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(Chest. 2002;121:103-111.)
© 2002 American College of Chest Physicians

Use of Biochemical Markers of Infarction for Diagnosing Perioperative Myocardial Infarction and Early Graft Occlusion After Coronary Artery Bypass Surgery*

Lene Holmvang, MD; Birgit Jurlander, MD, PhD; Christian Rasmussen, MD; Jens J. Thiis, MD; Peer Grande, MD, PhD and Peter Clemmensen, MD, PhD

* From The Heart Center, Rigshospitalet, Copenhagen University Hospital, Denmark.

Correspondence to: Lene Holmvang, MD, The Heart Center B-2141, Rigshospitalet, Copenhagen University Hospital, Blegdamsvej 9, 2100 Copenhagen Ø, Denmark; e-mail: lene.holmvang{at}dadlnet.dk


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Conclusion
 References
 
Study objectives: Perioperative myocardial infarction (PMI) during coronary artery bypass grafting (CABG) is an important clinical problem because it is closely associated with increased morbidity and mortality. The diagnosis of PMI is, however, associated with several problems. Due to the surgical trauma, the usual indicators of myocardial infarction (pain, ECG changes, and elevated biochemical markers of infarction) have uncertain diagnostic value. The primary aim of this study was to illustrate the levels of the biochemical markers after uncomplicated bypass surgery defined as no clinical or ECG evidence of PMI, and no graft occlusion at 7 days by repeat angiography; and secondarily, to establish biochemical diagnostic discrimination limits for detection of in-hospital graft occlusion.

Methods and results: One hundred three patients undergoing elective CABG were closely monitored by serial measurements of creatine kinase (CK)-MB mass, myoglobin, troponin T, and troponin I, and underwent a repeat angiography before discharge. Seven patients had ECG evidence of PMI. Peak troponin T and CK-MB values were significantly higher in these seven patients, although the diagnostic performances of the optimally chosen cutoff levels for diagnosing AMI were fair. Twelve patients had at least one occluded graft shown by repeat angiography. Peak values of CK-MB and troponin T were significantly higher in patients with graft occlusion (52.2 µg/L vs 24.7 µg/L, p = 0.01; and 3.7 µg/L vs 1.0 µg/L, p = 0.05, respectively). By multivariate analysis, a diagnostic discrimination level of 30 µg/L for CK-MB did not reach statistical significance; however, the independent diagnostic value of a cutoff level for troponin T at 3 µg/L reached a level of significance (p = 0.06).

Discussion: We have suggested normal values of four different biochemical markers of infarction after uncomplicated coronary bypass surgery. Patients with in-hospital graft occlusion had higher peak CK-MB and troponin T values. However, the overlap with patients without graft occlusion is substantial, and the patency status in the individual cannot be reliably predicted from these noninvasive tests.

Key Words: bypass surgery • ECG • grafting • troponins


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Conclusion
 References
 
Perioperative myocardial infarction (PMI) during coronary artery bypass grafting (CABG) is an important clinical problem because it is closely associated with increased morbidity and mortality.1 2 In the early postoperative phase, patients are usually monitored with frequent biochemical analysis and routine ECG recordings. Myocardial infarction diagnosed by these two methods is associated with adverse outcome.3 4 Several studies have addressed the issue of diagnosing PMI by serial blood sampling for determination of creatine kinase (CK)-MB5 6 7 8 troponin I,8 9 10 11 and troponin T,12 and various cutoff limits have been suggested.10 The diagnostic performance of these markers in the "nonsurgical" setting is very well described in numerous studies.13 14 15 Due to the surgical trauma, the diagnostic discrimination limits in patients after CABG remains arbitrary. ECG ST-segment and QRS changes have also been used to diagnose PMI,3 5 9 10 but in all these studies the diagnosis has not been confirmed by other tests.

