Quantitative analysis of fluid balance during abdominal surgery

Surgical injury causes acute sequestration of interstitial fluid in injured tissue. Fluid sequestration treated with IV fluid administration can lead to postoperative complications related to excessive intravascular volume. Quantitative prediction of interstitial fluid sequestration may foster a better understanding of the relationship between fluid administered and the resulting balance between intra- and extravascular fluid.

We developed a mathematical model describing the dynamic distribution and transport of fluid and proteins with the goal of quantifying the balance of fluid between intra- and extravascular compartments. Fluid volume changes in the plasma, interstitial and urine compartments were calculated for a simulated 4 h abdominal surgery in a 70 kg male. To validate the model, we compared the results obtained with those measured by segmental bioelectrical impedance on 30 patients undergoing elective abdominal surgery.

The model predicted that, compared to the normal state, surgical injury would result in the sequestration of 705 mL of interstitial fluid in injured tissue, whereas plasma volume would undergo a 356 mL decrease. During surgery, it was not possible to obtain a normal plasma volume, even with fluid replacement at a rate of almost 20 mL x kg(-1) x h(-1). Bias and limit of agreement on interstitial fluid volume changes in body segments between bioelectrical impedance and model prediction were -131 and 325 mL, respectively for limbs, and -157 and 834 mL for the trunk.

The model shows that increasing the fluid replacement rate above 10 mL x kg(-1) x h(-1) does not have the desired effect on plasma volume but instead increases the interstitial volume.