Mechanisms compensating for altered resistance to respiration during muscular exercise in man

Alterations in respiratory parameters following the substitution of a helium-oxygen (He−O₂) or sulfur hexafluoride-oxygen (SF₆−O₂) mixture for air were analyzed during the first 10 respiratory cycles in human volunteers exposed to either of these mixtures for 3 min at rest and during forced respiration. Both at rest and during moderate physical exercise neither the volume of pulrnonary ventilation nor the partial carbon dioxide pressure differed significantly in the subjects breathing air, He−O₂, or SF₆−O₂. When the He−O₂ mixture was substituted for air, the forces developed by the inspiratory muscles, the work of breathing, the activity of the parasternal intercostal muscles, and the central inspiratory activity were all reduced, whereas substitution of the SF₆−O₂ mixture for air led to significant increases in these four parameters. It is concluded that compensatory responses of the respiratory system to altered density of the gaseous medium develop on the basis of the afferent impulse traffic from mechanoreceptors of the lungs and respiratory muscles and also on account of segmental reflexes and intrinsic properties of the muscle fibers themselves. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 120, N ^o 9, pp. 247–251, September, 1995 Presented by A. D. Ado, Member of the Russian Academy of Medical Sciences