In order to suggest discrimination levels for diagnosing PMI, knowledge of the ranges of biochemical markers of infarction after uncomplicated CABG is essential. Early ischemia or infarction after CABG is most likely to be due to problems with the inserted grafts. Graft occlusion can be caused by thrombosis due to poor quality of the graft or recipient artery, by technical deficiencies related to the newly inserted graft, or by the size of the native coronary artery. A study16 of patients undergoing CABG has found graft patency to relate directly to prognosis. Thus, early detection of incomplete revascularization may prompt reintervention aimed at preventing or limiting myocardial damage and thus potentially improve prognosis. Coronary angiography remains the "gold standard" for assessment of graft patency, but this invasive procedure is not routinely performed after CABG.

No study has yet directly investigated the relationship between early graft occlusion, levels of biochemical markers, and ECG changes postoperatively. The present study investigates 103 patients undergoing elective CABG. The first aim of the study was to describe serial biochemical measurements in 64 of 103 patients who had an uncomplicated surgical procedure and in-hospital course, with patent grafts demonstrated by repeat angiography at discharge. The second aim was to compare the concentrations of the biomarkers in patients with and without ECG evidence of PMI in patients without early graft occlusion. Finally, the diagnostic abilities of biochemical markers of infarction, ECG changes, and procedure-related variables for identification of patients with in-hospital graft occlusion after CABG were evaluated.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Conclusion
 References
 
Setting
This was a single-center, prospective study performed at the Heart Center, Rigshospitalet, Copenhagen University Hospital, Denmark.

Patient Population
The initial study population consisted of 124 consecutive patients undergoing elective CABG who fulfilled the inclusion criteria and accepted participation in the study. Inclusion criteria were age > 18 years and need for elective myocardial revascularization for angina pectoris. A total of 108 patients underwent repeat angiography, and 103 of these patients had complete ECG and biochemical data sets.

Operative Procedures
The CABG was performed using cold-crystalloid cardioplegic arrest, moderate systemic hypothermia, topical cooling with ice slush, and single aortic cross-clamp for all distal anastomoses. The internal mammary arteries (IMAs) and the saphenous veins were the preferred conduits.

Postoperative Management
Aspirin therapy was restarted within 24 h after surgery, and all patients were monitored with continuous registration of arterial pressure, left atrial pressure, central venous pressure, and a 12-lead ECG.

Procedure-Related Data
Goldman et al17 reported several procedure-related variables to be associated with patent grafts 3 years after surgery: cross-clamp time <= 80 min, vein preservation solution temperature <= 5°C, bypass time <= 2 h, two or less proximal anastomoses, continuous vs intermittent cross-clamp technique, and recipient artery diameter. Thus, an attempt was made to collect these data on the patients in the present study. The cross-clamp technique was continuous in all cases, and the veins were preserved at room temperature. Demographic data and other baseline clinical characteristics, as well as data on the surgical procedure, were collected from the surgery reports and patient charts for inclusion in a multivariate analysis.

Angiographic Data
A diagnostic repeat angiography was performed on days 5 to 7 after surgery by an experienced invasive cardiologist. The patency of all inserted grafts and native coronary arteries were determined. Only a total occlusion of 100% was considered significant. An experienced cardiologist blinded to other patient data decided graft patency status.

Biochemical Data
During the postoperative period, 16 blood samples were drawn from each patient at every other hour during the first 20 h after surgery, at 24, 30, 36, and 48 h after CABG, and finally on days 3 and 5. The samples were analyzed separately for CK-MB mass, troponin I, and myoglobin using an Opus Magnum (Behring Diagnostics; Frankfurt, Germany) based on the principle of two-site immunoassay using polyclonal antibodies to recognize epitopes unique to CK-MB mass, troponin I, and myoglobin. Measurements of troponin T were analyzed using an ES 300 analyzer (Boehringer Mannhein GmbH; Mannheim, Germany) using a single-step sandwich principle with streptavidin-coated tubes as the solid phase and two monoclonal, antihuman troponin T antibodies. The release patterns for the cardiac markers in the patients with an uncomplicated course are depicted by time-concentration curves (median values). When comparing the group of patients with occluded and patent grafts, all comparisons had to be analyzed using peak concentrations. This simplification had to be adopted because comparisons at specific time points would be useless because the actual timing of graft occlusion was unknown.

