Static lung load and posture effects on pulmonary mechanics and comfort in underwater exercise

Static lung load (SLL), or transrespiratory pressure gradient, imposed by underwater breathing apparatus can affect breathing comfort and mechanics, especially during exertion. We examined the effects of body position and SLL on two factors known to affect or limit exertion: a) tidal flow-volume limitation, i.e., the percentage of the tidal volume that meets the boundary of the maximum expiratory flow-volume curve; and b) breathing discomfort. Eight healthy male scuba divers (28 +/- 4 yr) performed cycle ergometry to exhaustion during immersion in each of four combinations of body position and SLL: upright, prone, +10 cmH2O, -10 cmH2O. SLL was referenced to the sternal notch. Tidal flow-volume limitation was significantly greater with the negative SLL (P less than 0.05). In the prone position, higher expiratory flows were achieved (P less than 0.01) and flow limitation was not significantly increased. Respiratory discomfort was quantified with a psychophysical rating scale and increased significantly as exercise intensity increased (P less than 0.01). No effect of posture or SLL on discomfort was found. We conclude that, although respiratory comfort is unaffected, positive static lung loading and the prone body position minimize adverse changes in respiratory mechanics during exercise in immersion.