What A Heart Murmur Means
A heart murmur is an abnormal sound heard during cardiac auscultation — turbulent blood flow producing a sound between the normal S1 (“lub”) and S2 (“dub”) heart sounds, or sometimes continuously through the cardiac cycle.
Heart murmurs vary enormously in significance:
- Innocent puppy flow murmur — resolves spontaneously, no treatment
- Mild myxomatous mitral valve disease in Cavalier — monitor lifelong
- Severe subaortic stenosis in Boxer — limited lifespan
- Advanced congestive heart failure — multimodal treatment, hospice planning
This calculator combines Levine grade (I-VI), murmur location, timing, ACVIM stage, and breed signalment to interpret what your dog’s murmur means and guide workup.
The Levine Grading Scale
| Grade | Description |
|---|---|
| I/VI | Very soft, heard only after extended listening in a quiet environment |
| II/VI | Soft but heard within the first few minutes of auscultation |
| III/VI | Moderately loud, clearly audible |
| IV/VI | Loud but no palpable thrill |
| V/VI | Loud WITH PALPABLE THRILL (vibration felt by hand on chest wall) |
| VI/VI | Extremely loud, audible with stethoscope held off chest |
Important: grade INFLUENCES but does NOT determine clinical significance:
- Some grade II murmurs are clinically important (early MMVD progressing)
- Some grade IV murmurs are stable for years (mild subaortic stenosis)
The combination of grade + location + timing + breed + signs matters more than grade alone.
Murmur Timing
Systolic (Most Common In Dogs)
Murmur heard between S1 and S2 — most common pattern.
Differentials:
- Mitral regurgitation (MMVD) — left apex
- Subaortic stenosis — left base
- Pulmonic stenosis — left base
- Tricuspid regurgitation — right hemithorax
- VSD (ventricular septal defect) — right hemithorax
- Innocent flow murmur (puppies) — often left base
Diastolic (Less Common)
Murmur heard between S2 and S1 — less common, often clinically significant.
Differentials:
- Aortic regurgitation (endocarditis classic)
- Pulmonic regurgitation (pulmonary hypertension)
Continuous / Machinery
Murmur heard throughout the cardiac cycle — distinctive “machinery” quality.
Classic for Patent Ductus Arteriosus (PDA) — the most common congenital heart defect in dogs. Surgical or interventional closure is curative.
Location (Point Of Maximum Intensity)
The PMI (point of maximum intensity) suggests the affected valve:
Left Apex (5th Intercostal Space, Left)
Mitral valve area.
Differentials:
- Myxomatous mitral valve disease (MMVD) — most common acquired heart disease in dogs, particularly small breeds
- Mitral dysplasia (congenital)
- Endocarditis (rare)
Left Base (3rd-4th Intercostal Space, Left)
Aortic and pulmonic valve area.
Differentials:
- Subaortic stenosis (SAS) — congenital; Boxer, Newfoundland, Golden, Rottweiler classic
- Pulmonic stenosis — congenital; Bulldog, Beagle, Mastiff, Chihuahua, Boxer
- Innocent flow murmur in puppies and athletic dogs
- Aortic regurgitation (endocarditis)
Right Hemithorax (3rd-4th Intercostal Space, Right)
Tricuspid valve area.
Differentials:
- Tricuspid valve disease (often concurrent with MMVD)
- Tricuspid dysplasia (Labrador classic congenital)
- Pulmonary hypertension causing tricuspid regurgitation
- VSD (ventricular septal defect)
The Big Picture – Myxomatous Mitral Valve Disease (MMVD)
MMVD is the most common acquired heart disease in dogs — accounts for about 75% of all canine cardiac disease. Particularly affects:
- Cavalier King Charles Spaniel — >50% have murmur by age 5; >90% by age 10
- Miniature Poodle
- Miniature Schnauzer
- Dachshund
- Yorkshire Terrier
- Chihuahua
- Toy and small breeds generally
ACVIM Staging For MMVD
| Stage | Definition |
|---|---|
| A | At-risk breed (Cavalier, Mini Poodle etc.) but no murmur |
| B1 | Murmur present but heart NOT enlarged on imaging |
| B2 | Murmur with heart enlargement (LA:Ao ≥1.6 and/or LVIDDN ≥1.7 on echo, or VHS >10.5 on radiographs) |
| C | History of congestive heart failure |
| D | Refractory CHF on maximal therapy |
The EPIC Trial – Pimobendan Game-Changer
Boswood et al. 2016 JVIM — the EPIC trial showed pimobendan started at Stage B2 delays onset of CHF by approximately 15 months on average.
