Echocardiography offers the future for infarct size quantification

"Up until now infarct size has only been measured as part of clinical studies and not in routine clinical practice. The reason being that the reference method of gadolinium based contrast agents in MRI is expensive, takes a great deal of time to perform, and can only be undertaken by imaging specialists," explains EAE president Dr Luigi Badano, from the University of Padua, Italy. "The advantages of STE over MRI is that it's far quicker to use, cheaper, and can be used by cardiologists at the bedside with portable machines, and repeated serially when ever needed."

Additionally, STE can be applied even in patients with contraindications for MRI, such as metallic devices, claustrophobia, and severe renal failure that preclude use of contrast infusions. "The studies presented here open the way for every patient who is admitted to hospital with STEMI to undergo assessment of infarct size with echocardiography prior to discharge," says Badano.

Infarct size matters for determining how well patients will recover from STEMI. Statistics suggest that people who suffer damage to more than 30% of the left ventricle are twice as likely to die within a year of the event than people who suffer less damage. "It's well known that patients with larger infarcts are more likely to undergo alterations in the structure (dimensions, mass and shape) of the left ventricle, known as cardiac remodelling, which leads to heart failure," says Badano.

Evidence is mounting, he adds, that screening for patients with larger infract sizes enables identification of patients with a worse prognosis who benefit from more aggressive therapy and more frequent follow-up visits. "Nowadays many more options exist for STEMI patients deemed at high risk of adverse events, including prescription of ACE inhibitors and insertion of devices like CRT cardiac resynchronisation or ICDs", says Badano.

Speckle tracking echocardiography (STE) is a comparatively new non-invasive echocardiography technique well suited to quantifying infarct size. It works by tracking the movement of natural acoustic markers or "speckles" which are present on standard grey ultrasound tissue images. With the use of wall motion tracking software, speckle movement (and therefore myocardial tissue movement) can be visualised during the cardiac cycle. Speckle-tracking can be used to evaluate myocardial strain, which describes the myocardial deformation throughout the cardiac cycle. Reductions in measurement of strain, have been found to show direct relationships to the size of the infarct.

First use of 3D speckle-tracking to estimate infarct size following STEMI

Measuring circumferential strain with 3D speckle-tracking provides a good predictor of infarct size after STEMI , finds an Italian study¹.

In the study, which presents the first data on use of 3D speckle-tracking in measuring infarct size, Dr Denisa Muraru and colleagues from the University of Padua (Italy), estimated infarct size and necrosis transmurality in patients with recent STEMI, who had undergone successful treatment with primary PCI. One of the advantages of 3D speckle-tracking over 2D, say the authors, is that it allows the assessment of longitudinal (apex-to-base shortening), circumferential (shortening in the circumferential direction); radial strain (myocardialwall thickeningtowards LV cavity center) and area strain (a deformation parameter combining longitudinal and circumferential strain) at the same time. In the study, 49 patients with recent STEMI, successfully reperfused with primary PCI were assessed by 3D speckle-tracking, and the obtained LV strain parameters were compared with peak troponin I levels, as an estimate of the extent of myocardial cell injury. In a multivariable analysis, results showed that only circumferential strain emerged as a significant independent predictor of infarct size. Furthermore, in the subgroup of 27 patients who underwent additional assessment with delayed-enhancement MRI within 24 hours from the echocardiographic study, circumferential strain again showed the closest correlation with infarct size and the best predictive power to identify LV segments with transmural necrosis among all strain components.