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J Thorac Cardiovasc Surg 2003;125:855-862
© 2003 The American Association for Thoracic Surgery
Cardiopulmonary Support and Physiology |
From the Division of Cardiothoracic Surgery, New York Presbyterian Hospital and Columbia University, New York, NY.
Received for publication Sept 24, 2001. Revisions requested Jan 3, 2002; revisions received July 15, 2002. Accepted for publication July 23, 2002. Address for reprints: Vivek Rao, MD, PhD, FRCS(C), Toronto General Hospital, 200 Elizabeth St, EN 14-222, Toronto, Ontario, M5G 2C4 Canada (E-mail: vivek.rao{at}uhn.on.ca).
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Abstract |
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5). However, the revised score improved risk discrimination (operative mortality of 46% for a score >5 vs 12% for a score to 5). Statistical accuracy was comparable between scores, but the relationship between observed and predicted outcomes was improved with the revised score. |
Introduction |
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1-3 For the 2 most common long-term implantable left ventricular assist devices (LVADs), the vented electric HeartMate (Thoratec Corp, Pleasanton, Calif) and the Novacor (WorldHeart Corp, Ottawa, Ontario, Canada), survival to transplantation is greater than 70%. 2-4
We have previously reported a preoperative screening scale that predicts operative mortality in patients receiving the 1000IP or 1205VE HeartMate LVAD. 5 In developing this scale, we reviewed 56 consecutive patients who received a device either at our institution or at The Cleveland Clinic Foundation. All patients met the hemodynamic criteria for LVAD insertion proposed by Norman and colleagues 6: a cardiac index of less than 2.0 L · min-1 · m-2 with a left atrial or pulmonary capillary wedge pressure of greater than 20 mm Hg. A 10-point scale was developed by using 5 factors found to be statistically significant by means of univariate analysis (Table 1). Each factor was weighted according to its relative risk, and a summation score of greater than 5 was found to be associated with a prohibitive risk of death (67% operative mortality).
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7 In this high-risk subgroup of patients, we reported a survival to hospital discharge of 66% compared with a reported survival of less than 20% in patients supported with triple-inotropic therapy and intra-aortic balloon pump counterpulsation. 8 Since the publication of that report, we have noticed, in our institution, a substantial increase in the proportion of patients presenting for LVAD insertion with a temporary device in situ (bridge-to-bridge support). These patients have often undergone 2 previous sternotomies with cardiopulmonary bypass and present for a third bypass run in a span of less than 48 hours for insertion of a long-term device as a bridge to transplantation. Not surprisingly, these patients pose a challenge for perioperative management and have severe abnormalities of hepatic, renal, and pulmonary function.
Because of the changing nature of our patient population, we believed that a reassessment of our screening scale was appropriate. We now have the statistical power to develop a score on the basis of multivariable analyses of data obtained from a single institution. As suggested by Ivanov and colleagues, 9 periodic recalibration of existing indices and remodeling of screening scores are necessary to ensure that emerging risk factors are not inadvertently overlooked. The advantage of a single-center study is the uniformity of management practices, which removes this variable from the outcome measures in multicenter analyses. However, an admitted limitation is the potential lack of generalizability. Therefore, the purpose of this report is to validate the previous multicenter screening scale in a cohort of 130 consecutive patients receiving LVADs at our institution and to determine whether there are emerging risk factors that predict mortality in this challenging patient population.
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Methods |
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1,10
Statistical analysis
The SAS statistical software program version 8 for Windows (SAS Institute, Cary, NC) was used for all statistical analyses. Categoric data are expressed as their absolute or percentage frequency values and were compared with 
2or Fisher exact tests where appropriate. Continuous data are expressed as the means ± SD and were compared by using the Student t test.
Model derivation
The original summation score (Table 1
) was derived from a univariate analysis of 56 patients who received an LVAD as a bridge to transplant at Columbia Presbyterian Hospital or The Cleveland Clinic Foundation. 5 We validated this index by using it in our current data set, which contains no patient overlap with the previous study. A stepwise logistic regression model was developed by using the 5 criteria specified in the original scoring system, and the regression coefficients were then entered into an equation to yield the predicted probability of mortality for each patient. Patients were then grouped according to risk score (0-10), and the mean probability of mortality was calculated for each score.
The revised score was developed by reviewing all explanatory variables with a univariate P value of less than .25 for operative mortality after LVAD insertion. A revised score was then calculated by using the variables selected in a multivariable model, with weights assigned according to their odds ratios (ORs), univariate relative risks, or both (Table 2). Once again, a stepwise logistic regression model was developed by using the 5 criteria specified in the new summation score, and the probability of mortality was calculated for each score as above.
