The effect of race on coronary bypass operative mortality

General demographics

A comparison of clinical preoperative risk factors for blacks and whites (Table 1) shows significant differences between the two groups. Because of the large number of patients in this analysis, nearly all univariate risk factors differ significantly as a function of race (p < 0.01). Several differences are striking, however. In general, black CABG patients were more likely to be female than were white patients (44.4% vs. 27.9%, p < 0.001). Black patients were also younger, having an average age of 61.5 years versus 64.8 years for whites. Black patients were more likely to have hypertension (80.5% vs. 62.4%), diabetes (43.8% vs. 27.8%), morbid obesity (15.4% vs. 11.4%) and renal disease (6.4% vs. 2.3%) than whites. However, we did not find strong evidence that blacks were referred for bypass later or in more emergent states than whites. Specifically, blacks were less likely to have their cases done under urgent, emergent or salvage conditions and had less New York Heart Association (NYHA) class IV symptoms at the time of bypass. Additionally, black patients did not appear to have more severe coronary disease anatomy or markedly worse left ventricular function (Table 1).

Operative outcomes by race

Table 2 provides the acute outcomes of bypass surgery for black patients and white patients. The operative mortality was 3.83% for blacks and 3.14% for whites, p < 0.001, unadjusted black/white (B/W) OR 1.23 [1.15–1.310]. Similarly, blacks had higher rates of postoperative complications, including stroke (B/W OR 1.36 [1.25, 1.48]), reoperation for bleeding (B/W OR 1.10 [1.02–1.20], prolonged ventilation (B/W OR 1.34 [1.28, 1.40]), renal failure (B/W OR 1.42 [1.34, 1.51]) and deep sternal wound infection (B/W OR 1.22 [1.05, 1.40]). Postoperative length of stays were also slightly longer for black patients (Table 2).

Table 3 shows the operative mortality for blacks and whites among important clinical subsets. The operative mortality was higher for blacks in most risk categories. For one important risk factor, however, blacks had a significantly lower operative mortality. The operative mortality for blacks with renal failure (7.6%) was lower than the operative mortality for whites with renal failure (10.5%), p = 0.0003.

Risk model comparisons

Using stepwise logistic regression, we identified 27 preoperative characteristics associated with operative mortality. A summary of the results of the logistic regression model appears in Table 4. The risk factors in Table 4 have been arranged according to their contribution to the predictive power of the overall model (as assessed by the value of the Wald chi-square statistic). Thus, age, reoperation, the need for preoperative intraaortic balloon pump (IABP) and ejection fraction emerge as the strong multivariate predictors of operative mortality. The overall ability of this model to discriminate patients who would live from those who would die was excellent (area under the ROC curve or C-statistic = 0.78).

To test whether race was an important independent predictor of operative mortality, we added race to our overall multivariable model. After controlling for the 27 other known risk factors, blacks had 29% higher operative mortality risk compared with whites (adjusted B/W OR 1.29 [1.21, 1.38]). Thus, patient race is an independent risk factor for bypass mortality even after accounting for other differences in patient clinical profiles.

While race is an independent risk factor for procedural outcomes, other known preoperative mortality risk factors among blacks and whites were strikingly similar (Table 4). Specifically, when the multivariable model was applied in black patient and white patient subsets separately, the directionality and magnitude of the ORs for these risk factors were quite similar in blacks and whites (Table 4). We also more formally tested for race by risk factor interactions in the overall patient population. Of the interactions examined, only race by gender was significant (p < 0.05), indicating that the difference in mortality between blacks and whites was more pronounced for men than for women. In fact, there was no significant difference in unadjusted mortality between black and white women (4.49% vs. 4.41%; p = NS).

Finally, the relationship between race and overall mortality risk was examined. Specifically, black patients and white patients were grouped into equal-sized deciles by their calculated operative mortality risk (based on the multivariable model). As demonstrated in Figure 1, blacks had slightly higher observed mortality across all risk deciles. When examined as risk ratios (Fig. 2), however, it becomes clear that B/W mortality differences are greatest among those patients at lowest predicted operative risk. For example, in Decile 1 (lowest-risk patients), the B/W OR for observed mortality was 1.83, whereas among those in Decile 10 (highest risk) this observed B/W mortality risk was only 1.03. In fact, a formal test of race by risk interaction was significant (p < 0.05), indicating that the difference in mortality rates for blacks and whites is not the same across risk groups.

