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We studied the respiratory output in five subjects exposed to parabolic flights [gravity vector 1, 1.8 and 0 gravity vector in the craniocaudal direction (Gz)] and when switching from sitting to supine (legs bent at the knees). Despite differences in total respiratory compliance (highest at 0 Gz and in supine and minimum at 1.8 Gz), no significant changes in elastic inspiratory work were observed in the various conditions, except when comparing 1.8 Gz with 1 Gz (subjects were in the seated position in all circumstances), although the elastic work had an inverse relationship with total respiratory compliance that was highest at 0 Gz and in supine posture and minimum at 1.8 Gz. Relative to 1 Gz, lung resistance (airways plus lung tissue) increased significantly by 52% in the supine but slightly decreased at 0 Gz. We calculated, for each condition, the tidal volume changes based on the energy available in the preceding phase and concluded that an increase in inspiratory muscle output occurs when respiratory load increases (e.g., going from 0 to 1.8 Gz), whereas a decrease occurs in the opposite case (e.g., from 1.8 to 0 Gz). Despite these immediate changes, ventilation increased, going to 1.8 and 0 Gz (up to ≈23%), reflecting an increase in mean inspiratory flow rate, tidal volume, and respiratory frequency, while ventilation decreased (approximately -14%), shifting to supine posture (transition time ∼15 s). These data suggest a remarkable feature in the mechanical arrangement of the respiratory system such that it can maintain the ventilatory output with small changes in inspiratory muscle work in face of considerable changes in configuration and mechanical properties. [ABSTRACT FROM AUTHOR]

High-frequency ventilation was investigated in a model of the central airways of a human lung cast enclosed in a spherical cavity (alveolí). The experiments were performed at high frequencies (5, 10, 15 and 20 Hz) and tidal volumes (50, 90 and 120 cm) less than the anatomical dead space volume. The efficiency of gas mixing in the model was assessed by measuring the decay of carbon dioxide concentration in the sphere during high-frequency mechanical ventilation. The experimental results were then compared with those obtained from simulations on a theoretical model based on a diffusional resistance concept which could take account of either turbulent, steady laminar or oscillatory laminar dispersion coefficients. Our results showed that the Taylor turbulent dispersion mechanism had a significant effect and could be the principal factor for the efficiency of gas mixing during high-frequency ventilation at small tidal volumes. [ABSTRACT FROM AUTHOR]