Mitral regurgitation (MR) is a common finding in dogs, and most often secondary to degenerative mitral valve disease. Mitral valve degeneration affects smaller dogs disproportionately, and in some breeds (such as King Charles Cavalier Spaniels), prevalence approaches 100% with increased age (Pedersen & Häggström, 2000). The anterior leaflet or both leaflets together are overwhelmingly affected (Terzo et al., 2009). It is rare for the posterior leaflet to be affected alone.
When assessing MR, one can often be limited not only by available echocardiographic windows (it is not always possible or practical to lay the animal down, or to shave for apical windows), but also by the quality of the colour Doppler mapping on the ultrasound machine. On quite a lot of mid-range portable machines, sadly, colour Doppler is present in little more than name only. It might be useful for confirming the presence or direction of bloodflow in vessels (and this is likely to be what the sales rep will have demonstrated it on – the human carotid artery is a favourite!), but when it comes to cardiac work, many lack the frame rates required to provide useful information in small animals.
Above: Notice how, in the above feline cardiac scan, even on a high end machine the frame rate drops from 76 frames per second (76Hz) to 22 frames per second (22Hz) when colour Doppler is turned on.
At a heart rate of 200bpm, for example, the machine may only be able to obtain around three frames during the entire duration of systole, which does not give sufficient time resolution to track a mitral regurgitation jet clearly.
In the following case, we scanned a small dog with a fast heart rate. He had a visually enlarged left atrium, and also notice in the clip below how the mitral valve leaflets close behind the plane of the annulus, suggestive of bileaflet prolapse.
These three signs (compensatory fast heart rate, dilated left atrium, bileaflet prolapse) make the presence of significant MR likely, but Doppler is needed to confirm its presence. With the colour frame rate of the Sonoscape S2, however, it’s difficult to rule in or out. The flash of turbulence of the colour flow in the left atrium during systole is perhaps the only clue that something is amiss.
Even on a higher end Siui Apogee system, the fast heart rate of this nervous little dog presents a challenge, but the flow of blood back into the left atrium during systole is more easily appreciated at this machine’s faster colour frame rate. Freezing the image and going back through the frames using the trackball can also help to check timings.
In this case, Continuous Wave (CW) Doppler was used to confirm the diagnosis on the Siui system (it was not available on the Sonoscape S2). Even without parallel alignment (an apical view would have been better), the mitral regurgitation (MR) trace is dense. The sound coming from the speakers was harsh, and likely similar to a murmur the veterinarians would have heard on auscultation. Qualitatively, therefore, this dog appears to have at least moderate mitral regurgitation due to bileaflet mitral valve prolapse.
Pedersen, H., Häggström, J. (2000). Mitral valve prolapse in the dog: a model of mitral valve prolapse in man. Cardiovascular Research, Vol 47(2).
Terzo, E., Di Marcello, M., McAllister, H. et al. (2009). Echocardiographic assessment of 537 dogs with mitral valve prolapse and leaflet involvement. Vet Radiol Ultrasound, Jul-Aug, 50(4):416-22.