When investigating valve disease or outflow tract obstruction, it is really important to obtain a trace through the valve that accurately represents disease severity. The best way to ensure this is by obtaining traces from multiple views, as your alignment will vary slightly from each approach.
The subcostal (or subxiphoid) view provides a particularly good opportunity for catching the true peak (and mean) velocity through the aortic valve in dogs. In almost all patients, the alignment of the heart is such that, with very little manipulation of your image, your Doppler beam can be easily directed straight down the middle of your screen. In other words, the angle between your beam and the valve is 0°, giving the maximum Doppler shift and therefore capturing the highest and truest velocities possible.
In patients for whom you most need this trace, you are likely to want to be using Continuous Wave (CW) Doppler, because you will be expecting elevated velocities as a result of aortic stenosis or dynamic outflow tract obstruction (e.g. obstructive hypertrophic cardiomyopathy or systolic anterior motion of the mitral valve). Luckily for us, CW is surprisingly sensitive, and it is often possible to obtain a clear trace even when the imaging is poor. I have scanned many dogs where I actually cannot get an image at all, but I optimistically turn on my CW anyway, fan my hand very slowly, and a trace – often a very clear one – will usually appear. Turning your Doppler volume on can help with this, as you will often hear the feint murmur of the valve before you get a clear trace, and this alerts you to the fact that you are very close.
Scanning without an image isn’t as ridiculous as it sounds; in human echocardiography, it’s common to use a non-imaging “blind probe” (Pedoff probe) when investigating aortic stenosis. This probe has a tiny round footprint – about the size of a thumbprint – so can find gaps an imaging transducer could not, and also has higher Doppler sensitivity. Using this probe, you rely on your ears and a lot of patience to guide you to the valve. It can sometimes take 5 minutes or more to find the perfect alignment, trying 2-3 imaging windows, which demonstrates the importance echocardiographers place on obtaining the angle that gives the maximum velocity.
In the video clip below, you will see that image quality is suboptimal, and yet it is still possible to obtain a continuous wave trace. This clip is actually taken from a cousin of my own dog. She is not only related but also of the same sex and only four days younger, and therefore serves as an excellent healthy control to which I can compare my own dog’s measurements. If you’ve watched any of my videos before, you’ve probably figured out that my dog does not have a normal heart!
The video below shows you how to obtain this view:
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