Correction for haemodilution is necessary for properly interpreting laboratory results obtained perioperatively in open-heart surgery

Introduction

Extracorporeal circulation (ECC) induces an oxidative burst of free radicals. Free radical activity can be assessed, among others, by measuring plasma lipofuscin levels, lipofuscin being a product of lipid peroxydation by free radical activity. Thus, theoretically a rise in lipofuscin levels can be expected during ECC.

Haemodilution, using non-blood primes for extracorporeal circuits during open-heart surgery, is standard practice. Usually moderate haemodilution, with haematocrites in the range of 0.20-0.25, is aimed at. In blood coagulation studies the potential effects hereof on study results are taken into account, but whether this is necessary in other studies as well, is still controversial.

We investigated the effect of correcting for haemodilution by comparing corrected with uncorrected sets of results of perioperative plasma levels of lipofuscin in order to examine the effect of this practice on the interpretation of our results.

Methods and material

In nineteen consecutive patients undergoing elective, uncomplicated coronary-artery bypass grafting (creatinine clearance >50 ml/min, age <80 years, no hypotension (mean arterial pressure <60 mmHg for over 1 h), no congestive heart failure (NYHA III-IV), and no clinical evidence of infection) plasma lipofuscin levels are measured by spectrophotometry as units of quinine sulphate (pg/ml). Samples are taken A) 10 min prior to anaesthesia, B) 2 min after aortic clamp release, C) at admission to the ICU, D) 4, and E) 8 h postoperatively. Correction for haemodilution is calculated using the formula : 1-HtB/1-HtA. Mean lipofuscin levels at specified time points are compared with the mean preoperative initial value (paired t-test).

Results

Discussion

Uncorrected plasma lipofuscin levels show no rise during open-heart surgery, whereas corrected values, which are all statistically significantly higher in comparison to the initial preoperative value, do. This is in agreement with the theoretically expected, and thus offers the opportunity for correct interpretation.

The above underscores the importance of correcting for haemodilution. It is essential for a proper interpretation of perioperative laboratory results and consequent understanding of the pathophysiology of ECC in open-heart surgery.