Does Creatine Phosphokinase MB Predict Long-Term Cardiac Death or Atrial Fibrillation?

Objectives: Exploring the long-term consequences of elevated postoperative creatine phosphokinase MB as a surrogate measure of cardiac tissue damage subsequent to coronary artery bypass graft surgery (CABG). Methods: Prospective cohort study including 414 patients subjected to solitary CABG at Odense University Hospital from September 30, 2007, and backwards in time. According to individual postoperative peak creatine phosphokinase MB, patients were assigned to five subgroups. Kaplan Meier survival analysis and Cox proportional-hazard regression evaluated the predictive value of creatine phosphokinase MB on late death from cardiac cause and diagnosis of paroxysmal or persistent atrial fibrillation within 10-years after CABG. Results: There was a general numeric decrease in time to cardiac death with increasing postoperative creatine phosphokinase MB (Log-Rank 0.285). Creatine phosphokinase MB < 25 was significantly associated with both shortened extracorporeal circulation time (HR 0.42; 95% CI 0.30 - 0.57; p < 0.001) and aortic cross-clamp time (HR 0.54; 95% CI 0.40 - 0.73; p < 0.001) compared to creatine phosphokinase MB ≥ 100. No association between creatine phosphokinase MB and atrial fibrillation was identified. Postoperative atrial fibrillation occurred in 84 patients, whereof 21 later developed paroxysmal or persistent atrial fibrillation. There was a statistically significant 2.4-fold (HR 2.38; 95% CI 1.37 - 4.14; p = 0.002) increase in paroxysmal or persistent atrial fibrillation in patients with postoperative atrial fibrillation compared to patients in postoperative sinus rhythm. Conclusions: Postoperative creatine phosphokinase MB was not found predictive of late cardiac death or diagnosis of paroxysmal or persistent atrial fibrillation within 10 years after CABG. However, patients with postoperative atrial fibrillation had increased risk of later developing paroxysmal or persistent atrial fibrillation compared to patients in postoperative sinus rhythm.


Introduction
Annually, around 1400 on pump coronary artery bypass graft surgeries (CABG) are performed in Denmark [1]. Data demonstrate a postoperative mortality of 1.3% within 30 days increasing to 3.0% within 12 months in elective cases [2].

Creatine Phosphokinase MB
Postoperative biomarker measurements serve to assess effect of revascularization procedures and potential surgery induced damage to cardiac tissue. The bio- Myocardial infarction is defined as myocardial necrosis due to prolonged ischemia. However, myocardial injury with necrosis may be detected in conditions subsequent to non-ischemic myocardial damage including heart failure, myocarditis, arrhythmias, renal failure, pulmonary embolism and otherwise uncomplicated percutaneous or surgical interventions [3].

Mortality
Steuer et al. [4] found patients with CK-MB ≥ 61 μg/l after CABG were in increased risk of cardiac death within 30 days. This addresses the immediate clinical relevance of CK-MB more than long-term prognostic capability. Domanski et al. [5] documented a significant association between elevated CK-MB and increased cardiovascular mortality within 4 years after CABG. However, the study only included patients with diabetes mellitus, which impair extrapolation to a general CABG population. Costa et al. [6] showed elevated postoperative CK-MB to be significantly associated with late adverse outcome.
The duration of extracorporeal circulation time (ECC) and aortic cross-clamp time (CC) has been correlated with early postoperative adverse events but not long-term outcomes [7]. The stress of ECC and CC duration on cardiac tissue needs further quantification.

Atrial Fibrillation
Postoperative atrial fibrillation (POAF) occurs in around 35% of patients in the immediate period after solitary CABG [8] [9]. POAF develops most frequently on the second to third postoperative day [8].
The mechanisms responsible for POAF are not fully understood. It appears that preoperative degenerative changes in the atrial myocardium combined with perioperative circumstances changing a number of electrophysiological parameters create a myocardium prone to supraventricular arrhythmias [9]. The only factor consistently associated with greater risk of POAF is age, presumably due to development of fibrotic tissue and vulnerability to electrophysiological changes [9] [10].
POAF is often self-limiting and spontaneously converts to sinus rhythm regardless of treatment [11]. However, POAF increases the risk of thromboembolic events [12] [13], why it is standard practice to convert POAF to sinus rhythm immediately.
The mechanism responsible for late onset POAF have yet to be determined, given it is not merely a reflection of a general increase in arrhythmia with age.
As mentioned, CK-MB is a marker of myocardial ischemia, why elevated postoperative levels are an indicator of insufficient myocardial preservation, myocardial damage due to surgery performed or a result of insufficient coronary re-vascularisation. The potential of CK-MB as a parameter in atrial fibrillation risk prediction algorithms need evaluation.

