Echocardiography Basics 1: The Left Atrium to Aorta Ratio (LA:Ao)

The first article in a series on echo basics in veterinary patients. Hope it is helpful. However, echo is a practical skill, so please consider attending an in-person teaching course somewhere near you to develop your skills.

Any time I discuss a cardiac case with colleagues, the number one question we all want to know is “What size is the left atrium?”.

Left atrial size is a window into cardiac health, in both dogs and cats. Heart failure results from to much pressure in the left atrium, and size is directly linked to this pressure.

But how do we measure it? The left atrium is a very complex 3D structure and approximating it’s size with a single linear measurement is a challenge. In addition, veterinary patients vary very greatly in size, so reference ranges for echo measurements are correspondingly more tricky.

So enter the left atrial to aorta ratio. It solves most of these issues, by using the aorta as a benchmark. Even though, as we’ll see, it is quite a flawed and problematic measurement. But if you’re just getting started with echo, it’s where you need to begin.

The left atrial to aorta ratio (LA:Ao) is arguably the most important metric a first-opinion practitioner can master. It serves as our primary barometer for assessing the hemodynamic impact of mitral regurgitation and, crucially, it is the gatekeeper for initiating life-changing therapy.

Why the Left Atrium?

In MMVD, the left atrium is the “canary in the coal mine”. Because it is a thin-walled, compliant chamber, it begins to dilate as soon as the volume of regurgitant blood becomes significant. This enlargement is a direct reflection of chronically elevated left-sided filling pressures, even very mild ones.

The same happens in cats as they start to struggle with hypertrophic cardiomyopathy. In this case, diastolic failure starts to occur typically, but the result is the same - left atrial dilation.

We use the aorta as our benchmark because, in the absence of rare aortic root disease, its size remains relatively constant. By creating a ratio, we produce a body-weight independent index, allowing us to apply the same diagnostic thresholds to a Chihuahua as we do to a Spaniel.

The Technique: Finding the “Mercedes Benz”

To obtain an accurate LA:Ao, we use the right parasternal short-axis view at the heart base. I like the term ‘whale view’ as it looks a bit like a cartoon whale, with the right amount of imagination.

The goal is to obtain a perfectly transverse cross-section of the aortic valve.

How to obtain:

• Start with a right parasternal short axis view - ‘the mushroom’.
• Use a fanning movement to move your transverse view cranially through the heart, moving the beam towards the spine.
• Often need to slightly rotate and move the head of the probe to optimise the view.
• Aim for a completely symmetrical aorta, showing the 3 valve cusps as equally as possible.

Landmarks:

Look for the three cusps of the aortic valve forming a symmetrical Y-shape during diastole: the famous “Mercedes Benz” sign.

Timing:

This is the most frequent source of error. The measurement must be taken in early diastole, specifically on the first frame after the aortic valve closes.

Top Tip: Learn how to record a ‘loop’ on your machine and use it all the time. This usually shows 3-5 cardiac cycles if set correctly. This allows you to capture several attempts and get the optimum timing by measuring them later, at leisure.

Caliper Placement (The Swedish Method):

Originally popularised by Häggström:

• Aorta (Ao): Measure the internal diameter along the commissure between the non-coronary and right coronary cusps, and bisecting the left coronary cusp.

• Left Atrium (LA): Measure the internal diameter in a line extending from and parallel to the commissure between the non-coronary and left coronary cusps, extending to the distant wall of the atrium.

Technical Feature	Clinical Requirement	Potential Pitfall
View	Right parasternal short-axis	Foreshortening (aorta looks oval)
Timing	First frame after Ao valve closure	Measuring during atrial contraction
LA Border	Distant blood-tissue interface	Including the pulmonary vein ostia

Table 1: Technical requirements for accurate LA:Ao assessment.

Alternative techniques (American)

Mark Rhisniw, among others, has long advocated for a maximal left atrial diameter measured at end-systole. There is also a description of measuring the aorta at an angle matching the line you will take across the LA:

Landmarks: Measure the internal diameter of the aorta along the commissure between the non-coronary and right coronary cusps, and bisecting the left coronary cusp

Either method is fine, however consistency is key. Guidelines for echo measurements are long awaited and still not publised (as of April 2026), maybe they will end this debate!

Recent publications suggest that most cardiologists do some hybrid approach anyway…

Feature	Swedish Method (Häggström)	American Method (Rishniw)
Primary Timing	First frame after Ao valve closure (Early Diastole)	Frame just before MV opening (End-Systole) [2, 3]
Aortic Line (Ao)	Along the commissure between the non-coronary and right coronary cusps	Along the commissure of the non-coronary and left coronary cusps, bisecting the right coronary cusp
Atrial Line (LA)	Parallel to the non-coronary/left coronary commissure, extending to the distant wall	Extension of the aortic line across the LA to the distant wall
Used In...	Foundational MMVD trials (EPIC, DELAY)	Foundational 2D reference interval studies (2000)

Clinical Interpretation: The 1.6 Threshold

The landmark EPIC trial provided us with a clear evidence-based line in the sand. For a dog to be classified as Stage B2 (asymptomatic with significant remodelling), they must typically meet a specific echocardiographic threshold.

A dog is considered to have significant left atrial enlargement when the LA:Ao > 1.6.

However, intellectual honesty requires us to remember that we never treat a number in isolation. According to the ACVIM 2019 guidelines, we generally only initiate pimobendan if the dog meets both the atrial and ventricular enlargement criteria:

1. Atrial Enlargement: LA:Ao > 1.6.

2. Ventricular Enlargement: LVIDdN > 1.7.

The formula for the normalised left ventricular internal diameter is:

$$LVIDdn = \frac{LVIDd (cm)}{Weight (kg)^{0.294}}$$

LVIDdn = LVIDd (cm) Weight (kg)^0.294

Take a look at our echo calculator for easy calculation of all relevant echo normalisations and ratios.

Why you musn’t rely on the LA:Ao

Recent advice seems to have drifted to the point of advising clinicians to start to put dogs on pimobendan the moment the LA:Ao hits this 1.7 mark.

I strongly recommend you avoid this approach! Why?

• Many dogs seem to have a normal LA:Ao that sits at this kind of level.
• Many clinicians make mistakes in measuring an accurate LA:Ao
• It is a highly variable measurement anyway
• It takes very little time to measure the LV diameter as well, and do things properly.

Summary for the Clinic

If you find an LA:Ao of 1.7 but the ventricle remains within normal limits (LVIDdN < 1.7), the patient is likely in Stage B1. In these cases, our role is to monitor, not to medicate. We are looking for the point where the heart can no longer compensate, and that requires both chambers to show signs of eccentric hypertrophy.

Mastering the “Mercedes Benz” view and the Swedish measurement protocol ensures that your clinical decisions are rooted in evidence rather than guesswork. In the next part of this series, we will look at the nuances and controversies of this measurement, including why it often fails us, and how to improve our technique and clinical decision making.