Estimating Operative Mortality in Cardiogenic Shock Patients Undergoing CABG Surgery

Cardiogenic shock continues to be a major complication of acute myocardial infarction (MI), with in-hospital mortality approaching 70% to 80% for patients who are managed medically.¹ While both percutaneous and surgical approaches have shown improvement in mortality in this subset of patients,² many individuals with cardiogenic shock are referred for coronary artery bypass graft (CABG) surgery because their coronary anatomy does not allow for percutaneous coronary intervention (PCI) or they have mechanical complications, such as ventricular septal defects or papillary muscle rupture. Due to the paucity of information on the clinical and angiographic features and inhospital outcomes of patients with cardiogenic shock undergoing CABG surgery, we attempted to shed light on these issues by examining data from the Society of Thoracic Surgeons (STS) National Cardiac Database (NCD).

Subjects and methods

We analyzed data for 708,593 patients in the STS NCD with and without cardiogenic shock undergoing CABG surgery alone or in combination with aortic or mitral valve surgery or ventricular septal rupture repair between 2002 and 2005. Patients were considered to have preoperative cardiogenic shock if they were in a clinical state of hypoperfusion at the time of the procedure, which was indicated by a systolic blood pressure lower than 80 mm Hg, a cardiac index of less than 1.8 L/min per square meter despite maximal treatment or intravenous inotropes, the need for an intra-aortic balloon pump to maintain a systolic blood pressure higher than 80 mm Hg, a cardiac index of more than 1.8 L/min per square meter, or a combination of these findings. Operative mortality was defined as all deaths occurring in the hospital period in which the procedure was performed or those occurring within 30 days of hospital discharge. The chi-square test was used to compare categorical variables and the Wilcoxon rank sum test to compare continuous variables. Logistic regression modeling was used to estimate risk of operative mortality among patients with cardiogenic shock. In creating the bedside tool (Figure 1)*, regression coefficients from simplified model approximation (consisting of 11 predictive variables that explained most variation in outcomes) were then converted to whole integers by multiplying the coefficients by a fac- tor of 10 and rounding to the nearest integer. The c-index was used to determine the ability of the simplified model to discriminate between patients with and without in-hospital death.

Results

Preoperative shock was present in 14,956 patients (2.1%) undergoing CABG each year between 2002 and 2005, and this percentage was constant over this time period. Patients with and without cardiogenic shock were of similar ages, but those with cardiogenic shock had more comorbid conditions, especially acute MI within 24 hours before CABG surgery, left main coronary artery stenosis, and lower left ventricular ejection fractions. A preoperative intraaortic balloon pump was used in twothirds of these patients. Mitral valve or ventricular septal rupture repairs were more likely to be performed in patients with cardiogenic shock. Use of a ventricular assist device was 13-fold higher in patients with cardiogenic shock than in those without it; however, overall use of these devices was low, with only 3.0% of cardiogenic shock patients receiving them. In-hospital outcomes were worse in patients with cardiogenic shock than in those without cardiogenic shock (Table)*, and one-third of deaths occurred within 1 day after CABG surgery. Operative mortality was 7-fold higher in patients with cardiogenic shock and accounted for 1 of 7 overall CABG deaths. Mortality was highest in patients undergoing CABG surgery with ventricular septal rupture repair (58%) and lowest in those undergoing isolated CABG surgery (20%). Figure 2 shows the relationship between the risk score and observed outcomes. The risk score demonstrated reasonably good ability to identify those at low, moderate, or high risk for operative mortality (c-statistic of 0.74).

Discussion

While CABG surgery remains an important therapeutic option for revascularization in patients with cardiogenic shock,³ our data suggest that 14% of all CABG deaths occur in these patients even though they constitute a minority of patients undergoing CABG surgery.

There was also a high incidence of major morbidity in these patients, which prolonged hospitalization and resource use. Our data suggest that the risk of CABG death is lower in appropriately selected patients compared with the historically reported rates of 70% to 80% for those receiving medical management alone. Clinicians can use our bedside tool to estimate this risk of operative death (Figure 2)*.

