Estimation of total body fluid shifts between plasma and interstitium in man during extracorporeal circulation

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Abstract
Fluid transport between plasma and interstitium during extracorporeal circulation was studied in seven patients undergoing aortocoronary bypass grafting. The absolute shifts in plasma volume during hypothermia were determined as the difference between input and loss of fluid and the changes in blood volume. The change in haemoglobin concentration due to acute haemodilution when starting extracorporeal circulation was used to calculate the absolute blood and plasma volume. The Starling equation for exchange across the capillary wall was used to describe fluid shifts. The total fluid filtered during the 60‐ to 90‐min period of extracorporeal circulation averaged 34.1 ± 11.1 (s.d.) ml/min. The total body filtration coefficient from the Starling relationship averaged 0.046 ± 0.012 ml/kg ± mmHg ± min (0.354 ± 0.092 ml/kg ± kPa ± min). Haemodilution, reducing colloid osmotic pressure in plasma (COPP) by approximately 10 mmHg (1.3 kPa) will result in a loss of plasma fluid of around 2 1 per hour. When corrected for lower fluid viscosity due to hypothermia during extracorporeal circulation, CFC would be about 40% higher, and a filtered volume of nearly 31 in a normothermic 70‐kg person would be expected. Crystalloid haemodilution for shorter periods of time does not produce excessive oedema and thus may be well tolerated.