice.