Persistence of Infarct Zone T2 Hyperintensity at 6 Months After Acute ST-Segment–Elevation Myocardial Infarction

Supplemental Digital Content is available in the text.


ST-elevation myocardial infarction patients
We performed a longitudinal cohort study in a regional cardiac center between 11 May 2011 and 22 November 2012. Patients with acute ST-elevation myocardial infarction (STEMI) were consecutively screened for suitability and those recruited provided written informed consent. Inclusion criteria were an indication for primary percutaneous coronary intervention (PCI) or thrombolysis for STEMI 1 . Exclusion criteria were contraindications to contrastenhanced cardiac magnetic resonance (CMR) imaging. For the purposes of this analysis, STEMI patients who experienced a recurrent MI or had an additional PCI following the index procedure were not included since these events could influence myocardial T2 (ms) during the intervening period. STEMI management followed current guidelines 1,2 . The study was approved by the National Research Ethics Service (Reference 10-S0703-28) and was publically registered (NCT02072850). The flow diagram for the study is shown in Supplementary Figure 1.
Screening, enrolment, and data collection were prospectively performed by cardiologists in the cardiac catheterization laboratories of the Golden Jubilee National Hospital, Glasgow, United Kingdom. This hospital is a regional referral center for primary and rescue PCI. The hospital provides clinical services for a population of 2.2 million. A screening log was recorded, including patients who did not participate in the cohort study.

Coronary angiogram acquisition and analyses
Coronary angiograms were acquired during usual care with cardiac catheter laboratory X-ray (Innova®) and IT equipment (Centricity®) made by GE Healthcare.

Percutaneous coronary intervention
Consecutive admissions with acute STEMI referred for emergency PCI were screened for the inclusion and exclusion criteria. During ambulance transfer to the hospital, the patients received 300 mg of aspirin, 600 mg of clopidogrel and 5000 IU of unfractionated heparin 1,2 .
The initial primary PCI procedure was performed using radial artery access. A conventional approach to primary PCI was adopted in line with usual care in our hospital 1,2 . Conventional bare metal and drug eluting stents were used in line with guideline recommendations and clinical judgment. The standard transcatheter approach for reperfusion involves minimal intervention with aspiration thrombectomy only or minimal balloon angioplasty (e.g. a compliant balloon sized according to the reference vessel diameter and inflated at 4-6 atmospheres 1-2 times). During PCI, glycoprotein IIbIIIa inhibitor therapy was initiated with high dose tirofiban (25 µg/kg/bolus) followed by an intravenous infusion of 0.15 µg/kg/min for 12 hours, according to clinical judgment and indications for bail-out therapy 1,2 . No reflow was treated according to contemporary standards of care with intra-coronary nitrate (i.e. 200 µg) and adenosine (i.e. 30 -60 µg) 1,2 , as clinically appropriate. In patients with multivessel coronary disease, multivessel PCI was not recommended, in line with clinical guidelines 1,2 .
The subsequent management of these patients was symptom-guided.

Angiographic analysis
The coronary anatomy and disease characteristics of study participants were described based on the clinical reports of the attending cardiologist.

Outcome definitions
Coronary blood flow can be described based on the visual assessment of coronary blood flow revealed by contrast injection into the coronary arteries 1,2 . TIMI Coronary Flow Grade 0 is no flow, 1 is minimal flow past obstruction, 2 is slow (but complete) filling and slow clearance, and 3 is normal flow and clearance.

CMR acquisition
CMR imaging was performed on a Siemens MAGNETOM Avanto (Erlangen, Germany) 1.5-Tesla scanner with a 12-element phased array cardiac surface coil.

CMR image analyses
The images were analysed on a Siemens work-station by observers with at least 3 years CMR experience (N.A., D.C., I.M, S.R.). All of the images were reviewed by experienced CMR cardiologists (C.B., N.T.). LV dimensions, volumes and ejection fraction were quantified using computer assisted planimetry (syngo MR®, Siemens Healthcare, Erlangen, Germany).
All scan acquisitions were spatially co-registered.

