Introduction: Left atrial-to-aortic ratios (LA:Ao) provide a body weight independent estimate of left atrial size. However, reference intervals were established with small sample populations and for only single points in the cardiac cycle. More robust reference intervals are warranted. Animals: Two hundred and thirty eight apparently healthy adult dogs. Materials and methods: LA:Ao measurements were obtained at 3 points in the cardiac cycle – maximal dimension, at the closing of the aortic valve (or just before opening of the mitral valve) (LA:AoMAX); minimal dimension, at the onset of the QRS complex (LA:AoMIN) and at the onset of atrial systole (LA:AoP). LA:AoMAX was obtained from right parasternal short and long-axis views, and LA:AoMIN and LA:AoP were obtained from the right parasternal short-axis view. Dogs were excluded from analyses of reference intervals if weight-based left atrial and left ventricular diastolic dimensions exceeded reference interval limits. Effects of breed and body weight on LA:Ao measurements were examined. Results: Upper LA:Ao reference limits mostly agreed with previously published limits, although 10% of dogs had LA:AoMAX in the short-axis view exceeding 1.6. These dogs had smaller aortae than expected for their body weight, and included mostly boxers and English setters. Reference limits for LA:AoMIN and LA:AoP were smaller than those for LA:AoMAX in either view. No LA:Ao measurements were associated with body weight. Conclusions: Reference limits were either confirmed or established for the common two-dimensional methods of assessing relative left atrial size in healthy dogs. Clinicians should use caution when diagnosing mild left atrial enlargement in certain dog breeds and should examine the weight-based aortic dimensions in such cases.
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