Intensity describes sound
Grades I–VI describe how loud and widely radiating the murmur is. ACVIM stages A–D describe progression of confirmed myxomatous mitral valve disease.
Educational use only: A murmur is a physical-exam finding, not a diagnosis. Loudness alone cannot identify the cause or reliably determine disease severity. Breathing difficulty, collapse, or blue/gray gums require emergency veterinary assessment.
Quick reference
Timing + location narrow the list
Left-apical systolic murmurs often suggest mitral regurgitation; left-basilar ejection murmurs suggest outflow obstruction; continuous murmurs strongly suggest PDA.
Not every soft murmur is disease
A quiet left-basilar systolic murmur in a thriving young puppy may be physiologic, but loud, diastolic, continuous, persistent, or symptomatic murmurs need investigation.
Count sleeping breaths
Count chest rises for 30 seconds and double the number. A persistent upward trend or a sleeping rate above the threshold set by your veterinarian should prompt contact.
Condition and murmur-pattern matrix
Filter the 17 entries. Patterns are typical—not definitive—and overlap is common.
| Condition / state | Category | Typical profile | Timing | PMI | Common sound pattern | Associated clues | Useful confirmation | Priority |
|---|---|---|---|---|---|---|---|---|
| Physiologic / innocent puppy murmur | Functional | Young, thriving puppy; no clinical signs | Systolic | Usually left base | Usually quiet (I–II/VI), short, localized | Normal growth and activity; often disappears as the puppy matures | Repeat auscultation; echocardiogram if persistent, louder, atypical, or symptomatic | Scheduled recheck |
| Myxomatous mitral valve disease (MMVD) | Acquired | Usually middle-aged to older small-breed dogs; Cavalier King Charles Spaniels are predisposed | Holosystolic | Left apex | Blowing systolic murmur; may radiate widely as intensity rises | May be asymptomatic for years; later cough, increased sleeping respiratory rate, exercise intolerance, pulmonary edema | Echocardiogram with Doppler; thoracic radiographs for heart size and pulmonary edema | Stage promptly |
| Dilated cardiomyopathy with functional mitral regurgitation | Acquired | Often large or giant breeds; Doberman Pinschers, Great Danes, Boxers and some Cocker Spaniels | Systolic | Left apex | Often soft; murmur intensity may underestimate myocardial disease | Weakness, collapse, arrhythmia, rapid breathing, cough, abdominal distention | Echocardiogram, ECG and often 24-hour Holter monitoring | Prompt workup |
| Pulmonic stenosis | Congenital | Usually detected in puppies or young dogs; several breeds predisposed | Systolic ejection | Left base | Harsh ejection murmur; may have a palpable thrill | Exercise intolerance, poor growth, exertional collapse; severe cases can develop right-sided heart failure | Doppler echocardiogram to measure obstruction severity | Prompt cardiology |
| Subaortic stenosis | Congenital | Often young large-breed dogs; Newfoundlands, Golden Retrievers, Boxers, Rottweilers and German Shepherd Dogs | Systolic ejection | Left base; may radiate to the right base or neck | Harsh, crescendo–decrescendo ejection murmur | Weak pulses, exercise intolerance, syncope; severe disease carries arrhythmia and sudden-death risk | Doppler echocardiogram; ECG/Holter when arrhythmia risk is suspected | Prompt cardiology |
| Patent ductus arteriosus (PDA) | Congenital | Usually puppy or young dog; more common in females | Continuous | Left base / left axillary region | Classic continuous “machinery-like” murmur, often loud | Bounding femoral pulses; large shunts can progress to left-sided congestive heart failure | Doppler echocardiogram; thoracic radiographs as indicated | Timely referral |
| Ventricular septal defect (VSD) | Congenital | Usually puppy or young dog | Holosystolic | Often right chest | Small defects can be surprisingly loud; very large defects may be softer | Poor growth or exercise intolerance in significant shunts; may be incidental when small | Doppler echocardiogram to define size, direction and pressure gradient | Prompt workup |
| Atrial septal defect (ASD) | Congenital | Usually young; may remain unnoticed if small | Systolic ejection | Left base / pulmonic area | Often soft, caused by increased flow across the pulmonic valve | May have no signs; larger shunts can cause exercise intolerance or right-heart enlargement | Doppler echocardiogram | Prompt workup |
| Mitral valve dysplasia / congenital mitral regurgitation | Congenital | Puppy or young dog; severity varies | Holosystolic | Left apex | Blowing regurgitant murmur | Poor growth, exercise intolerance, cough or breathing difficulty when severe | Doppler echocardiogram | Prompt cardiology |
