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Ann Thorac Surg 1996;61:1740-1745
© 1996 The Society of Thoracic Surgeons
Heart Center, Deaconess Hospital, and National Public Health Institute, Helsinki, Finland
Accepted for publication February 6, 1996.
| Abstract |
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Methods. Data from 386 consecutive patients who underwent coronary artery bypass grafting in a single center were retrospectively collected. The relationship between the preoperative risk factors and the postoperative morbidity was analyzed by the Bayesian approach. Three risk indices (15-factor and seven-factor computed and seven-factor manual models) were developed for the prediction of morbidity. The criterion for morbidity was a prolonged hospital stay postoperatively (>12 days) because of adverse events.
Results. The best predictive preoperative factors for increased morbidity were emergency operation, diabetes, rhythm other than sinus rhythm on the electrocardiogram or recent myocardial infarction, low ejection fraction (<0.49), age greater than 70 years, decreased renal function, chronic pulmonary disease, cerebrovascular disease, and obesity. The sensitivity of the scoring methods ranged from 51% to 72% and the specificity, from 77% to 86%.
Conclusions. The results show that individual patients can be stratified according to postoperative risk for complications on the basis of preoperative information that is available for most patients.
| Introduction |
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Most studies addressing the risk factors of CABG were prompted by the need of quality control and fair comparison of the results in different hospitals. Several studies [29] with mortality as the end point have demonstrated preoperative risk factors for CABG and valve operations. The Coronary Artery Surgery Study group has developed a logistic risk equation for quality assurance [10]. The value of mortality as the sole end point has been questioned in the evaluation of clinical trials. Instead, morbidity has been suggested as a valid end point [11]. The aim of this study was to develop a simple, easily used preoperative risk model and to find the preoperative risk factors that best predict postoperative morbidity for CABG with special reference to prolonged hospital stay.
| Patients and Methods |
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A complicated postoperative short-term outcome was defined as a prolonged hospital stay (>12 days) after operation because of adverse events, transfer to another hospital for treatment of complications, or death during the hospital stay. The following complications led to a prolonged hospitalization: central nervous system problems including stroke and hemiparesis, postoperative infection in the sternum or a leg wound, pulmonary infection, arrhythmia requiring pacing or cardioversion, renal failure postoperatively and need of dialysis, inotropic support, or intraaortic balloon pump, or death during the hospital stay.
Statistical Methods
Univariate analysis was first accomplished using a
2 technique with 2 x 2 contingency tables to determine the relationship between the preoperative risk factors.
BAYESIAN ANALYSIS.
A stepwise Bayesian approach [3, 12, 13] was used to determine the posterior probabilities and likelihood ratios and to ascertain the sensitivity and the specificity of the rule. The Bayesian method is described in Appendix 1. A receiver operating characteristics (ROC) curve was also calculated. It gives a graphic representation of the relationship between true-positive and false-positive rates and can be used to study the effect of changing the decision rule. The area under the ROC curve is commonly used to measure the predictive power of a statistical model [14].
DERIVATION OF INDICES.
We have developed an optimization process to make the Bayesian analysis easier. The program goes through all the possible variables and selects first only one variable that will best predict the selected outcome (prolonged hospital stay in our study). Then the program goes through all the remaining variables and selects as a second variable the one that will best predict the outcome together with the first variable. Adding one variable after another to the model, the program goes through all the variables. The optimum is the number of variables that together give the smallest error rate (best sensitivity and specificity) in the study group. Usually the optimum does not include all the variables. At the end, the program provides the best combination of variables that will most accurately predict the outcome. It also gives the critical area for the total risk index, where the number of false-negative and false-positive results are minimal. The program also calculates the ROC curve. The program makes an assumption that the variables are independent of each other. The model ignores the interrisk factor correlations.
We used the Bayesian optimization process to find the combination of factors that will best predict morbidity. The program selected 15 of the 21 variables for the first model. Then we deleted one factor at a time from the model and tested the result. We ended up with a seven-factor short binary model. On the basis of the information in this short model, another model that needs no computing was developed. After weighing the factors for ``risk'' (1 to 2 points) and ``no risk'' (0 points) values, we ended up with a model that has a total score of 10 and an individual score for factors between 0 and 2.
| Results |
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The results of the shorter seven-factor model are presented in Table 3
. This version gave 44 false-positive and 39 false-negative results compared with the observed results (Tables 4, 5![]()
). The sensitivity of the rule was 51% and the specificity, 87%. The efficiency for the correct prediction was 79% and the total error rate, 22%. The ROC curve gave an area under the curve value of 0.787.
