Comparison with prior studies
While several studies have demonstrated that higher blood lactate levels at weaning from CPB [
18], chest closure [
18] and end of the operation [
19,
20], and maximum lactate level during the operation [
21] were associated with worse outcomes in pediatric cardiac surgery patients, there has been a few report on the predictability of intraoperative change of lactate concentration for postoperative outcomes.
Munoz et al. have shown that increased lactate concentration during CPB (defined as the difference between baseline and the first lactate value obtained after CPB) was associated with the risk of postoperative SAEs [
1] in 174 pediatric cardiac surgery patients who required CPB. In their multivariate logistic analysis, lactate change during CPB tended to be independently associated with mortality (p = 0.13) and risk of complication (p = 0.06). Park et al have shown that the lactate change from CPB weaning to chest closure was significantly associated with major adverse events [
18]. However, they did not assess its independent associations with patients’ outcomes.
To our knowledge, this is the first study to assess the independent predictability of intraoperative lactate change after CPB for worse outcomes in pediatric cardiac surgery patients. Although results did not conflict with the prior study, further studies are required to confirm the clinical utility of this measure.
Interpretation
It might be relevant to find biomarkers which can identify patients who are at higher risk of worse outcomes. Although our findings have a similarly with prior studies to show that the significant association of the increase of lactate concentration with increased risk of worse outcomes in other settings [
8-
10,
12,
13], there were still few studies to assess the association between intraoperative lactate changes and outcomes in pediatric cardiac surgery patients.
In compared with the lactate concentration in postoperative ICU, intraoperative lactate change had advantage to promote the re-assess the quality of operation, cardiac function, circulatory blood volume and ventilator setting in operation room. This fact gives physicians the time windows to early intervention including administration of inotrope, adjustment of circulatory blood volume and alternation of ventilator settings. We should note that such an association existed even after adjusting the value of LacFIRST and the duration between LacFIRST and LacLast. This may suggest that irrespective of initial lactate value after CPB and duration between measurements, the lactate change after CPB was useful for prediction of worse outcomes.
Limitation
Our study has several limitations. First, it is retrospective in design and thus potentially subject to systematic error and bias. However, the clinical and electronic data were collected prospectively, are numerical in nature and were measured independently, and were thus not amenable to unintended manipulation.
Second, the study was conducted in only one hospital. The findings might be different for studies conducted in other hospitals or in other countries with different styles of management. However, it should be noted that there has been no multicenter study in which the association of lactate indices with outcomes in pediatric cardiac surgery patients was assessed. A multicenter study should be conducted in the future to refute or confirm our findings.
Third, there is no protocol for lactate measurements, and the duration of post CPB (between end of CPB and surgery) varied among patients. Such a time distribution of LAC⊿ might affect our findings. However, we performed multivariate analysis adjusting the duration between LacFIRST and LacLast and found that higher LAC⊿ had independent association with increased the length of ICU stay. Furthermore, we calculated the time weighted change of lactate level after CPB and also found that the higher LAC⊿tw was independently associated with increased length of ICU stay.
Fourth, in current study, we used pre-planed subgroups according to each 1 mmol/L of LAC⊿. Thus, our study could not determine the threshold of LAC⊿ to maximize its predictability of worse outcomes. Accordingly, we performed post-hoc analysis to determine the threshold of LAC⊿ to have larger area under the receiver operator characteristic curve (AUROC curve). Then, we found that LAC≧1.6mmol/L had largest AUROC curve for the length of ICU stay and the incidence of SAEs (Additional files
1 and
2). As this is post hoc analysis, future study is necessary to address the threshold of intraoperative change of lactate level for greater predictability of postoperative outcomes.
Fifth, we included patients with wide range of age. As age may influence the metabolic rate and oxygen tissue oxygen requirements after surgery, the association with LAC⊿ and patients’ outcomes may varied in different ages. We found that there was significant association between LAC⊿ and length of ICU stay both in younger and elder patients. Nonetheless, we believed that future study is necessary to compare the magnitude of association between LAC⊿ and outcomes among patients with different ages.
Sixth, we included the 26 children required circulatory arrest during surgery. This fact may influence our findings. However, even after excluded these 26 patients, we found that the duration of ICU stay significantly increased according to each category of LAC⊿ (p = 0.017).
Finally, although we assessed the independent association of LAC⊿ with outcomes with adjustment for age, gender, lowest temp CPB, CPB time, RACHS-1 the duration between LacFIRST and LacLast and epinephrine use, there may be other confounders that influenced the association between LAC⊿ and outcomes. In this regards, future study should be collected on these possible confounders to avoid potential biases.