Potential Role of Plasma Myeloperoxidase Level in Predicting Long-Term Outcome of Acute Myocardial Infarction

Abstract

We investigated the prognostic importance of plasma myeloperoxidase levels in patients with ST-elevation myocardial infarction (STEMI) at long-term follow-up, and we analyzed the correlations between plasma myeloperoxidase levels and other biochemical values.

We evaluated 73 consecutive patients (56 men; mean age, 56 ±11 yr) diagnosed with acute STEMI and 46 age- and sex-matched healthy control participants. Patients were divided into 2 groups according to the median myeloperoxidase level (Group 1: plasma myeloperoxidase ≤68 ng/mL; and Group 2: plasma myeloperoxidase >68 ng/mL). Patients were monitored for the occurrence of major adverse cardiovascular events (MACE), which were defined as cardiac death; reinfarction; new hospital admission for angina; heart failure; and revascularization procedures.

The mean follow-up period was 25 ± 16 months. Plasma myeloperoxidase levels were higher in STEMI patients than in control participants (82 ± 34 vs 20 ±12 ng/mL; P=0.001). Composite MACE occurred in 12 patients with high myeloperoxidase levels (33%) and in 4 patients with low myeloperoxidase levels (11%) (P=0.02). The incidences of nonfatal recurrent myocardial infarction and verified cardiac death were higher in the high-mye-loperoxidase group. In multivariate analysis, high plasma myeloperoxidase levels were independent predictors of MACE (odds ratio = 3.843; <95% confidence interval, 1.625–6.563; P=0.003).

High plasma myeloperoxidase levels identify patients with a worse prognosis after acute STEMI at 2-year follow-up. Evaluation of plasma myeloperoxidase levels might be useful in determining patients at high risk of death and MACE who can benefit from further aggressive treatment and closer follow-up.

Fissure and rupture of coronary plaques trigger the development of acute myocardial infarction (MI). Rupture of the fibrous cap and damage to the intima release into the circulatory system such procoagulant substances from the subendocardial matrix as collagen and tissue factors. Subsequently, platelets are activated and thrombin production increases, which results in the formation of thrombus.¹

Myeloperoxidase (MPO)—a tetrameric, glycolysed, lysosomal enzyme—is secreted by leukocytes in response to oxidative stress.² In cases of inflammation, MPO is profuse in leukocytes. It acts to produce reactive oxidative substances, and it causes catalytic activity in atherosclerotic lesions.^3,4 In patients who have angiographically documented coronary artery disease (CAD), MPO levels are found to be increased in lesions that are prone to rupture.^5,6

Plasma MPO levels are useful in predicting the risk of MI, target-vessel revascularization, and death in almost all categories of CAD patients, but little is known about the possible role of MPO levels in predicting the long-term outcome of these patients.^7,8 We investigated the prognostic value of plasma MPO levels in patients diagnosed with STEMI by evaluating 2-year outcomes and by analyzing the correlations between plasma MPO levels and other biochemical values.

Patients and Methods

Study Population

We prospectively studied 73 consecutive patients with a mean age of 56 ± 11 years (range, 21–72 yr), 56 of whom were men (77%); all patients were admitted to the intensive coronary care unit of the Cardiology Department of Gazi University Hospital with a diagnosis of ST-elevation MI (STEMI) from April 2004 through February 2005. We defined STEMI in accordance with recent criteria.^9,10 Patients with valvular heart disease, inflammatory disease, malignancies, end-stage renal disease, immunologic disease, or chronic liver disease were excluded. The control group consisted of 46 age and sex-matched healthy volunteers (30 of whom were men [65%]) with a mean age of 53 ± 12 years (range, 24–69 yr). Members of the control group did not have any documented atherosclerotic vascular disease, nor did they have any cardiovascular conditions, and their electrocardiograms did not reveal any pathologic finding. Our local ethics committee approved the study, and the patients gave written informed consent.

Left ventricular ejection fraction (LVEF) was evaluated by means of transthoracic echocardiography, and coronary angiography was performed in all patients with use of a General Electric DLX Angiographic System (GE Medical Systems Europe; Sedex, France).

