The effect of gain in total body water on haemoglobin concentration and body weight following cardiac surgery.

Background: Haemodilution contributes to a low post-operative haemoglobin concentration in cardiac surgery patients. An assessment of the degree of haemodilution could contribute to the avoidance of red cell transfusion when such an act is based simply on a haemoglobin "transfusion trigger". We have recorded post-operative change in total body water along with body weight to assess the impact of haemodilution on haemoglobin concentration.

Methods: Total body water, measured by bio-electrical impedance analysis, haemoglobin and body weight were measured pre-operatively and on the 1st, 3rd, 5th and 10th post-operative days. The percentage peri-operative change in all three variables was used to examine the paired associations.

Results: Total body water and body weight underwent a fall from day 1, with both variables significantly associated up until day 10. Haemoglobin rose steadily from day 1 to 10. This rise was associated with falling total body water and body weight until day 5, but not from day 5 to 10.

Conclusion: Following cardiac surgery, an individual's fluid state should be considered in determining a patient's need for red cell transfusion. Monitoring body weight provides a simple estimate. Such an approach may reduce the incidence of unnecessary, and potentially counterproductive, transfusion in cardiac surgery patients.