This is one of the most evidence-based interventions in canine cardiology. Standard dosing: 0.25 mg/kg PO q12h. Add ACE inhibitor (benazepril) or telmisartan for some patients.
Echocardiography for staging is critical — knowing whether a dog is in Stage B1 (monitor) vs B2 (treat) substantially affects management and outcomes.
Congenital Heart Disease
For young dogs with murmurs, the major congenital defects:
Patent Ductus Arteriosus (PDA)
- Most common congenital defect in dogs
- Continuous machinery murmur at left base
- Female predominance
- Breeds: Maltese, Pomeranian, Toy/Miniature Poodle, Shetland Sheepdog, Chihuahua, Yorkshire Terrier
- Treatment: surgical ligation or transcatheter closure with Amplatz device — curative
Subaortic Stenosis (SAS)
- Boxer, Newfoundland, Golden Retriever, Rottweiler
- Loud left basilar systolic murmur in young dog highly suggestive
- Severity stratified by velocity on echocardiography (>3.5 m/s significant)
- Sudden cardiac death risk in severe disease
- Beta-blocker for symptomatic dogs; rare interventional options
Pulmonic Stenosis
- Bulldog, Beagle, Mastiff, Chihuahua, Boxer, Cocker, Samoyed
- Loud left basilar systolic murmur
- Balloon valvuloplasty is curative for severe cases
Mitral / Tricuspid Dysplasia
- Mitral dysplasia: Bull Terrier, German Shepherd, Great Dane, Newfoundland
- Tricuspid dysplasia: Labrador classic
- Often severe at presentation
- Limited treatment options — palliative management
Ventricular Septal Defect (VSD)
- Hole between left and right ventricles
- Right hemithorax systolic murmur
- Small VSDs — sometimes asymptomatic for life
- Large VSDs — congestive heart failure; surgical closure complex
Innocent Puppy Murmurs
About 5% of healthy puppies have a brief innocent / flow murmur:
- Grade I-II/VI
- Left basilar
- Systolic
- Resolves by 4-6 months in many cases
If puppy murmur persists past 4-6 months, echocardiography is recommended to differentiate from pathological murmurs (mild subaortic stenosis, mitral dysplasia, small VSD).
When CHF Develops – Treatment Toolkit
Stage C/D MMVD = established congestive heart failure. Multimodal treatment:
- Pimobendan 0.25 mg/kg q12h — inodilator
- Furosemide 1-4 mg/kg q8-12h — diuretic; titrated to resting respiratory rate and clinical signs
- ACE inhibitor (benazepril) or telmisartan — RAAS blockade
- Spironolactone 1-2 mg/kg q24h — additional RAAS blockade
- Sometimes amlodipine for hypertension
- Sometimes sildenafil for pulmonary hypertension
- Cardiology referral appropriate
Home Monitoring – The Critical Tool
For dogs with confirmed heart disease (Stage B2+), the single most useful home monitoring tool is resting respiratory rate (RRR):
- Counted at sleep, daily
- >30 breaths/min OR sustained 20-25% increase above personal baseline = early CHF warning — call vet
- See PuppaDogs Resting Respiratory Rate Calculator
Other home signs:
- Exercise tolerance — any sudden decrease
- Increased cough during/after exercise
- Appetite — reduced appetite often precedes overt CHF by days
- Weight — rapid gain may reflect fluid retention
Standard Cardiac Workup
For any meaningful murmur:
- Echocardiography — the most useful single test; assesses chamber sizes, valve function, contractility, congenital defects
- Thoracic radiographs — heart size (VHS – vertebral heart score, normal <10.5); pulmonary parenchyma for oedema
- ECG — rhythm assessment
- Blood pressure
- NT-proBNP — cardiac biomarker; elevated in significant disease
- CBC + biochemistry + thyroid — rule out concurrent disease
Breed-Specific Cardiac Predispositions
| Breed | Primary disease |
|---|---|
| Cavalier King Charles Spaniel | MMVD (>90% by age 10) |
| Doberman Pinscher | Dilated cardiomyopathy (DCM) |
| Great Dane | DCM, atrial fibrillation |
| Boxer | ARVC, subaortic stenosis |
| Newfoundland | Subaortic stenosis, DCM |
| Golden Retriever | Subaortic stenosis |
| Bulldog | Pulmonic stenosis |
| Maltese / Pomeranian | PDA |
| Labrador | Tricuspid dysplasia |
| Bull Terrier | Mitral dysplasia |
For at-risk breeds, annual cardiac auscultation from age 3-4, with echocardiography for any new murmur.
