A Novel Echocardiographic Index (Modified-Left-Atrium-to-Aorta Ratio) for Quantifying Left Atrial Size and Differentiating Stages in Dogs with Myxomatous Mitral Valve Disease

Abstract

Myxomatous mitral valve disease (MMVD) is the most common heart disease in small-breed dogs, and accurate assessment of the left atrial (LA) size is essential for diagnosis and management. The traditional echocardiographic method, LA-to-Aorta (LA/Ao) ratio, is widely used, but evaluates LA size in only one view. This study introduces a novel index-the modified-LA/Ao (M-LA/Ao) ratio-which combines two echocardiographic views to better reflect the true LA size. This study retrospectively analyzed thoracic radiographs and echocardiograms from 136 dogs, including healthy controls and dogs with MMVD classified into stages B1, B2, and C according to ACVIM guidelines. The ability of the M-LA/Ao ratio is compared to conventional indices using correlation analysis and receiver operating characteristic (ROC) curves. The M-LA/Ao ratio showed a strong correlation with existing indices and slightly improved discrimination between ACVIM stages B1 and B2, although performance between stages B2 and C was similar to the LA/Ao ratio. Intra- and interobserver variability were also acceptable. Our findings suggest that the M-LA/Ao ratio may provide a practical and sensitive method to evaluate LA enlargement in early-stage MMVD, helping clinicians detect subtle cardiac remodeling before progression to advanced disease.

Keywords: canine; echocardiography; left atrium; myxomatous mitral valve disease.

Figure 1

The figures demonstrate the measurements used to calculate the M-LA/Ao ratio. (A) illustrates the measurement of LAD (*) in the right parasternal long-axis four-chamber view, where LAD is measured parallel to the mitral valve annulus at the point of maximum LA diameter. (B) illustrates the measurements required to calculate the LA/Ao ratio in the right parasternal short-axis view. The aortic diameter (**) is measured along the commissure between the noncoronary cusp and the left cusp. The LA dimension (***) is measured by extending the same line used for the aortic measurement, carefully excluding the pulmonary vein. All measurements are obtained using the inner-edge-to-inner-edge method. LAD—Maximum left atrial dimension, LA/Ao ratio—Left-atrium-to-aorta ratio, M-LA/Ao ratio—Modified-LA/Ao ratio.

Figure 2

Scatter plots of the LADn, M-LA/Ao ratio and LA/Ao ratio across different ACVIM stages. The bars indicate the median values for each measurement. All measurements were statistically different between stage B1 and stage B2. In the M-LA/Ao ratio and LA/Ao ratio, significant differences were also noted between stage B2 and stage C. ACVIM—American College of Veterinary Internal Medicine, LADn—Normalized maximum left atrial dimension, LA/Ao ratio—Left-atrium-to-aorta ratio, M-LA/Ao ratio—Modified-LA/Ao ratio. An asterisk (*) denotes a statistically significant difference between ACVIM stages B1 and B2, whereas double asterisks (**) denote a statistically significant difference between stages B2 and C.

Figure 3

Illustrations of receiver operating characteristic (ROC) curves for LADn, LA/Ao ratio, and M-LA/Ao ratio distinguishing between ACVIM stage B1 and stage B2. The area under the ROC curve (AUC) for LADn was 0.924, for LA/Ao ratio, 0.979, and for M-LA/Ao ratio, 0.981. ACVIM—American College of Veterinary Internal Medicine, LADn—Normalized maximum left atrial dimension, LA/Ao ratio—Left-atrium-to-aorta ratio, M-LA/Ao ratio—Modified-LA/Ao ratio.