How to measure the left atrium in cats: the LA/Ao ratio
5-year-old male Maine Coon with no clinical signs and a grade 2/6 murmur. Echocardiography shows left ventricular hypertrophy and you want to assess left atrial size rigorously, as it will determine stage and prognosis.
| Parameter | Result | Reference |
|---|---|---|
| Interventricular septum in diastole | 7.5 mm | < 6 |
| LA/Ao | 2.1 | < 1.5 |
| LA diameter (right parasternal long axis) | 19 mm | < 16 |
Which statement about measuring the left atrium in cats is correct?
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A. LA/Ao is measured in the left apical four-chamber view, at end-diastole
In the apical view the aorta is not seen in cross-section next to the atrium. Moreover, at end-diastole the atrium is at its smallest and enlargement is underestimated.
B. M-mode at aortic level is the most reliable method in cats
The M-mode beam often cuts the atrium obliquely or passes through the auricle, so it may underestimate atrial size. 2D measurement is preferred.
C. In cats an LA/Ao of up to 2.0 is normal, so this atrium is not enlarged
The normal value in cats is below 1.5. With 2.1 and a diameter of 19 mm, the atrium is clearly enlarged.
D. It is measured in 2D in the right parasternal short-axis view at heart base level, in early diastole; > 1.5 indicates enlargement
Correct. Use the first frame after aortic valve closure, when the aorta and atrium are seen together (the “Mercedes-Benz” sign plus atrium). This cat has moderate to severe atrial enlargement, placing it in stage B2.
lightbulb Key takeaway
Left atrial size is the most important prognostic factor in cats with cardiomyopathy: complement LA/Ao with the diameter in the right parasternal long-axis view (≥ 16 mm, enlarged). With moderate to severe enlargement (stage B2) the ACVIM consensus recommends thromboprophylaxis and monitoring the home respiratory rate.
References: ACVIM consensus statement guidelines for the classification, diagnosis, and management of cardiomyopathies in cats (2020)
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.