Honest Caveats
- Murmur grade is subjective — vets can vary by 1 grade
- Grade does not always reflect severity — small VSD with loud murmur may be clinically silent; severe MMVD with soft murmur may be life-threatening
- Echocardiography is the gold standard for cardiac diagnosis and staging
- Owner-described signs are approximate — vet examination and imaging are diagnostic
- This calculator helps you understand the framework — not replace cardiac workup
Conclusion
Canine heart murmurs vary enormously in significance — from innocent puppy flow murmurs to severe congestive heart failure. The Levine grade (I-VI) + location (apex/base/right) + timing (systolic/diastolic/continuous) + breed signalment combination strongly suggests the underlying diagnosis. Echocardiography is the gold-standard workup for any meaningful murmur. ACVIM staging for MMVD guides treatment — Stage B2 (murmur with heart enlargement) is the EPIC trial pimobendan threshold, prolonging time to CHF by approximately 15 months. Resting respiratory rate monitoring at home is the single most useful tool for detecting early CHF in dogs with Stage B2+ disease.
Frequently Asked Questions
What does a grade 3 heart murmur mean in dogs?
Grade III/VI murmur is moderately loud, clearly audible on routine auscultation. Combined with LOCATION (left apex, base, or right) and TIMING (systolic, diastolic, continuous), it suggests the underlying diagnosis. Most common in dogs: LEFT APICAL SYSTOLIC GRADE III/VI = myxomatous mitral valve disease (MMVD), often Stage B1 or early B2 in small breeds. ECHOCARDIOGRAPHY essential for staging – assesses left atrial size and other parameters to determine if Stage B2 (warranting pimobendan per EPIC trial) or still Stage B1 (monitor).
What is ACVIM stage B2 heart disease?
ACVIM Stage B2 = myxomatous mitral valve disease (MMVD) with HEART ENLARGEMENT on imaging but NO CONGESTIVE HEART FAILURE yet. Defined as LA:Ao ratio more than 1.6 AND/OR LVIDDN ratio more than 1.7 on echocardiography, or VHS (vertebral heart score) more than 10.5 on thoracic radiographs. The EPIC trial (Boswood 2016 JVIM) showed PIMOBENDAN started at Stage B2 delays onset of CHF by approximately 15 months. Standard dosing: pimobendan 0.25 mg/kg PO q12h. Monitor resting respiratory rate at home for early CHF detection.
Is a heart murmur in my puppy serious?
Depends on grade, location, timing, persistence. About 5% of healthy puppies have a brief INNOCENT / FLOW MURMUR – typically grade I-II/VI, left basilar, systolic, resolves by 4-6 months spontaneously. PATHOLOGICAL CONGENITAL MURMURS to consider: PDA (continuous machinery murmur left base – surgical closure curative), SUBAORTIC STENOSIS (left basilar systolic, often grade III-IV in significant disease – Boxer/Newfoundland/Golden/Rottweiler), PULMONIC STENOSIS (Bulldog/Beagle – balloon valvuloplasty curative), VSD, mitral/tricuspid dysplasia. If puppy murmur PERSISTS PAST 4-6 MONTHS, echocardiography recommended.