Study Aims
The primary objective is to investigate if peak creatine phosphokinase MB after uncomplicated solitary CABG predicts late cardiac death within 10-year postoperative follow-up. Further, the effect of extracorporeal circulation time and aortic cross-clamp time on creatine phosphokinase MB is investigated.
The secondary objective is to investigate if peak creatine phosphokinase MB or postoperative atrial fibrillation after uncomplicated solitary CABG are predictors of later diagnosis of paroxysmal or persistent atrial fibrillation.

Study Design
Prospective cohort study.

Study Population
The

Descriptive Variables
Patients were assigned to one of five predetermined CK-MB intervals (<25, 25

Outcome Variables
The Danish Cause of Death register supplied data, which was transformed into a dichotomous variable by categorising the primary cause of death as of non-cardiac or cardiac origin. E.g. heart failure, ischemic heart disease, myocardial infarction, arrhythmia et cetera. The Danish National Patient register provided information on hospital contacts. Atrial fibrillation (AF) was defined as any diagnosis of paroxysmal or persistent atrial fibrillation more than 30 days after surgery. Diagnosis of atrial fibrillation within 30 days after surgery was considered postoperative atrial fibrillation (POAF).
Patients diagnosed with arrhythmia prior to CABG were excluded from analysis.

Statistical Analysis
Potential differences in binomial variables between CK-MB subgroups were tested with Pearson-x 2 coefficient or Fishers exact test. Significant test statistics were followed with calculation of z-score and associated p-value for each subgroup. The alpha level was adjusted according to the Bonferroni correction for multiple comparisons.
Potential differences in ordinal variables between subgroups were assessed with Kruskal Wallis analysis. Significant test statistics were supplemented with post hoc pairwise Mann-Whitney test with Bonferroni correction.
Distribution of quantitative variables was tested with histogram, q-q plot and Shapiro-Wilk test. Equality of variance was explored using Levene's test.
Potential differences between subgroups in quantitative variables with normal distribution and equality of variance were disclosed by One-Way ANOVA analysis. Significant test statistics were followed with Scheffe post hoc test. Quantitative variables with normal distribution but inequality of variance were tested with Brown Forsythe and Welch test and post hoc Games-Howell test. Open Journal of Thoracic Surgery Quantitative variables with a distribution other than normal were log transformed to normalise distribution to satisfy the One-Way ANOVA condition. If the log transformed distribution did not approximate normality Kruskal Wallis test was used. In case of significant test statistics, pairwise Mann-Whitney test with Bonferroni correction, was performed.
Kaplan Meier survival curves with time to event late cardiac death were drawn for each of the five subgroups. Differences in estimates between subgroups were tested with Log-Rank.
Variables found to be significantly different between subgroups were tested in regard to potential confounding. Each variable was tested using univariable Cox proportional-hazard regression with time to event late cardiac death and event AF as the dependent variables, respectively. Confounders were identified at a significance level of 5% (p-value < 0.05). The proportionality assumption for the Cox models was tested with Schoenfeld residuals.
Univariable Cox proportional-hazard regression was subsequently used to evaluate the relationship between CK-MB and both ECC and CC.
A multivariable Cox proportional-hazard regression model was produced for late cardiac death and AF, respectively. Potential confounders identified in the previous analysis were adjusted for in the two models.
Cumulative hazard functions assessed the correlation between POAF and AF. Differences in baseline characteristics between the POAF patients and patients in postoperative sinus rhythm were considered with Pearson-x 2 coefficient or Fishers exact test for categorical variables and independent sample T-test for normal distributed quantitative variables. Variables with a distribution other than normal were assessed with non-parametric Mann-Whitney test. A multivariable Cox proportional-hazard regression model with time to event AF adjusted for possible confounders was produced.
Cox proportional-hazard regression was subsequently used for evaluating the risk of all-cause death and cardiac death following POAF and AF.
The proportionality assumptions for the overall multivariable Cox models were tested with global Schoenfeld residuals test. The goodness-of-fit of the models were estimated by Martingale residuals and associated Cox-Snell residuals.
Values with a two-sided alpha level of p < 0.05 were considered statistically significant.
IBM SPSS 25.0 and STATA 15 statistical software were used for statistical analysis.

