Static respiratory muscle work during immersion with positive and negative respiratory loading. Taylor, Nigel A. S., and James B. Morrison. 1Department of Biomedical Science, University of Wollongong, Wollongong, NSW 2522, AUSTRALIA and 2School of Kinesiology, Simon Fraser University, Burnaby, B.C. V5A 1S6, CANADA.
APStracts 6:0273A, 1999.
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 manoeuvres 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 (PM; uncompensated); the pressure at the lung centroid (PLC); and at PLC [angstrom]a0.98 kPa. When breathing at pressures less than the PLC, subjects generally defended an expiratory reserve volume (ERV) greater than the immersed relaxation volume (VR(i)), minus residual volume, resulting in additional inspiratory muscle work. The resultant static, inspiratory muscle work, computed over a 1-litre tidal volume above the ERV, increased from 0.23 J.l-1, when breathing at PLC, to 0.83 J.l-1 at PLC -0.98 kPa (p<0.05), and to 1.79 J.l-1 at PM (p<0.05). Under the control state, and during the above experimental conditions, static expiratory work was minimal. When breathing at PLC +0.98 kPa, subjects adopted an ERV less than the VR(i), minus residual volume, resulting in 0.36 J.l-1 of expiratory muscle work. Thus, static inspiratory muscle work varied with respiratory loading, while PLC air supply minimised this work during upright immersion, restoring lung-tissue, chest-wall and static muscle work to levels obtained in the control state. 

Received 17 August 1998; accepted in final form 7 June 1999.
APS Manuscript Number A737-8.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1999 The American Physiological Society.
Published in APStracts on 25 June 1999