Abstract
OBJECTIVES
The model for end-stage liver disease score (MELD = 3.8*LN[total bilirubin] + 9.6*LN[creatinine] + 11.2*[PT-INR] + 6.4) predicts mortality for tricuspid valve surgery. However, the MELD is problematic in patients undergoing warfarin therapy, as warfarin affects the international normalized ratio (INR). This study aimed to determine whether a simplified MELD score that does not require the INR for calculation could predict mortality for patients undergoing tricuspid valve surgery. Simplified MELD 
METHODS
A total of 172 patients (male: 66, female: 106; mean age, 63.8 ± 10.3 years) who underwent tricuspid replacement (n = 18) or repair (n = 154) from January 1991 to July 2011 at a single centre were included. Of them, 168 patients in whom the simplified MELD score could be calculated were retrospectively analysed. The relationship between in-hospital mortality and perioperative variables was assessed by univariate and multivariate analysis.
RESULTS
The rate of in-hospital mortality was 6.4%. The mean admission simplified MELD score for the patients who died was significantly higher than for those surviving beyond discharge (11.3 ± 4.1 vs 5.8 ± 4.0; P = 0.001). By multivariate analysis, independent risk factors for in-hospital mortality included higher simplified MELD score (P = 0.001) and tricuspid valve replacement (P = 0.023). In-hospital mortality and morbidity increased along with increasing simplified MELD score. Scores <0, 0–6.9, 7–13.9 and >14 were associated with mortalities of 0, 2.0, 8.3 and 66.7%, respectively. The incidence of serious complications (multiple organ failure, P = 0.005; prolonged ventilation, P = 0.01; need for haemodialysis; P = 0.002) was also significantly higher in patients with simplified MELD score ≥7.
CONCLUSIONS
The simplified MELD score predicts mortality in patients undergoing tricuspid valve surgery. This model requires only total bilirubin and creatinine and is therefore applicable in patients undergoing warfarin therapy.
Keywords: Tricuspid valve, Comorbidity, Valve surgery, MELD score
INTRODUCTION
Patients undergoing tricuspid valve surgery have an operative mortality of 7–11% [1–3]. Advanced tricuspid regurgitation is associated with pulmonary hypertension and congestive liver dysfunction [4]. Therefore, despite the ease of surgical procedure, tricuspid valve surgery is associated with worse prognosis when compared with aortic valve and mitral valve surgery [5].
Although liver disease is cited as a risk factor for mortality and complications after cardiac surgery [6–8], existing risk-scoring systems for cardiac surgery do not account for liver dysfunction. For example, the Society of Thoracic Surgeons (STS) risk prediction model and the European System for Cardiac Operative Risk Evaluation (EuroSCORE) do not take into account the presence of liver dysfunction.
The risk of tricuspid surgery for patients with liver disease has been estimated via the Child-Turcotte-Pugh classification and asialoscintigraphy [9]. However, these systems have insufficient accuracy and usability. Recent reports suggest that the model for end-stage liver disease (MELD) score can predict mortality for patients undergoing cardiac surgery [10], especially for those undergoing tricuspid valve surgery [11]. The MELD score was originally devised to estimate the prognosis of patients with end-stage liver disease, and it takes into account three factors: serum total bilirubin, the international normalized ratio (INR) and creatinine levels. It is calculated as follows: MELD = 3.8*LN[total bilirubin in mg/dl] + 9.6*LN[creatinine in mg/dl] + 11.2*LN[INR] + 6.4, where LN = log normal.
However, many patients who have tricuspid disease are receiving warfarin therapy, often for atrial fibrillation or a history of prosthetic valve replacement. The MELD score is not applicable for these patients, as warfarin affects the INR. Therefore, we excluded a variable of INR from the MELD score; it had the same meaning as assessing INR to 1.0 (because if INR = 1.0, LN[INR] = 0). Thus, we devised a new scoring model: simplified MELD score, which was calculated as follows:
 |
The goal of this study was to determine whether a simplified MELD score that does not require the INR for calculation could predict mortality for patients undergoing tricuspid valve surgery.
