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Eur J Cardiothorac Surg 2005;28:114-119
© 2005 Elsevier Science NL
a Department of Anesthesiology, St. Vincent Mercy Medical Center, 2213 Cherry Street, Toledo, OH 43608, USA
b Department of Cardiovascular Surgery, St. Vincent Mercy Medical Center, 2213 Cherry Street, Toledo, OH 43608, USA
c Department of Medicine, Medical College of Ohio, Toledo, OH, USA
d Department of Cardiovascular Surgery, Medical College of Ohio, Toledo, OH, USA
e Department of Cardiovascular Surgery, St. Luke's Medical Center, Maumee, OH, USA
Received 21 December 2004; received in revised form 2 March 2005; accepted 4 March 2005.
* Corresponding author. Tel.: +1 419 251 4715; fax: +1 419 251 3859. (Email: engoren{at}pol.net).
| Abstract |
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Key Words: Coronary artery bypass surgery Survival analysis Mortality Myocardial infarction Myocardial injury
| 1. Introduction |
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3 years) term outcome [25]. However, these studies used a variety of arbitrary definitions of what constituted elevated CK-MB, used the highest value of several measurements, and were conducted exclusively in patients who underwent hypothermic cardiopulmonary bypass. Because CK-MB elevation generally peaks 2h after CABG surgery, but peaks 17h after a myocardial infarction [6], it should be possible to determine if a single appropriately timed CK-MB measurement could be used as a risk factor to help predict mortality in patients undergoing normothermic cardiopulmonary bypass and to (1) elucidate the length of time that this association, if any, persisted and (2) the clinically meaningful CK-MB threshold value above which long-term outcomes are adversely affected.
| 2. Methods |
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The Institutional Review Board approved this study. As it was a database review, the Board waived informed consent.
2.1. Subjects
All patients who underwent first-time, isolated CABG with cardiopulmonary bypass between January 1, 1995 and December 31, 1996 were considered for this study. Twenty-six patients were excluded for having myocardial infarction less than 24h preoperatively. Another 19 patients (1.6%) with operative mortalitydefined as in-hospital death or out-of-hospital death within 30 days of surgery, as per the Society of Thoracic Surgeons definitionwere analyzed only univariately due to the small number of events. The remaining 1161 patients comprised the study population to investigate 31 day to 75month mortality. All data had been prospectively entered in the database. CK-MB levels were drawn the morning after surgery in all patients. The upper limit of normal in our laboratory is 5ng/mL. Long-term patient survival data was secured from the social security death index database http://ssdi.genealogy.rootsweb.com, which was queried in September 2001 using patient name and social security number combinations for all patients. This corresponds to minimum and maximum follow-up times of 51 months (December 1996 patients) and 75 months (January 1995 patients), respectively. The cardiac surgery database was then updated for all deceased patients with the exact date of death.
2.1.1. Definitions
The Society of Thoracic Surgeons definitions were used for all entries in the database [7]. Briefly they are: Same day admit, patient admitted for scheduled elective procedure on same day as procedure. Ever smoked, a history confirming any form of tobacco use in the past. Diabetes mellitus, a history of diabetes, regardless of duration of disease or need for anti-diabetic agents. Renal failure, documented fasting serum creatinine level of >2.0mg/dL. Hypertension, blood pressure exceeding 140/90mmHg or a history of high blood pressure, or the need for antihypertensive medications. Stroke, a central neurologic deficit persisting more than 24. Cardiomegaly, a cardiothoracic ratio >0.5 demonstrated on a preoperative chest film. COPD, a patient who requires pharmacologic therapy for the treatment of chronic pulmonary compromise, or a patient who has a FEV1 <75% of predicted value. Peripheral vascular disease, a history of aneurysm and/or occlusive vascular disease with or without previous extracardiac surgery. Cerebrovascular disease, any transient ischemia attack, reversible ischemic neurologic deficit, cerebrovascular accident/stroke, or history of cerebrovascular surgery. Myocardial infarction, the appearance of a new Q wave in two or more contiguous leads on ECG, or...having clinical, angiographic, electrocardiographic, and/or laboratory isoenzyme evidence of myocardial necrosis with an ECG showing no new Q waves. Congestive heart failure, the patient has at least three of the following: presence of dyspnea, rales thought to represent pulmonary congestion, peripheral edema, cardiomegaly on chest X-ray, chest X-ray compatible with interstitial edema. Arrhythmia, the presence of atrial or ventricular ectopy which may or may not require therapy...abnormal rapid ventricular rhythm causing hemodynamic collapse (tachycardia) or diffuse chaotic ventricular depolarization unable to produce and effective blood pressure (fibrillation)...intermittent failure of AV conduction manifested by electrocardiographic evidence of intermittently nonconducted P waves at physiological heart rates...multiple atrial foci that discharge without a single uniform atrial depolarization. There are no P waves and AV nodal conduction occurs in a nearly random fashion producing an irregular ventricular response. Preoperative diuretics, diuretics taken by the patient on a chronic basis within 30 days of surgery within 24h preceeding surgery. Left main disease, When there is >50% compromise of vessel diameter in any angiographic view. Perfusion time, The total number of minutes on cardiopulmonary bypass. Transfusion, The patient received red blood cells, fresh frozen plasma, platelets, or cryoprecipitate intra- or postoperatively. Prolonged mechanical ventilation, Patient required mechanical ventilation for >24h postoperatively. Perioperative myocardial infarction, new Q waves in 2 or more contiguous leads in postop 12 lead ECG. Predicted mortality was calculated from the Society of Thoracic Surgeons' risk assessment model [8].
