Effect of gravity and posture on lung mechanics.

The volume-pressure relationship of the lung was studied in six subjects on changing the gravity vector during parabolic flights and body posture. Lung recoil pressure decreased by ∼2.7 cmH 2 O going from 1 to 0 vertical acceleration (G z ), whereas it increased by ∼3.5 cmH 2 O in 30° tilted head-up and supine postures. No substantial change was found going from 1 to 1.8 G z . Matching the changes in volume-pressure relationships of the lung and chest wall (previous data), results in a decrease in functional respiratory capacity of ∼580 ml at 0 G z relative to 1 G z and of ∼1,200 ml going to supine posture. Microgravity causes a decrease in lung and chest wall recoil pressures as it removes most of the distortion of lung parenchyma and thorax induced by changing gravity field and/or posture. Hypergravity does not greatly affect respiratory mechanics, suggesting that mechanical distortion is close to maximum already at 1 G z . The end-expiratory volume during quiet breathing corresponds to the mechanical functional residual capacity in each condition.