Heart Murmur in Dogs: Complete Owner's Guide
Heart murmur in dogs usually signals mitral valve disease. Learn ACVIM stages, the signs of congestive heart failure, and when pimobendan changes the prognosis.
Heart murmur in dogs is the most common finding that leads to a diagnosis of degenerative mitral valve disease (DMVD), the leading cardiac condition in the species. DMVD accounts for approximately 75% of all canine heart disease and is especially prevalent in small breeds, with 75% of Cavalier King Charles Spaniels developing a clinical murmur by age 10 (JVIM, 2019). Being told your dog has a heart murmur covers an enormous range of severity: from a barely audible Grade II sound that may remain stable for years to a Grade V murmur with a palpable chest vibration indicating significant valvular regurgitation.
What determines urgency is not the murmur alone but whether the heart has begun to compensate abnormally. This guide explains the five ACVIM stages of DMVD, how to recognize early signs of congestive heart failure (CHF), what diagnostic tests clarify next steps, and why the 2016 EPIC trial changed how subclinical mitral valve disease is treated.
Key Takeaways
- DMVD accounts for approximately 75% of canine cardiac disease; 75% of Cavalier King Charles Spaniels have a murmur by age 10 (JVIM, 2019).
- The EPIC trial showed pimobendan in preclinical Stage B2 DMVD delayed CHF onset by approximately 15 months compared to placebo (JVIM, 2016).
- A resting respiratory rate above 30 breaths per minute is a validated early-warning sign of impending CHF that owners can monitor at home.
- Accurate staging requires echocardiography; murmur grade alone is insufficient to determine whether treatment is indicated.
Contents
- What Is Degenerative Mitral Valve Disease in Dogs?
- What Does a Heart Murmur in Dogs Actually Mean?
- What Are the ACVIM Stages of Mitral Valve Disease?
- What Symptoms Signal Congestive Heart Failure in Dogs?
- How Is DMVD Diagnosed and Staged?
- What Treatments Extend Life for Dogs with Mitral Valve Disease?
- Frequently Asked Questions
What Is Degenerative Mitral Valve Disease in Dogs?
Degenerative mitral valve disease is the progressive thickening and structural distortion of the leaflets of the mitral valve, the two-leaflet valve between the left atrium and left ventricle. As the valve degenerates, its leaflets no longer seal completely during cardiac contraction, allowing blood to regurgitate backward into the left atrium with each heartbeat. This regurgitant fraction represents wasted output: the heart must pump harder to maintain adequate forward flow, eventually causing the left atrium and ventricle to dilate under sustained volume overload.
DMVD is overwhelmingly a disease of small and medium-sized breeds. The Cavalier King Charles Spaniel carries the highest documented genetic burden and has been the central breed in most landmark studies on the condition. Males are affected approximately 1-2 years earlier than females on average (Beardow and Buchanan, 1993, corroborated by subsequent CKCS health surveys). Other high-risk breeds include Dachshunds, Chihuahuas, Miniature and Toy Poodles, Miniature Schnauzers, and Cocker Spaniels. In large-breed dogs, DMVD still occurs but is far less common and tends to follow a different clinical trajectory.
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<figcaption>DMVD prevalence in CKCS escalates sharply with age, reaching approximately 78% by age 10 and near-universal by age 13. Data from CKCS Health Scheme surveys and published JVIM references.</figcaption> </figure>
The disease mechanism explains why DMVD is manageable for years before reaching a critical point. Early DMVD creates a small regurgitant fraction that the heart compensates for through modest chamber enlargement and increased stroke volume. The compensated phase can last months to years with no clinical signs. The challenge is identifying the point at which cardiac remodeling becomes significant enough that intervention alters the disease course.
What Does a Heart Murmur in Dogs Actually Mean?
A heart murmur in dogs is the audible turbulence of blood moving through the heart under abnormal conditions. In DMVD, turbulence originates from blood leaking backward through the faulty mitral valve during the heart's contraction phase, producing a systolic whooshing sound best heard over the left cardiac apex. The murmur is graded on a I-to-VI scale reflecting audibility: Grade I is barely detectable in a quiet room; Grade VI is audible with the stethoscope held off the chest wall.
Murmur grade correlates roughly with the volume of regurgitant blood, but it does not reliably predict when a dog will decompensate. The clinical mistake owners sometimes make is treating a high grade number as an emergency signal regardless of context. A Grade IV murmur unchanged on three successive echocardiograms in a dog with normal left atrial dimensions carries a different prognosis from the same grade murmur detected for the first time in a dog showing nocturnal cough and increased resting respiratory rate. Stage determines treatment decisions. Grade guides surveillance intensity.
DMVD murmurs in dogs under age 5 warrant additional investigation, as early-onset murmurs in young animals are more likely to represent congenital valve abnormalities or other structural defects rather than typical age-related degeneration.
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What Are the ACVIM Stages of Mitral Valve Disease?
