Erin Ruiz, MS4
The Article: Favot, Mark, et al. "Strain echocardiography in acute cardiovascular diseases." Western Journal of Emergency Medicine 17.1 (2016): 54.
The Idea: Cardiovascular disease remains the leading cause of death in the United States. Often, these patients are first seen in the Emergency Department (ED) needing acute intervention. Previously, comprehensive transthoracic echocardiography was used to assess the patient and guide treatment. However, this is often a time intensive process, requiring time to complete the study as well as having a cardiologist interpret it. In the acutely ill patient that presents to the ED, this leads to limited utility of comprehensive echocardiography, as the emergency physician (EP) often needs to make time sensitive decisions. Therefore, in 2010, guidelines were released to establish standards and core applications of ED based point of care echocardiography. Since then, many advances have been made to allow EPs to further assess cardiovascular function, and one of those is strain imaging using speckle tracking.
About Strain: Conventional techniques to assess systolic function used visual assessment of wall motion, while strain echocardiography allows for actual measurement of tissue deformation within the myocardium. Strain is calculated by finding the difference between two points at the beginning of movement compared to the end of that movement, and dividing by the starting distance. Speckle tracking software allows those points to be tracked over time. This can be measured in multiple planes, but when considering the left ventricle, the longitudinal plane is the first to show impairment, and therefore is often used to measure strain. Once images are obtained, the software divides the LV into many segments, and that data can be combined to get a global evaluation called global longitudinal peak systolic strain (GLS). One can then compare this to that of healthy subjects, of which GLS is usually -18 to -22%.
Applications:
1. Acute Myocardial Infarction: There are many uses for strain in MI. For STEMI, it is a predictor of post-discharge adverse outcomes. Further, decreased GLS within 24 hours after being revascularized predicts patients more likely to have any of the following: all-cause mortality, hospitalization with re-infarction, heart failure, or stroke at 6 month follow up. However, for EPs, it may be most valuable in helping determine which patients with NSTEMI may benefit from urgent revascularization. Early stages of MI do not show wall motion abnormalities, but they do show impaired function of the longitudinal fibers of the LV, which may be picked up by strain echo.
2. Acute Heart Failure: Heart failure (HF) is about more than just ejection fraction (EF). In early studies, patients with HFpEF still had longitudinal strain, at levels similar to that of patients with reduced EF. Further, GLS may be an independent predictor of adverse outcomes in patients with HF. For EPs, this becomes particularly important as strain is a more sensitive measurement of the condition of the heart than LVEF, which is traditionally used. LVEF does not change during treatment of HF, but diastolic parameters do, and this is when strain assessment of diastolic function may be useful. This allows an EP to monitor responsiveness to treatment in real time and make clinical decisions based on this data.
3. Right Heart Strain: Strain can also be used to look at RV function, in situations when a pulmonary embolism is suspected. There is subjectivity in determining free wall hypokinesis or McConnell’s sign, and speckle tracking would remove that. A recent study compared RV strain in patients with PE vs controls and found that free wall and septal wall strain were significantly decreased in those with PE compared to those without. The presence or absence of McConnell’s sign did not change this data. RV free wall strain may be a potential way to determine RV systolic function and thus help EPs risk stratify patients suffering from PE.
Limitations: There are a couple of potential limitations to speckle tracking that EPs should be aware of. Since the speckle tracking requires tracking specific points over time, it requires high frame rates. Low frame rates may lead to error from poor tracking. To increase the frame rate one may decrease the depth. Another limitation has to do with the heart rate. At heart rates >120, or rates that vary from beat to beat, the data becomes less reliable. For this reason, atrial fibrillation and multifocal atrial tachycardia are relative contraindications to strain imaging. Another limitation that currently exists is the lack of standardization among ultrasound manufacturers in regards to LV strain. Lastly, the use of speckle strain would require new equipment, as it is not possible to simply upgrade an already purchased product. The cost to buy this new ultrasound is about $50,000. This may cause delay in wide spread implementation of use of speckle tracking and strain imaging.
The Takeaway: Currently, echocardiography in the ED is primarily used for 5 reasons: to assess the pericardium, right heart, LV function, fluid status, and for procedural guidance. It is speculated that new technologies, including strain echo and speckle tracking, will assist with diagnosis and management of a variety of conditions including MI, HF, and PE.