EPSS stands for “E–Point to Septal Separation,” and refers to the distance between the tip of the septal (or anterior) mitral valve leaflet and the interventricular septum during diastole. It is measured from the right parasternal long- or short-axis view, when the anterior leaflet is closest to the septum. In some animals, it may not be measurable at all, because the leaflet may make contact with the septum. This is usually an indication of good systolic function.
The concept is that, as left ventricular systolic function declines, so the gap between the open septal mitral valve leaflet and the septum increases. There are a few diseases that can confound this relationship, most notably, significant aortic regurgitation and mitral stenosis (the latter being rare in dogs).
EPSS has not been widely used in human echocardiography since the 1990s, but is making a comeback in critical care echo. According to its proponents, this is due to its ease of use and reproducibility. A non-cardiologist, such a an anaesthetist or emergency physician, can make a better assessment of systolic function from EPSS than they can subjectively assess function or quantitatively measure ejection fraction – or, at least, the learning curve is shorter.
In veterinary echocardiography, EPSS has remained in consistent use for assessing dogs for dilated cardiomyopathy, and particularly the screening of Dobermans. The 2003 ESVE Taskforce on Canine DCM included an increased EPSS as one of their minor criteria for diagnosis of pre-clinical DCM.
EPSS is definitely easier for a beginner than ejection fraction. If using M-mode, the main requirement is that the M-mode line goes through the tips of the open mitral valve leaflets. Ejection fraction, on the other hand, requires an unforeshortened left ventricle and accurate tracing of the endocardial border; two things which beginners frequently struggle with. Achieving the full length of the left ventricle requires confident transducer manipulation as well as knowledge of how the left ventricle should look, and accurate tracing requires knowing to exclude mitral valve apparatus, papillary muscles and trabeculation from the left ventricular cavity, and experience in delineating that (often poorly defined) border.
In reality, EPSS is not purely a surrogate measure of systolic function. It’s measuring an element of diastolic function as well. After all, it is the reduced excursion of the mitral valve leaflets during early diastole that is being measured.
The reason the anterior/septal mitral valve leaflet does not open as wide in a patient with systolic and diastolic dysfunction is that the failure of the heart to efficiently eject blood during systole means that there is higher pressure than normal inside the left ventricle when it is trying to fill. The early filling phase of diastole relies on low pressures in the left ventricle so that the chamber can fill, because it’s not until late filling that the left atrium actively pushes blood into the ventricle with that atrial kick, or A-wave.
This reduced pressure gradient between the left atrium and left ventricle mean that the mitral valve does not snap open in early diastole with the enthusiasm we are used to seeing in healthy hearts. It is this muted excursion of the septal leaflet that we are measuring in EPSS.
EPSS is measured as the shortest distance from the top of the E wave to the blood-tissue boundary of the interventricular septum – i.e. measured at the point of maximum excursion of the septal mitral valve leaflet. It is important that you line your M-mode cursor up with the tips and not the body of the mitral valve.
Watch this video below on how to obtain and measure EPSS.
In the M-mode trace below in a dog, you can see at a glance that EPSS is increased (although it measures right at the 6mm cut-off).
Me neither, but EPSS can be measured from B-mode, too! And even if you’re not convert to EPSS, it is still useful to think about what it is and means. It reminds us of the fact that a small gap or even contact of the anterior/septal mitral valve leaflet and septum during diastole is normal. So often, I see people using M-mode in cats to check for systolic anterior motion (SAM), and mistakenly interpreting diastolic contact between the mitral valve and the septum as evidence of SAM: forgetting the ‘systolic’ in systolic anterior motion!
This is another reason why I am not the biggest fan of M-mode in general; it can isolate the observer from the overall picture. Simply going through the B-mode image frame-by-frame yields the same information, and the aortic valve snapping open and visible contraction of the walls are better visual clues to a beginner of when systole has begun, than two lines moving closer together.
EPSS’ strength in assessing LV function is with inexperienced users: you might call it a substitute for proper training! Wouldn’t it be better to gain the confidence and support you need to be able to assess systolic function through a range of complementary methods?
You can get lifelong access to our echocardiography training programme and community here; or, if you’re not sure if this is for you, book a call to discuss where you’re at with echocardiography right now, and how we help you reach the next level.
References
Dukes-McEwan et al. (2003). Proposed guidelines for the diagnosis of canine idiopathic dilated cardiomyopathy.
Wess (2022). Screening for dilated cardiomyopathy in dogs.
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