PATIENTS AND METHODS
A retrospective review was performed for all patients who underwent tricuspid valve surgeries at our institution from January 1991 to July 2011. This study was approved by the institutional ethics committee in our hospital. A total of 172 patients (male: 66, female: 106; mean age, 63.8 ± 10.3 years) underwent tricuspid valve surgery. The surgical procedure included tricuspid valve replacement (TVR) or tricuspid valvuloplasty (TVP). Different types of prosthetic valves and methods of valvuloplasty were not distinguished.
Demographics, preoperative comorbidities and operative variables were compared between patients who survived to hospital discharge and patients who died prior to discharge. Preoperative laboratory data, clinical findings and medical history were used to determine Child-Turcotte-Pugh classification and the simplified MELD score.
Simplified MELD score could be calculated in 168 patients (97.7%). A total of 129 patients (75%) were on warfarin preoperatively, most commonly for atrial fibrillation or history of prosthetic valve replacement. Univariate and multivariate analysis were performed to identify the risk factors related to mortality.
To examine the significance of the preoperative simplified MELD score in the postoperative course, patients were stratified according to simplified MELD <7 (low group) or ≥7 (high group). Patients were followed up after tricuspid valve operation until their discharge. Prolonged ventilation was defined as mechanical ventilation >48 h. Preoperative variables, intraoperative variables and complications, including mortality, were evaluated.
Data are represented as mean ± standard deviation. Univariate analysis was performed by Fisher's exact tests for categorical variables and by Wilcoxon's rank-sum test for continuous variables. Multivariate logistic regression analysis was performed to identify independent risk factors for hospital mortality after tricuspid valve surgery. Variables entered into the model included those risk factors that had a significant (P < 0.05) or near significant (P < 0.20) influence on mortality by univariate analysis. All statistical analysis was performed using SPSS statistics 19 (SPSS, Inc., Chicago, IL, USA).
RESULTS
Patients characteristics are listed Table 1. Isolated tricuspid valve operation was performed in 6.4% (11 of 172) of patients. TVP (n = 155) was more common than TVR (n = 17). The rate of in-hospital mortality was 6.4% (11 of 172 patients). Causes of death were cerebral stroke (n = 3), sepsis (n = 3), heart failure (n = 2), acute liver failure (n = 2) and gastrointestinal haemorrhage (n = 1). Due to lack of data, simplified MELD score could not be calculated for 1 patient who died.
Table 1:
General patient characteristics
| Total surgical population (n = 172) | |
|---|---|
| Preoperative variables | |
| Age | 63.8 ± 10.1 |
| Male | 66 (38.3%) |
| Cerebrovascular disease | 27 (15.7%) |
| Diabetes mellitus | 12 (7.0%) |
| Peripheral vascular disease | 4 (2.3%) |
| Renal failure | 11 (6.4%) |
| Haemodialysis | 3 (1.7%) |
| Heart failure | 140 (81.4%) |
| Child-Pugh B or worse | 25 (14.5%) |
| On warfarin | 129 (75.0%) |
| Ejection fraction (%) | 55.3 ± 9.6 |
| Tricuspid valve regurgitation grade (1,2,3,4) | 9,21,58,36 |
| Intraoperative variables | |
| Tricuspid valve replacement | 17 (9.9%) |
| Reoperation | 91 (52.9%) |
| Concomitant operation | 161 (93.6%) |
| Aortic valve replacement | 26 (15.1%) |
| Mitral valve plasty or replacement | 161 (93.6%) |
| Maze | 26 (14.5%) |
| Coronary artery bypass graft | 3 (1.7%) |
| Cardiopulmonary bypass time (min) | 157.7 ± 44.0 |
| Postoperative outcomes | |
| Sepsis | 9 (5.2%) |
| Stroke | 9 (5.2%) |
| Myocardial infarction | 1 (0.6%) |
| Atrial fibrillation | 53 (30.9%) |
| Gastrointestinal complication | 10 (5.8%) |
| Multisystem organ failure | 7 (4.0%) |
| Prolonged ventilation (>2 days) | 26 (15.1%) |
| Haemodialysis | 8 (4.7%) |
| In-hospital mortality | 11 (6.4%) |
Risk factors related to mortality
Univariate analysis of all risk factors related to operative mortality was performed (Table 2). There was no significant difference in age, incidence of heart failure, tricuspid regurgitation grade or need for reoperation when comparing survivors and patients who died prior to discharge. Simplified MELD score (odds ratio (OR), 1.25; 95% confidence interval (CI), 1.09–1.42; P < 0.001), renal failure (OR, 14.4; 95% CI, 3.29–62.8; P = 0.002), need for haemodialysis (OR, 39.2; 95% CI, 3.21–479.3; P = 0.009) and Child-Turcotte-Pugh classification B or worse (OR, 6.90; 95% CI, 1.83–25.9; P = 0.007) were common among patients who died. The OR and 95% CI are per +1 increase in simplified MELD score.
