CHEST-WALL MECHANICS IN SUSTAINED MICROGRAVITY

We assessed the effects of sustained weightlessness on chest wall mech anics in five astronauts who were studied before, during, and after th e 10-day Spacelab D-2 mission (n = 3) and the 180-day Euromir-95 missi on (n = 2). We measured flow and pressure at the mouth and rib cage an d abdominal volumes during resting breathing and during a relaxation m aneuver from midinspiratory capacity to functional residual capacity. Microgravity produced marked and consistent changes (Delta) in the con tribution of the abdomen to tidal volume [Delta Vab/(Delta Vab + Delta Vrc), where Vab is abdominal volume and Vrc is rib cage volume], whic h increased from 30.7 +/- 3.5 (SE)% at 1 G head-to-foot acceleration t o 58.3 +/- 5.7% at 0 G head-to-foot acceleration (P < 0.005). Values o f Delta Vab/(Delta Vab + Delta Vrc) did not change significantly durin g the 180 days of the Euromir mission, but in the two subjects Delta V ab/(Delta Vab + Delta Vrc) was greater on postflight day 1 than on sub sequent postflight days or preflight. In the two subjects who produced satisfactory relaxation maneuvers, the slope of the Konno-Mead plot d ecreased in microgravity; this decrease was entirely accounted for by an increase in abdominal compliance because rib cage compliance did no t change. These alterations are similar to those previously reported d uring short periods of weightlessness inside aircrafts flying paraboli c trajectories. They are also qualitatively similar to those observed on going from upright to supine posture; however, in contrast to micro gravity, such postural change reduces rib cage compliance.