Variability of closing volume measurements in normal man.

Single breath oxygen measurements of closing volumes were performed in triplicate in 15 healthy nonsmoking young adults 3 times per day for 5 consecutive days. The purpose of the investigation was to determine the degree of variability in this test of lung function and, if possible, to specify its cause. The results demonstrated that sizable differences in the absolute volume for closing volume and for closing volume as a percentage of vital capacity can be present from trial to trial in any given person even though the technical aspects of the procedure are seemingly highly standardized. The reasons for this were found to be a combination of (1) the inherent variation in the expired volume that marked the onset of airway closure, (2) reader difficulties in detecting the onset of phase IV, and (3) variations in the expired vital capacity due to incomplete filling and/or emptying of the subject's lungs. The differences were not due to daily or diurnal rhythms or to a training effect.