Clinical Characteristics and In-Hospital Outcomes of Patients With Cardiogenic Shock Undergoing Coronary Artery Bypass Surgery
Insights From the Society of Thoracic Surgeons National Cardiac Database
Background— There exist few studies that characterize contemporary clinical features and outcomes or risk factors for operative mortality in cardiogenic shock (CS) patients undergoing coronary artery bypass grafting (CABG).
Methods and Results— We evaluated data of 708 593 patients with and without CS undergoing CABG enrolled in the Society of Thoracic Surgeons National Cardiac Database (2002–2005). Clinical, angiographic, and operative features and in-hospital outcomes were evaluated in patients with and without CS. Logistic regression was used to identify predictors of operative mortality and to estimate weights for an additive risk score. Patients with preoperative CS constituted 14 956 (2.1%) of patients undergoing CABG yet accounted for 14% of all CABG deaths. Operative mortality in CS patients was high and surgery specific, rising from 20% for isolated CABG to 33% for CABG plus valve surgery and 58% for CABG plus ventricular septal repair. Although mortality for CABG surgery overall declined significantly over time (P for trend <0.0001), mortality for CS patients undergoing CABG did not change significantly during the 4-year study period (P=0.07). Factors associated with higher death risk for CS patients undergoing CABG were identified by multivariable analysis and summarized into a simple bedside risk score (c statistic=0.74) that accurately stratified those with low (<10%) to very high (>60%) mortality risk.
Conclusions— Patients with CS represent a minority of those undergoing CABG yet have persistently high operative risks, accounting for 14% of deaths in CABG patients. Estimation of patient-specific risk of mortality is feasible with the simplified additive risk tool developed in our study with the use of routinely available preprocedural data.
Received July 21, 2007; accepted December 7, 2007.
Cardiogenic shock remains an important complication of acute myocardial infarction, with in-hospital mortality approaching 70% to 80% among those managed medically.1–4 The Should We Emergently Revascularize Occluded Coronaries for Cardiogenic Shock (SHOCK) Trial showed significant improvement in survival from immediate coronary revascularization in patients with cardiogenic shock, with similar beneficial effects observed with both percutaneous and surgical approaches.5 These results are reflected in the American College of Cardiology/American Heart Association guidelines that recommend immediate revascularization with either strategy as a class I indication in patients with cardiogenic shock.6
Clinical Perspective p 885
Many patients with cardiogenic shock are referred for consideration of coronary artery bypass grafting (CABG) as a result of unsuitable coronary anatomy for percutaneous coronary intervention (PCI) and/or because of mechanical complications such as ventricular septal or papillary muscle rupture. Despite this, limited information exists on the clinical and angiographic features and outcomes in large number of patients with cardiogenic shock undergoing cardiac surgery in the community. To date, most studies of cardiac surgery in the setting of cardiogenic shock are based on limited case series from major academic or high-volume centers and may not be representative of those seen in wider community practice.7–9
The Society of Thoracic Surgeons (STS) National Cardiac Database (NCD) provides an ideal opportunity to study a national sample of patients undergoing cardiac surgery among a diverse cross section of US hospitals (in this case, those with cardiogenic shock).10,11 Using these data, we sought to (1) characterize the clinical and angiographic features and in-hospital outcomes of patients undergoing CABG with cardiogenic shock; (2) track trends in outcomes over time; (3) identify preoperative clinical variables associated with operative mortality; and (4) create a bedside tool to estimate the operative mortality in patients with cardiogenic shock undergoing cardiac surgery.
Study Sample and Data Collection
The Society of Thoracic Surgeons National Cardiac Database
The formation, rationale, and methodology of the STS NCD have been published previously.10,11 Briefly, this registry was initiated in 1986 with a goal to provide participants with their risk-adjusted outcomes compared with the national experience. This information is used by member institutions as part of their continuous quality improvement efforts to help design strategies to improve their outcomes. This data set contains detailed clinical information on >2.5 million patients undergoing cardiac surgery from >600 academic, private, military, and Veterans Affairs hospitals from 50 US states and 5 Canadian provinces.
We analyzed data on patients with and without cardiogenic shock undergoing CABG either alone or in conjunction with aortic or mitral valve surgery or ventricular septal rupture repair during 2002–2005 at participating sites. Patients undergoing isolated valve, congenital heart, or aortic surgery were excluded from the analysis.
