Changes in colloid osmotic pressure in patients undergoing cardiothoracic surgery.

Measurements of plasma colloid osmotic pressure (COP) were made during and for 24 h after normothermic continuous flow cardiopulmonary bypass (CPB) in 20 adults exposed to sudden haemodilution by a crystalloid pump prime solution. Haematocrit, albumin and total protein were also measured. Within 5 min of starting bypass the mean COP fell to 45% pre-bypass value and remained at this level until bypass was discontinued. Following bypass, the oncotic pressure rose rapidly during the first hour to 84% of the prebypass value, and then more slowly until it reached the pre-bypass value 24 h later. The trend seen for plasma proteins and haematocrit was similar but the fall was smaller, 65-70% of the pre-bypass values. This significant difference in fall between plasma protein concentration and directly measured COP is explained by their non-linear relationship. Three patients died soon after bypass. They all showed significantly lower values of COP than the mean for the group as a whole at some stage (P less than 0.05).