ECV measurement
LV contours were delineated on the best spatially matched raw T1 image and copied onto color-coded spatially co-registered maps. Regions of interest were drawn in infarcted myocardium surrounding core, remote myocardium and LV blood pool. Hematocrit (HCT) was measured at the time of scanning. Extracellular volume (ECV) was calculated as a ratio of corresponding T1 values measured pre-and post-contrast in each of the regions of interest. ECV was calculated using ECV = (1-HCT) × λ, where Lambda (λ)=ΔR1myocardium/ΔR1blood, ΔR1=R1post-contrast-R1pre-contrast and R1=1/T1 11,12 .

Infarct definition and size
The territory of infarction was delineated using a signal intensity threshold of >5 standard deviations (SD) above a remote reference region and expressed as a percentage of total LV mass 13 . Infarct regions with evidence of microvascular obstruction were included within the infarct area and the area of microvascular obstruction was assessed separately and also expressed as a percentage of total LV mass.

Reference ranges
Reference ranges used in the laboratory were 105 -215 g for LV mass in men, 70 -170 g for LV mass in women, 77 -195 ml for LV end-diastolic volume in men, 52 -141 ml for LV end-diastolic volume in women, 19 -72 ml for LV end-systolic volume in men and 13 -51 ml for LV end-systolic volume in women.

Electrocardiogram
A 12 lead ECG was obtained before coronary reperfusion and 60 minutes afterwards with Mac-Lab® technology (GE Healthcare) in the catheter laboratory and a MAC 5500 HD recorder (GE Healthcare) in the Coronary Care Unit. The ECGs were acquired by trained cardiology staff. The ECGs were de-identified and transferred to the local ECG management system. The ECGs were then analysed by the University of Glasgow ECG Core Laboratory which is certified to ISO 9001: 2008 standards as a UKAS Accredited Organisation.

Biochemical measurement of infarct size
Troponin T was measured (Elecsys Troponin T, Roche) as a biochemical measure of infarct size. The high sensitive assay reaches a level of detection of 5 pg/ml and achieves less than 10%variation at 14 pg/ml corresponding to the 99th percentile of a reference population. A blood sample was routinely obtained 12 -24 hours after hospital admission, and again between 0700 -0900 hours during the first two days of the index hospitalization.

Biochemical measurement of LV remodeling
Serial systemic blood sample were obtained immediately after reperfusion in the cardiac catheterization laboratory, and subsequently between 0600 -0700 hrs each day during the initial in-patient stay in the Coronary Care Unit.
NT-proBNP, a biochemical measure of LV wall stress, was measured in a research laboratory using an electrochemiluminescence method (e411, Roche) and the manufacturers calibrators and quality control material. The limit of detection is 5 pg/ml. Long-term coefficient of variations of low and high controls are typically <5%, and were all within the manufacturers range.

Research Management
The study was conducted in line with Guidelines for Good Clinical Practice (GCP) in Clinical Trials 14 .

Trial management included a Trial Management Group, and an independent Clinical Trials
Unit. Day to day study activity was coordinated by the Trial Management Group who was responsible to the Sponsor which was responsible for overall governance and that the trial was conducted according to GCP standards.

Health outcomes
We prespecified adverse health outcomes that are pathophysiologically linked with STEMI.
The primary composite outcome was major adverse cardiac events (MACE) defined as cardiac death, non-fatal myocardial infarction or heart failure hospitalization following the 6month CMR scan. All-cause death or heart failure (heart failure hospitalization or defibrillator implantation) following the 6-month CMR scan was a secondary outcome.
Research staff screened for events from enrolment by checking the medical records and by contacting patients and their primary and secondary care physicians as appropriate. Each serious adverse event was reviewed by a cardiologist who was independent of the research team and blinded to all of the clinical and CMR data. The serious adverse events were defined according to standard guidelines 15 .