Use of CABG surgery to treat acute cardiogenic shock offers some unique benefits, including reducing the energy requirement of the heart, providing immediate protection of the myocardium through cardioplegia and unloading the ventricle during cardiopulmonary bypass, and providing more complete revascularization than can be done percutaneously. Despite these benefits, very few patients with cardiogenic shock and 3-vessel disease are referred for CABG surgery, ranging from 3.2% to 8.8%.⁴ In addition to the fear of high operative mortality, the hesitation in referral for CABG surgery may be a reflection of the logistical challenges in arranging timely emergency surgery, particularly during off hours, when the alternative strategy of primary PCI can be undertaken promptly and in more hospitals.

Our study suggests some potential strategies and opportunities to improve outcomes of patients with cardiogenic shock undergoing CABG surgery. Intra-aortic balloon pump support was used in only two-thirds of these patients, yet greater use of preoperative intra-aortic balloon pump support has the potential for improving operative outcomes. Shorter pump and cross-clamp times could decrease the impact of prolonged cardiopulmonary bypass on outcomes. Very few patients in the cardiogenic shock group had a ventricular assist device placed during their hospital stay. Recent data from STS NCD suggests that up to 60% of patients with persistent shock after CABG can be saved with the use of ventricular assist devices,⁵ and increasing the availability of this technology at centers offering CABG surgery may be an opportunity to decrease mortality.6

Conclusions

Patients with cardiogenic shock represent a small subset of those undergoing CABG surgery. While mortality in these patients is high, more than 75% of properly selected patients (even those aged ≥75 years) survive their operation. Our simple bedside tool allows clinicians to estimate patient-specific risk of operative mortality using preprocedural data, ensuring proper patient selection and that those who ultimately undergo these procedures are properly counseled regarding their risk for morbidity and mortality.

A more detailed discussion of this topic can be found in Mehta RH, Grab JD, O’Brien SM, et al; Society of Thoracic Surgeons National Cardiac Database Investigators. 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. Circulation. 2008;117(7):876-885. Figures and Tables reprinted with permission.

*Click here to view Table, Figure 1, and Figure 2*

References

1. Goldberg RJ, Samad NA, Yarzebski J, Gurwitz J, Bigelow C, Gore JM. Temporal trends in cardiogenic shock complicating acute myocardial infarction. N Engl J Med. 1999;340(15):1162- 1168.

2. Hochman JS, Sleeper LA, Webb JG, et al; SHOCK Investigators. Early revascularization in acute myocardial infarction complicated by cardiogenic shock. Should we emergently revascularize occluded coronaries for Cardiogenic shock. N Engl J Med. 1999;341(9):625-634.

3. Antman EM, Anbe DT, Armstrong PW, et al. ACC/AHA guidelines for the management of patients with STelevation myocardial infarction; A report of the American College of Cardiology/ American Heart Association Task Force on Practice Guidelines (Committee to Revise the 1999 Guidelines for the Management of patients with acute myocardial infarction). J Am Coll Cardiol. 2004;44(3):E1- E211.

4. Babaev A, Frederick PD, Pasta DJ, Every N, Sichrovsky T, Hochman JS; NRMI Investigators. Trends in management and outcomes of patients with acute myocardial infarction complicated by cardiogenic shock. JAMA. 2005;294(4):448-454.

5. Hernandez AF, Grab JD, Gammie JS, et al. A decade of short-term outcomes in post cardiac surgery ventricular assist device implantation: data from the Society of Thoracic Surgeons’ National Cardiac Database. Circulation. 2007;116(6):606-612.

6. Tayara W, Starling RC, Yamani MH, Wazni O, Jubran F, Smedira N. Improved survival after acute myocardial infarction complicated by cardiogenic shock with circulatory support and transplantation: comparing aggressive intervention with conservative treatment. J Heart Lung Transplant. 2006;25(5):504-509.