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To assess the utility of speckle tracking global longitudinal systolic strain (GLS) compared with traditional echocardiographic indices including left ventricular ejection fraction (LVEF), wall motion score index (WMSI), and end-systolic volume index (ESVI), in estimating the infarct size (IS) following a ST-elevation myocardial infarction (STEMI).The study includes 227 patients with STEMI and day 1 and day 30 echocardiograms, and myocardial perfusion imaging (MPI) only at day 30 to assess IS. IS was modelled by linear regression with echocardiographic parameters using MPI as reference. Resulting echocardiographic IS estimates were compared by ratios of standard deviations of model residuals (RSD). To estimate the resultant day 30 IS 1 day after a STEMI, GLS was more precise than LVEF (RSD: 0.91, P = 0.014) and ESVI (RSD: 0.88, P = 0.002), and comparable with WMSI (RSD 0.99, P = 0.86). To estimate IS from a day 30 echocardiogram, GLS was comparable with LVEF (RSD: 0.98, P = 0.68) and ESVI (RSD: 1.04, P = 0.40), but WMSI was more precise (RSD: 0.89, P = 0.006). Multiple linear regression revealed that on day 1 after STEMI, GLS significantly complemented the standard parameters separately (P-values all models <0.001) or combined [multivariable model: GLS (P = 0.001), WMSI (P = 0.03), LVEF (P = 0.40)]. On day 30, GLS significantly complemented LVEF and ESVI, but when WMSI was in the model, GLS's association with IS was not significant.On day 1 after revascularization for STEMI, GLS contains additional information about final IS compared with standard echocardiographic systolic function indices. Studies are needed to clarify whether this has prognostic implications.