ECG Data
Each patient had a standard 12-lead ECG recorded before surgery. During the initial 24 h after surgery, each patient underwent three or four standard 12-lead ECGs; in addition, the patients underwent ECG on days 3 and 5 after surgery. Each ECG obtained before repeat angiography was analyzed regarding QRS changes, ST-segment deviations, and T-wave amplitude to determine the occurrence of PMI. The QRS complex was scored according to a previously described scoring system for assessment of myocardial injury.18 New Q waves in more than two leads and new persistent bundle branch block were registered. In all available postoperative ECGs, the number of leads with ST-segment elevation or depression (>= 1 mm, measured 60 ms after the J point) as well as the sum of deviation in all leads except aVR and the number of leads with inverted T waves (>= 0.1 mV, except for V1) were determined. The ECG analysis was performed without knowledge of biochemical and angiographic data.

PMI was present if the patient met at least one of the following criteria: (1) new Q waves >= 40 ms in two consecutive leads on at least two post-CABG ECGs; (2) new R waves >= 40/50 ms in V1/V2 on at least two post-CABG ECGs; or (3) new, persistent, complete bundle-branch block compared to the pre-CABG ECG. Additionally, the ECG finding was considered abnormal if an increase in QRS score of >= 3 points was found at day 5 compared with the ECG obtained before surgery. These criteria have previously been validated for diagnosing "spontaneous" myocardial infarction but not myocardial infarction related to cardiac surgery.18

Statistical Tests
Values are given as medians followed by 25 to 75% quartiles. Mann-Whitney U test was performed to compare median values between patients with or without graft reocclusion and with or without PMI, determined by ECG criteria. The {chi}2 test was used to compare frequencies between subgroups. A Cox regression model using backward elimination strategy was constructed for multivariate analysis of the variables that reached a level of significance of 0.20 by univariate analysis.

Ethics
The local ethical committee approved the study, and the patients participated after giving informed consent.


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Conclusion
 References
 
Baseline Characteristics
The 124 patients had a total of 292 grafts inserted. The grafts inserted were 112 left IMA/right IMA and 180 vein grafts. The demographic data as well as procedure-related data are depicted in Table 1 .


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  		Table 1.. Baseline Characteristics and Procedure-related Variables*

 
Angiographic Results
In six patients, the graft status could not be assessed because of the patient’s refusal to undergo repeat angiography or because of poor procedure quality. One hundred eight patients underwent repeat angiography as outlined in the protocol. In 10 patients (8%), the procedure was performed earlier at the discretion of the attending surgeon. The reasons for this decision are depicted in Table 2 .


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  		Table 2.. Reasons for Early, Nonscheduled Repeat Angiography*

 
Fifteen patients (12%) had a total of 16 occluded grafts. Thirteen patients had an occluded vein graft (8% of inserted grafts), and 2 patients had an occluded left IMA (2% of inserted conduits). One patient had two occluded vein grafts. A subtotal occlusion > 70% was found in an additional three patients with left IMA grafts. Less severe stenosis of > 50% but < 70% were found in four patients with left IMA grafts and five patients with vein grafts. Angiographic and serial biochemical data were available in a total of 103 patients, of whom 12 patients had graft occlusion.

Biochemical Data and the "Uncomplicated" Course
Table 3 outlines the criteria defining complications related to the surgical procedure or the postoperative period. A total of 64 patients did not fulfill any of these criteria and were considered to have had an uneventful surgical procedure. Table 4 depicts peak biochemical values in the 64 patients who underwent an uneventful procedure. Figure 1 illustrates the biochemical values measured during the postoperative hours and postoperative days.


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  		Table 3.. Criteria Defining a Complicated Surgical Procedure and/or Postoperative Course

 

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  		Table 4.. Peak Biochemical Values in Patients With an Uneventful Surgical Course (n = 64)

 


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  		Figure 1.. Biochemical values measured during the postoperative hours and days in 64 patients with an uncomplicated surgical procedure (median values [25–75% percentiles]).

