Most textbooks tell us that the most common cause of aortic regurgitation (AR) in small animals is bacterial endocarditis. My own experience in UK first opinion practice has been that AR is most frequently congenital in origin, and often accompanies perimembranous ventricular septal defects or aortic stenosis. In these patients, even if the aortic valve leaflets have developed normally, the abnormal turbulent flow through them causes changes or damage to the leaflets, resulting in leakage.
There are also a number of published case reports describing AR in association with morphologically abnormal, quadricuspid aortic valves, in both dogs and cats, although this is admittedly very rare.
Chronic vs acute aortic regurgitation
When it comes to acute aortic regurgitation (also referred to as aortic insufficiency), endocarditis is undoubtedly the leading cause. You can distinguish acute from chronic AR because, provided there are no obvious signs of any other significant cardiac disease, the left ventricle will be normal in size and its function will be hyperdynamic.
In chronic aortic regurgitation, the left ventricle has had plenty of time to make adaptive changes and enlarge (concentric hypertrophy prevents the walls from thinning), to accommodate the increased volume of blood. Systolic function will appear normal to the eye, or in later stages of the disease, impaired.
Grading aortic regurgitation
Grading the severity of AR can be challenging, and the best way to improve confidence in your grading is to combine a number of different quantitative and semi-quantitative methods.
Jet width to LVOT ratio
From the right parasternal long axis (5 chamber) view, zoom in over the left ventricular outflow tract (LVOT) and measure its diameter during diastole, i.e. when the valve is leaking. Then measure the width of the regurgitant jet. The short clip below talks you through this process.
If the jet occupies 2/3 or more of the diameter of the LVOT, this is suggestive of severe regurgitation. Like everything with colour Doppler, though, it can be affected by machine settings (particularly your scale), so it is not wise to rely solely on this to make your diagnosis. Also bear in mind that eccentric jets may result in an underestimation of severity.
This measurement is taken from the subcostal or apical 5 chamber view – whichever you can obtain the best alignment and clearest trace from. The video below demonstrates how to obtain a continuous wave Doppler trace from the subcostal view.
In general, shorter pressure half-times correlate with greater regurgitation severity, which Matthew explains in the video below.
As with almost every rule in echocardiography, however, there are exceptions. Pressure half-time is affected by left ventricular compliance (it is, after all, a measure of how quickly pressures equalise between the aorta and left ventricle), and acute moderate aortic regurgitation could have a shorter pressure half-time than chronic severe regurgitation, because in the latter case, the left ventricle has had time to adapt. You need to interpret your pressure half-time in the context of everything else you can see on the echo.
Methods such as vena contracta width tend not to be used routinely in clinical veterinary echocardiography, and require some form of indexing to bodyweight or aortic dimensions. The presence of a large flow convergence zone is further semi-quantitative information that you can use. It is again heavily influenced by colour Doppler settings, but a good rule of thumb is that if you can see a flow convergence zone before you’ve even adjusted your scale, regurgitation is likely to be significant. You’d be hard pushed to be able to get a flow convergence zone from mild aortic regurgitation!
If you’d like more information on these or any other methods discussed in this article, please get in touch.