Static respiratory muscle work during immersion with positive and negativerespiratory loading

Upright immersion imposes a pressure imbalance across the thorax. This stud y examined the effects of air-delivery pressure on inspiratory muscle work during upright immersion. Eight subjects performed respiratory pressure-vol ume relaxation maneuvers while seated in air (control) and during immersion . Hydrostatic, respiratory elastic (lung and chest wall), and resultant sta tic respiratory muscle work components were computed. During immersion, the effects of four air-delivery pressures were evaluated: mouth pressure (unc ompensated); the pressure at the lung centroid (PL,c); and at PL,c +/-0.98 kPa. When breathing at pressures less than the PL,c, subjects generally def ended an expiratory reserve volume (ERV) greater than the immersed relaxati on volume, minus residual volume, resulting in additional inspiratory muscl e work. The resultant static inspiratory muscle work, computed over a 1-lit er tidal volume above the ERV, increased from 0.23 J.l(-1), when subjects w ere breathing at PL,c, to 0.83 J.l(-1) at PL,c -0.98 kPa (P < 0.05), and to 1.79 J.l(-1) at mouth pressure (P < 0.05). Under the control state, and du ring the above experimental conditions, static expiratory work was minimal. When breathing at PL,c +0.98 kPa, subjects adopted an ERV less than the im mersed relaxation volume, minus residual volume, resulting in 0.36 J.l(-1) of expiratory muscle work. Thus static inspiratory muscle work varied with respiratory loading, whereas PL,c air supply minimized this work during upr ight immersion, restoring lung-tissue, chest-wall, and static muscle work t o levels obtained in the control state.