The American College of Veterinary Internal Medicine (ACVIM) 2019 Consensus Statement on Degenerative Mitral Valve Disease established the five-stage classification that governs diagnosis and treatment decisions globally. The staging system is based on three clinical variables: whether a murmur is present, whether cardiac enlargement is detectable on imaging, and whether the dog has experienced CHF. Each stage maps to specific management recommendations.
<figcaption>ACVIM 2019 DMVD staging: treatment begins at Stage B2, when murmur is accompanied by echocardiographically confirmed cardiac enlargement. Stages B1 and A require monitoring, not medication.</figcaption> </figure>
The critical distinction is between B1 and B2. Both involve a murmur with no CHF, but B2 requires confirmation of cardiac enlargement by echocardiography or thoracic radiography using specific numerical thresholds from the ACVIM 2019 consensus:
- Left atrium-to-aortic root ratio (LA:Ao) of 1.6 or greater on echocardiography
- Vertebral heart score (VHS) of 10.5 or greater on thoracic radiograph
- Normalized left ventricular internal diameter in diastole (LVIDDN) of 1.7 or greater on echocardiography
A dog in Stage B1 can appear and feel entirely normal for several years. Many owners are surprised to learn their clinically healthy dog requires echocardiographic surveillance every 6-12 months simply because a murmur is present. The reason is that B1-to-B2 transition is the single most important clinical threshold in DMVD management: it is the point where early intervention with pimobendan is supported by trial evidence.
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What Symptoms Signal Congestive Heart Failure in Dogs?
Congestive heart failure in DMVD develops when mitral regurgitation is severe enough that the left atrium can no longer compensate, backing pressure up into the pulmonary veins and causing fluid accumulation in the lungs (pulmonary edema) or around them (pleural effusion). This transition from compensated to decompensated disease can occur over days, which is why recognizing early CHF is critical. Median survival for dogs with DMVD-related CHF on appropriate medical therapy is approximately 9-15 months across multiple published JVIM and JAVMA case series, making early detection and prompt treatment initiation a meaningful contributor to outcomes.
Signs of CHF in dogs with DMVD:
| Sign | What to Observe | Urgency |
|---|---|---|
| Increased resting respiratory rate | Over 30 breaths/min during sleep; count 30 sec, multiply by 2 | Call vet within hours |
| Nocturnal or positional cough | Coughing at night or when lying flat; different in character from kennel cough | Call vet same day |
| Exercise intolerance | Tires faster on walks; reluctant to climb stairs; slows significantly | Schedule recheck |
| Orthopnea | Can only rest sitting upright or standing; refuses to lie flat | Emergency |
| Panting or open-mouth breathing at rest | In a calm, cool environment with no exertion | Emergency |
| Abdominal distension | Fluid accumulation in the abdomen (less common in DMVD than pleural effusion) | Call vet same day |
| Syncope or collapse | Sudden brief loss of consciousness, sometimes preceded by weakness | Emergency |
The resting respiratory rate is the most objectively trackable early warning sign. Healthy dogs breathe at 15-30 breaths per minute during sleep. A consistent reading above 30 breaths per minute on two separate occasions indicates early fluid accumulation and warrants immediate veterinary contact. Several mobile applications (Cardalis, VetMeasure) support structured RRR logging and allow owners to share trending data directly with their veterinary team.
Cough in Stage B2 can result from left atrial enlargement compressing the mainstem bronchus mechanically, not from pulmonary edema. This means some B2 dogs cough without having CHF. Clinical assessment combining RRR, radiographic findings, and the character of the cough is needed to distinguish the two scenarios. Treating cough alone with suppressants without addressing the underlying stage is inappropriate.
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How Is DMVD Diagnosed and Staged?
Diagnosing and staging DMVD accurately requires three complementary tools: physical examination for murmur detection and grading, thoracic radiographs for cardiac size and pulmonary vasculature assessment, and echocardiography for left atrial and ventricular measurements. The first is available at any general practice; the second is routine; the third often requires referral to a board-certified veterinary cardiologist or a general practitioner with echocardiographic training.
A thoracic radiograph reveals cardiac silhouette enlargement and assesses for pulmonary infiltrates or pleural fluid, but it cannot provide the LA:Ao ratio or LVIDDN measurements that define Stage B2. A dog that appears radiographically enlarged may still be B1 on echo, and vice versa; echocardiography remains the definitive staging tool. For CKCS and other high-risk breeds, an initial staging echocardiogram with a cardiologist is valuable even in apparently asymptomatic dogs.
The vertebral heart score (VHS) of 10.5 or greater is the alternative B2 criterion when echocardiography is not immediately accessible, but echo confirmation before initiating pimobendan is recommended wherever possible.
Ongoing monitoring for dogs in each stage typically includes:
- Thoracic radiographs every 6-12 months (B1 and B2) or every 3-6 months (C and D)
- Echocardiography at cardiologist-directed intervals
- Owner-recorded RRR logs submitted at regular check-ins
- Renal panel and blood pressure if on diuretics (Stage C and D)
NT-proBNP (N-terminal pro-B-type natriuretic peptide) is a blood biomarker that rises with increased cardiac wall stress. It is not a substitute for imaging, but it can support the clinical picture and assist with screening in situations where echocardiography is not yet available or as an adjunct to radiographic findings.
