Quantification of left ventricular systolic function is arguably even more difficult in small animals than it is in humans. Even simple grading by eye can be challenging, particularly in the presence of disease such as significant hypertrophy or moderate-severe regurgitation. Strain could help, because it has been shown to be able to detect systolic dysfunction before it can be detected by eye or by conventional quantitative measures – but what is it, and does it have a role in veterinary echocardiography?
What is strain?
The myocardium changes shape during the cardiac cycle, and this can be measured by strain (magnitude of deformation) and strain rate (rate of deformation). The most commonly used strain parameter is Global Longitudinal Strain (GLS) – simply, the longitudinal shortening of the left ventricle.
Strain has been shown to be more sensitive to systolic dysfunction than conventional methods (namely, ejection fraction), even picking up subclinical stages of disease (Wess et al.,2010) or unmasking hidden dysfunction (Zois et al., 2012).
Images are usually obtained from the left apical four chamber view, and there are a number of methods for measuring longitudinal strain; M-mode (MAPSE), unidimensional measurements (measuring from both hinge points of the mitral annulus to the apex in four, two and three chamber / long axis views), tissue Doppler imaging (TDI) and, most recently, speckle tracking.
What is speckle tracking derived GLS?
Speckle tracking takes advantage of the speckle pattern of the myocardium on ultrasound. The ultrasound machine literally ‘follows’ or tracks the motion of the speckles, because these patterns are relatively stable between frames.
Speckle tracking derived GLS has shown a number of advantages over the angle-dependent methods of M-mode and TDI. It has also been shown to be more reproducible than ejection fraction, particularly in novice users (Chan, Shiino, Obonyo et al., 2017; Medvedofsky et al., 2017), due to its semi-automated nature: the user need only adjust the curve drawn by the software, and not trace the entire endocardium from scratch. However, besides cost, it suffers from three key practical barriers to uptake:
- There is too much variation in results between vendors
- It requires high frame rate, high quality imaging
- There is a huge lack of normal values for different breeds
Variability between vendors and the need for very high frame rate imaging are the most significant issues for veterinary echocardiography, but this is changing.
The future of strain in veterinary echocardiography
Recently, we have seen the emergence of an increasing number of vendor-neutral software packages. Using artificial intelligence to automatically select frames, they often require very little human input, and also have a far lower dependence on image quality or frame rate. The greater attention given to point-of-care ultrasound in the human world has facilitated this shift away from this being a technique only available on top end ultrasound machines from one of the ‘big three’ vendors.
Thus, strain is becoming ever more accessible, more practical and more affordable to integrate into clinical practice. Far from being the exclusive domain of researchers, strain could feasibly have a place in the general veterinary practice within the next few years, bringing confidence to veterinarians in the assessment of left ventricular systolic function.
Above: An example of GLS on the Siui Apogee 2300
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Chan, J., Shiino, K., Obonyo, N., et al. (2017). Left Ventricular Global Strain Analysis by Two-Dimensional Speckle-Tracking Echocardiography: The Learning Curve. J Am Soc Echocardiogr, 30(11):1081-1090.
Medvedofsky, D., Kebed, K., Laffin, L. et al. (2017). Reproducibility and experience dependence of
echocardiographic indices of left ventricular function: Side-by-side comparison of global longitudinal strain and ejection fraction. Echocardiography (34):365-370.
Wess, G., Sarkar, R., Hartmann, K. (2010). Assessment of left ventricular systolic function by strain imaging echocardiography in various stages of feline hypertrophic cardiomyopathy. J Vet Intern Med 24(6):1375-82.
Zois, N., Tidholm, A., Nagga, K. (2012). Radial and Longitudinal Strain and Strain Rate Assessed by Speckle‐Tracking Echocardiography in Dogs with Myxomatous Mitral Valve Disease. Journal of Veterinary Internal Medicine.