The STS NCD collects information on preoperative cardiogenic shock, defined as whether the patient was, at the time of procedure, in a clinical state of hypoperfusion according to either (1) systolic blood pressure <80 mm Hg and/or cardiac index <1.8 L/min per square meter despite maximal treatment or (2) intravenous inotropes and/or intra-aortic balloon pump necessary to maintain systolic blood pressure >80 mm Hg and/or cardiac index >1.8 L/min per square meter. Operative mortality was defined as (1) all deaths occurring during the hospital period in which the operation was performed and (2) those deaths occurring after hospital discharge but within 30 days of the procedure. Major morbidity was defined by the presence of any 1 of the following: need for reoperation (for bleeding, cardiac tamponade, graft occlusion, valvular dysfunction, or other cardiac or noncardiac problem), sternal wound infection, permanent stroke, renal failure, and/or prolonged ventilation. Information on all other adverse events was ascertained as in-hospital events. Remaining definitions of risk factors were as provided on the STS Web site (http://www.sts.org).
Summary statistics are presented as frequency and percentage or as median and interquartile range. We used the χ2 test for comparing categorical variables and the Wilcoxon rank sum test for continuous variables. Few records had missing data. Records with missing values of age, gender, and race (<1%) were excluded from all data presented here; records with missing elective operative status (<1%) were excluded from multivariable analysis of the cardiogenic shock subpopulation. Missing values of body mass index (<1%) were imputed to gender-specific median values. Missing values of information of valvular stenosis or insufficiency (<3%) and the remaining risk factors (<1%) were defaulted to their most common value (not present).
Logistic regression modeling was used to estimate risk of operative mortality among patients with cardiogenic shock. The model development was restricted to patients with cardiogenic shock undergoing salvage, emergent, or urgent cardiac surgery. The full model included a comprehensive list of 33 predictor variables including interaction terms between serum creatinine and body mass index and gender. Continuous variables were either assumed to have a linear relationship (age [in 5-year increments], body mass index) or categorized (serum creatinine, left ventricular ejection fraction). The full model was used as a gauge by which to measure the accuracy of a subsequent simpler model approximation and bedside risk scoring system.
Simplified Model Approximation
To reduce the number of variables, a parsimonious subset of predictor variables was identified that together explained >95% of the variation in the predicted log-odds of death as estimated by the aforementioned full model. Our simplified model is the one that explained the most variation of all models with only 11 variables. Regression coefficients were estimated by treating the predicted log-odds from the full model of the outcome variable in a least-squares regression model.12 The c index was used to determine the ability of the simplified model to discriminate between patients with and without in-hospital death.13 Both the full model and the process for determining the simplified model were internally validated with the use of the bootstrap resampling method. Calibration of the model was assessed by the Hosmer-Lemeshow test statistic.
Regression coefficients from the simplified model approximation were then converted to whole integers by multiplying the coefficients by a factor of 10 and rounding to the nearest integer to create a bedside risk prediction tool.14 For each possible risk score, the risk of operative death was estimated by averaging the predicted values from the original full model among all patients having the same risk score. The heterogeneity of mortality risk among patients having the same risk score was assessed by plotting the distribution of predicted risk within risk score subgroups. Finally, we assessed whether our risk score for mortality could also be used to predict the combined end point of mortality and/or major morbidity. Discrimination was quantified by the c index, which was adjusted for optimism by the use of bootstrapping. The risk of mortality and/or major morbidity was estimated within each risk score subgroup in 2 ways: (1) by calculating the average observed rate of mortality and major morbidity in each risk score subgroup and (2) by averaging the predicted values from a logistic regression model within the same risk score subgroup. The variables used in this logistic model were identical to those in the full mortality model. Because information on left ventricular ejection fraction was missing in 11% of patients, we explored 2 strategies to derive the risk scores: record exclusion versus imputing ejection fraction to median values within subgroups defined by gender, congestive heart failure, and operative status. These 2 sets of risk scores (with and without the imputation of ejection fraction) were highly correlated (correlation coefficient=0.98). Thus, we elected to exclude patients with missing ejection fraction from our multivariable modeling.
All analyses were performed with the use of S-Plus 6 (Insightful Corporation, Seattle, Wash) and SAS 8.1 (SAS Institute, Cary, NC) software.
The authors had full access to and take full responsibility for the integrity of the data. All authors have read and agree to the manuscript as written.