Biochemical Analysis

A single peripheral blood sample was collected from each study patient within 6 hours of the onset of symptoms. Plasma MPO enzyme levels were measured by the enzyme-linked immunosorbent assay (ELISA) method with an immune diagnostic MPO kit (Immundiagnostik AG; Bensheim, Germany). Plasma MPO levels were detected in the baseline plasma samples of all STEMI patients, with a median of 68 ng/mL (range, 8–358 ng/mL). These patients with STEMI were divided into 2 groups, on either side of the median MPO value of the entire cohort (≤68 vs >68 ng/mL).

Troponin T levels were evaluated by means of the ELISA method, through use of Cardiac Reader® cardiac troponin T kits (Roche Diagnostics; Indianapolis, Ind). Troponin T values from 0–0.1 ng/mL were considered negative; values above 0.1 ng/mL were accepted as positive. Creatinine kinase (CK) and CK–MB (myocardial band) fractions were measured by the immunoassay method (CK, Abbott Alcyon®, Abbott Laboratories; Abbott Park, Ill; and CK–MB, Pointe Scientific, Inc.; Canton, Mich). Normal reference levels for CK were 25–175 IU/L and for CK–MB were 0–24 IU/L. Levels of C-reactive protein (CRP) were quantified by nephelometry, with a reference interval of 0–6 mg/dL. Fibrinogen levels were evaluated by the Clauss method, with a reference interval of 180–350 mg/dL.

Follow-Up and Major Adverse Cardiac Events

Follow-up data were collected from hospital records. Patients with unclear event data were contacted by telephone. All events reported by telephone had to be verified by examination ofobjective hospital notes. Follow-up time was defined as the time from admission to the first cardiac event or, for patients without an event, from admission to 30 months.

Major adverse cardiac events (MACE) were defined as cardiac death; reinfarction; new hospital admission for angina; heart failure; and revascularization procedures by means of coronary artery bypass grafting or percutaneous coronary intervention (PCI). Any reported cardiac death had to be verified in hospital records as death due to MI, cardiac arrest, or other cardiac cause. Reinfarction was defined as recurrent chest pain or development of new electrocardiographic changes accompanied by a new rise (≥20%)in cardiac biomarkers, measured after the recurrent event. Additional revascularization procedures included PCI and coronary artery bypass surgery of the infarct-related artery or other coronary vessels. Angina was defined as the occurrence of chest pain, typically lasting a few minutes at rest or during physical effort. Heart failure was diagnosed in accordance with the Framingham criteria.¹¹

Statistical Analysis

Statistical analysis was performed using SPSS for Windows version 13.0 (IBM Corporation; Armonk, NY). The Kolmogorov-Smirnov test was performed for checking the distribution of numeric values. Continuous variables were presented as mean ± SD if normally distributed; otherwise, they were presented as median with 25th and 75th percentiles. For comparison of the independent continuous variables, Student's t test or the Mann-Whitney U tests were used, where appropriate. Categorical data were compared using the Fisher exact test or the χ² test. Pearson or Spearman rank correlation was used for examining the relationships between MPO levels and the levels of N-terminal pro-brain natriuretic peptide (NT-proBNP), troponin T, and white blood cell count. For the construction of patients' survival curves in the absence of MACE, the survival function of the Cox regression analysis was used. For all statistics, a P value <0.05 was considered statistically significant.

Results

The baseline clinical characteristics of STEMI patients and control subjects are summarized in Table I. Age, sex, and coronary artery risk factors did not significantly differ between the STEMI patients and members of the control group. However, plasma MPO levels were higher in STEMI patients (82 ± 34 vs 20 ± 12 ng/mL; P=0.001), as were serum CRP concentrations (9.9 ± 5.2 vs 3.6 ± 1.4 ng/mL; P=0.001).

