Effect of forward acceleration and negative pressure breathing on pulmonary diffusion

¹ Acceleration Section, Biophysics Branch, Aerospace Medical Laboratory, Wright Patterson Air Force Base, Ohio

Forward acceleration decreases lung volumes, resembling negative pressure breathing (NPB). At 4 g the relaxation pressure curve is shifted downward and to the right 15 mm Hg. Pulmonary gas exchange and diffusion capacity were measured in nine human subjects during NPB (–15 mm Hg) and forward acceleration (4 g). Pulmonary ventilation increased approximately 40% in each condition. The O₂ uptake increased with NPB (from 261 to 293 ml/min) and was slightly decreased or unchanged at 4 g. Carbon dioxide elimination increased in both experimental conditions. The apparent steady-state Dco was unchanged by NPB but decreased from an average control value of 21 to 12 ml/min mm Hg at 4 g. Since lung volumes are decreased by comparable amounts in both conditions, it is believed that the deleterious effects observed with forward acceleration are associated with the increased hydrostatic gradient from chest to back.

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(With the Technical Assistance of Justin Taylor)