Baseline Clinical Characteristics of Patients With Cardiogenic Shock Undergoing Cardiac Surgery
Preoperative shock was present in 14 956 (2.1%) of the total of 708 593 patients undergoing CABG enrolled in the STS NCD during 2002–2005. The proportion of patients with CABG having cardiogenic shock remained constant over this time period, constituting 2.1%, 2.1%, 2.1%, and 2.2% of overall CABG patients in 2002, 2003, 2004, and 2005, respectively.
Table 1 demonstrates the baseline, angiographic, and operative characteristics of the study patients. Despite the similar age of those with and without cardiogenic shock, those with cardiogenic shock generally had more comorbid conditions. In particular, a much higher percentage of patients with cardiogenic shock suffered an acute myocardial infarction within 24 hours before CABG versus those without shock (45% versus 7%; P<0.0001). Additionally, significant left main stenosis was 1.5 times more common and median left ventricular ejection fraction was 15% lower in patients with cardiogenic shock than in those without it.
Preoperative intra-aortic balloon pump was used in >70% of patients with cardiogenic shock as opposed to in 5.8% patients without cardiogenic shock. Operative procedure in patients with cardiogenic shock was more likely to include mitral valve surgery (usually for mitral regurgitation) or ventricular septal rupture repair and less likely to be an isolated CABG or CABG with aortic valve procedure. Use of an internal thoracic artery graft was only 58.1% in patients with preoperative cardiogenic shock compared with 85.0% in those without it. Median pump and cross-clamp times were modestly longer for shock patients. Although the use of a ventricular assist device was 13-fold higher in the cardiogenic shock patients than in those without it, <3% of cardiogenic shock patients received these devices.
Patients with cardiogenic shock undergoing CABG had worse in-hospital cardiac outcomes and longer lengths of intensive care unit and hospital stays than those without it (Table 2). Patients with cardiogenic shock had a shorter time to death, with a third of these deaths occurring by the next day of CABG. Major morbidity occurred in 61%. Operative mortality was >7-fold higher in patients with cardiogenic shock than in those without it undergoing CABG and accounted for 1 of ≈7 deaths in CABG patients (14%). Mortality varied accordingly to the type of surgical procedure performed among patients with cardiogenic shock and was lowest in patients undergoing isolated CABG and highest in those undergoing CABG plus ventricular septal rupture repair (Figure 1). Operative mortality was higher for patients with cardiogenic shock operated within 24 hours of their myocardial infarction (26%) compared with those operated from 1 to 7 days (20%), 8 to 21 days (20%), and >21 days (18%) after their myocardial infarction. Whereas the operative mortality in overall patients undergoing CABG declined over time (3.0%, 3.1%, 2.9%, and 2.8% for years 2002, 2003, 2004, and 2005, respectively; P for trend <0.0001), this did not change significantly in patients with cardiogenic shock undergoing urgent, emergent, or salvage procedures during the 4-year study period (22.9%, 22.5%, 21.9%, 21.2% for years 2002, 2003, 2004, and 2005, respectively; P for trend=0.0675). Increasing age was also associated with higher operative mortality. In patients ≥75 years of age, mortality increased 1.7-fold compared with those <75 years of age (31% versus 18%).
Multivariable analysis was performed on 12 339 of the 14 956 cardiogenic shock patients who met the inclusion criteria and did not have the aforementioned exclusions. The initial full model identified 18 factors independently associated with increased risk of death (model not shown) (c statistic=0.75). Figure 2 displays a comparison between observed and predicted rates of mortality based on this full model. Although the Hosmer-Lemeshow probability value for lack of fit was 0.04 (related to a very large sample size), from a clinical perspective good concordance existed between predicted and observed outcomes across the risk continuum (Figure 2). Furthermore, bootstrap validation of the full model with the use of 200 replicates showed modest internal validation (c statistic=0.746; 95% confidence interval [CI], 0.745 to 0.747).
The simplified model (Table 3) retained much of the predictive power of the full model (c statistic=0.74; for bootstrap of variable selection process, c statistic=0.741; 95% CI, 0.740 to 0.741) and showed reasonable fit when we compared deciles of observed versus predicted rates of death (Hosmer-Lemeshow P=0.09 for lack of fit).
