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.MSD Vet Manuals+1 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.Journal of Vete…
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.Journal of Vete… Dogs meeting none or only some of these criteria are Stage B1, with little risk of congestive heart failure.Journal of Vete… 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.Journal of Vete…
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 turbulence during the rapid-growth phase and resolve by 6 months of age in dogs and cats.MSD Vet Manuals For such a murmur, further diagnostic workup is not mandatory but warrants a client discussion that accounts for breed, intended use (breeding, field trials), and owner concerns.Journal of the…
Workup is indicated — not optional — for puppies with any of the following murmur characteristics regardless of grade: a continuous murmur, a diastolic murmur, a murmur that obscures the second heart sound, a murmur accompanied by a split second heart sound, a murmur radiating to the carotid region, a systolic murmur best heard at the left apex over the mitral valve area, or any murmur best heard on the right hemithorax.Journal of the… For systolic murmurs with a point of maximal intensity over the left heart base, workup is indicated at grade 3/6 or louder.Journal of the… Any murmur in a dog directly related (parent, offspring, or sibling) to a dog with known congenital heart disease warrants workup even if the murmur characteristics appear nonpathological.Journal of the…
Workup is also indicated regardless of murmur grade when the murmur is accompanied by jugular vein distension or abnormal pulsations, femoral pulse abnormalities (bounding, or parvus et tardus), poor peripheral perfusion, abnormal mucous membrane color, or any arrhythmia other than respiratory sinus arrhythmia.Journal of the… A right-to-left shunting patent ductus arteriosus (PDA), for example, can present with a grade 4/6 right systolic murmur and a grade 2/6 right diastolic murmur with mildly weak femoral pulses — findings that mandate full echocardiographic evaluation.Journal of the…
Echocardiography is the preferred diagnostic modality and should be performed by a cardiologist for greatest accuracy. Thoracic radiography identifies the correct diagnosis in only 37%–40% of dogs with congenital heart disease, whereas high-resolution Doppler echocardiography reliably identifies congenital defects, assesses severity, detects multiple concurrent defects, and guides treatment planning.Journal of the… Electrocardiography and radiography provide useful ancillary information but cannot provide a definitive etiologic diagnosis.Journal of the… In young animals, auscultation should specifically include the left axillary region for a continuous murmur (PDA), the right base, and along the neck.MSD Vet Manuals
| Murmur Grade | Thrill Present | Typical Workup Threshold | Key Caveat |
|---|---|---|---|
| I–II/VI | No | Defer if left-base systolic in puppy; mandatory if any high-risk feature present | Innocent murmurs resolve by 6 months MSD Vet Manuals |
| III/VI at left base | No | Mandatory workup in young dogs Journal of the… | Grade ≥3 is a B2 MMVD criterion in adults Journal of Vete… |
| III/VI at left apex, right hemithorax, or diastolic | No | Mandatory workup at any grade Journal of the… | Right-sided murmurs suggest congenital defect Journal of the…+1 |
| IV–VI/VI | Yes (IV–VI) | Mandatory workup; echocardiography by cardiologist Journal of the… | Grade VI audible off chest wall MSD Vet Manuals+1 |
Would you like guidance on the specific echocardiographic criteria used to differentiate Stage B1 from Stage B2 MMVD and when to initiate pimobendan?