Table 2:
Preoperative and intraoperative data in survivors versus patients with in-hospital death
| Survivors (n = 158) | Died (n = 10) | P-value | |
|---|---|---|---|
| Preoperative variables | |||
| Age | 63.6 ± 10.2 | 68.0 ± 5.0 | 0.55 |
| Male | 59 (37.3%) | 6 (60.0%) | 0.14 |
| Cerebrovascular disease | 26 (16.5%) | 1 (10.0%) | 0.50 |
| Diabetes mellitus | 10 (6.3%) | 2 (20.0%) | 0.15 |
| Peripheral vascular disease | 4 (2.5%) | 0 | 0.78 |
| Renal failure | 7 (4.4%) | 4 (40.0%) | 0.002 |
| Haemodialysis | 1 (0.6%) | 2 (20.0%) | 0.009 |
| Heart failure | 132 (83.5%) | 8 (80.0%) | 0.40 |
| Child-Pugh B or worse | 20 (12.7%) | 5 (50.0%) | 0.007 |
| On warfarin | 119 (75.3%) | 4 (40.0%) | 0.02 |
| Ejection fraction | 55.6 ± 9.6 | 55.1 ± 13.3 | 0.91 |
| TR grade (1,2,3,4) | 8,21,54,35 | 1,0,4,1 | 0.40 |
| Simplified MELDs | 5.75 ± 4.0 | 11.3 ± 4.1 | <0.001 |
| Intraoperative variables | |||
| TVR | 13 (8.2%) | 4 (40.0%) | 0.01 |
| Reoperation | 91 (57.6%) | 4 (40.0%) | 0.22 |
| Concomitant operation | 152 (96.2%) | 9 (90.0%) | 0.34 |
| AVR | 25 (15.8%) | 1 (10.0%) | 0.52 |
| MVP or R | 144 (91.1%) | 8 (80.0%) | 0.24 |
| Maze | 25 (15.8%) | 0 | 0.19 |
| CABG | 2 (1.3%) | 1 (10.0%) | 0.17 |
| CPB time (min) | 157.4 ± 44.2 | 189.2 ± 103.2 | 0.20 |
AVR: aortic valve replacement; CABG: coronary artery bypass graft surgery; CPB: cardiopulmonary bypass; MVP or R: mitral valve plasty or replacement; TR: tricuspid valve regurgitation; TVR: tricuspid valve replacement.
Multivariate logistic regression analysis was performed to identify the risk factors for mortality. Variables entered into the model included those risk factors with significant or near significant (P < 0.2) influence on hospital mortality and consisted of male gender, diabetes mellitus, Child-Turcotte-Pugh classification B or worse, simplified MELD score, TVR and coronary artery bypass graft surgery (CABG) (Table 3). Renal failure and haemodialysis were excluded from the model, because they directly affected the simplified MELD score. Based on this analysis, independent predictors of hospital mortality were simplified MELD score (OR, 1.25; 95% CI, 1.10–1.42, P = 0.001) and TVR (OR, 5.96; 95% CI, 1.28–27.76, P = 0.023).
