If your feline patient has a severely dilated and/or impaired left atrium, it is always wise to check the left atrial appendage. This can be seen from the right parasternal short axis view, apical 2 chamber view, and modified apical 2 chamber view. Since Doppler alignment is better from the apical views in the majority of cats, this article is going to focus on this.
If your patient has had longstanding disease of the left atrium, it will eventually begin to lose its contractile function. You will see this in cats with hypertrophic cardiomyopathy, dogs with chronic myxomatous mitral valve disease or dilated cardiomyopathy – in fact, nearly all of the most common cardiac pathologies we see in first opinion veterinary practice affect the left atrium. Assessing left atrial function is diagnostically and prognostically useful for most of your patients.
The most reproducible way to assess your patient’s left atrial function is by measuring it in the right parasternal 4 chamber view in diastole and systole, which you can do in either B-mode or M-mode. From this, you can calculate a fractional shortening % (or your machine will probably do it for you!). Watch this video to learn how:
Unlike cats and humans, dogs with poor left atrial function do not tend to exhibit very low flow or suffer from thromboembolic events, but if your feline patient has a low FS% then it is worth checking the auricle for signs of thrombus and to measure the velocity of blood flow entering and leaving it.
The left atrial auricle is the ‘comma shaped’ appendage that is attached to the left atrium. There has been a lot of debate around what its purpose actually is: is it simply an embryological remnant we don’t really need, or does it aid left atrial function? Increasingly, we are understanding its importance to overall left atrial function and ventricular filling, but we do also know it is not essential: in humans with very low flow in their left atrial appendage, the left atrial appendage is sometimes blocked off using a closure device, to reduce their risk of thromboembolic events.
In cats, the best we can do right now is to closely monitor these patients, and anticoagulate where risk is deemed to be high.
To find the left atrial appendage from the apical 4 chamber view, you need to rotate your hand clockwise to obtain a 2 chamber. Once the right ventricle is fully out of view, the appendage should come into view on the right of your screen. By shifting your probe cranially by one rib space, the appendage should really open up. This will be a lot easier in cats with dilated left atriums (the pleural of “atrium” is “atria” but since a single individual cannot have more than one left atrium, “left atria” does not seem correct!).
In an appendage with very low flow, you will be able to see blood stasis (“smoke”). You can also measure velocities in and out of the appendage using pulse wave Doppler. PW has to be the method of choice because it allows you to precisely pinpoint from where you wish to measure velocities from. Aim for the main body of the appendage, and avoid going too high up towards the pectinate muscles right at the top. You want to look at velocities both above and below the baseline, so keep your baseline in the centre of your scale. You will probably need to reduce your scale/PRF in order to properly see your waveform, as blood flow velocities will be lower than in other areas of the heart, and you will probably need to decrease your wall filter for the same reason.
A normal LAA flow velocity is >0.25m/s.
Watch the video below to see how to do this in a normal and diseased left atrium:
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