Variability of the closing volume measurement in normal subjects.

The repeatablilty of the measurement of closing volume (nitrogen method) in healthy, normal subjects was investigated. At least 7 consecutive measurements at 5-min intervals were made in each of 22 subjects. The mean coefficient of variation for the absolute volume of phase IV was 36.1 per cent and for the percentage ratio of phase IV to vital capacity, 36.1 per cent. Repeated measurements by the same observer of the same records on various days did not differ significantly (P greater than 0.5). There was a significant difference (P less than 0.05) between 2 observers measuring the same records, but there was no significant difference between the two observers in the variance of the closing variability in the same subject, it may be necessary to perform several closing volume maneuvers in each subject to establish his or her range before changes in the closing volume can be interpreted.