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Clinical Chemistry 51: 40-46, 2005; 10.1373/clinchem.2004.041103
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(Clinical Chemistry. 2005;51:40-46.)
© 2005 American Association for Clinical Chemistry, Inc.

Evidence-based Laboratory Medicine and Test Utilization

Cardiac Troponin I Concentrations, but Not Electrocardiographic Results, Predict an Extended Hospital Stay after Coronary Artery Bypass Graft Surgery

Robert F. Salamonsen1, Hans-Gerhard Schneider2,3,a, Michael Bailey4 and Andrew J. Taylor5

1 Intensive Care Unit, 2 Clinical Biochemistry Unit, and 5 Department of Cardiology, Alfred Hospital, Melbourne, Australia.
Departments of 3 Medicine and 4 Epidemiology and Preventive Medicine, Monash University, Melbourne, Australia.

aAddress correspondence to this author at: Clinical Biochemistry Unit, Alfred Hospital, Commercial Road, Melbourne 3004, Australia. Fax 61-3-9276-3781; e-mail schneiderh{at}alfred.org.au.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Background: Cardiac damage in coronary artery graft (CABG) surgery is an important contributor to postoperative cardiac dysfunction and delayed hospital discharge. Currently, no simple method exists for its quantification.

Methods: In a prospective study of 300 patients having routine CABG surgery, we compared cardiac troponin I (cTnI) concentrations at 6 and 24 h after surgery with electrocardiographic (ECG) results as predictors of an extended postoperative stay in the intensive care unit (ICU) and in the hospital. We stratified outcome variables by tertiles of cTnI concentration and studied the significance of differences between outcome variables across tertiles.

Results: Multivariate analysis showed that 24-h cTnI is a significant predictor of increased postoperative ICU stay (P = 0.012) and postoperative hospital stay (P = 0.024). For 6-h cTnI, corresponding significance values were P = 0.29 and 0.9. ECG was of no value (P = 0.39 and 0.47). Differences in 24-h cTnI were highly significant, particularly for lowest vs highest tertiles, and allowed stratification of risk into "low" (<10 µg/L), "equivocal" (10–20 µg/L), and "high" (>20 µg/L).

Conclusions: Use of a single 24-h cTnI value to quantify perioperative myocardial damage identifies patients who are at greater risk of extended ICU and hospital stays. This strategy could assist in allocation of patients to different management streams after CABG surgery.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Coronary artery bypass graft (CABG)1 surgery is a cornerstone in the management of coronary artery disease (1), but myocardial damage still complicates the postoperative course for a significant number of patients (2), necessitating longer in-hospital stays and leading to increased morbidity. To further streamline postoperative management and to balance financial constraints with clinical goals, identification of those patients at high risk of a complicated postoperative course is highly desirable.

Although the influence of preoperative factors on adverse patient outcome is well defined (3), it is also recognized that the CABG procedure may itself induce perioperative myocardial ischemia and consequent myocardial damage (4), which in itself is an important determinant of increased postoperative morbidity, leading to longer postoperative stays in the intensive care unit (ICU) and the hospital. The identification of new Q-waves on postoperative electrocardiography (ECG) is a reliable sign of infarction (5). In many instances, however, perioperative ischemia leads to nontransmural infarction, which presents with less striking ECG abnormalities that are difficult or even impossible to differentiate from nonsignificant postoperative changes (6), leading to a sensitivity as low as 50% for ECG in the diagnosis of post-CABG infarction (7). This problem is compounded by the fact that postoperative ischemia is often silent, making the diagnosis even more challenging (2). As a result, the efficacy of routine screening with perioperative ECGs for postoperative infarction and hence risk stratification is severely limited.

In contrast to ECG, measurement of cardiac troponin I (cTnI) permits the identification of even minor cardiac damage with high diagnostic accuracy (8). cTnI measurement has already been shown to be highly effective for identifying higher-risk patients in several clinical scenarios, including after percutaneous coronary interventions (9) and peripheral vascular surgical procedures (10). The high diagnostic accuracy of this test and its predictive power have been demonstrated in studies examining early use of platelet glycoprotein IIb/IIIa inhibitors in unstable angina (11) and has led to the routine use of cTnI status in chest pain evaluation centers worldwide for the diagnosis of unstable angina (12)(13) and risk stratification.

