Diurnal variations and reproducibility of the N2 closing volume test in healthy subjects

To assess the diurnal variation of closing volume measurements, 11 non-smokers and 10 somkers, all healthy, were tested with the single-breath nitrogen test. In each subject, 3 satisfactory tracings were recorded at 9:a.m., 11:a.m., 1:p.m., 3:p.m. and 5:p.m. on each of two consecutive days. Duplicate copies of the tracings were read in radom order by two independent observers. The "best+ and the mean values of closing volume to vital capacity ratio (CV/VC or phase 4/VC) and of the slppe of phase 3 were calculated. The study shows that: (1) the time of the day may be a source of variation of the closing volume measurements. Meals and cigarette smoke did not appear to be responsible for this diurnal variation, (2) values obtained with the "best" tracing method can, at least in some readers, give systematic differences with the mean of several traces, (3) individual variations in CV/VC and in the slope of phase 3 are the highest with the "best" trace analysis, and (4) the vlaues obtained by two independent readers may significantly differ. The differences observed between hours, although significant, were nevertheless small in magnitude and did not explain most of the variation of the measurements. Variations in trace aspect were small in some subjects. The reproducibility of the test was remarkable in them, at least when the junction of phase 4 with phase 3 was well defined. In other subjects, the coefficient of variation was high mainly because of varying curve shape and/or poorly defined departure of phase 4. This explains for a great part the intra-and interreader variations observed in this study.