Effect of central vascular engorgement and immersion on various lung volumes.

The purpose of this study was to determine the relative contribution of central vascular engorgement (CVE) and increased hydrostatic pressure on various lung volumes during head-out immersion in water. Residual volume (RV) and vital capacity (VC) were determined on 12 male volunteers under three randomly assigned conditions: control, CVE, and immersion. CVE was produced via G-suit inflation. There were significant (P less than 0.01) mean decreases, compared with the control value, of 4.9% (280 ml) and 5.9% (340 ml) in VC during CVE and immersion, respectively. RV was not significantly changed across the three conditions. It was concluded that more than 80% of the decrease in VC during immersion can be attributed to CVE. However, the mechanism by which CVE decreased VC is still unclear. In addition, these data suggest that RV is relatively insensitive to the increase in CVE normally associated with immersion. Therefore, during immersion, RV is not simply the result of the balance of these opposing forces (i.e., CVE and hydrostatic pressure), as previously suggested.