Table 3:
Multivariate analysis of factors related to mortality after tricuspid valve surgery
| Variables | Odds ratio | 95% Confidence interval | P-value |
|---|---|---|---|
| Male | 0.78 | 0.15–3.72 | 0.711 |
| Diabetes mellitus | 1.11 | 0.10–12.45 | 0.934 |
| Simplified MELDs | 1.25 | 1.10–1.42 | 0.001 |
| Child-Pugh B or worse | 1.44 | 0.21–9.97 | 0.710 |
| TVR | 5.96 | 1.28–27.76 | 0.023 |
| CABG | 13.42 | 0.84–215.63 | 0.067 |
CABG: coronary artery bypass graft surgery; TVR: tricuspid valve replacement.
Effect of simplified MELD score on tricuspid valve surgery
A cut-off of simplified MELD score as a predictor of in-hospital mortality was defined based on receiver operating characteristic (ROC) analysis (Fig. 1). Youden index showed that the optimal simplified MELD score cut-off point was 7.42. Therefore, we set one major cut-off point to be 7 (sensitivity = 0.80 and specificity = 0.71) and the second cut-off point to be 14 (sensitivity = 0.30 and specificity = 0.99) for the purpose of being easy to use as a preoperative scoring model.
Figure 1:
ROC curve for simplified MELD score as a predictor of in-hospital mortality after tricuspid valve surgery. Areas under the curve (AUC) are 0.849. Youden index shows that the optimal cut-off point was 7.42.
Patients were stratified into the low group (simplified MELD score <7) or the high group (simplified MELD score ≥7), and preoperative, intraoperative and postoperative variables were compared between the two groups (Table 4). Patients in the high group were more likely to be male (P = 0.04) and have diabetes mellitus (P = 0.01). TVR was performed more commonly in the high group (P = 0.001). Concomitant operations were significantly more frequent in the low group (P = 0.04). In terms of postoperative complications, multisystem organ failure (P = 0.003), prolonged ventilation (P = 0.01) and need for haemodialysis (P = 0.002) were more common among patients in the high group. Sepsis and cerebral stroke occurred with similar frequency when comparing the two groups.
Table 4:
Preoperative, intraoperative and postoperative data from patients in the low group (simplified MELD score <7) compared with patients in the high group (simplified MELD score ≥7)
| Outcome | Low | High | P-value |
|---|---|---|---|
| Preoperative variables | |||
| Age | 63.8 ± 10.0 | 63.9 ± 10.2 | 0.26 |
| Male | 38 (33.3%) | 27 (50.0%) | 0.04 |
| Cerebrovascular disease | 20 (17.5%) | 7 (13.0%) | 0.45 |
| Diabetes mellitus | 4 (3.5%) | 8 (14.8%) | 0.01 |
| Peripheral vascular disease | 3 (2.6%) | 1 (1.9%) | 0.61 |
| Renal failure | 1 (0.9%) | 10 (18.5%) | <0.001 |
| Haemodialysis | 1 (0.9%) | 2 (3.7%) | 0.24 |
| Heart failure | 93 (81.6%) | 47 (87.0%) | 0.43 |
| Child Pugh B or worse | 4 (3.5%) | 21 (38.9%) | <0.001 |
| On warfarin | 83 (72.8%) | 40 (74.1%) | 0.99 |
| Ejection fraction (%) | 55.8 ± 8.8 | 55.2 ± 11.7 | 0.78 |
| TR grade (1,2,3,4) | 9,16,40,23 | 0,5,18,13 | 0.18 |
| Intraoperative variables | |||
| TVR | 4 (3.5%) | 13 (24.1%) | <0.001 |
| Reoperation | 67 (58.8%) | 28 (51.9%) | 0.40 |
| Concomitant operation | 112 (98.2%) | 49 (90.7%) | 0.04 |
| AVR | 18 (15.8%) | 8 (14.8%) | 0.87 |
| MVP or R | 107 (93.9%) | 45 (83.3%) | 0.03 |
| Maze | 21 (18.4%) | 4 (7.4%) | 0.06 |
| CABG | 2 (1.8%) | 1 (1.9%) | 0.69 |
| CPB time (min) | 162.7 ± 46.2 | 150.6 ± 51.3 | 0.32 |
