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INTRODUCTION
Serum lactate level has been a useful tool for assessing severity of various diseases and is associated with poor outcomes in critically ill patients, such as septic, cardiogenic, hypovolemic shock, and liver failure [1–3]. Patients with end-stage liver disease are prone to develop acute decompensation with progressive organ failures, which may lead to developing the acute-on-chronic liver failure (ACLF) [4].
However, relationships between pre-liver transplant (LT) lactate and liver disease severity with organ failures are poorly studied in LT candidates.
In the current study, we sought to comprehend the distribution of lactate in LT candidates and searched the risk factors for hyperlactatemia. Specifically, to identify the most susceptible organ failures for the development of hyperlactatemia, we employed random survival forest (RSF) analysis and definitions from the Chronic Liver Failure Consortium ACLF score (CLIF-C ACLFs) [5]. Random forest is the machine learning technique using ensemble learning method for classification, regression, survival and identifying important risk factors [6–8]. Additionally, we evaluated association between pre-LT hyperlactatemia and early mortality after LT.
MATERIALS AND METHODS
Study population
The study design and a waiver of informed consent for participants were approved by the Institutional Review Board (no. 2021-0711). Data was extracted from the institution’s LT Registry, which prospectively registered patients who underwent LT. From January 2008 to February 2019, there were 4,406 potentially eligible LT recipients. Patients with toxic hepatitis (n = 141), fulminant hepatic failure (n = 134), were excluded. The patients who did not measure lactate within 7 days before LT (n = 1,822) and those with incomplete data (n = 307) were also excluded. After exclusion, 2,002 LT recipients were finally included. We included the majority of heterogenous LT recipients except acute toxic or fulminant hepatitis because we aimed to identify which comorbidities, or organ failures were most susceptible to pre-LT hyperlactatemia.
Data collection and definition of organ failures
Patient demographics, medical history Model for End-stage Liver Disease score (MELDs), and laboratory variables were obtained automatically using a fully computerized data extraction software. Mortality data were obtained from patients’ electronic medical records and the updated record of the institution’s LT registry. To identify risk factors associated with hyperlactatemia, we thoroughly investigated the association of hyperlactatemia with comorbidities and six organ failures (liver, kidney, brain, coagulation, circulation, and lungs) incorporated in definitions from the CLIF-C ACLFs. Briefly, they were defined as follows; Liver failure: bilirubin level of > 12 mg/dl, Kidney failure: creatinine > 2.0 mg/dl or renal replacement, Brain failure: hepatic encephalopathy grade by West-Haven, 3–4, Coagulation failure: INR ≥ 2.5, Circulatory failure: use of vasopressor, Respiratory failure: PaO2/FiO2 ≤ 200; SpO2/FiO2 ≤ 214; or on ventilator treatment [5].
Classification of patients with Lactate > 4.0 mmol/L and ≤ 4.0 mmol/L
Among patients with > 1 pre-transplant lactate measurements, the most proximate to the date of transplant within seven pre-transplant days was used. Lactate levels greater than 2 mmol/L represent hyperlactatemia, whereas lactic acidosis is generally defined as a serum lactate concentration above 4 mmol/L.
We chose threshold of lactate as 4.0 mmol/L according to the Surviving Sepsis Campaign guidelines, in which A lactate > 4 mmol/L qualifies for administration of early quantitative resuscitation therapy [9].
Statistical analysis
Data were expressed as mean with standard deviation (SD) or median with interquartile range (IQR) for continuous variables, and numbers and percentages for categorical variables. Analyses between groups were performed using Student’s t-test, the Mann–Whiney U test, analysis of variance, or the Kruskal–Wallis test for continuous variables and the χ2 test or Fisher’s exact test for categorical variables, as appropriate. Kaplan–Meier survival curve analysis with log-rank test was used to evaluate the risk of 90-day mortality.
Identifying important variables for preoperative hyperlactatemia
Variable importance (VIMP) was defined in classification and regression trees of RSF using a measure involving surrogate variables. Since VIMP is the difference between Out-of-Bag prediction error before and after permutation, a large VIMP value indicates that misspecification detracts from the variable predictive accuracy in the forest.
A random forest for pre-LT serum lactate levels was generated by creating 1,000 trees using ggRandomForest R-software package. The following variables were included for predicting preoperative hyperlactatemia: MELDs, left ventricular ejection fraction, hemoglobin, sepsis, and six organ failures defined by CLIF-C ACLFs.
All statistical analyses were performed using R version 3.6.1 (R Foundation for Statistical Computing, Austria). P value of less than 0.05 was considered statistically significant.
RESULTS
Of 2,002 LT recipients included, their mean age was 53.0 (48.0, 58.0) years and 1,489 (74.4%) were men. The mean MELDs was 16.0 (10.0, 26.0). The primary causes of liver disease were B or C virus-related liver cirrhosis (69.2%), alcoholic liver disease (19.8%) and others (11.0%) (Table 1). Prevalence of organ failures defined by CLIF-C ACLFs [5] were liver failure (24.0%), respiratory failure (10.8%), kidney failure (8.3%), coagulation failure (9.3%), circulatory failure (5.6%), and brain failure (2.6%), respectively (Table 2).
