Sample sizes per procedure ranged from 1 to 4868. The 140 procedures with at least 10 cases are listed in along with their sample sizes, raw and model-based morbidity estimates, and Morbidity Scores and Categories.
Model-estimated risk of major complications ranged from 1.0% for atrial septal defect repair to 38.2% for truncus arteriosus with interrupted aortic arch repair. Procedure-specific estimates of average PLOS ranged from 2.9 days for implantable cardioverter defibrillator procedure to 34.6 days for a stage 1 Norwood procedure and 42.6 days for a combined atrial switch and Rastelli procedure for congenitally corrected transposition. The Spearman rank correlation between raw rates of major complication and average PLOS was 0.63 () in procedures with at least 10 cases and 0.82 in procedures with at least 200 cases. This degree of correlation suggests that complication rates and PLOS provide related, but not redundant, information about morbidity.
FIGURE 1 Unadjusted average PLOS (days) and unadjusted rate of major complications (percentage) are measured on the horizontal and vertical axes, respectively. Squares represent the 140 procedure types with n greater than 10. PLOS, Postoperative length of stay. (more ...)
Procedure-specific overall morbidity was defined as 0.141 × percentage rate of major complications + 0.162 × average PLOS in days. The numbers 0.141 and 0.162 were calculated as the reciprocals of the standard deviations of the percentage rate of major complications and average PLOS, respectively. The STS Morbidity Score was obtained by rescaling this overall morbidity measure to lie in the interval 0.1 to 5.0. Thus, by design it ranged from 0.1 to 5.0. Procedures with the least morbidity (STS Morbidity Score = 0.1) include atrial septal defect repair and implantable cardioverter defibrillator procedures. The procedure with the greatest morbidity (STS Morbidity Score = 5.0) was repair of truncus arteriosus with interrupted aortic arch.
STS Morbidity Categories were obtained by grouping procedures into 5 unequally sized categories (1 = least bidity, 5 = most morbidity) chosen to be maximally homogeneous with respect to overall morbidity. The number of procedures assigned to categories 1, 2, 3, 4, and 5 were 36, 43, 36, 21, and 4, respectively. The rate of major complication ranged from 3.2% in category 1 to 30.0% in category 5. The aggregate average PLOS ranged across categories from 6.3 days in category 1 to 34.0 days in category 5.
Several analyses were performed to address potential methodological concerns with the composite measures used in this analysis. First, we addressed potential issues related to “Major Complication,” which is a composite designating the occurrence of any 1 or more of 6 individual complications. The observed rate of discharge mortality for patients who experienced at least 1 major complication was 23.5% () in comparison with 2.0% among patients who experienced none of the major complications. When end points in a composite occur with differing frequencies, the more frequent end points may sometimes dominate.6
As shown in , the aggregate rate of major complications ranged from 0.8% for “postoperative neurologic deficit persisting at discharge” to 4.7% for “unplanned reoperation.” To verify that each individual complication contributed statistical information but did not dominate the composite, we calculated the Spearman rank correlation coefficient between procedure-specific rates of each individual complication and rates of any major complication. These correlations ranged from 0.37 for heart block to 0.79 for unplanned reoperation. Thus, although unplanned reoperation explained much of the variation in the major complication end point, no single item dominated. All 6 complications contributed statistical information.
Second, we assessed the impact of modifying the list of major complications to include mortality. Although mortality was ultimately excluded, we thought it was important to know whether results would be similar or different had mortality been included. To address this, we calculated 2 versions of the major complication end point (1 including and 1 excluding mortality) and compared them. As shown in , the 2 major complication end points were highly correlated but not perfectly related. The rank correlation coefficient between them was 0.97.
The proportions (%) of Any Major Complication and of Any Major Complication or Mortality are measured on the horizontal and vertical axes, respectively. The squares represent the 140 procedure types with n greater than 10.
Third, although morbidity was calculated as an equally weighted combination of complication rate + average PLOS, strong consideration was given to an þalternative composite consisting of the rate of major complications and the average time on ventilator. Rank correlation between these 2 composite morbidity measures was 0.93, suggesting that the 2 methods tend to give similar, but not completely identical, results. The version using PLOS was preferred in part because PLOS was collected with high (>99.9%) completeness, whereas ventilation time was more than 15% missing. Moreover, during the time period of this study, the STS definition of time on ventilator only included the time until the first extubation and did not include the additional time on ventilator for patients who were subsequently reintubated.
Fourth, we assessed the reliability (ie, statistical precision; see “Materials and Methods”) of the various measures that were used for ranking procedures in this study. For major complications, average PLOS, and the composite morbidity measure, the estimated reliability values were 0.80 (95% CrI, 0.71-0.87), 0.88 (95% CrI, 0.82-0.92), and 0.90 (95% CrI, 0.85-0.94), respectively. Thus, reliability was greatest for composite morbidity, which was the basis of the proposed Morbidity Score and Categories. The estimated reliability of composite morbidity increased to 0.95 (95% CrI, 0.92-0.97) when considering only procedures with at least 30 cases (N = 115 procedures) and to 0.99 (95% CrI, 0.98-0.99) when considering only procedures with at least 200 cases (N = 67 procedures).
Finally, we assessed the degree of association between the proposed Morbidity Score and the existing STS-EACTS Mortality Score. A weak association would suggest poor content validity because, conceptually, we know that morbidity and mortality are closely related. On the other hand, a perfect association would suggest that the morbidity score is redundant with mortality and thus is unneeded. To address these issues, the proposed Morbidity Score and the STS-EACTS Mortality Score were plotted and compared. As shown in , they are closely related (rank correlation = 0.79), but far from being redundant.
FIGURE 3 The relationship between the STS-EACTS Mortality score2 and the STS Morbidity Score. Squares represent the 140 procedure types with n greater than 10.
Descriptive characteristics of the 5 Morbidity categories are shown in . The association between Morbidity Categories and Mortality Categories is summarized in . The Morbidity and Mortality Categories were identical for 74 procedures, differed by 1 or fewer positions for 135 procedures, and differed by 2 or fewer positions for 139 procedures. One procedure (pulmonary artery debanding) was in category 4 for mortality but category 1 for morbidity.
Summary of morbidity categories
Association between morbidity categories and mortality categories