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Results |
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, with a comparison of survivors and nonsurvivors. Most of the previously identified risk factors continue to be associated with higher mortality, with the exception of renal insufficiency as defined by poor urine output. Redo surgery (defined as remote sternotomy) no longer predicts death. New univariate predictors of mortality include previous LVAD or right ventricular assist device (RVAD) placement (bridge-to-bridge support), postcardiotomy shock, acute myocardial infarction, and ischemic cardiomyopathy. A stepwise linear regression model identified a ventilated patient (OR, 5.7; 95% confidence interval, 1.8-17.5) and a previous LVAD (OR, 2.8; 95% confidence interval, 1.0-7.9) as independent predictors of mortality after device insertion. Table 3 shows the risk factors used to devise the revised summation score and the relative risk ratios for the variables excluded from the new score.
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Discussion |
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11 As the use of mechanical assistance to support this population increases, the indications for device implantation need to be clarified. Although individual patients will undoubtedly benefit from this technology, the cost-benefit ratio of this therapy might not justify device insertion in all potential recipients. 12 The increased interest in public reporting of surgical results has led to the development of many risk indices for mortality after isolated coronary bypass surgery. 9,13-15 Unfortunately, the relatively small clinical experience in single centers has made derivation of similar risk indices for LVAD insertion difficult.
To provide a common denominator with which to begin discussions with referring physicians, we devised a risk factor summation scale that successfully predicted operative mortality in HeartMate recipients. 5 To achieve the necessary statistical power, we combined the clinical experience of Columbia Presbyterian Hospital and The Cleveland Clinic Foundation to analyze a total of 56 LVAD recipients. The summation score derived from this earlier study performed satisfactorily in the present analysis, which included a single center's experience with 130 consecutive patients.
There are advantages and disadvantages of deriving a risk index from a single center's experience. The major advantages include uniformity of care and relevance to that center's patient population. The disadvantage lies in its potential lack of generalizability to larger patient populations and to centers with different management algorithms. Regardless of the data sources, it is useful to periodically update and remodel existing risk indices to detect emerging hazards and to assess the relative effect of known risk factors.
The purpose of the original scale was to provide LVAD teams with a quick screening tool to determine operative risk in potential device recipients. At our center, we found this tool to be extremely useful when initiating dialogues with referring physicians from "spoke" centers (ie, referral centers without the capability of providing long-term mechanical assistance or transplantation). 7 However, our clinical observation that the increasing proportion of patients presenting with postcardiotomy shock faced increased operative risk prompted us to reexamine our risk factor summation score. Our analysis provided some expected results but also revealed surprising trends in previously important variables.
Effect of postcardiotomy shock
The 3 most important variables in the new scoring scale all reflect the high-risk nature of a patient with postcardiotomy cardiogenic shock. Most series report a survival to hospital discharge of only 20% to 40%. 7,8,16,17 A ventilated patient presenting with a temporary LVAD in situ has already achieved a score of 8 (predicted mortality of 37%). The fact that mechanical ventilation emerged as the most important multivariable predictor of death (OR, 5.7) reflects the underlying status of a patient who is deemed to require endotracheal intubation. Although measurements of inspired oxygen concentration, oxygen saturation, or Po2 might provide valuable clinical information, the deliberate act of endotracheal intubation is a binomial criterion with high discriminatory powers. As a result of these considerations, mechanical ventilation earned 4 points on the revised scoring scale and represents the single most important risk factor for mortality after LVAD insertion.
The technical aspects of redo surgery, including the increased risk of bleeding and myocardial injury during sternal reentry, are no longer statistically significant, with a relative risk of only 1.2 compared with 1.7 in the old scale. Postcardiotomy shock conferred a relative risk of 3.3 and replaced redo surgery as a variable in the new scale. Most patients with a temporary LVAD suffered from postcardiotomy shock. All but 4 patients (1 device failure, 1 episode of cardiogenic shock after acute myocardial infarction, and 2 episodes of acute myocarditis) presented with a device after a failed cardiac surgical procedure. The presence of a temporary device is an indicator of the degree of hemodynamic instability and is an important risk factor in the new scale, with a relative risk of 3.3.
