Anterior leaflet prolapse and bileaflet prolapse are by far the most common expressions of mitral valve disease in canines (in humans, posterior leaflet prolapse is the most common expression of degenerative mitral valve disease). This small dog had been diagnosed with a heart murmur, and needed an echocardiogram to pinpoint the problem. Even on 2D imaging, the source of the murmur was immediately obvious. Look at the clip below:
2D signs:
In the next clip, we add Colour Doppler, and the backflow of blood into the left atrium can be seen.
Disease progression would be monitored through serial measurements of the left ventricular diastolic and systolic dimensions, which is why it is so important to be reproducible, performing measurements at the same level through the ventricle every time. The favoured method for veterinarians is from the short axis view, at the papillary muscle level just as the chordal attachment begin, due to the availability of reference values from this approach (although it should be noted that most of these were obtained with M-mode and without the use of tissue harmonic imaging, making an update of reference values taken on more modern machines badly needed).
Systolic dimensions are particularly important as an increased systolic diameter alerts the vet to a fall in contractility. An ejection fraction within the normal range can provide false reassurance in the case of mitral regurgitation (MR). This is because end diastolic volumes increase with significant mitral regurgitation (the mechanism for which is explained beautifully by Melanie Hezzell in this article here), but systolic volumes remain unchanged. Hence, the difference between the diastolic and systolic measurements becomes greater with significant MR, making the ejection fraction higher. Ejection fraction can often mask underlying systolic dysfunction until it becomes severe enough that even the effect of an increased end diastolic volume cannot compensate for the lack of systolic contraction in the vet’s EF calculation. For this reason, a worsening EF is not a good marker of early disease progression, as a fall in measured ejection fraction may only occur at the very late stages of the disease. In earlier stages, paradoxically, EF may actually increase. Calculated EF in a canine patient with significant mitral regurgitation should always be well above 50% – almost supranormal.
Disease progression
Unfortunately, the dilatation of the left-sided chambers due to the increased volumes only exacerbates the problem over time, causing stretching of the mitral annulus and further loss of coaptation between the leaflets. The annulus also loses its saddle shape and contractility, becoming more circular and planar. Medical management for mitral regurgitation can therefore never be curative, with surgical or percutaneous intervention (e.g. MitraClip) being the only way to halt disease progression and encourage remodeling. Application of these methods to canine heart disease are currently only in their experimental stages, and are not routinely offered even in expert centres.
As MR severity inevitably worsens, the left atrium cannot dilate any further to compensate for the high volumes of regurgitant blood flow. Left atrial pressures increase, which feeds back to the pulmonary system, resulting in congestive heart failure and the development of pulmonary hypertension, right ventricular dilatation and tricuspid regurgitation, as these elevated pressures feed back into the right side of the heart.
References
Hezzell, M., 2018. Pathology and prognosis of canine myxomatous mitral valve disease. In Practice (BMJ).
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