EFFECTS OF AGE ON CLOSING VOLUME DURING HEAD-OUT WATER IMMERSION

Previous studies during head-out immersion have shown closing volume ( CV) to either increase or remain unchanged. We hypothesized that these inconclusive results might be related to differences in the ages of t he subjects tested. To elucidate this we studied single-breath argon w ashout tests performed by a younger group (n=8, age 23-26) and an olde r group (n=8, age 40-54) of males every 5 min during 30 min of seated, thermoneutral head-out immersion. No temporal changes in CV during im mersion were observed in either group, therefore values within each gr oup during immersion were combined. In the younger group, CV increased 77% (dry, 0.26+/-0.11 L; wet, 0.46+/-0.10 L; Delta=0.20 L) (P<0.001) but remained less than the tidal breathing range upper limit [expirato ry reserve volume + tidal volume (ERV + VT)]. In the older group, CV i ncreased 34% (dry, 0.83+/-0.29 L; wet, 1.11+/-0.19 L; Delta=0.28 L) (P <0.05) and was not different from ERV + VT, The absolute increase in C V during immersion did not differ between the groups. ERV decreased du ring immersion in both groups and was lower in the older than younger group (P<0.001). Alveolar plateau (phase III) slope became steeper in the younger (P<0.001) but not in the older group. We conclude that dur ing immersion: (1) The absolute increase in CV is independent of age, and (2) in subjects over 40, CV approaches the highest lung volumes re ached during tidal breathing. Older subjects may be more likely to dev elop atelectasis during immersion and/or under other conditions in whi ch CV is increased and tidal breathing occurs at low lung volumes.