2.2. Data analysis
The effect of CK-MB on survival in the 1161 operative survivors was tested in three ways: (1) Subdividing patients according to arbitrary CK-MB cutoffs may not allow a clear identification of a critical value above which outcomes are (or are not) adversely affected. We, therefore, computed KaplanMeier survival for CK-MB-based patient quintile groups with 67% overlapping ranging from the lowest to highest values as described by us previously [9]. In addition, we similarly analyzed the highest CK-MB decile group. From each of these curves, we extracted the survival values corresponding to 12, 24 and 48 months postCABG. This approach allows for a finer and more gradual description of the relation between the dependent (CK-MB) and independent variables (survival) and minimizes the possible effects of CK-MB heterogeneity within each analyzed subgroup. These results were then used to construct hazard functions depicting the rate of death per month for each of the groups. These functions are useful to identify the between-group variation in survival trends and the most critical period determining postoperative survival. Death hazard analysis versus time after CABG was done based on Blackstone's multiphase model via a custom made nonlinear regression routine (SigmaPlot 2000, SPSS Inc., Chicago, IL) [10]. CK-MB levels were then used to create Receiver Operator Characteristic curves. (2) Binary logistic regression for (a) all deaths and (b) deaths occurring in the first year and (3) Cox proportional hazard model for (a) all deaths and (b) all deaths occurring after the first year. In toto, 72 demographic, preoperative, intraoperative, and postoperative variables from the database were analyzed.
2.3. Statistical methods
Continuous variables were analyzed with either the unpaired t-test and presented as mean±SD or the nonparametric MannWhitney rank sum test and presented as median (interquartile range) depending on normality. Categorical variables were analyzed with the Chi-square test or the Fisher Exact tests depending on applicability. Binary logistic regression and Cox proportional hazard modeling was done by backwards selection and confirmed by forward selection. Because CK-MB values were not normally distributed, CK-MB values were logarithm-transformed (LN of CK-MB) and these values were used in the logistic regression and Cox proportional hazard modeling. Variables were entered if they were univariably significant at P<0.25 and were retained at P<0.05. Separate models were created to predict all deaths, those dying within their first postoperative year, and, after excluding all those who died in that year, on the remaining patients. When LN of CK-MB was not statistically significant, the multivariable models were re-calculated with LN-MB forced to remain in the models. However, the results of the other parameters are presented from the nonforced models. A P-value less than 0.05 indicated significance for all statistical tests. (SPSS 11 for Windows, SPSS, Chicago, IL).
| 3. Results |
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40ng/mL and died within 1 year of surgery, those who had CK-MB>40ng/mL and died within 1 year of surgery, and those who were alive 1 year after surgery.
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40ng/mL were not associated with any increase risk of death (Table 3-bottom). When forced into this model, logarithm of CK-MB was not a statistically significant predictor of late mortality: P=0.839 with a wide confidence interval that included 1.0. Next, Cox proportional hazard modeling was used on all those patients who survived at least 1 year after surgery. This found that CK-MB levels were not associated with death rate in patients who survived at least 1 year (Table 4
). Again, when logarithm of CK-MB was forced into this model, it was not statistically significant: P=0.203 with a wide confidence interval that included 1.0. Instead, it found that transfusion, longer perfusion time, and preoperative renal failure, COPD, congestive heart failure, and arrhythmia were predictive of death after 1 year.