 
Proposed Diagnostic Levels of Biochemical Markers for PMI
Ten patients had either new Q waves or new left bundle-branch block on the post-CABG ECGs, which are established criteria for PMI.5 6 The diagnosis was angiographically confirmed in three of these patients with occluded grafts. In the remaining seven patients, the biochemical markers were compared with the values in the patients with neither ECG evidence of PMI nor occluded grafts (n = 84). Peak CK-MB and troponin T values were significantly higher in the seven patients with ECG signs of PMI (p = 0.04 and p = 0.02, respectively; Fig 2 ). The distributions of the biochemical values are depicted in Figure 3 . Using the ECG criteria as "gold standard," the best discrimination levels for diagnosing PMI was determined by receiver operating characteristic (ROC) analysis (data not shown). The optimal discrimination level for CK-MB was 30 µg/L (sensitivity 0.57, specificity 0.67) and for troponin T was 1.75 µg/L (sensitivity 0.71, specificity 0.68). Thus, 32% of the patients without ECG signs of PMI and with open grafts had troponin T values above the suggested cutoff level. Of the 64 patients with peak troponin T values < 1.75 µg/L, 4 patients showed graft occlusion at repeat angiography and 2 patients had new Q waves on the post-CABG ECG (9.5%).



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  		Figure 2.. Left: ROC curve of sensitivity and specificity of various concentrations of CK-MB mass for prediction of early graft occlusion. Right: ROC curve of sensitivity and specificity of various concentrations of troponin T for prediction of early graft occlusion.

 


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  		Figure 3.. Top: distribution of peak CK-MB mass values in patients with ECG evidence of PMI (n = 7) and without PMI (n = 84). Bottom: distribution of peak troponin T values in patients with ECG evidence of PMI (n = 7) and without PMI (n = 84). Min = minimum; Max = maximum.

 
Noninvasive Detection of In-hospital Graft Occlusion
The ECG changes in the patients with and without early graft occlusion are depicted in Table 5 . No difference in postoperative ST-T segment changes could be determined. The peak biochemical values in patients with and without graft occlusion are shown in Table 6 . The peak values of CK-MB mass and troponin T were significantly higher in patients with graft occlusion, while no differences were seen regarding troponin I and myoglobin. To determine the optimal discrimination values for detection of graft occlusion, ROC analyses for the two significant markers revealed the optimal discrimination values at 30 µg/L for CK-MB (sensitivity 67%, specificity 65%) and 3 µg/L for troponin T (sensitivity 67%, specificity 76%).


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  		Table 5.. ECG Changes in Relation to Graft Patency Status*

 

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  		Table 6.. Peak Biochemical Values in Patients With or Without Graft Occlusion Within 7 Days After Surgery*

 
The independent predictive value of the serum markers and other important clinical variables is presented in Table 7 . Univariate analysis revealed peak troponin T >= 3 µg/L and the presence of more than two proximal anastomoses to be significantly associated with early graft occlusion. In a multivariate statistical model, only troponin T >= 3 µg/L was found to carry independent diagnostic information at a near significant level (p = 0.06). Identification of graft occlusion based on "clinical criteria" (Table 2) obtained a sensitivity of only 20% compared to a sensitivity of 75% for troponin T alone.


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  		Table 7.. Multivariate Analysis of Risk Factors Related to In-hospital Graft Occlusion*

 

   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Conclusion
 References
 
Until the present, the value of obtaining biochemical markers after CABG has been uncertain because the interpretation of the data was questionable. In the present study, we suggest "normal" ranges of biochemical values after CABG in 64 patients who had an uneventful course with no in-hospital graft occlusion and a very low likelihood of PMI. The values reflect the myocardial injury caused by the surgical procedure alone and could be used in the future as normal biochemical values to be expected after CABG. Until the present, our institution has used a discriminative CK-MB mass value of 10 µg/L for diagnosing acute myocardial infarction in nonoperated patients. This discrimination value cannot be used for detection of PMI or graft occlusion because incomplete myocardial protection during cardiopulmonary bypass, use of extracorporal circulation, autotransfusion, or skeletal muscle trauma during surgery, or direct-current shock may cause release of biochemical markers of infarction. As demonstrated, a CK-MB mass value > 35.8 µg/L can be detected in 25% of post-CABG patients with otherwise no indications of ischemic complications. Of course, PMI in the absence of ECG changes is possible. However, since pain and elevation of biochemical markers are very uncertain parameters after cardiac surgery, ECG changes, clinical observations, and graft status are the best available data for excluding PMI in the present setting. Previous studies have suggested a threshold level of 30 µg/L for CK-MB mass6 or threefold the upper normal limit for the reference laboratory19 after CABG; however, these thresholds were arbitrarily selected.