Comparison with previous studies

Prior to the present study, there have been relatively few published investigations of the effect of race on CABG operative mortality or postoperative long-term survival (15–21). Gray et al. (21) reviewed the records of 3,113 white and 115 black patients who underwent CABG at Cedars-Sinai Medical Center from 1984 through mid-1994. The operative mortality was 5.5% for blacks and 4.6% for whites, p = 0.48. Although there was no significant difference in operative mortality in this study, a multivariate analysis was performed only for long-term survival (21). Higgins et al. (19) analyzed the results of 2,282 white and 494 black patients undergoing CABG from 1990 to 1996 at Henry Ford Hospital in Detroit. The overall operative mortality was 5.5% for black patients and 2.5% for white patients (p < 0.003). However, using multivariate logistic regression, race was not a significant predictor of mortality (19).

Maynard et al. (18) using the Coronary Artery Surgery Study (CASS) database found no statistically significant difference in the five-year survival of patients treated surgically as a function of race. Taylor et al. (20) also using the CASS database, examined the records of 22,714 white and 571 black patients who underwent angiography between 1974 and 1979. Long-term (>10-year follow-up) survival for surgically treated patients was significantly lower for black patients than for white patients, with black race emerging as a significant adverse predictor of long-term survival in a multivariate Cox proportional hazards model.

The search for alternative risk factors

The reasons for the higher operative mortality observed in black patients in this study are not clear. It is difficult to understand how race per se should have a significant impact on operative mortality once all known risk factors have been considered in a multivariate model. Our results suggest several possible hypotheses. First, we found that the average age of the black CABG patients in our population (61.5 years) is significantly lower than the average age of the white patients (64.8 years; p = 0.0001). Yet if a multivariate analysis is performed excluding race as a variable, the predicted mortality for black patients is 3.12%, whereas the predicted mortality for whites is 3.14%. Despite younger age, black patients have similar predicted mortality independent of race. Thus, black patients were generally younger and slightly sicker for their age than were white patients. Therefore, race in this study may be a marker for patients with more rapid rate of progression of coronary artery disease (CAD), in particular, and vascular disease, in general.

This finding of potentially more aggressive atherosclerosis in black patients is also consistent with the observed higher rates of cerebrovascular disease, hypertension, and renal disease in blacks relative to whites. The factors that correlate with vascular disease progression, such as elevations in C-reactive protein, are likely to be more prevalent in these patients (22). The associated inflammatory milieu is likely to exacerbate end organ function to a greater extent after cardiopulmonary bypass and to increase the risk of cardiac, neurological, pulmonary and renal complications after surgery, as we observed (23)(Table 2). These and other serum factors may also be important in determining CABG operative mortality, considering that the removal of inflammatory mediators using conventional and modified ultrafiltration has recently been shown to improve pulmonary, hemostatic and cardiac function after cardiac surgery in adults (24). Endothelial dysfunction has also been raised (25) as a risk factor for hypertension, and perhaps it could lead to complications in patients after CABG.

Blacks, in the absence of angiographic evidence of CAD, have a significant reduction in endothelium-independent coronary flow reserve compared with whites (25). Additionally, the depressed coronary vascular relaxation in response to both acetylcholine and adenosine in black patients has been associated with the higher prevalence of left ventricular hypertrophy (LVH) seen in blacks (26). For a given degree of hypertension, black patients are more likely to have significant LVH (27). In this analysis, although LVH was not specifically studied, we did, however, note that racial differences in operative mortality appeared to be limited to those patients with hypertension (Table 3). Specifically, although hypertensive blacks had significantly higher operative mortality than whites, race was not a significant predictor of surgical outcome among normotensive patients. The LVH may be more prevalent in the black patients studied with hypertension, perhaps increasing the incidence of postoperative arrhythmias or compromising myocardial protection during cardiopulmonary bypass.

Finally, as previous researchers have indicated, the study of racial differences in care and outcome is complex (28). Dichotomous variables based on skin tone do not capture the ethnic and biological diversity inherent among humans. Additionally, race is often a surrogate marker for underlying socio-economic, educational, behavioral and cultural differences. Unfortunately, these factors were not collected within our study, but the potential influence of these and other nonbiologic factors on both the short- and long-term outcomes of CABG should be the focus of future studies.

Conclusions

In summary, the finding that race is an independent predictor of operative mortality after CABG should prompt a more careful search for additional risk factors for CABG operative mortality. The unexpected finding of a significant race/gender interaction is important. In the general population, female gender is an independent predictor of operative mortality after CABG (29,30)(Table 4). Thus, further analysis of the differential effects of race and gender on outcome, as functions of gender and race, respectively, is required. Although the operative mortality after CABG is significantly higher for blacks than for whites, the differences are relatively small in absolute terms. Therefore, patients should continue to be referred for CABG based on established clinical criteria, independent of race.