Abstract
Calculation of left ventricular ejection fraction (LVEF) is a decades-old cornerstone of clinical cardiac practice. This parameter had been the basis for current clinical data, and criteria for pharmacological and device management are based on the LVEF thresholds. The LVEF has formed the foundation of longitudinal tracking and prognostication of several clinical issues. Since it is a volumetric and load-dependent metric, it can be reported as normal despite baseline myocardial impairment. The influence of ventricular geometry and considerable inter-observer and intra-observer variability are the main limitations. Global longitudinal strain imaging is typically derived from the speckle tracking mode of echocardiography to estimate the left ventricular (LV) strain. Over the last 2 decades, it has been established as superior to ejection fraction (EF) in detecting even subtle LV dysfunction. Since ischemia and ventricular pressure-overloading have a predominant and preferential effect on the sub-endocardial longitudinal muscle fibers, global longitudinal strain imaging (GLS) scores over the assessment of wall motion abnormalities.
Nadkarni et al. published their observations on 101 patients with a diagnosis of chronic stable angina with a positive treadmill test result. Echocardiogram showed LVEF of more than 55% and no obvious wall motion abnormality in any of them. All were subjected to coronary angiograms. The resting GLS was significantly reduced in the high-risk coronary artery disease (CAD) (left main or triple vessel disease or its equivalent anatomy) patients compared to the rest. An optimal cutoff for such identification derived in their study was −17.25%, with a sensitivity of 71.9% and specificity of 97.7%, comparable to nuclear imaging or stress echo. 1 Choi et al. and Bala et al. had suggested similar cutoffs for detecting high-risk CAD patients.2–4 Global longitudinal strain imaging appears as a promising tool in this direction; it is cost-effective, reproducible, and, being a non-invasive and simple test, it should become a choice in resource-limited settings.
Global longitudinal strain imaging can detect early sub-clinical LV dysfunction. It had added feathers as a predictor of mortality in acute heart failure and hypertrophic cardiomyopathy beyond what LVEF can do. 5 In cases of heart failure with preserved ejection fraction, it can pick up the high-risk sub-group for refining their medical management. Among patients with advanced chronic kidney disease, GLS predicted overall mortality and cardiovascular mortality, with higher sensitivity than EF. 6 Global longitudinal strain imaging is proving to be a favorable tool to bring out subtle LV dysfunction in cases of asymptomatic aortic stenosis and to detect the cardiotoxicity of cancer chemotherapy. In a recent study in post-coronary artery bypass grafting patients, an epicardial (layer-specific) GLS was performed, and it was found to have incremental prognostic value compared to conventional echocardiographic parameters for risk stratification. This type of novel assessment cannot be expected from EF, 7 which reflects the global function of the LV chamber.
With a sizable number of studies claiming superiority of GLS over EF, it is often pondered if the time has come to phase out the decades-old EF in clinical practice. Compared to EF, it has lower inter-observer and intra-observer variability and better reproducibility among echocardiographers of varying levels of expertise. In practical terms, GLS estimation needs special software, good image quality, a bit longer procedure time, and some initial learning curve is involved to get accurate results. The cutoff values to detect LV dysfunction are different in different studies. Some values also vary with the equipment and the vendor who manufactures the machines. Current evidence and guidelines suggest a complementary approach and recommend continuing to use both parameters in practice. Global longitudinal strain imaging reclassifies patients who have normal function by EF but are at higher risk.