TABLE I. Baseline Characteristics of STEMI Patients and Control Subjects

In our study, the median plasma MPO level at presentation was 68 ng/mL (range, 8–358 ng/mL) in STEMI patients. Patients were divided into 2 groups according to the median MPO value of the entire cohort (Group 1: plasma MPO ≤68 ng/mL, 26 men, mean age 56 ± 11 yr; and Group 2: plasma MPO >68 ng/mL, 30 men, mean age 57 ± 13 yr). The clinical characteristics and coronary angiographic and echocardiographic findings of the study groups are summarized in Table II. Patients with high plasma MPO levels were more likely to have anterior-wall MI (47% vs 31%; P=0.043), low LVEF (0.46 ± 0.09 vs 0.52 ± 0.08; P=0.012), and multivessel CAD (61% vs 31%; P=0.013) than were those with low plasma MPO levels. Thrombolysis, primary PCI, and rescue angioplasty were performed in 28 patients (38%), 38 patients (52%), and 7 patients (10%), respectively. The MPO did not differ significantly according to whether a patient was treated by means of thrombolysis or PCI (71 ± 28 ng/mL vs 63 ± 23 ng/mL; P=0.112). Kaplan-Meier survival curves in these 2 subgroups were also similar at long-term follow-up.

TABLE II. Patients' Characteristics in Accordance with Their Plasma Myeloperoxidase Levels

Long-term medical therapy was managed in accordance with clinical findings and the physician's judgment. Briefly, aspirin, angiotensin-converting enzyme inhibitors, statins, and β-blockers were administered (Table II).

In the STEMI patients, plasma MPO levels correlated with levels of CRP (r=0.451, P=0.004), troponin T (r=0.390, P=0.004), and NT-proBNP (r=0.445, P=0.002), but not with white blood cell counts (r=0.166, P=0.189) (Fig. 1).

Fig. 1 Correlations between plasma myeloperoxidase (MPO) levels and those of A) C-reactive protein (CRP), B) N-terminal pro-brain natriuretic peptide (NT-proBNP), and C) troponin T. See also the lack of correlation with D) white blood cell (WBC) counts.

Clinical Follow-Up

All patients were monitored for the development of MACE, including death from any cause, MI, cerebrovascular events, new hospital admission for chest pain or congestive heart failure, or target-vessel revascularization. The clinical outcomes of the study groups are summarized in Table III. Follow-up lasted at least 8 months in all patients; the mean follow-up period was 25 ± 16 months (range, 8–48 mo). Composite MACE occurred in 12 patients (33%) with high MPO levels and in 4 patients (11%) with low MPO levels (P=0.02) (Fig. 2). The STEMI patients who experienced MACE during the follow-up period also had significantly higher plasma MPO levels than did those who were MACE free (118 ± 46 vs 56 ± 24 ng/mL; P=0.001). The incidence of nonfatal recurrent MI was 6% in the high-MPO group and 0 in the low-MPO group (P=0.146). All-cause death at follow-up was 3% in patients with low MPO levels and 14% in those with high MPO levels (P=0.082). Verified cardiac death was 3% in patients with low MPO levels and 11% in patients with high MPO levels (P=0.155). Plasma MPO levels were higher in the patients who died (106 ± 56 ng/mL) than in survivors (62 ± 34 ng/mL; P=0.001). Among patients with a poor prognosis (low LVEF, multivessel disease, and anterior MI), MACE was higher in those patients with high MPO levels at the 2-year follow-up (36% vs 16%; P=0.012). Finally, the time to event was 12 ± 6 months in patients with low MPO levels. This was remarkably and significantly longer than in patients with high MPO levels (8 ± 5 mo) (P=0.032).

TABLE III. Clinical Outcomes Classified in Accordance with Plasma Myeloperoxidase Levels

Fig. 2 Kaplan-Meier curves for long-term MACE-free survival. Composite MACE occurred in 12 patients (33%) with high MPO levels and in 4 patients (11 %) with low MPO levels (P=0.02).

MACE = major adverse cardiac events; MPO = myeloperoxidase

Figure 2 shows the Kaplan-Meier curves for long-term, MACE-free survival in patients with high MPO levels compared with patients with low MPO levels. When a Cox proportional hazard model was constructed to include all variables representing a significant correlation with MACE in univariate analysis, high plasma MPO levels were independent predictors of MACE (odds ratio [OR]=3.843; <95% confidence interval [CI], 1.625–6.563; P=0.003), as were CRP (OR=2.863; 95% CI, 1.337–6.452; P=0.012) and LVEF <0.40 (OR=3.225; 95% CI, 1.434–6.554; P=0.001).