Although the routine postoperative measurement of cTnI represents a promising means for risk stratification of CABG patients, previous studies in this area have focused largely on its ability to predict postoperative Q-waves on ECG, which in itself is highly insensitive in predicting infarction. Traditionally, the combination of ECG and increases in cardiac marker concentrations has been used to define acute myocardial infarction (5)(6)(14). Recently, cardiac markers, particularly troponins. have become the new standard in diagnosing acute myocardial infarction (8) because of their superior sensitivity and specificity for cardiac tissue. Because there is no true "gold standard" for perioperative infarction in CABG patients, we examined the relationship between cTnI and ischemic changes in the ECG as predictors of postoperative morbidity in terms of extended ICU and hospital stay. In doing so, we propose a method for the identification of patients at higher risk after CABG surgery, which in the future could enable more effective stratification of postoperative care.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
This study was approved by the institutional ethics committee, and all procedures followed were in accordance with institutional guidelines. All patients presenting for routine coronary artery bypass surgery with cardiopulmonary bypass who gave informed consent were included in the trial. Of the 300 consecutive patients recruited, 31 had to be excluded because of a preoperative troponin I value above the upper limit of the reference interval (≥0.6 µg/L), indicating preoperative cardiac damage. Management of the anesthesia and the cardiopulmonary bypass procedure was standardized to conform with protocols established in our institution. At the time this study was performed (2000–2002), cTnI concentrations were not recognized as powerful indicators of postoperative risk in CABG patients and were not part of our protocols for discharge from the ICU or hospital after surgery.

Patients were designated by the ICU physician as fit for transfer from the ICU as soon as predefined criteria were satisfied: extubated and able to maintain breathing independently, acceptable blood pressure maintained without inotropic support, a stable cardiac rhythm, sufficient mental function to allow step-down ward care, and acceptable renal function (urine output >40 mL/h, with stable serum creatinine, potassium, and acid-base status). The time of being fit for discharge from ICU was recorded in the ICU chart and is taken as the ICU discharge time.

This study was divided into two sections. We first compared the ability of 6- and 24-h cTnI concentrations with the ECG results to predict extended ICU and hospital stays. We then stratified cTnI concentrations into ranges predictive of various degrees of extended ICU and hospital stays to form the basis of clinically relevant indicators of cardiac damage. Troponin I concentrations were measured 6 and 24 h after surgery (measured as arrival in ICU—within 1–2 min of discharge from operating theater).

ecg analysis
ECGs were recorded immediately preoperatively, on admission to the ICU, and on first postoperative day and were subsequently examined by a cardiologist blinded to outcome, who used the Minnesota Criteria for scoring ischemic changes (15). ECGs were scored as negative, nonspecific changes, and "Q-wave" changes (I–III).

cTnI was determined on the AxSYM analyzer (Abbott Laboratories), a frequently used assay (16). We have previously shown that cTnI concentrations >2.0 µg/L with this assay predict 30-day mortality in patients presenting to the emergency department (17). Samples were collected in lithium heparin tubes and processed immediately, and the results were available to the clinician within 1 h after receipt of the sample. The assay has a detection limit of 0.24 µg/L, and the 99th percentile in 989 healthy volunteers has been determined as 0.38 µg/L (18).

statistical analysis
All analyses were conducted with SAS, Ver. 8.02 (SAS Institute Inc.). Outcome variables were assessed for normality and were log-transformed when appropriate.

Univariate analysis was conducted with Student t-tests, Pearson correlation coefficients, {chi}2 tests for equal proportion, and Kruskal–Wallis tests. Multivariate analysis was conducted with multiple linear regression with a stepwise selection procedure and validated by use of a backward elimination procedure. Results for continuous variables are presented as the mean (SE) or as a geometric mean (95% confidence interval) when log-transformations were required. Categorical variables are reported as prevalences (95% confidence intervals). P values <0.05 were considered significant.

Because indices of myocardial damage can be easily categorized as negligible, equivocal, and marked risk, ranked cTnI concentrations and associated outcome indices were stratified into tertiles for further analysis. The magnitude of differences in values for ICU and hospital stay together with measurements chosen to indicate end-organ dysfunction across the three strata of cTnI values are reported as medians.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
We enrolled 269 consecutive patients admitted to hospital for elective CABG surgery in this prospective study; 22 of those presented for a redo operation. Perioperative mortality was 0.7%, and there were no operations other than those required for early postoperative management of acute bleeding during the hospital stay.

The patients’ risk factors are listed in Table 1 , which also shows statistical significance of P ≤0.2 for the two main outcome indices, postoperative ICU stay and total postoperative hospital stay, on univariate analysis. cTnI concentrations at 6 and 24 h after surgery were superior to the ECG. The latter barely achieved significance for intensive care stay (P = 0.03) and was not a significant predictor of postoperative hospital stay (P = 0.21). cTnI concentrations at 24 h were significant predictors for both outcome indices (P <0.0001 for both) and also showed higher significance than cTnI concentrations at 6 h (P = 0.005 and 0.0006). The ratio of 24-h cTnI to 6-h cTnI achieved a significance intermediate between the 24-h cTnI and the 6-h cTnI values taken in isolation. Other variables not achieving the required significances were body mass index, diabetes, and number of distal anastomoses. Preoperative cTnI concentrations >0.5 µg/L were associated with prolonged postoperative stay in both the ICU and hospital (P <0.05), whereas lower concentrations were not, thus confirming the appropriateness of the cutoff value for acceptable preoperative cTnI concentrations (data not shown).


