THE MEASUREMENT OF THE PULMONARY DIFFUSING CAPACITY IN THE PRESENCE OF LUNG DISEASE

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Abstract
Data collected during estimates of the pulmonary diffusing capacity on 151 patients have been analysed to clarify the validity of the end tidal sampling method of measuring the mean alveolar CO concentration in lung disease. During exercise, there is good correspondence between the Dco calculated from such samples, and the Dco estimated by using the Bohr equation to compute a value for the mean alveolar CO concentration. On exercise, measurement of the rate of uptake of CO alone would be sensitive enough to use in the assessment of abnormality in pulmonary diffusion. This measurement would not be adequate under resting conditions. Comparisons between the Dco calculated from the arterial pCO2 and the Deo calculated from the measured end tidal CO concentration show that the former is probably measuring a "highest likely" Deo and the latter probably the Deo of predominantly ventilated lung. A comparison of results obtained with the end tidal sampling technique in patients with asthma and emphysema indicates that the use of CO in these two conditions enables an estimate to be made of the relative normality of the lung parenchyma. In spite of the many contemporary uncertainties in the measurement of the pulmonary diffusing capacity by any technique, the general form of reported results in 3 different lung diseases suggests that all methods are giving an approximately similar range of results.