The simplified model coefficients were then converted to an additive risk score (Figure 3). The average absolute difference between estimates calculated from the full model compared with estimates based on the risk scores was 2.5%. This risk score demonstrated excellent ability to identify those with low, moderate, and high risk for operative mortality (Figure 4). Bootstrap validation of the risk score for death predicting mortality or morbidity showed a modest concordance between predicted outcome with the use of the mortality risk score and observed mortality plus morbidity (c statistic=0.726; 95% CI, 0.719 to 0.733; Hosmer-Lemeshow P=0.07 for lack of fit) (Figure 5). Finally, excellent correlation was present between the risk score based on the simplified model and that based on the full model (Pearson correlation coefficient=0.9394 or R2=0.88).
CABG represents an important therapeutic option for revascularization in patients with cardiogenic shock.6,15 Our study suggests that, although patients with cardiogenic shock constitute a minority of patients undergoing CABG, they account for 16% of all CABG deaths. Even more striking was the high incidence of major morbidity that prolonged hospitalization and resource use. Despite improvements in surgical and anesthesia techniques and newer technologies (ventricular assist devices), the risks with CABG in cardiogenic shock have remained stable over time. Our study provides a bedside tool that makes prediction of the risks of mortality and the combined end point of mortality plus major morbidity possible on the basis of simple preoperative factors.
CABG offers some unique potential benefits in the treatment of patients with cardiogenic shock. It provides immediate protection of myocardium with cardioplegia, the ability to cool the heart to reduce the energy requirement, unloading of ventricle during cardiopulmonary bypass, and the ability to achieve more complete revascularization than percutaneous revascularization. However, nationally, very few patients with cardiogenic shock and 3-vessel disease are referred for CABG, ranging from 3.2% to 8.8%.16 This reservation in referral for CABG may be a reflection of the logistical challenges in arranging timely emergency surgery, especially during off hours, when the alternative strategy of primary PCI can be undertaken expeditiously and in more hospitals than those offering CABG, as well as the fear of high operative mortality.
Our study summarizes the national results of CABG performed in the setting of cardiogenic shock. Overall, we found that even when faced with an unarguably sick and critically ill population, operative mortality was not prohibitive in appropriately selected patients. Additionally, although age was strongly associated with operative mortality, a full 70% of patients even in the oldest age group (≥75 years) survived this major surgery. Therefore, it is reasonable for clinicians to consider surgical options in appropriate functional elderly patients with cardiogenic shock.
Although not designed to test various treatment options, our study suggests some potential opportunities to improve outcomes of patients with cardiogenic shock undergoing CABG. Intra-aortic balloon pump support was utilized in only two thirds of such patients. Although direct trials of intra-aortic balloon pump in this setting are not available, it appears that greater use of preoperative intra-aortic balloon pump support has the potential for improving operative outcomes. Shorter pump and cross-clamp times could also decrease the impact of prolonged cardiopulmonary bypass on outcomes. The majority of procedures were done with the use of cardiopulmonary bypass, and the role of off-pump surgery merits evaluation in this cohort. In addition, very few patients in the cardiogenic shock group had a ventricular assist device placed during their hospital stay. This seems low, given the fact that many of these may have been candidates for this therapy as a bridge to cardiac transplant. Recent STS NCD data suggest that up to 60% of patients with persistent shock after CABG can be salvaged with the use of ventricular assist devices.17 Improving the availability of this technology at centers offering CABG may represent an opportunity to save more lives.18
Although emergent revascularization has been shown to improve outcomes in patients with cardiogenic shock,5 the optimal revascularization strategy in patients with cardiogenic shock needs to be determined. The 22% overall mortality (18% versus 31% in those aged <75 and ≥75 years, respectively) observed for the cohort with cardiogenic shock undergoing CABG may appear to be low but is a stable estimate among a large number of such patients at a national level. For example, this rate is very similar to that observed for years 2002–2004 in the National Registry of Myocardial Infarction, which reported in-hospital mortality between 16.7% and 28.6% in a very small number of patients with cardiogenic shock undergoing CABG.16 Similarly, in the SHOCK trial registry of 884 patients with predominant left ventricular pump failure, in-hospital mortality occurred in 24% of patients treated with CABG.19
Finally, we also identified important clinical factors that were associated with increased risk for operative mortality (and mortality plus morbidity) in patients with cardiogenic shock undergoing CABG. The prognostic importance of advanced age, female gender, type of surgery, timing of surgery in relation to acute myocardial infarction, prior cardiac surgery, preoperative serum creatinine, lower left ventricular ejection fraction, preoperative resuscitation, and use of immunosuppressive therapy has been reported previously and is intuitive, meriting little further explanation. In contrast, the use of the intra-aortic balloon pump in patients with cardiogenic shock undergoing CABG may be a marker of other unmeasured confounders or worse hemodynamic status that confers higher risk, accounting for the association of the intra-aortic balloon pump with an increased risk of operative mortality and mortality plus morbidity.