Statistics
Continuous variables are described as mean±SD, if normally distributed, and median (Q1, Q3) otherwise. Categorical variables are described as n (%). Variables are described overall and by presence or absence of persistent T2 hyperintensity. Patient and angiographic characteristics and CMR findings were compared between groups with presence or absence of persistent T2 hyperintensity using independent sample t-tests or Mann-Whitney tests, as appropriate. Binary logistic regression was used to identify associates of persistent T2 hyperintensity. Multivariable linear regression analyses using the enter method were performed to identify associates of the change in infarct zone T2 and LV parameters. Linear regression assumptions were verified using standardized residual plots.
Random effects models were used to compute inter-rater reliability measures (inter-class correlation coefficient (ICC)) for the reliability of infarct zone T2 values measured independently by 2 observers in 20 randomly selected patients from the cohort.
Cox proportional hazards regression was used to explore potential associations between persisting T2 hyperintensity and health outcome. The proportional hazards assumption was verified using log-minus-log plots. For these plots, continuous variables were categorized as above and below the median.
All p-values were 2-sided. A p-value >0.05 indicated the absence of a statistically significant effect. The natural log was used in transformations of variables. Analyses were performed using SPSS version 22 for Windows (SPSS, Inc., Chicago, Illinois), or R v3.3.0.

CMR findings
The full list of CMR findings are summarized in Supplementary Table 1.
The association between change in LV ejection fraction and persisting T2 hyperintensity and the change in infarct zone T2 is shown in Supplementary Table 2.

Infarct zone T2 inter-observer reliability
Infarct zone T2 in a subgroup of 20 randomly chosen patients was independently measured by two observers. The intra-class correlation coefficient for reliability of infarct zone T2 was 0.92 (95% confidence interval (CI): 0.75, 0.97); p<0.001. Bland-Altman plots (Supplementary Figure 2) showed no evidence of bias. The coefficient of variation for infarct zone T2 was 7.4%.

Persistent T2 hyperintensity and extracellular volume
Infarct zone ECV was measured in 127 patients at baseline and 124 patients at follow-up (n=124 paired measurements). The characteristics of these patients were similar to the whole cohort (data not shown). Infarct zone ECV and T2 were associated at baseline (0.14 (0.04, 0.24); p=0.007) which likely reflects the early increase in infarct zone extracellular water content. Additionally, infarct zone ECV at 6 months was higher in patients with persisting T2 hyperintensity ( Table 2). There was an association between the change in infarct zone ECV at 6 months compared to baseline and the change in T2 in the infarct zone (0.15 (0.05, 0.24); p=0.002; n=124). The directions of change in infarct zone T2 and infarct zone ECV were independent (Chi square; p=0.420). In the majority of patients, infarct zone T2 decreased over time, which means those who had a smaller decrease in infarct zone T2 had a larger increase in infarct zone ECV.
Since ECV may also reflect extracellular collagen volume fraction 16 , progressive extracellular fibrosis within the infarct zone may lead to an increase in ECV in the chronic phase post-STEMI. After adjustment for infarct zone ECV, infarct zone T2 was no longer a multivariable associate of LV remodeling. Accepting some loss of statistical power in this subset analysis, the potential explanations for this result may include 1) progressive infarct zone fibrosis is associated with persistent T2 hyperintensity at 6 months; 2) extracellular rather than intracellular edema is prominent in pathological remodeling post-MI and; 3) measurement error, since hematocrit, which is required to calculate ECV, may not be uniformly distributed in systemic blood and injured capillaries may allow varying amounts of formed blood elements to occupy the microvascular compartment.

Associates with adverse remodeling at 6 months
The clinical characteristics that were included in the multivariable model with adverse remodeling at 6 months were BMI (p=0.589), age (p=0.502), male sex (p=0.812), previous MI (p=0.708), previous PCI (p=0.347), diabetes mellitus (p=0.432), previous angina The multivariable predictors are described in Table 3 in the main paper.