What is mitral valve disease in dogs?
Myxomatous mitral valve disease (MMVD, also called endocardiosis) is the MOST COMMON ACQUIRED HEART DISEASE in dogs – about 75% of all canine cardiac disease. Progressive degeneration of the mitral valve causing regurgitation. Particularly affects SMALL BREEDS – Cavalier King Charles Spaniel classic (more than 50% have murmur by age 5, more than 90% by age 10), Miniature Poodle, Miniature Schnauzer, Dachshund, Yorkie, Chihuahua. ACVIM staging A through D. Treatment by stage – Stage B2 (with heart enlargement) prompts PIMOBENDAN per EPIC trial. Stage C/D (CHF) multimodal – pimobendan + furosemide + ACE inhibitor + spironolactone.
Should I worry about a soft heart murmur in my old dog?
Grade I-II/VI soft murmur in older dog is most commonly EARLY MMVD – particularly in small breeds. ECHOCARDIOGRAPHY recommended to confirm and stage. Stage B1 (no heart enlargement) typically monitored every 6-12 months. Stage B2 (with heart enlargement) prompts PIMOBENDAN treatment per EPIC trial. Even soft murmurs can be early significant disease – the grade does not necessarily reflect severity. Combined with breed predisposition (Cavalier, small toy breeds especially), workup is appropriate.
What is the EPIC trial for dog heart disease?
The EPIC trial (Effect of Pimobendan in Cardiomegaly, Boswood et al. 2016 JVIM) was a landmark veterinary cardiology study showing PIMOBENDAN started at ACVIM STAGE B2 myxomatous mitral valve disease (murmur WITH heart enlargement but no CHF yet) DELAYS ONSET OF CHF by APPROXIMATELY 15 MONTHS on average. This is one of the most evidence-based interventions in canine cardiology. Standard pimobendan dosing: 0.25 mg/kg PO q12h. Stage B2 is defined by LA:Ao more than 1.6 and/or LVIDDN more than 1.7 on echo, or VHS more than 10.5 on radiographs. Echocardiography for staging is therefore critical in MMVD.
Related PuppaDogs Calculators
Continue building your dog’s personalised care plan with these related PuppaDogs calculators:
- Dog Pregnancy / Whelping Due-Date Calculator
- Puppy Weight Predictor (Adult Weight Calculator)
- Heatstroke Risk Calculator for Dogs
- Bloat (GDV) Risk Calculator for Dogs
- Dog Life Expectancy Calculator (Breed, Body Condition, Lifestyle)
- Spay/Neuter Timing Calculator for Dogs (Breed-Specific)
References & Further Reading
The dosing ranges and safety information on this page are drawn from the following veterinary references. Always defer to your own veterinarian and the manufacturer’s label for your specific product.
- Keene BW, Atkins CE, Bonagura JD, et al. ACVIM consensus guidelines for the diagnosis and treatment of myxomatous mitral valve disease in dogs. JVIM, 2019.
- Boswood A, Haggstrom J, Gordon SG, et al. Effect of pimobendan in dogs with preclinical myxomatous mitral valve disease and cardiomegaly: The EPIC Study. JVIM, 2016.
- Atkins C, Bonagura J, Ettinger S, et al. Guidelines for the diagnosis and treatment of canine chronic valvular heart disease. JVIM, 2009.
- Borgarelli M, Buchanan JW. Historical review, epidemiology and natural history of degenerative mitral valve disease. Journal of Veterinary Cardiology.
- ACVIM Forum proceedings on canine cardiology.
- Plumb’s Veterinary Drug Handbook – pimobendan, furosemide, ACE inhibitors.
- PuppaDogs. Resting Respiratory Rate Calculator and Mitral Valve Disease ACVIM Stage Calculator. puppadogs.com.
