THE EFFECTS OF BREATHING PATTERN IN THE ESTIMATION OF PULMONARY DIFFUSING CAPACITY

Abstract

The diffusing capacity of the homogeneous lung for carbon monoxide differs when measured by the steady state and single breath methods. Jones and Meade [1961] showed that measured single breath DLCOSB is too large by about 20% if time delays caused by breathing pattern effects are ignored. Here it is shown that measured DCLOSS using the Bohr method is too small by as much as 25% because of similar breathing pattern effects. Steady state and single breath experiments on normal subjects produce close agreement between adjusted values, suggesting that the homogeneous lung at rest is characterized by the same ‘actual’ DLCO parameter in both methods. The adjustment to DLCOSS depends on two important features of the periodic breathing pattern: (1) an ‘effective’ breath‐holding time and (2) the diffusing capacity per unit volume averaged over a cycle. These correspond to T and k of the single breath method. The theoretical conclusions are based on steady state analysis of a homogeneous lung model. The dead space has fixed volume and plug flow. A lumped diffusion parameter DLCO characterizes the alveolar compartment. Alveolar CO concentration and volume are time dependent. Breathing patterns which were modelled were the square‐wave (the periodic equivalent of the single breath method), the ramp and the ramp with a pause at the end of expiration. The latter closely approximates spontaneous breathing patterns which were measured. © 1974 The Physiological Society