Time course variations of haemodynamics, plasma volume and microvascular fluid exchange following surface cooling: an experimental approach to accidental hypothermia.

OBJECTIVE: To describe how surface cooling influences fluid distribution, vascular capacity and haemodynamic variables.
METHODS: Seven anaesthetised pigs, following normothermic stabilization for 60 min, were cooled to 27.8+/-1.6 degrees C. Fluid balance, haemodynamics, colloid osmotic pressures (plasma/interstitial fluid), haematocrit [s-albumin/protein] were recorded and plasma volume measured together with tissue perfusion during normothermia, cooling and stable hypothermia (coloured microspheres). Fluid shifts and changes in albumin and protein masses were calculated. At the end tissue water content was assessed.
RESULTS: Haemodynamic variables changed with the start of cooling in parallel with a decreasing cardiac output. During hypothermia the haematocrit increased from 0.31+/-0.01 to 0.35+/-0.01 (P < 0.01). Plasma volume decreased from 1139.0+/-65.4 ml at start of cooling to 882.0+/-67.5 ml 3 h later (P < 0.05). In parallel the plasma albumin and protein masses decreased from 37.8+/-2.5 g and 54.6+/-4.0 g to 28.0+/-2.7 g (P < 0.05) and 41.2+/-4.1 g (P > 0.05), respectively. The main changes occurred 120-180 min after start of each experiment. In this period the fluid extravasation rate was elevated (P < 0.05) without influencing the colloid osmotic pressure of plasma/interstitial fluid. The increased fluid filtration was reflected by an increase in tissue water content.
CONCLUSION: Our results are in favour of a shift of plasma from circulation to the interstitial space during surface cooling. This conclusion is based on the parallel losses of fluid and proteins from circulation with unchanged colloid osmotic pressures (plasma/interstitial fluid). Inflammation may be involved.