In this study population, we tried to determine biochemical diagnostic discrimination levels for PMI, using conventional ECG criteria as the "gold standard." The diagnostic performance of the best biochemical markers (CK-MB and troponin T) was fair, with sensitivities at 0.57 and 0.71 and specificities at 0.67 and 0.68, respectively. Thus, one third of the patients without ECG evidence of PMI had biochemical values above the suggested cutoff level. A recent study by Carrier et al10 suggested a diagnostic discrimination level for troponin T at 3.4 µg/L for diagnosing PMI after CABG. The population in that study consisted of 493 patients, and a high sensitivity and specificity was obtained by using that cutoff level when CK-MB, ECG criteria, and myocardial scan was used as "gold standard." The authors10 also showed that elevated troponin T after CABG correlated with a higher rate of in-hospital mortality and postoperative morbidity. Repeat angiography to determine graft status was not performed. In our study as well as in other studies,8 10 the use of only ECG criteria and CK-MB values as "gold standard" for PMI implies a risk of overestimating the diagnostic ability of the newer biochemical markers, because increased levels of CK-MB (for any reasons) and increased levels of other biochemical markers hardly are independent parameters. Conversely, by using only new Q waves as diagnostic criteria for PMI, subendocardial infarctions will be missed.

A review by Califf et al20 recommend preprocedural and postprocedural ECGs combined with serial measurements of CK-MB for identification of patients with procedure-related myocardial infarction. However, the published consensus document21 of the Joint European Society of Cardiology/American College of Cardiology Committee for the Redefinition of Myocardial Infarction points out the difficulties in diagnosing PMI defined as myocardial damage due to coronary artery occlusion, because myocardial damage can be caused by different mechanisms, including direct trauma during the surgical procedure.22 Nevertheless, the consensus report states that the higher the value for the cardiac biomarker, the greater the amount of damage to the myocardium, irrespective of the mechanism.

Preoperative characteristics of patients are known to influence the outcome after CABG. Age at the time of surgery, left ventricular function, ventricular arrhythmia, duration of surgery, and type and number of grafts are influencing factors.23

A previous retrospective study24 found that the majority of patients presenting with myocardial ischemia after CABG had either graft failure, or incomplete or even inadequate revascularization demonstrated by repeat angiography. The present prospective study confirms that early (within 7 days) graft occlusion is not uncommon, occurring in 8% of vein grafts and 2% of IMA conduits. These occlusion rates are in accordance with previous findings.25 26 Importantly, these early graft occlusions are potentially detectable because they are associated with a rise in serum concentration of biochemical markers of infarction.

Several studies16 17 27 28 have investigated the various factors influencing graft patency after surgery. The management consequences of identifying patients with early, in-hospital graft occlusion remain controversial. In patients with catastrophic events, such as sustained ventricular tachyarrhythmias or acute heart failure, prompt reoperation seems the only option.24 However, in patients with evidence of myocardial infarction but without heart failure, there are few data to support a decision of conservative vs aggressive management. A conservative strategy would imply intensive medical treatment during hospitalization and observation of the patient after discharge for symptoms of recurrent angina or congestive heart failure before deciding on reintervention. An aggressive approach would dictate acute repeat angiography to determine whether reoperation or percutaneous transluminal coronary angioplasty (PTCA) can correct the underlying problem.24 Due to the lack of randomized trials comparing these two approaches, the decision is left to the surgeon. Reliable, noninvasive diagnostic tools must be available for identification of patients with abrupt graft occlusion, since routine repeat angiography after CABG is inappropriate due to its inherent risk and costs.

