Nature of the disturbance in the body fluid compartments during and after surgical operations.

Haematocrit and plasma protein concentrations have been measured in patients undergoing surgical operations in order to estimate relative changes in the volumes of the extracellular and plasma compartments. That these provide valid estimates of volume changes has been shown by applying them to patients known to have lost extracellular fluid. Further confirmation that changes in haematocrit, as indices of falls in plasma volume, are valid even after surgery is provided by measurements of red cell mass, using red cells labelled with 99Tcm. When these methods were applied to patients undergoing abdominal and non-abdominal operations, only the abdominal group showed a significant fall in plasma volume. This fall was caused not by a loss of crystalloid from the extracellular space as a whole, but by a loss of plasma; the magnitude of this loss correlated significantly with the duration of operation. These changes were also demonstrated using the radioisotope dilution indicators 125I (RISA) and 131I (RISA).