Study Design and Participates
This prospective cohort study was performed in Peking University First Hospital and the Affiliated Hospital of Xuzhou Medical University. Patients who diagnosed with STEMI and treated with primary PCI comprising stent implantation in one of the two participating centers from February 2015 to September 2016 were eligible for this study.
Exclusion criteria included: 1) atrial fibrillation, frequent premature contractions, persistent ventricular tachycardia, or other tachyarrhythmia; 2) previous cardiac surgery or myocardial infarction; 3) severe liver and/or kidney dysfunction; 4) malignancy; 5) life expectancy of less than 1 year; 6) pregnancy; and 7) contraindications to magnetic resonance imaging (e.g. contrast agent allergy, ferromagnetic objects in the body, claustrophobia).
CMR Imaging Protocol
CMR imaging was performed before discharge (generally 5–7 days after the index event). All patients were examined with a 1.5 Tesla GE magnetic resonance imaging scanner. Three long-axis views (four-, three-, and two-chamber orientation) and short-axis stacks were acquired using a balanced steady-state free-precession imaging technique for functional cardiac analyses. Native T2, T2-weighted, and post-contrast T1-weighted image sequences were used for the assessment of edema, infarction size, microvascular obstruction (MVO), and intramyocardial hemorrhage. T1-weighted images were obtained 15 minutes after the administration of gadolinium-based contrast agent.
CMR Analysis
The analysis was performed offline by two experienced radiologists. Infarct size, edema, MVO, and intramyocardial hemorrhage were quantified using CVI 42 software (Circle Cardiovascular Imaging Calgary, Canada) [17]. CMR-FT strains (global longitudinal strain (GLS), global circumferential strain (GCS), and global radial strain (GRS)), LV end-diastolic volume (LVEDV), LV end-systolic volume, and LV ejection fraction (LVEF) were determined using the TomTec Imaging System (2D CPA MR, Cardiac Performance Analysis, version 1.1.2, TomTec Imaging Systems, Germany) [18, 19]. Briefly, LV contours were drawn semi-automatically at the end of diastole and systole. Subsequently, image features throughout an entire cardiac cycle were determined by the automatic border tracking algorithm of the software. Accurate tracking was confirmed by visual review of all borders and manual adjustments with consequent reapplication of the algorithm if necessary.
Follow-up and Endpoints
The incidence of major adverse cardiac events (MACE), consisting of cardiac death, reinfarction, unplanned revascularization, and heart failure within 6 months after STEMI was obtained from hospital records or face-to-face visits. Heart failure manifestations were defined as the exacerbation of exertional dyspnea or pulmonary edema requiring hospital admission, initiation of diuretics, or an increase in an existing diuretic regimen. Follow-up CMR imaging was performed at 6 months after STEMI with the patients’ willing. Adverse LV remodeling was defined as an LVEDV of > 15% greater than the LVEDV before discharge from the hospital.
Statistical Analysis
Patients were divided into groups based on infarct size (group IS < 10%, group 10% ≤ IS < 20%, and group IS ≥ 20%) or whether the endpoints (MACE or LVAR) occurred. Continuous variables were denoted as mean ± standard deviation, and the independent t test or Fisher exact test was used to evaluate differences between groups. Variables that were not normally distributed (as determined by Kolmogorov–Smirnov tests) were expressed as medians with 25th and 75th percentiles, and were compared using the Mann-Whitney U test. Categorical variables were presented as counts and corresponding percentage. Differences between groups were examined by chi-square test. Uni- and multivariate logistic backward stepwise regression analyses were performed to identify potential predictors of MACE and LVAR. Concerning the sample size and number of events, the regression model only included variables showing significant differences between endpoint and non-endpoint groups and most clinically important. Receiver operating-characteristic (ROC) curve analysis was used to determine the cutoff values of the GLS for predicting MACE. All statistical analyses were performed with a test significance level of 0.05 using SAS version 9.4 (SAS Institute, Inc., Cary, NC).