Heart murmurs are detected in up to 31% of dogs presenting for routine veterinary examination. While some murmurs are innocent (especially in puppies), others indicate significant structural heart disease. Degenerative mitral valve disease (DMVD) accounts for approximately 75% of acquired heart disease in dogs. Use the ECG Reader AI for rhythm analysis and the Cardiology Specialist for workup guidance.
The Levine grading scale standardizes murmur intensity assessment. Accurate grading requires a quiet environment, proper stethoscope placement, and systematic auscultation of all valve areas.
| Grade | Description | Clinical Significance |
|---|---|---|
| I/VI | Very soft; heard only after prolonged auscultation in a quiet room | May be innocent; further evaluation based on context |
| II/VI | Soft but readily audible; localized to a specific valve area | Often significant; warrants monitoring or workup |
| III/VI | Moderate intensity; immediately audible upon auscultation | Usually pathologic; diagnostic workup recommended |
| IV/VI | Loud; may radiate widely but no precordial thrill | Pathologic; full cardiac workup indicated |
| V/VI | Very loud with a palpable precordial thrill | Significant disease; echocardiography essential |
| VI/VI | Audible with the stethoscope lifted off the chest wall; palpable thrill | Severe disease; often indicates large shunts or severe regurgitation |
Murmur grade does not always correlate directly with disease severity. A grade II/VI murmur from severe mitral regurgitation in a decompensated patient can indicate worse cardiac function than a grade IV/VI murmur in a well-compensated dog.
The point of maximal intensity (PMI) helps localize the source of the murmur. Left apex (mitral area) murmurs suggest mitral valve disease, while left base murmurs suggest aortic stenosis or pulmonic stenosis. Right-sided murmurs may indicate tricuspid regurgitation or ventricular septal defects.
Timing classification is equally important. Systolic murmurs (between S1 and S2) are most common and include regurgitant murmurs (mitral, tricuspid) and ejection murmurs (SAS, PS). Diastolic murmurs are rare and suggest aortic insufficiency. Continuous (machinery) murmurs are characteristic of patent ductus arteriosus (PDA), heard best at the left base dorsally.
Puppies and young dogs: Innocent puppy murmurs (typically grade I-III/VI, soft, left basilar, resolve by 4-5 months), congenital defects including subaortic stenosis (SAS), pulmonic stenosis (PS), patent ductus arteriosus (PDA), and ventricular septal defect (VSD). Certain breeds are predisposed: German Shepherds and Golden Retrievers to SAS, Bulldogs and Terriers to PS, Miniature Poodles and German Shepherds to PDA.
Adult and senior dogs: Degenerative mitral valve disease (DMVD, also known as myxomatous mitral valve disease) is by far the most common cause, particularly in small breeds (Cavalier King Charles Spaniels, Dachshunds, Miniature Poodles). Dilated cardiomyopathy (DCM) is the second most common acquired heart disease, primarily affecting large and giant breeds (Doberman Pinschers, Great Danes, Irish Wolfhounds).
Warning: A newly detected murmur in a previously murmur-free adult dog should prompt a full cardiac workup. In Doberman Pinschers, occult DCM screening with Holter monitoring and echocardiography is recommended annually starting at 3-4 years of age due to the breed's high incidence of sudden cardiac death.
The appropriate workup depends on murmur characteristics and clinical context. A minimum cardiac database includes thoracic radiographs (right lateral and DV or VD), electrocardiogram (ECG), and blood pressure measurement. The definitive diagnostic tool is echocardiography (ultrasound of the heart), which provides structural and functional assessment.
Additional diagnostics include: NT-proBNP (N-terminal pro-B-type natriuretic peptide), a blood biomarker that helps differentiate cardiac from respiratory causes of dyspnea and identifies occult heart disease; cardiac troponin I for myocardial injury assessment; Holter monitoring (24-hour ECG) for intermittent arrhythmia detection, particularly important in Dobermans and Boxers with suspected occult cardiomyopathy.
Referral to a veterinary cardiologist is recommended in the following scenarios: congenital heart disease suspected in a puppy (for definitive diagnosis and potential interventional procedures such as PDA ligation or balloon valvuloplasty), any dog with clinical signs of congestive heart failure (cough, dyspnea, exercise intolerance, syncope), grade IV/VI or higher murmurs, DMVD patients approaching or exceeding ACVIM Stage B2 criteria (left atrial enlargement on radiographs or echo), and Dobermans or Boxers with suspected occult cardiomyopathy.
The American College of Veterinary Internal Medicine (ACVIM) consensus staging system guides treatment for DMVD: Stage A (at risk, no murmur), Stage B1 (murmur present, no cardiac remodeling), Stage B2 (murmur with cardiac remodeling but no clinical signs), Stage C (current or prior congestive heart failure), and Stage D (refractory CHF). Pimobendan is indicated starting at Stage B2 based on the landmark EPIC trial, which demonstrated a 15-month delay in onset of CHF.
- Heart murmurs are graded I-VI; grade alone does not determine disease severity.
- PMI and timing (systolic, diastolic, continuous) help localize the lesion.
- Puppies: consider innocent murmurs vs congenital defects (SAS, PS, PDA, VSD).
- Adults: DMVD is the most common cause, especially in small breeds; DCM in large breeds.
- Echocardiography is the gold standard for definitive cardiac diagnosis.
- ACVIM staging guides treatment; pimobendan at Stage B2 delays CHF onset by 15 months.