Electrocardiographic screening for left atrial enlargement in dogs: derivation and validation of an algorithm combining lead II P-wave duration and the Morris index

Combining electrocardiographic measurements may improve identification of dogs requiring echocardiographic assessment. This retrospective diagnostic accuracy study derived a three-category algorithm in 150 dogs and evaluated it in 50 separate validation dogs. Three observers independently measured lead II P-wave duration and the V1 Morris index, with observer A providing the primary analysis. Left atrium-to-aorta ratio (LA/Ao) ≥ 1.6 was the primary outcome; normalized left ventricular internal diameter in diastole (LVIDDN) ≥ 1.7 was evaluated independently as a secondary outcome. Thresholds from 114 reference dogs were 48 and 52 ms for P duration and 3.04 mV·ms for Morris. Positive triage comprised intermediate and high categories. Primary-outcome sensitivity and specificity were 90.6% and 50.0% in derivation and 100% and 85.7% in validation. Validation sensitivity had a 95% confidence interval of 88.1%–100%. The ordinal algorithm area under the curve was 0.961 in validation, compared with 0.868 for P and 0.844 for Morris; paired differences were not significant after Holm adjustment. Secondary-outcome validation sensitivity and specificity were 88.0% and 60.0%. The algorithm provided complementary detection of left atrial enlargement and reproducible measurements across observers, supporting further evaluation as an adjunct to echocardiographic triage.