Echocardiography Basics 3: Controversy and Nuance in the LA:Ao Ratio

In our first article, we established that left atrial size is really important in veterinary cardiology, and that LA:Ao > 1.6 is the threshold for enlargement, including cats and dogs in defining Stage B2 MMVD and HCM. If cardiology were simple, we would stop there. However, any clinician who has spent time behind a probe knows that the heart base view is rarely perfect, and the “rules” often seem to conflict.

Measurement controversies:

• Pulmonary vein drop out (“The Invisible Wall”)
• Timing issues (“Swedish vs American”)
• Aortic valve movement ‘out of plane’ (“The Moving Target”)
• Breed variations (“The cavalier exception”)

As a result of these inconsistencies and other challenges, the LA:Ao ratio measurement has been shown to have high variability when measuring even when done by experts. Which is not good.

The Invisible Wall: Pulmonary Vein “Dropout”

One of the most frequent challenges in the short-axis view is the “disappearing” posterior wall of the left atrium. The pulmonary veins enter the atrium at its caudolateral aspect, and because the wall here is incredibly thin, it often suffers from ultrasound dropout.

When this happens, the blood in the atrium and the blood in the vein appear to merge into one large anechoic space. If you simply place your caliper at the first visible tissue interface, you will inadvertently include the pulmonary vein in your measurement, leading to an artificially high LA:Ao and a potential misdiagnosis of Stage B2 MMVD.

Here are 2 examples of a normal LA with a pulmonary vein, and a very enlarged LA also having pulmonary vein issues:

IMAGE - PULM VEIN comparison

The Clinical Workaround: If you see a vein entering the atrium, do not measure into the vessel. There are multiple described approaches for coping with this, and as yet no consensus on the best approach:

• Option 1: “approximate” the atrial border by connecting the visible edges of the atrium with an imaginary curved line that follows the natural contour of the chamber. If the dropout is so severe that you are guessing, it is time to move to a different view.
• Option 2: Angle your measure to the little ‘bump’ that sits just to the right of the vein.
• Option 3: Angle your measure to the wall where it first becomes visible to the left of the vein

IMAGE - PULM VEIN comparison annotated

Personally, I’m a fan of option 2. But set guidelines if published will maybe solve this debate for good…

The Great Timing Debate: Swedish vs. American

We also must acknowledge that “The LA:Ao” is not a single measurement, but a choice between two primary methodologies: the Swedish (Häggström) method and the American (Rishniw) method. There are 2 ways to measure the aorta, and 2 time points to choose. As the LA size and aortic size both vary a lot during the cardiac cycle, this creates a lot of potential error.

IMAGE different AO measures

The most common source of confusion in first-opinion practice is a “hybrid” approach: practitioners often measure the largest atrium they can find (which is end-systolic) but then apply the EPIC trial threshold (which is early-diastolic).

Best practice: The “bisecting” line used in the American method often results in a slightly larger aortic diameter. Because the Swedish method was used to validate the LA:Ao > 1.6 threshold in the EPIC trial, I strongly recommend sticking to the early-diastolic timing for staging purposes .

We are hopefully going to see the first set of veterinary echocardiographic guidelines published this year.

The Moving Target: Through-Plane Motion

The heart is not a static object; it moves cranio-caudally and “bobs” within the chest with every cycle. This root motion (which can be up to 1.5 cm per cycle) creates a specific frustration for the LA:Ao. Remember the ultrasound is slicing the heart transversely, and this means it is moving back and forth through the plane with every heart beat and breath.

The Swedish method requires us to measure the first frame after the aortic valve closes. Paradoxically, this is often the exact moment the heart shifts position, causing the aortic cusps to “vanish” out of frame, or the aorta to appear oval rather than circular.

If the “Mercedes Benz” sign is not visible or looks skewed on that specific frame, your measurement is technically invalid. A common pitfall is to “settle” for a frame that is close enough. If the aorta is elongated, your Ao diameter will be too large, and your ratio will be too low, potentially masking a heart that actually needs medication.