| Tricuspid valve dysplasia / tricuspid regurgitation | Congenital | Usually young; some large breeds predisposed | Holosystolic | Right apex | Right-sided regurgitant murmur | Jugular pulsation, abdominal fluid, exercise intolerance in significant disease | Doppler echocardiogram | Prompt cardiology |
| Mitral stenosis | Congenital | Rare; usually congenital and detected in younger dogs | Diastolic | Left apex | Often low-grade diastolic murmur; concurrent dysplasia can add a systolic murmur | Exercise intolerance, pulmonary edema or syncope in significant obstruction | Doppler echocardiogram | Cardiology referral |
| Tetralogy of Fallot | Congenital | Young dog; rare cyanotic congenital disease | Systolic ejection | Often left base | Ejection murmur related to right-ventricular outflow obstruction | Blue/gray mucous membranes, weakness, poor growth, exercise intolerance, collapse | Doppler echocardiogram; CBC may show erythrocytosis | Emergency if cyanotic |
| Infective endocarditis | Acquired | Any age; risk varies with bacteremia and underlying valve disease | Variable; systolic or diastolic depending on valve | Variable | New or changing murmur; aortic involvement may create a diastolic murmur | Persistent or intermittent fever, lethargy, weight loss, shifting-leg lameness, embolic signs | Echocardiogram plus multiple blood cultures and laboratory evaluation | Urgent evaluation |
| Anemia / high-output functional murmur | Functional | Any age; context depends on cause of anemia or high-output state | Systolic | Often basilar or diffuse | Usually soft flow murmur; may resolve when the underlying state is corrected | Pale gums, rapid heart rate, weakness, reduced appetite, bleeding or systemic illness | CBC with reticulocyte count; search for blood loss, hemolysis or reduced production | Same-day if weak/pale |
| Advanced heartworm disease with tricuspid regurgitation | Acquired | Dog with exposure risk or inadequate prevention; murmur more typical in advanced disease | Systolic | Right side / right apex | Right-sided systolic murmur from tricuspid insufficiency | Cough, exercise intolerance, pulmonary hypertension, jugular distention, ascites; caval syndrome can be critical | Heartworm antigen testing, microfilaria testing, thoracic radiographs and echocardiography | Urgent if symptomatic |
| Aortic regurgitation / insufficiency | Acquired | Uncommon in dogs; may occur with aortic-valve infective endocarditis | Early diastolic | Left base | Decrescendo diastolic murmur immediately after S2 | Clinical clues depend on cause; fever or systemic illness raises concern for endocarditis | Doppler echocardiogram; blood cultures when infection is suspected | Prompt workup |
| Canine hypertrophic cardiomyopathy / dynamic outflow obstruction | Acquired | Rare in dogs; diagnosis requires imaging | Systolic | Often left base; may also involve mitral-regurgitation sounds | Soft to prominent systolic murmur; gallop sound may be present | Exercise intolerance, weakness, syncope or heart-failure signs in clinically important disease | Echocardiogram and ECG | Prompt workup |
Data compiled from and expanded on this guide:
heart murmur stages, symptoms and treatment overview.
Murmur intensity grades
Intensity is one descriptor. Timing, location, quality, radiation, patient profile and imaging findings are also essential.
| Grade | Auscultation description | Palpable precordial thrill? | Interpretation guardrail |
|---|---|---|---|
| I / VI | Extremely quiet, focal, heard only in a quiet setting | No | Can be physiologic or mild disease; context and recheck matter |
| II / VI | Soft and focal, but consistently heard | No | Low intensity does not automatically equal “harmless” |
| III / VI | Moderate intensity and regional, heard across the affected side of the chest | No | Usually merits diagnostic characterization, especially in a young dog |
| IV / VI | Loud and widely radiating | No | Current Merck scale places the first palpable thrill at Grade V |
| V / VI | Loud, consistently heard and widely radiating | Yes | Suggests substantial turbulence; cause and physiologic effect still require imaging |
| VI / VI | Very loud, widely radiating and audible with the stethoscope lifted about 1 cm from the chest | Yes | Urgent diagnostic workup is appropriate, but loudness alone is not a diagnosis |
ACVIM MMVD stage quick-lookup
This staging system applies to myxomatous mitral valve disease—not to every murmur or every cardiac condition.
