The mechanisms behind poorer cardiac outcomes in underweight patients with acute coronary syndrome (ACS) are not understood and features of coronary culprit lesions in underweight ACS patients have not been fully examined. A total of 1,683 patients with ACS were divided into 4 groups according to body mass index (BMI): <18.5 (n = 73), 18.5 to 24.9 (n = 995), 25 to 29.9 (n = 488), and ≥30 (n = 117). Angiography and optical coherence tomography (OCT) images were analyzed for 1,428 of these patients who had primary percutaneous coronary intervention (PCI) and 838 who had primary PCI with OCT guidance, respectively. Diabetes (p <0.001), hypertension (p <0.001), and dyslipidemia (p <0.001) were less prevalent in BMI <18.5. Statin prescription at discharge was less frequent in the BMI <18.5 group (p <0.001). Quantitative coronary angiography analyses revealed smaller reference vessel (p = 0.001) and minimum lumen diameters after PCI (p = 0.019) and OCT revealed longer lipidic plaque length (p = 0.029) in the BMI <18.5 group. Kaplan-Meier analyses revealed higher rates of cardiac mortality (p <0.001) and major bleeding (p = 0.034) during the 2-year follow-up in the BMI <18.5 group. After adjusting for traditional cardiovascular risk factors, BMI <18.5 independently predicted 2-year cardiac mortality (hazard ratio 1.917 [95% confidence interval [1.082 to 3.397], p = 0.026). In conclusion, being underweight contributed to poorer cardiac outcomes in established ACS population. Smaller minimum lumen diameter after PCI and further progressed atherosclerosis at the culprit lesions despite their lower prevalence of comorbid metabolic risk factors may be related partly to poorer cardiac outcomes.
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