By Jeffrey Zimmet, MD, PhD

Associate Professor of Medicine, University of California, San Francisco; Director, Cardiac Catheterization Laboratory, San Francisco VA Medical Center

Dr. Zimmet reports no financial relationships relevant to this field of study.

SYNOPSIS: This largest-to-date analysis of six randomized studies of ST-elevation myocardial infarction revealed no association between body mass index and infarct size, one-year mortality, or heart failure hospitalization.

SOURCE: Shahim B, Redfors B, Chen S, et al. BMI, infarct size, and clinical outcomes following primary PCI: Patient-level analysis from 6 randomized trials. JACC Cardiovasc Interv 2020;13:965-972.

The “obesity paradox” refers to the observation that although obesity contributes to many risk factors that make cardiovascular disease more likely, obese people may fare better than their normal weight counterparts in acute exacerbations of disease, such as acute myocardial infarction (MI). In ST-elevation MI (STEMI) in particular, the evidence in this area has been mixed. Yet a host of data have been built up supporting the putative obesity advantage. Some, but not all, prior studies have revealed smaller infarct sizes among overweight patients. Experimental models have demonstrated the ability of hormones produced by adipose tissue to reduce infarct size in mice.

To examine this question, Shahim et al combined patient-level data from six contemporary randomized trials of patients receiving primary percutaneous coronary intervention (PCI) for STEMI. The authors of each trial measured infarct size at a median of four days after an event, with five using cardiovascular magnetic resonance (MR) and one using 99mTc-sestamibi SPECT/CT. All collected data on body mass index (BMI), and each reported clinical endpoints that were adjudicated by independent clinical events committees.

Among the six trials, data were available for 2,238 patients with STEMI undergoing primary PCI. Analyses were performed using BMI as a continuous variable and with patients stratified according to the World Health Organization (WHO) definitions of normal weight, overweight, and obese. Among the patients analyzed for the study, 658 were classified as normal weight, 1,008 were overweight, and 586 were obese. Regarding baseline characteristics, overweight and obese patients were more likely than normal weight patients to have diabetes, hypertension, and hyperlipidemia, but were less likely to smoke. No significant differences were seen among the BMI groups when it came to presentation of the acute MI itself.

Regardless of whether BMI was treated as a continuous variable or a categorical one, Shahim et al found no association between BMI and infarct size, microvascular obstruction, or ejection fraction (EF). During a median follow-up of 350 days, there was no unadjusted or adjusted association between BMI and the rates of death or heart failure hospitalization. Similarly, when stratified by WHO classification, these outcomes were similar among normal, overweight, and obese individuals. The authors concluded that among patients undergoing primary PCI for STEMI, there was no protective effect of increased BMI on infarct size, microvascular obstruction, EF, or one-year rates of death and heart failure hospitalization.

COMMENTARY

In several ways, this study was specific, dealing only with STEMI patients undergoing primary PCI and weighted toward anterior infarct and larger infarcts. Unfortunately, there was no insight regarding procedural complications, which elsewhere have been observed less often in overweight patients. By design, patients with poor early outcomes who did not make it to imaging evaluation were excluded from the analysis.

The authors acknowledged BMI is a highly imperfect measure of obesity, and that other measures, such as waist circumference and fat mass index, may be relatively advantageous. Regardless, this was by far the largest study to date of the relationships between obesity (as measured by BMI) and infarct size and hard clinical outcomes after STEMI. Most of the 2,238 subjects were evaluated by cardiac MR, which is likely the best modality for this evaluation. Three prior reports that used cardiac MR for infarct size included 89, 193, and 426 patients. Overall, Shahim et al conducted an excellent study. Their work represents the best effort to date to provide a definitive conclusion on this issue. For now, the best available evidence suggests overweight and obese patients have no advantage in terms of outcomes in STEMI — but no detriment, either. In this regard, the obesity paradox remains a point for discussion.