Discussion

In this study, we investigated the prognostic importance of plasma MPO levels in 73 consecutive STEMI patients during a mean follow-up period of more than 2 years, and analyzed the correlations between plasma MPO levels and other biochemical values. We found that plasma MPO levels were an independent predictor of 2-year survival and freedom from MACE; MPO levels correlated with levels of CRP, troponin T, and NT-proBNP.

Previous studies have reported that, in patients with CAD and acute MI, plasma MPO levels were higher than levels in healthy subjects.^7,12,13 In accordance with previous studies, plasma MPO levels in STEMI patients were significantly higher than in a matched control population in our study.

Plasma MPO levels have been shown to predict the risk of MI, target-vessel revascularization, and death in almost all categories of CAD patients, but little is known concerning the possible role of MPO level in predicting long-term outcomes in these patients.7,8,12,13 Brennan and colleagues⁷ showed the prognostic usefulness of MPO in patients who presented with chest pain at emergency departments. They found MPO to be helpful as an independent predictor of MACE (MI, reinfarction, the need for revascularization, or death) in the following 30 days and at 6 months. Cavusoglu and associates⁸ investigated the predictive value of baseline MPO levels for the development of MI at 2 years in 193 male patients who had been diagnosed with acute coronary syndrome and referred for coronary angiography. They found a significant independent association between increased baseline MPO levels and the development of MI during the subsequent 2 years. In that study, STEMI was diagnosed in only 23 patients (12%). However, most of the patients developed nonSTEMI (43%) and unstable angina (45%). Chang and colleagues¹⁴ showed that baseline plasma levels of MPO were significantly higher in STEMI patients who underwent primary PCI. At short-term follow-up (30 d), patients with higher plasma MPO levels had significantly lower LVEFs and a higher incidence of 30-day composite MACE (reinfarction, repeat PCI, Killip score ≥3, or death) than did those with low plasma MPO levels.

In our study, patients with high plasma MPO levels on admission were older, more hypertensive, and more likely to be diabetic; they were also more likely to have an anterior-wall MI, a low LVEF, and multivessel CAD than were those with low plasma MPO levels. Most deaths and instances of MACE were detected in patients with higher MPO levels, and MPO was an independent predictor of MACE development during the 2-year follow-up. The Kaplan-Meier survival curves for long-term, MACE-free survival supported the prognostic importance of the MPO.

One important finding of our study was the relationship between MPO level and the extent of CAD. Zhang and co-authors¹⁵ compared the MPO levels of 158 CAD patients with those of control participants proved healthy by angiographic study. They showed that blood and leukocyte MPO activities were higher in patients with CAD than in the control participants and that this increased activity was significantly associated with the extent of CAD. In accordance with previous studies, we found that in the higher-MPO group multivessel disease was common (61% of the group). However, in the lower-MPO group, this figure was only 31% (P=0.013).

Inflammation plays an essential role in the pathogenesis of atherosclerosis.¹⁶ Several studies have shown that markers of inflammation, especially an elevated level of CRP, are associated with increased risk of cardiovascular events in patients with CAD and acute MI.^17–21 Canale and colleagues¹⁷ showed that determination of serum CRP level alone can predict an outcome in STEMI patients. Yip and associates¹⁸ found a relationship between initially high CRP level and the risk of cardiac events in patients diagnosed with STEMI who underwent primary PCI in the first 6 hours after the onset ofpain. Pietila and co-authors¹⁹ also studied CRP levels after acute MI: in that study, the baseline CRP value predicted death in the first 6 months, but was not predictive at 2 years. Anzai and colleagues²⁰ showed that peak CRP levels predict the risk of cardiac death for up to 1 year after a first Q-wave acute MI. In addition, Dibra and associates²¹ noticed increased myocardial damage and an elevated 18-month mortality rate in a particular population of STEMI patients who had higher CRP levels before primary PCI. Brennan and co-authors⁷ found that plasma MPO levels correlated weakly with CRP levels (r=0.1, P=0.01) in patients who presented with chest pain at the emergency department. This suggests that the evaluation of CRP in acute coronary syndrome could help to accurately identify patients at high risk of future cardiovascular events. In our study, plasma MPO levels correlated to a great extent with CRP (r=0.45, P=0.004), and in the higher-MPO group, serum CRP levels were also significantly higher (6.4 vs 13.9 mg/dL; P=0.004).