In our study, ST-segment deviation and T-wave inversion, usually associated with acute ischemia, were not related to graft occlusion. Peak troponin T values > 3 µg/L were the only independent predictor of in-hospital graft occlusion, and the levels of the other biochemical markers of infarction were generally higher in the patients with occluded grafts compared with the patients with patent grafts. Surprisingly, procedure-related variables, including the administration of aprotinin, showed no significant prognostic value.

Even though the present study indicates that ECG, clinical, and serial postoperative biochemical data can identify a subgroup of patients with a high rate of early graft occlusion after CABG, there are several problems. Noninvasive discrimination between graft occlusion and PMI is difficult. Interpretation of the ECG data are hampered because pericardial involvement and change in heart position might cause ECG changes without concomitant graft occlusion. Chest pain as an indicator of reocclusion is a dubious parameter after CABG (compared to PTCA patients) due to the surgical trauma, and elevated concentrations of biochemical markers may also be related to suboptimal cardioplegia, extracorporal circulation, direct surgical trauma, and other factors.22

Results of a 1998 study of restenosis after PTCA28 and our data indicate, however, that symptoms of reocclusion differ between PTCA patients and CABG patients. At our institution, repeat angiography shortly after CABG is performed in all patients fulfilling one or more of the following criteria: new localized changes in the ST-segment, CK-MB values > 80 U/L, new Q waves in the ECG, recurrent or sustained ventricular tachyarrhythmia, ventricular fibrillation, or hemodynamic deterioration with symptoms of left ventricular failure despite inotropic support. This procedure has previously demonstrated graft failure or incomplete revascularization in the majority of the patients fulfilling the criteria.24

Some of the limitations associated with the ECG data might be solved if reliable continuous ECG ischemia monitoring in multiple leads could be performed in the ICU.30 In the near future, newer noninvasive imaging techniques like contrast echocardiography or ultrahigh-speed MRI might overcome some of the limitations associated with invasive angiography.31


   Conclusion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Conclusion
References
 
The present study is unique because all patients underwent angiography after surgery regardless of clinical symptoms, thus giving the opportunity to estimate the overall graft occlusion rate with greater accuracy. The study concludes that noninvasive identification of early graft occlusion and especially PMI after CABG remains difficult. There are, at this time, no reliable, independent markers of PMI available. Thus, the collection of multiple blood samples for diagnosing PMI may not be cost-effective. However, our results indicate that serial postoperative biochemical data—preferably CK-MB mass and troponin T—can identify a subgroup of patients with a high rate (20 to 27%) of early graft occlusion. The selection of patients for repeat angiography based on simple clinical judgment has poor sensitivity. Patients with early, angiographically documented graft occlusion have significantly higher incidence of new Q waves and higher peak CK-MB and troponin T values. Despite these subtle differences in ECG changes and markers of myonecrosis, the overlap with patients without graft occlusion is substantial and the patency status in the individual cannot be reliably predicted from these noninvasive tests.

Study Limitations
The number of patients limits generalizability of the findings in the present study. However, 124 patients is a fairly high number for a single institution and it ensures the inclusion of a broad population of patients undergoing elective bypass surgery.

When there are a large number of potential explanatory variables related to a certain event (in this case, graft occlusion), some of the variables could be expected to be significant just by chance. To reduce this possibility, we carefully chose only variables for univariate analysis that, based on previous findings, were expected to be of importance. Suggesting cutoff values for identification of patients with graft occlusion and subsequently testing the predictive values of the discrimination limits in the same population is not optimal. The suggested cutoff values for identification of a population with a high likelihood of graft occlusion must be tested prospectively in another population.


   Footnotes
 
Abbreviations: CABG = coronary artery bypass grafting; CK = creatine kinase; IMA = internal mammary artery; PMI = perioperative myocardial infarction; PTCA = percutaneous transluminal coronary angioplasty; ROC = receiver operating characteristic

The study was supported by The Lykfeldt Foundation and The Danish Heart Foundation through a grant from the Jens Anker Andersen Foundation.

Received for publication November 2, 2000. Accepted for publication July 17, 2001.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Conclusion
 References
 

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