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  		Table 1. Univariate statistics for predictors of extended ICU and hospital stays.

The strength of cTnI concentrations at 24 h as predictors of both ICU stay and hospital stay, as demonstrated by multivariate analysis, is shown in Table 2 . Only 24-h cTnI was significant (P = 0.012 and 0.024). The negative slope coefficient (ß) for cTnI at 6 h is explained by co-linearity errors with the 24-h cTnI. Repeat analyses with 6-h cTnI alone corrected ß, but it remained nonsignificant for both ICU and hospital stay. Repeat analyses of ECG score alone also did not predict these variables. The ratio of 24-h cTnI to 6-h cTnI also was not significant in its own right and tended to reduce the significance of the 24-h value. The partial r2 listed in Table 2 allows estimation of percentage contribution of the predictor variable to the total variation in the outcome variables accounted for by the model. For ICU stay, 24-h cTnI accounted for 4.0% of the model variance out of a total contribution of 20.3% for all identified predictors. For hospital stay, 24-h cTnI accounted for 2.2% of the model variance out of a total of 22.2% for all identified predictors. The strongest predictor was bypass time, which accounted for 10.0% of variation in ICU stay and 12.7% variation in hospital stay.


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  		Table 2. Multivariate statistics for predictors of extended ICU and hospital stays.1

To further categorize patients with different cTnI concentrations after CABG surgery, we divided our population into tertiles according to the absolute cTnI concentrations at 6 and 24 h after CABG surgery. As shown in Table 3 , 24-h cTnI showed highly significant differences in medians of outcome variables across low vs high tertiles. The differences were less significant for 6-h cTnI. The distribution of individual values for the hospital stay is shown in Fig. 1 . Despite the wide scatter, particularly in the high tertiles, the difference in medians achieved statistical significance for both the 6-h (P = 0.008) and 24-h (P <0.0001) cTnI concentrations. Differences between high vs middle and middle vs low tertiles were less impressive and are not detailed in Table 3 . This also applied to measurements indicating postoperative organ dysfunction. The differences in median values for ICU stay and hospital stay could be used to predict differences in postoperative ICU and hospital residence times across high vs low tertiles. For the 24-h cTnI concentrations, there was a difference of 6 h in ICU stay and 1.5 days in hospital stay. For 6-h cTnI concentrations, the differences were reduced to 3 h and 0.5 days, respectively.


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  		Table 3. Significance of differences in outcome variables for high vs low tertiles of cTnI at 6 and 24 h after surgery.1



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  		Figure 1. Length of postoperative hospital stay across tertiles of ranked cTnI concentrations.

Error bars indicate the 10th and 90th percentiles. Top and bottom limits of each box indicate the 75th and 25th percentiles, {blacksquare} indicates the mean, and the line inside each box indicates the median value.

We also tested the difference in the ratio between 24-h and 6-h cTnI concentrations across high vs low tertiles. Although achieving significance for ICU and hospital stay (P <0.004), the 6-h cTnI was a mediocre predictor of organ dysfunction and is not reported in detail.


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Our data indicate that cTnI concentrations 6 and 24 h after CABG surgery consistently outperform ischemic ECG changes as indicators of extended ICU and hospital stay outcome in both univariate and multivariate analyses, with cTnI measurements 24 h after surgery achieving the greatest predictive power. This superiority was demonstrated not only for ICU and hospital stays; the 24-h cTnI concentrations also indicated organ dysfunction postoperatively. cTnI peaks around 24–36 h have been reported for patients suffering perioperative myocardial infarctions (19)(20)(21)(22) and patients who developed postoperative organ dysfunction (20)(23)(24)(25), but to our knowledge, this study is the first to prospectively assess the relationship of increased postoperative cardiac troponin concentrations and serial ECGs with postoperative morbidity in CABG patients, permitting a direct comparison of their predictive power. Furthermore, the measurement of cTnI both 6 and 24 h after CABG surgery in our study provides useful data on the likely pathophysiology of cTnI release after cardiac surgery.

In our multivariate analysis (Table 2Up ), a portion of the length of stay in ICU was predicted by length of surgery, 24-h cTnI, and preoperative creatinine, and the length of stay in hospital was predicted by length of surgery, 24-h cTnI, and preoperative hemoglobin. Although the length of surgery was the most powerful predictor of length of stay in the ICU and hospital, this might also indicate the degree of difficulty of the surgical approach.

