The importance of plasma colloid osmotic pressure for interstitial fluid volume and fluid balance after elective abdominal vascular surgery.

The relationships between plasma colloid osmotic pressure (COPp) and interstitial fluid volume (IFV) as well as postoperative fluid balance were investigated in a prospective study involving 53 patients undergoing elective abdominal aortic reconstruction. The patients were divided into four groups according to pre- and postoperative blood replacement and fluid therapy programs whereby a continuum of postoperative COPp-values between 33 and 16 mmHg was obtained. Measurements were done before the operation and on days 1 and 4 after surgery. After surgery, COPp below 20 mmHg led to increased IFV. On day 1, COPp was linearly correlated to the total amount of fluid retained during the day of operation. A positive fluid balance of 3 L on this day ensured unchanged extracellular fluid volume (ECV). Of the 3 L, 1.5 L was insensible water loss and 1.5 L had moved into the cells. On day 4 after surgery, COPp below 22 mmHg was associated with increased plasma volume. The authors suggest that COPp be maintained above 20 mmHg after major surgery, and positive fluid balance should not exceed 5 L during the day of operation.