| Stage | Core definition | Typical status | What distinguishes it |
|---|---|---|---|
| A | At risk for MMVD | No current structural disease or murmur attributable to MMVD | Breed or age risk; routine surveillance |
| B1 | Preclinical MMVD | Murmur/valve disease present, but no heart enlargement meeting B2 criteria | No current or previous congestive heart-failure signs |
| B2 | Preclinical MMVD with remodeling | Valve disease plus cardiac enlargement meeting defined imaging criteria | Still no current or previous congestive heart-failure signs |
| C | Current or previous congestive heart failure | Clinical signs caused by MMVD-related congestion | History or evidence of pulmonary edema/CHF requiring stage-based treatment |
| D | End-stage / refractory heart failure | Clinical signs persist or recur despite standard therapy | Requires specialist-guided, individualized management |
Diagnostic test selector
| Test | Best for | What it can answer | Important limitation |
|---|---|---|---|
| Auscultation | Initial detection and characterization | Grade, timing, duration, quality, radiation and point of maximal intensity | Cannot define anatomy or disease stage by itself |
| Doppler echocardiogram | Structural and flow diagnosis | Valve anatomy, chamber size, shunts, regurgitation, stenosis gradients, myocardial function | Operator expertise and image quality matter |
| Thoracic radiographs | Heart size and lung assessment | Cardiac silhouette, pulmonary vessels, pulmonary edema and alternative respiratory causes | Does not directly show valve motion or measure flow velocity |
| ECG | Rhythm assessment | Heart rate, rhythm, conduction abnormalities and some chamber-enlargement clues | A brief ECG may miss intermittent arrhythmias |
| Holter monitor | Intermittent or breed-associated arrhythmias | Rhythm burden over 24 hours or longer, including ventricular ectopy | Does not replace echocardiography for structural disease |
| Blood pressure | Systemic pressure assessment | Identifies hypertension or hypotension that can alter cardiac workload and management | Stress and cuff technique can affect readings |
| CBC | Anemia, infection and systemic clues | Red-cell count, inflammatory pattern, platelets and evidence supporting high-output murmurs | Does not diagnose the cardiac lesion |
| Serum chemistry + urinalysis | Comorbidity and treatment baseline | Kidney, liver, electrolyte and metabolic status | May be normal despite important heart disease |
| Cardiac biomarkers | Adjunctive screening or risk assessment | NT-proBNP may support cardiac stress assessment; troponin may indicate myocardial injury | Not specific enough to replace imaging and clinical interpretation |
| Heartworm testing | Exposure-risk dogs or compatible right-heart/pulmonary findings | Antigen and microfilaria evidence of infection | Test interpretation depends on infection stage and prior prevention/treatment |
| Blood cultures | Suspected infective endocarditis | Identifies bloodstream bacteria and guides antimicrobial selection | Multiple samples and careful timing are usually needed |
Owner-facing urgency guide
When to escalate
Emergency assessment now
Labored or open-mouth breathing, blue/gray gums or tongue, collapse, inability to settle, severe weakness, or marked respiratory distress.
Contact a veterinarian the same day
A persistent rise in sleeping respiratory rate, a rate above the veterinarian’s action threshold, new cough with reduced energy, pale gums, abdominal swelling, or fainting that has resolved.
Schedule a prompt diagnostic visit
A newly detected murmur; any persistent puppy murmur; a Grade III–VI systolic murmur; or any diastolic or continuous murmur, even without obvious symptoms.
Sleeping respiratory-rate method
- Wait until the dog is asleep or resting calmly—not panting, hot, stressed, or recently active.
- Count one chest rise-and-fall as one breath.
- Count for 30 seconds and multiply by two.
- Record the number, date and context. Trends are more useful than a single isolated count.
- Follow the individual threshold supplied by the treating veterinarian. Many veterinary references describe 15–30 breaths/minute as a common resting range and advise veterinary contact for persistent elevations.
Interpretation guardrails
Can a Grade I or II murmur still be important?
Yes. Quiet systolic murmurs can be physiologic, but timing, location, patient age, persistence and symptoms matter. Diastolic or continuous murmurs are not considered “innocent” simply because they are quiet.
Does a louder murmur always mean worse heart failure?
No. Murmur intensity can correlate with some lesions, but it is an unreliable stand-alone measure across diseases. Myocardial disease such as DCM may produce a soft murmur despite important dysfunction.
Does coughing prove that the murmur has caused heart failure?
No. Airway disease is common in dogs, including older small-breed dogs. Thoracic radiographs and the full clinical picture help distinguish pulmonary edema from respiratory causes of cough.
What test most directly identifies the cause?
A Doppler echocardiogram is generally the most informative test for valve lesions, shunts, stenosis, chamber enlargement and myocardial function. Other tests answer complementary questions.
Clinical references
- Merck Veterinary Manual: Abnormalities of the Cardiovascular System in Animals
- ACVIM consensus guidelines for diagnosis and treatment of canine MMVD
- Merck Veterinary Manual: Cardiac Shunts in Animals
- Merck Veterinary Manual: Semilunar Valve Stenosis
- Merck Veterinary Manual: Dilated Cardiomyopathy
- Merck Veterinary Manual: Infectious Endocarditis
- Merck Veterinary Manual: Heartworm Disease
- Cornell University College of Veterinary Medicine: Heart Murmurs
- VCA: Home Breathing Rate Evaluation