The mediocre performance of the 6-h cTnI value, also reported by others (19)(26), is disappointing because its earlier availability would better assist planning of postoperative care (27). However, the greater predictive power of 24-h cTnI for extended ICU and hospital stays suggests that the later released cTnI is more indicative of irreversible myocardial damage than the earlier released fraction. It is likely that postoperative increases in cTnI represent a continuum of reversible and irreversible damage to the myocardium (25)(27)(28). Other workers have shown that in uncomplicated cardiac surgery, there is an early increase around 6 h followed by a rapid decrease, giving concentrations that are substantially lower at 24 h (20)(22)(29)(30)(31). Troponin flushed from the microvasculature after release of the cross-clamp should have a short half-life because renal elimination itself appears quite rapid (32). This is in contrast to the fraction gradually released from disintegrating contractile apparatus after cell death, which should provide a longer apparent half-life in plasma. cTnI released after direct cellular trauma from surgery also contributes to the 6-h value because the 6-h peak is very high in some cases. Ischemia from cross-clamping of the aorta and reperfusion injury also plays a major role because the amount of cTnI released after off-pump CABG surgery is very small compared with on-pump cases (33). We tested the hypothesis that significant myocardial necrosis is particularly indicated by cTnI concentrations that increase from 6 to 24 h, as suggested by Benoit et al. (20), by examining the ability of the ratio of 24-h to 6-h cTnI values to predict extended ICU and hospital stay. However, the ratio did not achieve significance in multivariate analysis in its own right and impaired performance of the 24-h value as well. We thus conclude that the 6-h cTnI concentration has little value either by itself or in combination with the 24-h value.

Heparin has been reported to decrease results for both troponin T and I in ~10% of patient samples in some troponin assays, but the AxSYM assay used does not seem to be affected by this (34).

Stratification of 24-h cTnI values into tertiles allows more clinically relevant interpretation of cTnI values because these fall naturally into low-, equivocal-, and high-risk categories. The appropriateness of this strategy is supported by the data presented in Table 3Up , which confirm high significances across the low and high tertiles. On the basis of the threshold cTnI values used for three-way stratification of outcome indices, we can make the following recommendations. At 24 h after surgery, cTnI concentrations <10 µg/L could be described as indicative of low risk, concentrations between 10 and 20 µg/L as equivocal, and concentrations >20 µg/L as indicative of high risk of extended ICU and hospital stays attributable to perioperative myocardial damage. Although less precise, the equivalent thresholds for cTnI at 6 h would be <20 µg/L for low risk, 20–40 µg/L for equivocal risk, and >40 µg/L for high risk.

Comparison with results reported by other workers is restricted by the fact that concentrations measured by different assays are not strictly comparable (28). The two reports based on the AxSYM assay used for the present study do give comparable results. Fellahi et al. (23), in a recent prospective study of 202 patients, divided their patients into two groups and used a cutoff of 13 µg/L to differentiate CABG patients with and without complications. Swaanenberg et al. (33), studying only CABG patients who experienced no complications, reported somewhat lower concentrations: median concentrations were 12 µg/L at 6 h and 8 µg/L at 24 h in 36 patients.

The use of a single biochemical test at 24 h after CABG surgery to identify cardiac damage and predict adverse outcome has several benefits. Although post-CABG ECG testing is routine in many centers, our data would suggest that ordering of post-CABG ECGs should be limited to specific indications, such as the assessment of rhythm disturbances. This could reduce costs and possible confusion. From the median 24-h cTnI values, it can be calculated that there was a mean prolongation in postoperative ICU stay of 6 h (27%) and in total hospital stay of 1.5 days (also 27%) with progression from lowest to highest tertile. The identification of such patients within 24 h of CABG surgery could permit substantial cost savings in terms of hospital days saved through appropriate and efficient discharge planning. Conversely, those patients in the highest tertile at 24 h may warrant closer clinical attention to improve outcomes in this higher-risk group.

The emergence of CABG surgery as a highly effective means of treating symptomatic coronary artery disease coupled with advancing technology and ever-increasing operator skill, has led to an enormous number of bypass surgeries being performed per year. Although seen as an almost routine procedure in many Western countries, minimizing adverse outcomes and reducing the length of hospital inpatient stays associated with this procedure remain of paramount importance. The high efficacy and simplicity of a single cTnI measurement 24 h after CABG surgery in identifying adverse outcomes in our study support its routine use along with other similar variables in the future streamlining of post-CABG care.


   Acknowledgments
 
We are thankful to Robbie Brown, research nurse, for data collection and processing, and to Pall Australia Pty. Ltd. (Melbourne, Australia), which provided her salary.


   Footnotes
 
1 Nonstandard abbreviations: CABG, coronary artery bypass graft; ICU, intensive care unit; ECG, electrocardiography; and cTnI, cardiac troponin I.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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