How do I grade a cardiac murmur in a dog and what workup is warranted at each grade?

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Canine cardiac murmurs are graded on a I–VI scale, and the grade — combined with murmur timing, location, and signalment — determines whether workup can be deferred, is optional, or is mandatory. The grading scale runs from grade I/VI (softest audible murmur, heard only after careful auscultation in a quiet room) through grade VI/VI (audible with the stethoscope held off the chest wall). Grades I–II are soft murmurs without a palpable thrill; grade III is easily heard but still without a thrill; grade IV is loud and accompanied by a palpable precordial thrill; grades V–VI are very loud with a thrill, with grade VI audible without full stethoscope contact.413 The current grading scheme has been critiqued as unnecessarily complex with redundant information between adjacent grades, and a simpler scheme has been proposed, though the six-grade system remains the standard in clinical communication.13 Murmur intensity correlates with disease severity in myxomatous mitral valve disease (MMVD), pulmonic stenosis, and subaortic stenosis. In preclinical MMVD, a murmur grade ≥3/6 is one of the criteria — alongside vertebral heart size (VHS) >10.5, normalized left ventricular internal diastolic dimension (LVIDdN) ≥1.7, and left atrial-to-aortic root ratio (LA:Ao) ≥1.6 — that classifies a dog as Stage B2, which carries higher risk of congestive heart failure and warrants pimobendan administration to delay its onset.11 Dogs meeting none or only some of these criteria are Stage B1, with little risk of congestive heart failure.11 A machine-learning algorithm trained to predict murmur grade from electronic stethoscope recordings differentiated Stage B1 from B2 preclinical MMVD with an area under the curve of 0.861, a sensitivity of 81.4%, and a specificity of 73.9% for detecting loud or thrilling murmurs.3 In puppies, a grade 1/6–2/6 soft systolic murmur localized to the left heart base is typically nonpathological (innocent). Innocent murmurs are caused by mild physiological turbule