1. The effects of acute intravenous infusion of 2 litres of saline/120 min on pulmonary capillary blood flow (Q̇c), diffusing capacity per unit of alveolar volume (DL/VA), functional residual capacity (FRC), and pulmonary tissue plus capillary blood volume (VTPC) were compared with the changes induced by water immersion to the neck for 4 h. Serial measurements were made at 30 min intervals in five normal subjects, utilizing a non-invasive rebreathing method with a gas mixture containing 0·5% acetylene, 0·3% C180, 10% He, 21% O2 and 68·2% N2.

2. Infusion of saline produced a rise in Q̇c which was similar to that induced by immersion. This increment in Q̇c persisted for the 3 h of observation after stopping the infusion, in contrast to the prompt decrease in Q̇c to pre-study values after cessation of immersion.

3. DL/VA was unaffected by saline administration in contrast to the marked and prompt increment induced by immersion.

4. Pulmonary tissue plus capillary blood volume was unchanged during both saline administration and immersion, suggesting that neither gradual saline administration nor immersion induces major extravasation of fluid into the pulmonary interstitial space.

5. The present data indicate that the ‘volume stimulus’ of immersion is similar to that of saline-induced extracellular fluid volume expansion in normal seated subjects. Immersion may be a preferred investigative approach for assessing the effects of volume expansion in subjects in whom rapid reversibility of the ‘volume stimulus’ is desirable.

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