Lung volumes measurements were performed on 30 healthy young volunteers of both sexes at three different times of the day (8 a.m., 4 p.m., 12 a.m.) on the same day. They included total lung capacity, functional residual capacity, vital capacity, residual volume, inspiratory and expiratory reserve volumes, and inspiratory capacity, and were technically performed according to recently stated international standards.

Volume and capacities mean values generally resulted comprised within the predicted ranges, which take into account sex, age and height of the subjects.

As predicted, the mean value of inspiratory reserve volume resulted higher than mean expiratory reserve volume. No significant daily variations were observed for the mean values of vital capacity, inspiratory reserve volume and inspiratory capacity. Expiratory reserve volume mean value at 4 p.m. resulted higher than at 12 a.m. (p<0.05, Student t-test for paired data).

We found a significant increment of the mean value of the residual volume at 4 p.m. with respect to 12 a.m., and the increment with respect to 8 a.m. resulted almost significant (p=0.08), probably due to a relative lack of statistical power. These differences were of the order of 10%, and were associated with a significant increase of lung capacities including residual volume, namely total lung and functional residual capacities, both of which resulted higher at 4 p.m. with respect to both 8 and 12 a.m. (p<0.05).

The possible mechanisms explaining these findings are addressed, including daily variations of respiratory system compliance, alveolar surfactant activity, and abdominal wall and diaphragmatic muscle tonic activity.