Our predictive model is derived from a very large national patient sample with a large number of events, unlike any prior studies. As a result, we were able to develop and internally validate a simplified risk tool that can be useful to physicians in risk assessment across a wide spectrum of cardiogenic patients undergoing CABG and/or other concomitant operative procedures and in counseling of such patients and their family. This simplified risk tool may be useful to clinicians to appropriately provide patients and their families with empirical estimates of the patient’s risk. Although such risk tools should be used as a guide, one should also consider individual patient risk factors, the skills of the surgeons, and the quality of the hospitals when individuals with cardiogenic shock are considered for CABG. Additionally, it is important to stress that such a tool may have value in determining the prognosis of patients with cardiogenic shock selected to undergo CABG but not necessarily in decision making for referral for CABG.
This observational study describes the characteristics and outcomes of patients with cardiogenic shock undergoing CABG in community practice. It should be emphasized that the decision to refer patients with shock for CABG was made by physicians taking care of the patients. Thus, extrapolation of our findings to shock patients treated medically or with PCI should be made with caution. Additionally, this also has the potential of resulting in an inherent bias favoring survival after CABG over patients treated conservatively or with PCI. In particular, patients referred for CABG, a relatively greater invasive procedure, may be those who are more likely to survive with lesser severity of shock, younger age, and fewer comorbid conditions. In contrast, PCIs may be preferred for the relatively sicker, older patients with more severe shock. We also do not have the universe of all patients with coronary artery disease and cardiogenic shock in the STS NCD, which only includes patients with cardiogenic shock selected for CABG. Thus, we are unable to directly perform a comparative analysis of outcomes of shock patients undergoing CABG versus those shock patients treated with PCI or no revascularization procedures. Although the STS now represents the majority of US CABG centers, results from larger centers and those with better overall outcomes are slightly overrrepresented. Second, as is true in any registry study, we are only able to analyze risk factors collected and cannot account for the influence of any unmeasured factors influencing operative risk. The current database only allows for evaluation of short-term risk because we do not yet have any information on long-term outcomes. Nonetheless, long-term survival and quality of life are excellent among 30-day survivors of cardiogenic shock who underwent successful revascularization.20,21
Patients with cardiogenic shock represent a minority of patients undergoing CABG. Although mortality is high, this does not appear to be prohibitive, and >75% of selected patients (even those aged ≥75 years) survive their operation. Estimation of the patient-specific risk of operative mortality and mortality plus morbidity is feasible with the simplified additive risk tool developed in our study with the use of routinely available preprocedural data. We believe that physicians will find this tool applicable and useful in routine clinical practice. Finally, no significant improvement in operative mortality has been observed in recent times in patients with cardiogenic shock, underlying the importance of evaluating newer strategies for improving outcomes of patients with cardiogenic shock undergoing CABG.
Sources of Funding
This study was supported by the STS NCD and Duke Clinical Research Institute.
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Because few patients with cardiogenic shock (CS) undergo coronary artery bypass grafting (CABG), the contemporary clinical features and outcomes or risk factors for operative mortality in these patients are not well known. Our study of 708 593 patients with and without CS undergoing CABG enrolled in the Society of Thoracic Surgeons National Cardiac Database (2002–2005) found that patients with preoperative CS constituted 14 956 (2.1%) of patients undergoing CABG yet accounted for 14% of all CABG deaths. Operative mortality in CS patients was high and surgery specific, rising from 20% for isolated CABG to 33% for CABG plus valve surgery and 58% for CABG plus ventricular septal repair. Although overall CABG mortality improved during the 4-year study period, that for CS patients undergoing CABG did not. Finally, we identified factors associated with higher death risk for CS patients undergoing CABG using multivariable analysis and summarized these into a simple bedside risk score (c statistic=0.74) that accurately stratified those with low (<10%) to very high (>60%) mortality risk. We believe that estimation of patient-specific risk of mortality is feasible with the simplified additive risk tool developed in our study with the use of routinely available preprocedural data and that physicians will find this tool applicable and useful in routine clinical practice.
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