Distribution of Lactate in LT candidates
Median lactate level was 1.9 mmol/L (range: 0.4–24.6, IQR: 1.4, 2.4 mmol/L) and 107 (5.3%) patients showed lactate > 4.0 mmol/L. When patients were grouped with MELDs of < 15, 15–35, > 35, their median lactate level were 1.7 (1.4, 2.2), 1.9 (1.5, 2.5), and 2.4 (1.9, 3.6) mmol/L, respectively (P < 0.001) (Fig. 1).
Important variables for pre-LT hyperlactatemia
When we ignored the negative value of VIMP, the four most important variables for hyperlactatemia were MELDs, circulatory failure, hemoglobin, and respiratory failure, respectively (Fig. 2).
Survival analysis on 30-day and 90-day mortality
The 30-day and 90-day mortality rates were 2.7% and 5.1%, however, patients with lactate > 4.0 mmol/L showed increased morality rate of 15.0% and 19.6%, respectively. Kaplan-Meier analysis showed clear separation of survival curve of 90-day mortality (P < 0.001, Fig. 3).
DISCUSSION
Lactate levels is known to be a useful and rapid tool for assessing severity of disease in critically ill patients [10]. It has been shown that increased lactate levels and reduced lactate clearance are associated with mortality in critically ill patients with liver cirrhosis [1]. Therefore, lactate is believed to be a simple and accurate prognostic marker, and its incorporation improved performance of CLIF-C ACLFs significantly [10]. The Asian Pacific Association for the Study of the Liver (APASL) also incorporated lactate levels in its APASL ACLF research consortium-ACLF score [11].
Conventionally, lactate has been considered as a marker for tissue hypoxia, as lactate is overproduced and underutilized under anaerobic glycolysis due to impaired mitochondrial oxidation [12]. Therefore, increased serum lactate is usually the result from an imbalance of increased lactate production or reduced consumption [12]. Given that the liver is responsible for about 70% of whole-body lactate clearance, especially acute and fulminant hepatic impairment is associated with increased lactate levels and poor outcomes [13]. In contrast, under stable conditions, even severe liver cirrhosis has rarely been known to be associated with relevantly elevated lactate levels [12].
In the current study, after exclusion of toxic and fulminant hepatic failure, we investigated the important variables for preoperative hyperlactatemia in LT candidates and the relation between preoperative hyperlactatemia and early outcome after LT by using random forest survival analysis. We found that lactate monitoring is important in LT candidates because patients with lactate > 4.0 mmol/L showed increased 30-day morality rate of 15.0% and 90-day mortality rate of 19.6%, respectively.
Although the lower limit of the normal range for the blood lactate level, 0.5 mmol/L, is generally accepted among clinical laboratories, the upper limit can vary considerably, from 1.0 mmol/L to 2.2 mmol/L [12]. However, a plasma lactate concentration that exceeds 4 mmol/L generally defines lactic acidosis, even among patients without systemic acidemia. In the current study 5.3% of LT candidates fell into this category of lactate > 4 mmol/L.
The important variables related to hyperlactatemia were MELDs, circulatory failure, hemoglobin level, and respiratory failure in the order of importance. MELD score is useful for estimating overall disease severity and predicting survival in the patient with liver disease [14]. Therefore, the highest VIMP of MELD score in the current study may be related to hepatic dysfunction and reduced metabolism of lactate. Another important thing to be considered in the VIMP analysis is that circulatory failure and hemoglobin were chosen as VIMP in the current study. These results suggest that not only liver disease severity but also circulatory disturbance with anemia is an important contributor of pre-LT hyperlactatemia.
Circulatory failure has been defined typically as a condition in which the circulation is insufficient to deliver adequate oxygen to match the needs of the oxidizing tissues. However, the correlation between cardiac index and hyperlactatemia is not noticeably clear. A study reported that lactate levels were normal in about 75% of the patients with advanced heart failure [15], however, in LT candidates, circulatory failure defined by CLIF-C ACLFs remained one of important risk factors for pre-LT hyperlactatemia in the current study.
Both anemia and hypotension may induce tissue hypoxia due to reduced tissue oxygen delivery. In our study, patients who received transfusions or vasopressors would have higher plasma lactate concentrations due to anemia or hypotension. However, the retrospective nature of the current study cannot discriminate that high lactate was associated with either transfusions or vasopressors, or both.
Variables of importance for pre-LT hyperlactatemia was identified by machine learning using RSF in the current study. One of advantages of this analysis is that missing data are also less of a concern when using RSF. RSF performs excellently even with heavy missingness and when missing data are not missing completely at random [16,17]. Of 2,002 cases of lactic acid analysis, missing values were organ failures of 4, sepsis of 1, and left ventricular ejection fraction of 53 cases in the current study, respectively.
In this analysis, VIMP close to zero indicates the variable contributes nothing to predictive accuracy, and negative values indicate the predictive accuracy improves when the variable is misspecified. Therefore, we ignored variables with negative and near zero values of VIMP.
This study has several limitations. The enrolled patients are from the observational cohort study in a single center and retrospective observational design. Therefore, further prospective randomized control studies are warranted to validate our results.
In conclusion, about 50% and 5% of LT candidates showed pre-LT hyperlactatemia of > 2.0 mmol/L and > 4.0 mmol/L, respectively and those with lactate > 4.0 mmol/L was associated with increased early post-LT mortality. Four the most important variables for hyperlactatemia were MELD score, circulatory failure, hemoglobin, and respiratory failure. Our results suggest that future study of correcting modifiable risk factors, such as anemia and hypotension correction, may play a role in preventing hyperlactatemia and lowering early mortality after LT.
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