Effects of immersion water temperature on whole-body fluid distribution in humans

Aim: In this study, we quantified acute changes in the intracellular and extracellular fluid compartments during upright neutral- and cold-water immersion. We hypothesized that, during short-term cold immersion, fluid shifts would be wholly restricted to the extracellular space. Methods: Seven males were immersed 30 days apart: control (33.3 degrees SD 0.6 degreesC); and cold (18.1 degrees SD 0.3 degreesC). Posture was controlled for 4 h prior to a 60-min seated immersion. Results: Significant reductions in terminal oesophageal (36.9 degrees +/- 0.1 degrees-36.3 degrees +/- 0.1 degreesC) and mean skin temperatures (30.3 degrees +/- 0.3 degrees-23.0 degrees +/- 0.3 degreesC) were observed during the cold, but not the control immersion. Both immersions elicited a reduction in intracellular fluid [20.17 +/- 6.02 mL kg(-1) (control) vs. 22.72 +/- 9.90 mL kg(-1)], while total body water (TBW) remained stable. However, significant plasma volume (PV) divergence was apparent between the trials at 60 min [12.5 +/- 1.0% (control) vs. 6.1 +/- 3.1%; P < 0.05], along with a significant haemodilution in the control state (P < 0.05). Plasma atrial natriuretic peptide concentration increased from 18.0 +/- 1.6 to 58.7 +/- 15.1 ng L-1 (P < 0.05) during cold immersion, consistent with its role in PV regulation. We observed that, regardless of the direction of the PV change, both upright immersions elicited reductions in intracellular fluid. Conclusion: These observations have two implications. First, one cannot assume that PV changes reflect those of the entire extracellular compartment. Second, since immersion also increases interstitial fluid pressure, fluid leaving the interstitium must have been rapidly replaced by intracellular water.