Although a previous RVAD emerged as a significant univariate predictor of mortality (relative risk, 3.2; P = .007), the small sample size (n = 10) precluded meaningful analysis. In addition, all 10 patients presented with both a left and right-sided device. Therefore RVAD in situ was excluded as a variable in the new screening scale.
When postcardiotomy shock and previous LVAD insertion were presented to the multivariable model, only previous LVAD insertion emerged as an independent predictor of mortality (OR, 2.83). However, because of the challenges of LVAD insertion in a patient with recent cardiac surgery, we elected to award 2 risk points for postcardiotomy shock in addition to the 2 risk points awarded for a previous LVAD in situ.
Acute myocardial infarction and a diagnosis of ischemic cardiomyopathy were weakly interrelated univariate predictors. Because of the significant overlap between these 2 variables and their failure to emerge as independent predictors in the multivariable analysis, we elected to exclude them in the revised screening scale.
Effect of end-organ function
The most striking finding in the present analysis is the lack of effect of preoperative renal insufficiency. Although measurements of serum creatinine might provide additional information, it is insensitive and lags behind changes in renal function. In addition, interpretation of isolated values is difficult because the context might involve recovery from a remote injury or worsening function from prolonged shock. We have previously found that urine output in the immediate preoperative hours is a sensitive indicator of renal function and was the most important predictor of mortality in the old screening score. However, a more aggressive approach to oliguria and the earlier institution of ultrafiltration or hemodialysis has attenuated the effect of this risk factor. As a result, immediate preoperative urine output is no longer represented on the risk factor summation score. However, it is clear from Table 2 that the development of postoperative renal failure is still a strong predictor of outcome after LVAD insertion. Therefore continued attention to the perioperative management of renal function is imperative, despite the fact that it is no longer a preoperative predictor of mortality.
Similarly, the effect of hepatic and right-heart failure has attenuated with time. In the old scale the relative risk of a prolonged prothrombin time (>16 seconds) was 2.4 compared with 2.1 in the new scale. The relative risk of an increased central venous pressure (>16 mm Hg), a marker of poor right heart function used to be 3.1 and in the present study has fallen to 2.1. Again, the aggressive treatment of volume overload has decreased the relative effect of both of these risk factors. Earlier institution of continuous venovenous hemodialysis leads to less hepatic congestion and improved right heart function. Decreasing hepatic congestion improves the synthetic function of the liver, thereby improving coagulation and decreasing the need for blood product transfusions. The inflammatory cytokine response to heterologous blood transfusion is associated with worsening right-heart function. 18,19 Because both increased central venous pressure and prothrombin time were associated with increased risk by means of univariate analysis, they were each awarded 1 point in the revised scoring scale.
Comparison of screening scores
The results of our analysis validate the previous summation score, which was derived from a different cohort of patients from 2 separate institutions. The old score satisfactorily predicted survival, with a threshold of 5 discriminating high-risk patients (38% vs 13%, P = .001). The new score provided improved discrimination, with a threshold of 5 representing a 46% risk compared with the 12% risk in patients with scores of less than 5 (P < .001). The area under the ROC curves were similar, with slightly higher accuracy in the new scale (0.77 vs 0.73). The relationship between observed and predicted values was greatly improved in the new scale. The old scale overestimated mortality in patients with predicted survival of greater than 70% and underestimated mortality in high-risk patients with predicted survivals of less than 70%. In contrast, there was excellent correlation between observed and predicted outcomes when the revised scoring scale was used (slope = 1.00, r = 0.87).
Summary
As the effect of mechanical circulatory assistance for the treatment of acute and chronic heart failure increases, a greater proportion of patients will present for device consideration at high risk for perioperative mortality. Identification of the preoperative variables, which accurately predict risk, is important for both resource use and for revising management strategies for these challenging patients. A dramatic example of this concept is illustrated in the present report, which documents the changing effect of preoperative renal status on perioperative mortality. Identification of poor urine output in our previous analysis led us to adopt a more aggressive approach to the treatment of impending renal failure. As a result of this paradigm shift, we have improved our perioperative management of overall volume status, and thus poor urine output, increased central venous pressure, and prolonged prothrombin time no longer increase operative risk significantly.
The present analysis suggests that the predominant predictor of risk in contemporary patients receiving LVADs is the preoperative requirement for mechanical ventilation. New strategies aimed at improving pulmonary function might help to mitigate the effect of this variable and improve the clinical results of mechanical circulatory support in an increasingly high-risk patient population. The current study is based on a single center's experience with only one of the available long-term implantable devices. A prospective multicenter experience is needed to validate the conclusions of this report.
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