We have shown a positive correlation between MPO and CRP, and CRP has proved a proficient marker for the inflammatory process. However, we did not find any relationship between MPO and white blood cell levels, as was shown in the study of Chang and colleagues.²² The absence of correlation between MPO and white blood cell levels could be due to the small size of our study population. The discrepancy between the 2 trials might also be explained by differences in the interval between onset of symptoms and blood sampling: blood was drawn in the first 6 hours in our study and in the first 12 hours in the study by Chang and col-leagues.²²

N-terminal pro-brain natriuretic peptide—a neurohormone synthesized in the ventricular myocardium and released in response to pressure overload and ventricular dilation²²—was first established as a diagnostic and prognostic marker in patients with congestive heart failure.²³ Several studies have shown that NT-proBNP provides prognostic information and predicts death and morbidity in patients with STEMI.^24,25 Mega and associates²⁴ showed that elevated levels of BNP at initial presentation were associated with impaired reperfusion after fibrinolysis and with increased risk of short-term death in patients with STEMI. Omland and co-au-thors²⁵ showed that NT-proBNP is a powerful indicator of long-term death in patients with acute coronary syndromes. Another, more current study¹² revealed that, like NT-proBNP, MPO is a strong predictor of death or nonfatal MI in patients with STEMI. In addition, the dual rise in MPO and NT-proBNP levels was associated with a significantly higher mortality rate than was the elevation of just one peptide level. Similarly, in our study, plasma MPO levels correlated with NT-proBNP levels (r=0.45, P=0.002); and in the higher-plasma-MPO group, serum NT-proBNP levels were also higher (666 vs 351 μg/mL). Unlike Khan and associates,¹² we found a negative relationship between MPO levels and left ventricular systolic function.

A single determination of the MPO in acute MI is a limitation of our study. We have shown that MPO level negatively correlates with LVEF and positively correlates with other prognostic biomarkers. Hence, we can conclude that MPO level on admission might reveal prognostic information regarding the course of acute MI. We collected blood samples to measure MPO levels from each study patient within 6 hours of the onset of symptoms. Previous studies showed that MPO levels were higher during the first 12 hours after STEMI than afterward. Nilsson and co-authors²⁶ found that plasma MPO levels were at their highest level (1,305 ng/mL) immediately after STEMI, then dropped to 321 ng/mL at 12 hr, 512 ng/mL at 24 hr, and 309 ng/mL at 48 hr (P <0.001). The other important paper, published by Khan and colleagues,¹² investigated the role of MPO as a predictor of death or MI in patients with STEMI and compared it with NT-proBNP. Three hundred eightyfour post-STEMI patients were studied. In order to determine plasma MPO and NT-proBNP levels, serial blood measurements were made at 0–24, 25–48, 49–72, 73–96, and 97–120 hours after onset of chest pain. Finally, these authors showed plasma MPO levels to be highest during the first 24 hours, then to decrease significantly over the 5 days after STEMI (P <0.001).

The main limitation of our study is the relatively small number of patients in the sample. However, we showed that plasma MPO levels were higher in the STEMI patients than in the healthy control group. Because our study was not designed to investigate serum MPO kinetics in acute MI, the MPO level on admission was used for prognostic evaluation. Our study showed that even a single determination of plasma MPO level on hospital admission can predict outcome in STEMI patients at the 2-year follow-up. Nonetheless, repeated evaluations of MPO levels at subsequent intervals could render our study still more useful for prognostic purposes.

We conclude that the study of plasma MPO levels might be useful in identifying STEMI patients who are at high risk of death and the development of MACE.