Variations of respiratory mechanics in the rat during the day

We applied the end-inflation occlusion method to measure resistive and elastic properties of the rat's respiratory system at two different hours of the day, 8.00 a.m. and 4.00 p.m. Contextually lung hysteresis surface areas were also measured. We found a significant reduction of resistive pressure necessary for the movement of air in the airways and for the movement of lung and chest wall tissue, hence of respiratory and lung resistances, in the afternoon with respect to morning hours (Rrs, min 0.037 vs 0.117 cm H(2)O/ml sec(-1), p = 0.008, Rl, min 0.021 vs 0.068 cm H(2)O/ml sec(-1), p = 0.001). The areas of hysteresis were significantly higher in the afternoon compared to the morning (3.71 vs 2.51 cm H(2)Oml, p = 0.039). No other significant differences in respiratory mechanical parameters were found. Thus we show in the rat that airways and lung and chest wall resistances exhibit variations during the day, confirming previously reported data describing a circadian rhythm of resistance in other animal species. Hysteretic behaviour of the lungs also changes during the day, suggesting diurnal variations of alveolar surfactant activity.