| Postoperative outcomes | |||
| Sepsis | 4 (3.5%) | 5 (9.2%) | 0.12 |
| Stroke | 5 (4.4%) | 4 (7.4%) | 0.31 |
| Myocardial infarction | 1 (0.9%) | 0 (%) | 0.67 |
| Atrial fibrillation | 40 (35.1%) | 13 (24.1%) | 0.15 |
| Gastrointestinal complication | 6 (5.3%) | 4 (7.4%) | 0.40 |
| Multisystem organ failure | 1 (0.9%) | 6 (11.1%) | 0.005 |
| Prolonged ventilation (>2 days) | 12 (10.5%) | 14 (25.9%) | 0.01 |
| Haemodialysis | 1 (0.9%) | 7 (13.0%) | 0.002 |
| In-hospital mortality | 2 (1.8%) | 8 (14.8%) | 0.002 |
AVR: aortic valve replacement; CABG: coronary artery bypass graft surgery; CPB: cardiopulmonary bypass; MVP or R: mitral valve plasty or replacement; TR: tricuspid valve regurgitation; TVR: tricuspid valve replacement.
Mortality increased along with increasing simplified MELD score (Fig. 2). Patients with simplified MELD <0, 0–6.9 and 6.9–13.9 had mortalities of 0, 2.0 and 8.3%, respectively. In patients with simplified MELD >14, the mortality increased markedly to 66.7%.
Figure 2:
Operative mortality as a function of preoperative simplified MELD score.
The distribution of simplified MELD scores for this study population is shown Fig. 3. Only 2 patients with simplified MELD score >14 survived. One was a 40-year old male on chronic haemodialysis who underwent mitral valvuloplasty and tricuspid annuloplasty. The other patient was a 55-year old woman who had previously undergone mitral valve replacement. She suffered from acute prosthetic paravalvular leakage and underwent emergent mitral valve repair and tricuspid annuloplasty.
Figure 3:
Distribution of the simplified MELD score in the study population.
DISCUSSION
The MELD score was initially developed for patients undergoing transjugular intrahepatic portosystemic shunt placement [12, 13]. Subsequently, it has been validated as a predictor of survival in patients with end-stage liver disease. Since February 2002, the United Network for Organ Sharing (UNOS) has used a MELD score for liver transplantation organ allocation based on its utility. Although designed for patients with primary liver disease, the MELD score has also been shown to predict mortality for patients with liver dysfunction undergoing non-transplant surgery [14, 15].
Indeed, the MELD score has been utilized to assess the risk of cardiac surgery in patients with liver disease. Suman et al. [10] reported a strong correlation between Child-Turcotte-Pugh classification, MELD score and mortality (AUC = 0.84 and 0.87, respectively) in 44 patients undergoing cardiac surgery. Ailawadi et al. [11] concluded that the EuroSCORE and the MELD score had a similar ability to predict mortality for patients undergoing tricuspid valve surgery (AUC = 0.78 and 0.79, respectively) in 168 patients. Of note, the MELD score is the most convenient among these three scoring systems. While the Child-Turcotte-Pugh classification is generally more reliable, it contains subjective parameters, such as ascites and hepatic encephalopathy, that may affect categorization. The EuroSCORE is a complex risk-stratification scheme that utilizes 17 patient-related, cardiac-related and operation-related factors to calculate a predicted mortality rate. In contrast, the MELD score is relatively simple and is calculated based on three common laboratory tests (total bilirubin, creatinine and INR).
