Daily variations in lung volume measurements in young healthy adults

Lung volume measurements were performed on 30 healthy young volunteers of both genders at three different times of the day (8 am, 4 pm, and 12 am). They included total lung capacity, functional residual capacity (FRC), vital capacity, residual volume (RV), inspiratory and expiratory reserve volumes, and inspiratory capacity, and were technically performed according to well-known, recently stated, international standards.We found statistical increments of the mean values of total lung capacity, FRC, and RV at 4 pm with respect to both 8 am and 12 am 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.