Interobserver Analysis
An interobserver analysis was performed to evaluate the validity of the information from The Western Denmark Heart Register. From each CK-MB subgroup, 10% of patients were chosen at random. Two researchers (KKJ and LPR) independently reviewed patient records. Discrepancies were resolved by consensus. The information from The Western Denmark Heart register was compared to Open Journal of Thoracic Surgery the information obtained from patient records by calculating Cohen's κ -coefficient, variable sensitivity and specificity.

Approvals
The Danish Data Protection Agency and the Danish Patient Safety Authority approved the study. The Regional Committees on Health Research Ethics for Southern Denmark waived consent requirements.

Results
The study evaluated 414 patients. Baseline characteristics for the five CK-MB subgroups were balanced with respect to preoperative baseline characteristics (Table 1).

CK-MB and Late Cardiac Death
In the 10-year follow-up period 105 of 414 patients died, whereof 33 deaths were of cardiac origin. Three patients died from unknown causes.
The Kaplan Meier survival estimates of time to late cardiac death generally decreased with an increasing level of CK-MB especially pronounced after 5 years in subgroups with CK-MB > 75 μg/l ( Figure 1). However, Log-Rank test did not identify any statistically significant difference between subgroups.
Neither univariable nor multivariable Cox proportional-hazard regression analysis identified any statistical significant association between postoperative CK-MB level and late cardiac death ( Table 2). Subsequent analysis showed a statistically significant decrease in both ECC and CC in subgroup CK-MB < 25 compared to subgroup CK-MB ≥ 100 (Table   3).

CK-MB and AF
AF occurred in 54 out of 414 patients, within 10 years. No association between postoperative CK-MB level and time to AF was identified in univariable or multivariable Cox proportional-hazard analysis (Table 4).
No differences between CK-MB subgroups regarding postoperative CNS injury, sternal infection or reoperation for bleeding were found. An increased postoperative CK-MB level was associated with a significantly higher risk of postoperative acute myocardial infarction, new Q-wave, higher EuroSCORE, reoperation due to ischemia and prolonged intensive care unit admission time.

AF and late Cardiac Death
Cardiac death was more frequent in POAF and AF patients compared to patients in sinus rhythm (Table 6).
Median admission time of patients in the POAF group was 175 hours (95% CI Open Journal of Thoracic Surgery

Interobserver Analysis
The interobserver analysis showed a general underreporting in The Western Denmark Heart Register. The underreporting of comorbidities was moderate with Cohens κ -coefficient ranging from 0.53 in reporting of chronic obstructive pulmonary disease to 1.0 for diabetes mellitus. Notification regarding medicinal usage was varying from unreliability of hypertensive treatment ( κ = 0.05) to the well performing beta-blocker treatment ( κ = 0.79) and ACE inhibitor treatment ( κ = 0.80). Underreporting also applied for postoperative variables such as CNS injury, sternal infection and new Q-wave on ECG. Open Journal of Thoracic Surgery   and specificity of 100%.
Two patient records reported CABG to be acute and one reported a preoperative AF diagnosis, why they were excluded from analysis confer exclusion criteria.

CK-MB and Late Cardiac Death
There was a numeric increased cardiac mortality among higher CK-MB subgroups, progressively pronounced beyond 5 years postoperative. However, the result must be interpreted with caution. As time progress after the CABG procedure, there is a growing risk of residual confounding influencing the result.
Furthermore, a higher postoperative CK-MB level was significantly associated with an increased EuroSCORE, which indicates a more vulnerable cardiac risk profile in these patients. Hence, a shorter lifespan due to co-morbidities is to some extent expected in these patients.

