Closing volumes in man immersed to the neck in water

Closing volumes (CV), along with residual volume (RV), vital capacity (VC), along with residual volume (RV), vital capacity (VC), and expiratory reserve volume (ERV) were determined in 10 subjects in the dry and while immersed to the neck in water. Closing volumes during immersion increased 41.3% (P less than 0.001) over dry values while RV decreased 9.35% (P less than 0.001) and VC decreased 9.94% (P less than 0.001). The large decrease of 71.3% (P less than 0.001) in ERV resulted in the impingement of closing capacity (CV + RV) on the tidal volume in 9 out of 10 subjects. We interpret this to mean that airway closure occurs during tidal ventilation in immersed subjects and may result in impaired gas exchange. When tourniquets were applied to all four limbs during immersion closing volumes increased only 32.1%, but increased to 64.3% when they were removed. If engorgement of peribronchial vessels predisposes airways to collapse, a reduction of plasma volume during an extended period of immersion might lessen this possibility. In a series of long term (2.5-h) immersion experiments where moderate reductions (-10 to -7%y in plasma volume were observed, we found, however, no correlative changes in closing volume.