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What Treatments Extend Life for Dogs with Mitral Valve Disease?
The 2016 EPIC trial (Evaluating Pimobendan In Cardiomegaly; Boswood et al., JVIM) is the most cited clinical trial in small animal cardiology in the past 20 years. The trial enrolled dogs in Stage B2 DMVD and demonstrated that pimobendan delayed the onset of CHF or cardiac-related death by approximately 15 months compared to placebo, with a 37% reduction in the risk of reaching the primary endpoint (JVIM, 2016). Before EPIC, there was no evidence-based treatment for preclinical DMVD; the field took a "watch and wait" approach through the entire subclinical phase.
<figcaption>EPIC trial: pimobendan in Stage B2 DMVD delayed onset of CHF by approximately 15 months and reduced the primary endpoint risk by 37% (JVIM, 2016). Bar lengths represent approximate median time to CHF onset or cardiac-related death.</figcaption> </figure>
Medical treatment by ACVIM stage:
Stage B2: Pimobendan (0.2-0.3 mg/kg twice daily) is recommended when ACVIM echocardiographic criteria are confirmed. No other cardiac medications are indicated at this stage under the current consensus.
Stage C: Pimobendan plus furosemide forms the core of Stage C management. Spironolactone and an ACE inhibitor (enalapril or benazepril) are commonly added as disease progresses. Suppressing cough alone without addressing the underlying fluid status is inappropriate.
Stage D: Refractory CHF requires intensification of diuretic therapy, often through a second diuretic agent (torasemide or hydrochlorothiazide) or combination protocols. Some dogs benefit from referral to a board-certified veterinary cardiologist for advanced management.
Mitral valve repair surgery: Surgical valve repair or replacement is available at a small number of specialized academic centers, primarily in Japan and the United States. The procedure is technically demanding and costs approximately $8,000-$15,000 USD; in experienced hands it can produce durable correction of regurgitation. Most candidates are Stage C or above at the time of evaluation.
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Frequently Asked Questions
What is the life expectancy of a dog diagnosed with a heart murmur?
Life expectancy depends on the underlying stage of DMVD, not on the murmur grade itself. A dog in Stage B1 with a stable Grade III murmur may live many additional years without CHF. A dog newly diagnosed in Stage C has a median survival of approximately 9-15 months with appropriate treatment. The EPIC trial (JVIM, 2016) demonstrated that treating Stage B2 dogs with pimobendan extends the time before CHF by approximately 15 months, which meaningfully shifts the overall trajectory.
How do I know if my dog's heart murmur is getting worse?
Murmur grade changes are not the most reliable progression marker because grading varies between examiners. Better indicators of progression include echocardiographic increases in left atrial size at recheck appointments, a resting respiratory rate that consistently exceeds 30 breaths per minute, new exercise intolerance or nocturnal cough, and radiographic evidence of increasing cardiomegaly or pulmonary infiltrates. Scheduled surveillance every 6-12 months for B1 and B2 dogs allows objective tracking.
Should my dog start pimobendan right after a murmur is first detected?
Not automatically. Pimobendan is indicated for Stage B2 dogs, which requires confirmation of cardiac enlargement using ACVIM 2019 echocardiographic thresholds. Dogs with a murmur but no cardiac enlargement (Stage B1) do not yet have a demonstrated indication for pimobendan based on current trial evidence. Starting the drug without confirming B2 status extrapolates beyond the EPIC data and the ACVIM 2019 consensus.
Can my dog with DMVD still exercise normally?
In Stages A, B1, and early B2, most dogs live entirely normal lives with no activity restriction. Moderate regular exercise is appropriate and generally beneficial. As disease advances to Stage C, activity may need to be moderated and guided by the cardiologist based on the dog's response. Avoidance of heat and prolonged strenuous exercise becomes more important once CHF is present.
Which breeds are most at risk for degenerative mitral valve disease?
DMVD is most prevalent in Cavalier King Charles Spaniels, with near-universal disease penetrance by age 13. Other high-risk small breeds include Dachshunds, Chihuahuas, Miniature and Toy Poodles, Miniature Schnauzers, and Cocker Spaniels. Males in these breeds tend to develop murmurs approximately 1-2 years earlier than females. Among large-breed dogs, DMVD occurs but at lower rates and with generally later onset.
Next Steps
For any dog with a newly detected heart murmur:
- Request a staging echocardiogram with a board-certified veterinary cardiologist or an experienced echocardiographer to determine B1 vs B2 before any treatment decision is made.
- Start resting respiratory rate monitoring immediately after any DMVD diagnosis: count breaths for 30 seconds during sleep, multiply by 2. Two consecutive readings above 30 warrants a same-day call to your veterinarian.
- Follow ACVIM recheck intervals: every 6-12 months for B1; every 6 months for B2; as directed by your cardiologist for C and D.
- Discuss EPIC eligibility criteria with your veterinarian if the dog is classified as B2 and confirm whether echocardiographic thresholds (LA:Ao of 1.6 or greater or LVIDDN of 1.7 or greater) are met before starting pimobendan.
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