CK-MB and AF
The study did surprisingly not identify any association between CK-MB and late AF. As elevated CK-MB correlated well with impaired cardiac function and arguably insufficient cardioplegia of the myocardium, then CK-MB would theoretically have been associated with increased risk of arrhythmia [15]. This finding suggests that development of AF is less dependent on the invasiveness of surgery and more so on pre-existing pathological substrate. sinus rhythm compared to patients with POAF is presumably a manifestation of the well known antiarrhythmic property of beta-blocker therapy [16].

POAF and AF
The association between POAF and AF remained significant after adjusting for age. Furthermore, the hazard function for patients experiencing POAF increases at a higher rate from around 5 years postoperative compared to that of patients in postoperative sinus rhythm. If age were the main cause of AF, one would expect a more constant increase in AF with age.
These findings are consistent with the findings of Ahlsson et al. [17], who showed POAF to be associated with late AF and speculated if heart surgery could be viewed as a 'stress test' that unmasks a predisposition for AF. Melduni et al. [18] shared the hypothesis of a predisposing pathological substrate and showed that likelihood of recurrence of AF was coupled to the extent of underlying disease quantified by left ventricular diastolic dysfunction. Although non-significant, patients with POAF were more likely to die from cardiac cause within 10 years after CABG compared to patients in sinus rhythm. This correlates well with previously found increased all-cause mortality due to POAF in large cohort studies [19] [20]. Ahlsson et al. [17] and Schwann et al. [21] found POAF to be correlated with increased cardiovascular mortality.
Melduni et al. [18] did not find POAF significantly associated with all-cause mortality but did however find AF to be. Therefore, the deficient significance regarding this matter in the present study is most likely due to underpower. However, accumulation of events in POAF patients could merely be a surrogate measure of the significantly higher Euro-SCORE in POAF patients.
A number of other postoperative characteristics were considered. POAF patients were admitted significantly longer to both the regular hospital ward and intensive care unit. Furthermore, there was a higher incidence of postoperative CNS injury in POAF patients. POAF has previously been proved to increase the length of hospital stay in CABG patients [22] [23] and accumulate costs [24]. POAF prophylactic regimes have shown to significantly reduce risk of POAF and length of stay [25].

Limitations and Strengths
Data was collected prospectively. It is the first study to consider the long-term predictive capability of CK-MB on the risk of death confined to cardiac cause.
Knowledge of an association between POAF and AF is sparse, why this study reveals new data.
The cohort originates from a single-centre, which impair the external validity. Though it is noted, that the CABG procedure is relatively standardized. Therefore, extrapolation of the results might be reasonable.

Data
Missing register data were to a large extent acquired from patient files, reaching a low percentage of missing values. Register data are subjected to interobserva-

Population
The requirement of including more patients to increase power and prevent type 2 errors was weighed against the resulting higher risk of residual confounding if patients from earlier surgical years were included in the study. If so, prompting a higher risk of confounding from differences in patient demographics and procedural factors.

Descriptive Variables
Postoperative CK-MB measurement is a standardized procedure and blood sample analyses were performed at the same laboratory for all patients. In addition, it has to be taken into consideration that even though CK-MB is considered specific to cardiac tissue, some amount of the CK-MB is produced in the brain tissue and striated skeletal muscle. As mentioned, elevated CK-MB can also be measured in other clinical conditions [3].

Outcome Variables
The Western Denmark Heart Register does not distinguish between atrial flutter and atrial fibrillation and the occurrence is only registered during hospital admission. However, a study by Zebis et al. [26] showed no occurrences of atrial flutter after CABG in a similar population and only a risk of 1% in developing atrial fibrillation after hospital discharge.
Munkholm et al. [27] evaluated the POAF variable in The Western Denmark Heart Register and calculated variable sensitivity 75.2% (95% CI 72.2 -78.7), specificity 90.9% (95% CI 88.8 -92.6) and concluded that the POAF variable was reliable to be used in epidemiological studies. These findings correspond well with the results from the interobserver analysis.
The Danish Cause of Death Register was fully updated, why all patients had a complete 10-year followed-up. All patients were accounted for in The National Patient Register.

Conclusions
No association between elevated creatine phosphokinase MB level after solitary CABG and late cardiac death within 10 years was evident. However, creatine phosphokinase MB level ≥100 was significantly associated with both prolonged extracorporeal circulation time and aortic cross-clamp time compared to a creatine phosphokinase MB level < 25.