Static respiratory muscle work during immersion with positive and negative respiratory loading

1 Department of Biomedical Science, University of Wollongong, Wollongong, New South Wales 2522, Australia; and 2 School of Kinesiology, Simon Fraser University, Burnaby, British Columbia Canada, V5A 1S6 Upright immersion imposes a pressure imbalance across the thorax. This study examined the effects of air-delivery pressure on inspiratory muscle work during upright immersion. Eight subjects performed respiratory pressure-volume relaxation maneuvers while seated in air (control) and during immersion. Hydrostatic, respiratory elastic (lung and chest wall), and resultant static respiratory muscle work components were computed. During immersion, the effects of four air-delivery pressures were evaluated: mouth pressure (uncompensated); the pressure at the lung centroid (P L ,c); and at P L ,c ±0.98 kPa. When breathing at pressures less than the P L ,c, subjects generally defended an expiratory reserve volume (ERV) greater than the immersed relaxation volume, minus residual volume, resulting in additional inspiratory muscle work. The resultant static inspiratory muscle work, computed over a 1-liter tidal volume above the ERV, increased from 0.23 J · l 1 , when subjects were breathing at P L ,c, to 0.83 J · l 1 at P L ,c 0.98 kPa ( P