The aim of this study was to estimate echocardiographic elevated mean left atrium pressure (MLAP) based on measurements from thoracic radiographs and to determine a cut-off value for each radiographic measurement that suggests a high MLAP. A retrospective cross-sectional study was performed to include cases admitted from January 2015 to December 2016. Thoracic radiographic examinations from 93 dogs with and without a high MLAP were included. Specific measurements were made from thoracic

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... phs and compared with echocardiographic variables known to indicate high MLAP. This comparison was used to generate equations that allowed the estimation of echocardiographic surrogates from the radiographic measurements. The values indicative of high MLAP were obtained using a regression curve. Formulas that indicated high MLAP were generated using a number of radiographic measurements. Positive echocardiographic findings of high MLAP were used as the gold standard. These formulas helped to predict high MLAP in myxomatous mitral valve disease (MMVD) without the need for echocardiographic examination. The best formula was left atrium (LA):aorta (Ao)echo = 0.03×(vertebral heart score,VHS) + 0.14×(LA) + 0.27×(LA:caudal vena cava (CVC)rad). Values ? 12.2v for VHS, ? 4.5cm for LA, ? 3.3 for LA:Aorad and ? 3.2 for LA:CVCrad suggested high MLAP. Thus, we propose equations, based on measurements from thoracic radiographs, to identify high MLAP. Simple radiographic thoracic measurements, such as LA:CVCrad, can be used to define overload and a high MLAP in dogs with MMVD.