PULMONARY DIFFUSING CAPACITY: A COMPARISON OF BREATH-HOLDING AND STEADY STATE METHODS USING CARBON MONOXIDE

Abstract

In 16 normal subjects and 125 patients with cardiac or respiratory disease, measurements of the diffusing capacity of the lungs for carbon monoxide (DL) were made by both the single breath and the steady state methods with end-tidal samples as a measure of alveolar CO. In 16 normal subjects at rest the steady state DL was about 75% of the single breath DL. The steady state DL increased with increasing tidal volume so that at maximal tidal volume the steady state DL was approximately the same as the single breath DL The increase of DL on exercise was greater than could be accounted for by the increased ventilation on exercise. An increase of respiratory rate without an increase in tidal volume did not increase DL. The patients with cardiac or respiratory disease were considered in three groups. Group 1 (74 patients with normal pulmonary gas mixing) the steady state DL was 74% of the single breath DL at rest. Group 2 (26 patients with impaired pulmonary gas mixing but no clinical evidence of emphysema) the steady state DL was 66% of the single breath figure. The difference between groups 1 and 2 may be due, in part, to the errors in end-tidal sampling in patients with uneven pulmonary ventilation. Group 3 (25 patients with emphysema) there was no relationship between the single breath and steady state DL. The reasons for this are discussed.