Biochemical markers for post-operative fatigue after major surgery.

Objective: To establish a link between tryptophan (a precursor for 5-hydroxytryptamine (5-HT) or serotonin, involved in sleep and fatigue) and post-operative fatigue after major surgery.

Background: There is a link between tryptophan (the precursor for the neurotransmitter 5-hydroxytryptamine), and its competitive binding with non-esterified fatty acids (NEFA) to albumin in the blood. An increase in the plasma concentration of free tryptophan can lead to an increased rate of synthesis of 5-HT in the brain. Free tryptophan competes with the branched chain amino acids (BCAA) for the same port of entry across the blood-brain barrier. It is suggested that the plasma concentration of these amino acids could be a marker of post-operative fatigue. In a previous study undertaken in this laboratory on patients undergoing two different types of major surgery, similar post-operative increases were observed in the plasma concentration of free tryptophan and the plasma concentration ratio of free tryptophan to branched chain amino acids. However, the study was retrospective and no measure of fatigue had been made.

Methods: In the present study, this deficiency has been addressed by administering a modified Profile of Mood States questionnaire to patients undergoing reconstructive or colorectal surgery. In addition, blood samples were measured for plasma free tryptophan, albumin, NEFA and branched chain amino acids before and on 2 days after surgery.

Results: There was a significant correlation between fatigue scores and plasma free tryptophan (P<0.000), and the plasma concentration ratio of free tryptophan/BCAA (P<0.016) after surgery in all the patients studied (n=34). This correlation was more marked in the colorectal-surgery patients, in whom surgery was more severe. In the three categories of patients receiving elective reconstructive surgery (n=24), those having breast reductions (n=6) had a lower plasma concentration of NEFA and appeared to recover from fatigue more quickly than those with pre-tibial lacerations or malignant melanoma.

Conclusions: These data provide further evidence of a possible biochemical mechanism for central fatigue which involves a precursor of 5-HT. The provision of branched chain amino acids may help to combat the surge in free tryptophan that occurs during stress such as major surgery.