The LA:Ao ratio is one of the most utilised measurements in canine and feline echocardiography. Its purpose is to quantify left atrial dilatation, using the animal’s own aorta as a comparison, rather than indexing to something like bodyweight. In animals with no aortic valve disease, the aortic root should be proportional to the animal’s body size, and remain relatively constant in diameter throughout the animal’s life. I have discussed the most common causes of left atrial dilatation previously.
This ratio is most commonly obtained from the basal short axis view, from the right parasternal approach – but not always! Some may prefer the long-axis over the short-axis, and some may use a combination of approaches; some will use B-mode, others will choose M-mode.
How do people measure LA:Ao from the basal short axis?
When it comes to performing the measurement, there is yet more disagreement. From the most popular view, many vets with an interest in echo will be aware of the two main methods (the “American” and “Swedish” methods), but the reality in practice is often a hybrid.
I am guilty of this, and have to confess – at risk of incurring the wrath of both sets of authors if they ever read this – that I use the Hansson/Häggström/Kvart method in how I align my measurement points through the aorta, but my interpretation of the Rishniw/Erb method when choosing at which border I place my points (I’ll explain this further later on).
My reason for doing this is that my focus is on reproducibility, both with respect to myself, and between me and veterinarians who might measure the same patient on future occasions. The environments in which I scan differ from those of cardiologists working in referral and research centres, and I have found from my years of teaching general practice vets that placing measurement points at the blood-tissue interface is simply more intuitive and memorable for them, and most importantly, saves a lecture on lateral resolution in ultrasound imaging!
Hence, while inner edge to inner edge may slightly underestimate the diameter of the aortic annulus and left atrium, switching between the two during serial follow-up of a patient would be more damaging. Consistency is the key. It is also worth noting that the resolution of ultrasound machines has improved significantly in the last two decades, such that following a leading edge to leading edge method may no longer be as important as it once was.
A final, and possibly even more significant problem, is correct frame selection for measurement. There is great variation in left atrial size throughout the cardiac cycle, particularly in a healthy left atrium.
A millimetre or two when measuring a severely dilated left atrium, like that pictured above, is neither here nor there; but, measuring a healthy left atrium incorrectly, and perhaps even starting a patient on medication because of it, is a worrying thought. The reverse scenario is even more terrifying. It seems sensible to base identification of the correct frame on the imaging rather than on ECG timing, given that not every vet performing a cardiac ultrasound has access to ECG. However, even if everyone is attempting to identify this frame in the same way, there will be differences in machine frame rates universally, and visibility of the aortic valve leaflets from the short-axis specifically.
Should we still use LA:Ao?
Despite all of these difficulties, LA:Ao is considered a staple of veterinary echocardiography. Indeed, from my personal experience visiting hundreds of vet practices over the years, most vets who attempt any cardiac ultrasound – regardless of skill or experience – will attempt this measurement. It’s almost as if any cardiac scan is time wasted unless it produces this number.
My focus when teaching beginners is always on encouraging subjective assessment, but when vets are put under such pressure to produce this measurement by senior colleagues who “always” report an LA:Ao ratio, regardless of image quality (or whether or not they’ve had any training in cardiology in the last 20 years!) – and even by treatment guidelines – it is difficult to convince them that their echocardiogram has as much, if not more, value without this measurement.
To give an example of the importance of this measurement to GP vets, the ACVIM consensus guidelines for the diagnosis and treatment of myxomatous mitral valve disease in dogs are extremely helpful and widely referred to; however, correctly staging one’s patient depends (from an echocardiography point of view) on the following:
- LA:Ao > 1.6 from the short axis view - LV diastolic diameter ≥ 1.7 when normalised for body weight
If a general practice or emergency vet wishes to stage their patient without referral to a cardiologist (which many owners will not be in a position to afford) in order to decide a treatment plan, they are under immense pressure to produce these numbers if they perform a cardiac ultrasound.
What’s the answer?
The idea that only cardiologists should perform measurements on echocardiography is outdated and unrealistic. General practice veterinarians using cardiac ultrasound will always want to quantify left atrial dilatation in some way, but it may be that there are more reproducible methods that can be unambiguously explained, such as measuring from the long-axis instead of the short-axis.
Interestingly, when I asked a cardiology professor about how she assesses left atrial size in her feline patients, an LA:Ao ratio from the basal short axis was not at the top of her list.
It is no secret that the entire community desperately needs clear guidelines on how to perform this measurement. Experts are sure to want to continue to use whatever method is prevalent within their institution, but a universal method for beginners needs to be considered. This should prioritise intra- and inter-operator reproducibility, accepting that, at beginner level, this is actually more important than the accuracy of any single study.
We also need to harness the power of artificial intelligence a lot more effectively than we currently do. Again, the key here is reproducibility. AI may never measure exactly as a single expert would, but as long as it performs within the range of a consensus of experts, it is a technology worth introducing in order to assist non-specialists with measurements in echocardiography (Howard et al., 2021).
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Hansson, K., Häggström, J., Kvart, C. Left atrial to aortic root indices using two-dimensional and M-mode echocardiography in Cavalier King Charles Spaniels with and without left atrial enlargement.
Howard et al. (2021). Automated Left Ventricular Dimension Assessment Using Artificial Intelligence Developed and Validated by a UK-Wide Collaborative. Circulation: Cardiovascular Imaging.
Keene, B. et al. (2019). ACVIM consensus guidelines for the diagnosis and treatment of myxomatous mitral valve disease in dogs. Journal of Veterinary Internal Medicine.
Rishniw, M. & Erb, H. (2000). Evaluation of Four 2-Dimensional Echocardiographic Methods of Assessing Left Atrial Size in Dogs. Journal of Veterinary Internal Medicine.
Strohm et al. (2018). Two-dimensional, long-axis echocardiographic ratios for assessment of left atrial and ventricular size in dogs. Journal of Veterinary Cardiology.