One drawback of the MELD score is the interlaboratory variability in all three variables. The most clinically relevant variability is the INR [16–18]. The Clinical and Laboratory Standards Institute has established guidelines for the INR. The confidence range of INR is from 1.5 to 4.5, and reproducibility and inter-laboratory results may have a ±15% variation [19]. However, INR carries the most weight among the three variables when calculating the MELD score. Thus, inappropriate determination of INR can result in an incorrect MELD score. In fact, the median difference in the MELD score is around 3–12 points due to the variability of INR.
We hypothesized that the exclusion of the INR could still result in a reliable scoring system. Thus, we devised the simplified MELD score, and assessed the ability of this system to predict hospital mortality for patients undergoing tricuspid valve surgery. This study demonstrated that the simplified MELD score was an independent risk factor for hospital mortality and morbidity. Patients with a simplified MELD score ≥7 had a significantly increased risk of a poor outcome. Conversely, patients with simplified MELD score <7 had favourable outcomes. Interestingly, CTP, which is conventionally used for risk assessment, was an independent predictor of hospital mortality in this study, which further highlights the superiority of the simplified MELD score. Thus, the present data suggest that exclusion of the INR from the MELD score calculation results in a relevant clinical scoring system.
In comparison with the original MELD score, the simplified score has several advantages. First, it is not confounded by warfarin therapy and therefore does not require cessation of warfarin therapy (and the associated risks of thrombosis or hospitalization) to calculate an accurate score. Secondly, the simplified MELD score uses a scoring table (Fig. 4), while a personal computer or calculator is required for calculation using the original MELD score. Thus, the simplified MELD score is more convenient than the original MELD score.
Figure 4:
Simplified MELD scoring table. The horizontal axis shows creatinine (mg/dl), and the vertical axis shows total bilirubin (mg/dl). At the intersection, the corresponding simplified MELD score can be found. Pale pink indicates higher risk of death following tricuspid valve surgery, and orange indicates highest risk of death following tricuspid valve surgery.
The simplified MELD score reflects the reserve capacity of the liver and kidneys. According to previous reports, liver dysfunction and renal failure are important risk factors for mortality and morbidity in patients undergoing cardiac surgery [6, 20]. Future studies by our group will determine if risk assessment using the simplified MELD score applies to cardiac surgeries other than tricuspid valve surgeries.
The current study has several limitations. First, administrative and clinical databases are limited by their retrospective nature, and the total number of patients is admittedly small. Thus, the lack of statistical power results. Secondly, the time period from which patient data were obtained was relatively long, and our institution utilized three different chief surgeons in the Department of Cardiovascular Surgery at our hospital over this time period. Further, indications for tricuspid valve surgery have evolved over time according to the opinions of these different surgeons, and there have been advances in operative procedures during this time period. All of these factors may have biased patient selection. Thirdly, it was not possible to determine the number and course of patients with tricuspid valve disease who were not candidates for operation due to liver dysfunction, because our operative database did not include the patients who did not undergo surgery. Therefore, we could not determine whether surgery or medical treatment was more appropriate for patients with high simplified MELD scores. Finally, it is difficult to separate the effect of end-stage renal failure from the significance of the simplified MELD score, as the simplified MELD score is directly dependent on serum creatinine. Thus, the simplified MELD score is likely overestimated in patients undergoing haemodialysis. To avoid preferential or unfavourable treatment of patients in terms of liver transplant allocation on the basis of renal function, UNOS has made several changes to how the original MELD score is calculated. For example, the lower limit for serum creatinine was fixed at 1.0 mg/dl, and the upper limit was capped at 4.0 mg/dl. Many patients undergoing haemodialysis have creatinine >4.0 mg/dl. Thus, the protocol of this study might have been improved had we distinguished between those patients who required haemodialysis and those who did not.
In conclusion, this simplified MELD score can predict mortality and morbidity in patients undergoing tricuspid valve surgery. This score requires total bilirubin and creatinine and is therefore applicable in patients undergoing warfarin therapy.
Acknowledgements
We thank Yoshiyuki Nakamura for statistics analysis, Mako Sugimura for data collection and Yukiko Tsuda for total assistance with this manuscript.
Conflict of interest: none declared.
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