Diurnal variation in pulmonary diffusing capacity for carbon monoxide.

Abstract

The pulmonary diffusing capacity for carbon monoxide (DLCO), measured in 24 normal subjects at 2-hr. intervals, fell progressively throughout the day at a rate of 1.2%/hr between 9.30 AM and 5.30PM, and at 2.2%/hr between 5.30 and 9.30 PM. This fall was apparently not caused by initial apprehension, practice in the measurement technique or ambient change, nor was it associated with maintenance of the erect posture since a comparable fall (1.65%/hr.) was shown between 10.00 AM and 4.00 PM by 5 subjects who lay in bed and were measured in the recumbent posture. The change in DLCO appears to be a diurnal rhythm resembling that in hematocrit and urinary catecholamine excretion.