However I also frequently see an aorta that has come out too small, due to being measured too high up the aortic root. Which results in an overestimate of the LA:Ao ratio.

Breed Nuances: The Cavalier “Exception”

We are using the aortic diameter as a reference point, to compare the LA size to. But is it reliable? It turns out there are lots of variations in aortic size, even when looking at the same sized dogs.

So we must be cautious when applying a generic 1.6 cut-off to every breed. The Cavalier King Charles Spaniel (CKCS) is a prime example of where linear ratios can mislead. Research suggests that healthy Cavaliers often have smaller aortae relative to their body weight than other breeds, which can artificially inflate the LA:Ao ratio .

In fact, up to 10% of healthy dogs - particularly Bulldogs, Boxers and English Setters - may have an LA:Ao that exceeds 1.6 without having any heart disease . This is why we never treat the number alone. If the LA:Ao is 1.7 but the ventricle is not enlarged (LVIDdn < 1.7), you are likely looking at breed variation or very early Stage B1 disease . Or even better, normalise the LA size to body weight and measure it in long axis too, so that these situations become obvious. Use our echo calculator if this isn’t set up on your echo machine.

The Feline Factor: Why Cats are Different

When we pivot to cats, the rules of the game change entirely. The feline heart base is technically demanding due to high heart rates and narrow intercostal spaces, making the “Mercedes Benz” sign notoriously difficult to time.

Cats can have their heart ‘tipped’ a long way forwards, creating a lot of variation in the position of the ‘long axis’ and ‘short axis’. This is especially pronounced in older cats. In addition a rotation effect often means the LA:Ao measurement ends up being quite a vertical one:

IMAGE VERTICAL LA:AO

Also cats sometimes enlarge their atrium in length rather than width. In cats, we use the “Rule of Thirds” for a quick subjective check: on a long-axis view, the left atrium should occupy roughly one-third of the total length of the heart silhouette. If the atrium looks like it is dominating the image, or if it has lost its “square” shape and become globular, the cat is at high risk for blood stasis and ATE.

Even accounting for all of this, there are some cats that seem to have congestive heart failure issues with very minimal LA enlargement. This is largely due to the fact they tend to go into ‘diastolic’ failure. Learning to measure the LA fractional shortening seems to be a good way of helping to spot this potential issue.

Moving Beyond the Short Axis

The controversies surrounding the short-axis LA:Ao - from the “Mercedes Benz” symmetry to breed-specific aortic root sizes - have led many specialists to advocate for a more repeatable alternative.

It turns out that the simple idea of using the aorta as a ‘fixed’ reference point is maybe ok when looking for dramatic enlargement, but doesn’t hold up so well when looking for more subtle changes or monitoring the same patient repeatedly over time.

Many cardiologist now are suggesting we move toward the long-axis left atrial dimension (LAD).

In the next part of this series, we will explore why the long-axis measurement is often easier to master, more repeatable between different vets, and why it might be the future of first-opinion heart monitoring.

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Clinical Takeaways for this section:

• Don't measure the "void": If the LA wall has dropped out due to a pulmonary vein, don't guess - use the imaginary curved line or change views.

• Consistency is Key: If you are using the EPIC 1.6 threshold, you must use the Swedish (early-diastolic) timing.

• The “Rule of Two”: Never initiate pimobendan based on a high LA:Ao if the LVIDdn is still normal (< 1.7).

• Normalise your LA short axis measurement to body weight as well, it is an extra check in case you have mismeasured the Ao or if the dog has a naturally small one.

• Learn the LAD: Its much more reliable

• Cat Risk: In cats, errors can compound, so I strongly recommend that LAD measurement as well. In theory, an LA:Ao of 1.5 is already the upper limit of normal; anything approaching 2.0 requires immediate consideration of anti-thrombotic therapy like clopidogrel. But in practice check the LAD.