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| 4. Discussion |
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Steuer et al. studied 4911 patients with CK-MB drawn the morning after CABG [3]. They examined the relationship between CK-MB and aspartate aminotransferase (AST) and postoperative mortality using Cox proportional hazards model. Outcome was examined over four time ranges: 030 days and 03, 36, >6 years of follow-up. Both elevated AST and CK-MB predicted several-fold increases in death within 30 days of CABG. Beyond 30 days, they found that patients with AST>2.35µ/L had 1.5-fold (1.11.9, 95% confidence interval) increase in death over all years of follow-up. CK-MB was dichotomously divided into
or >61µg/L. CK-MB>61µg/L was associated with a 1.5 P-fold (1.12.0, 95% confidence interval) increase in death over all years of follow-up. When examined over each time interval, CK-MB>61µg/L was associated with a nonsignificant increase in death: 1.3 fold (0.82.2) over 3 years, 1.4 fold (1.02.0) from 3 to 6 years, and 1.3 fold (0.82.2) beyond 6 years. By using 03 years, rather than 01 year as we did, without explaining why they chose that time interval, they may have missed an increased risk in the first year by averaging it with a longer time interval in which CK-MB is not a risk factor for death.
Brener et al. followed 3812 patients after first time CABG [4]. Follow-up was 1035±516 days after CABG. They found that unadjusted risk of death was constant for CK-MB
10 times the upper limit of normal (ULN), but was nearly threefold higher when CK-MB was >10 times ULN for 3 year outcome. After correction for other predictors, CK-MB>10 times ULN was associated with a 1.3 fold (1.11.5) increase in mortality. They did not evaluate risk over shorter time frames. We found a similar increased risk for CK-MB>8 times ULN. But by failing to examine risk over shorter time frames they may have missed important time-dependent effects of elevated CK-MB.
Costa et al. studied 496 patients with CK-MB obtained 6, 12, and 18h after CABG or longer if these values were elevated [5]. Of 57 patients with CK-MB levels>5 times ULN, 4 died within 30 days and 2 more within the next 11 months. Of 37 patients with CK-MB levels 35 times ULN, 2 died within 30 days and 0 within the next 11 months. The increased one-year risk that they found from higher CK-MB values can be explained entirely on deaths within 30 days.
There are four features specific to our analysis that distinguishes it from the four prior studies. These are we: (1) objectively identify the CK-MB threshold via the overlapping ranges method [9], (2) determined the CK-MB main effect interval (1-year) via CK-MB specific death hazard functions (Fig. 4) to find if and where the mortality rates became similar between patients with high and low CK-MB levels and used that time to divide our patient into early (<1 year) and late (>1 year) deaths [10], (3) confirmed the effect with ROC curves that elevated CK-MB is associated with 1-year mortality but not later mortality and 4) conducted this study in patients undergoing normothermic cardiopulmonary bypass.
The other studies, by being only six months duration [2], by being done in 3 and 6 year intervals [3], by using only one 3 year interval [4], and by using a small population with most of the deaths occurring within the first 30 postoperative days [5] may have missed important time-dependent changes in the risk of an elevated CK-MB.
The gradually decreasing risk with time that we found after CK-MB elevation is similar to that seen after a traditional myocardial infarction. There, patients have an increased risk of dying within the immediate postMI period, but over the succeeding several weeks to months the risk gradually decreases [1116]. The increased risk is probably related to myonecrosis. After PTCA, contrast-enhanced MRI showed discrete areas of myonecrosis only in the distribution of the target vessel and only in patients with elevated CK-MB [17]. Recently, Selvanayagam et al. showed a similar association in patients undergoing CABG [18]. Using contrast-enhanced MRI, they showed that 40% of patients undergoing CABG developed new areas of myonecrosis and that the size of myonecrosis correlated with enzyme levels.
There are several limitations to our study. First, CK-MB levels were drawn the morning after surgery rather than a constant number of hours after surgery. This would tend to blur the predictive power of CK-MB. Yet, the 1018h time interval after surgery before CK-MB determination should provide sufficient time for CK-MB to rise after myocardial injury [6]. Second, we do not have preoperative CK-MB levels on most patients and some of the postoperative elevation may have been present preoperatively. However, we doubt that this was a significant factor. We excluded all patients who had a myocardial infarction immediately preoperatively. We did include patients who had a myocardial infarction preoperatively when the CK-MB levels had returned to normal. Third, we did not confirm the presence of myocardial necrosis by other means, such as electrocardiograms or pyrophosphate scans. Previous studies have shown that enzyme elevations predictive of myocardial damage or nonoperative all cause mortality frequently occur without electrocardiographic or even pyrophosphate scan changes [5,1922]. Last, while we had no defined protocol for postdischarge medications, the same group of surgeons treated all patients and no major changes were made in therapy over the study duration. Specifically, all patients received aspirin and beta blockers unless clinically contraindicated. Other medicines were used as clinically appropriate.
In conclusion, a single determination of CK-MB the morning after CABG showing an elevation >40ng/mL is a risk factor for death. However after 1 year the increased risk loses its statistical significance. Patients with CK-MB>40ng/mL should be treated as if they had a myocardial infarction and should be followed closely.
| References |
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