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As an example of the manual CABDEAL, 1 of our patients had a preoperative creatinine level of 150 µmol/L (1.7 mg/dL) (2 points). He was 75 years old (1 point), and the body mass index was 29 (1 point). He also had diabetes (2 points). The operation was elective (0 point). The patient had chronic atrial fibrillation (1 point) and chronic obstructive pulmonary disease with a forced expiratory volume in 1 second of less than 50% of normal (1 point). The total score for this patient was 8. He had a prolonged hospitalization because of a wound infection in the leg and disorientation postoperatively. The patient also returned to atrial fibrillation in the intensive care unit on the first postoperative day. He was discharged from the hospital on postoperative day 15.
| Comment |
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The information about risk can be used for several purposes. The observation of high risk in an individual patient can influence the timing of the operation, the planning of the surgical procedure, the type of anesthesia (fast-track versus non-fast-track), and the resource allocation for postoperative observation and treatment or hospital discharge plan (``7-day package'' versus longer stay). It may help in comparing the results over time to estimate the impact of changes in therapeutic procedures. The index can also be used to compare the immediate results of CABG in different institutions. This kind of comparison is not possible without normalizing the risk factors in the patient populations treated in individual hospitals. The risk index may also provide an opportunity to estimate the treatment costs of a patient population.
The validity of the present findings for CABG patients in other hospitals and countries remains to be tested. Our study population consisted of patients with a wide variety of factors that influence outcome. The risk factor profile of the patients was typical for patients referred for CABG. Patients were treated by a small, experienced team, and this diminished the variability resulting from perioperative factors. The influence of local factors on the results cannot be excluded, and therefore the index has to be validated in other patient populations.
Many previous studies [210, 18, 19] have focused on postoperative mortality, whereas only a few studies have focused on short-term morbidity and complications. One previous study [20] measured outcome in patients after a cardiac operation on the basis of length of stay in the intensive care unit. One recent review article [21] concluded that there are several indices available to predict mortality from different institutions. However, there is a lack of easy-to-use models predicting individual risk for complications (morbidity). Our study gives an option for individual risk stratification of morbidity. Table 7
compares the present results with those of four previous studies. All risk factors except the electrocardiographic abnormalities were found in at least two of the other studies. Emergency operation, renal dysfunction, diabetes, and pulmonary disease were identified as strong risk factors in at least three studies, including ours. Using univariate analysis and logistic regression analysis to relate risk factors for perioperative morbidity and mortality, Higgins and associates [22] studied a large CABG patient group. Their outcome measures were mortality and severe complications. Several of the risk factors were found in their study and our study: emergency procedure, elevated serum creatinine level, chronic pulmonary disease, and diabetes mellitus. Higgins and associates [22] also used an additive scoring system comprising 14 weighted factors that were graded from 1 through 6 giving a maximum total score of 33 points. Our manual scoring system had only seven factors that were graded 0 through 2 to give a maximum score of 10. The specificity and sensitivity of the two simplified risk scores were comparable. However, the CABDEAL model is easier to use at bedside for the evaluation of individual patients.
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In conclusion, patients selected for CABG can be stratified according to their risk of postoperative complications. This knowledge can be used both for patient groups and for individual patients.
| Appendix 1. |
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L=
i[P(xi'D)/P(xi'D`)]
Posterior probability = L(x)/[1+L(x)]
The likelihood ratios are expressed in the tables as 10 x 10 based logarithms of the original values. A constant 10 has been added: 10 x [1 + log (L)]. If any risk factor value is missing, an estimate of 10 has been used. That will equal a likelihood value of 1.0. Thus, a missing value is interpreted as ``no risk.'' Similarly, missing values were given a value of 0 in the manual CABDEAL model.
| Acknowledgments |
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| Footnotes |
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| References |
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