Heart Murmur in Dogs: What the Finding Means and How Vets Investigate It

This article is educational and is not a substitute for veterinary diagnosis or treatment.

A heart murmur is an abnormal sound heard during the heartbeat cycle, caused by turbulent blood flow within the heart or great vessels. In dogs, this finding is not a disease itself but a clinical sign that signals an underlying structural or functional cardiac abnormality. The detection of a heart murmur in dogs requires a systematic investigation to determine its cause, severity, and clinical significance. This article provides a comprehensive, evidence-based review of what a canine heart murmur means, how veterinarians approach its diagnosis, and what dog owners need to know about the condition.

At a Glance: Heart Murmur in Dogs

What Is a Heart Murmur in Dogs?

A heart murmur in dogs is an audible vibration produced by turbulent blood flow within the cardiovascular system. Normally, blood flows smoothly through the heart chambers and valves, producing little to no audible sound beyond the normal heart sounds (S1 and S2). When flow becomes turbulent due to structural abnormalities, increased velocity, or altered vessel geometry, it generates a "whooshing" or "hissing" sound that can be heard with a stethoscope.

Murmurs are characterized by several features:

• Timing: Systolic (between S1 and S2), diastolic (between S2 and S1), or continuous
• Location: Point of maximum intensity (PMI) on the thoracic wall
• Intensity: Graded I/VI (barely audible) to VI/VI (audible without stethoscope)
• Configuration: Crescendo, decrescendo, or plateau
• Quality: Harsh, blowing, musical, or rumbling

The presence and intensity of heart murmurs are sensitive indicators of several cardiac diseases in dogs, particularly myxomatous mitral valve disease (MMVD) [2]. However, not all murmurs indicate significant disease. Innocent or physiologic murmurs can occur in puppies, in dogs with high sympathetic tone, or in conditions like anemia or fever.

Prevalence and Clinical Significance

Heart murmurs are among the most common abnormal findings on physical examination in dogs. In a large registry study of 6,102 dogs with MMVD, the median age at presentation was 10 years (range 1-19 years), and mixed-breed dogs were the most common breed represented (22%) [12]. Among apparently healthy Chihuahua dogs, mitral regurgitation was found in 38% of individuals, with a systolic murmur audible in 22% [14].

The prevalence of murmurs increases with age. In the Chihuahua study, dogs with mitral regurgitation were significantly older (median 7.6 years) than those without (median 3 years) [14]. This age-related increase reflects the progressive nature of degenerative valve disease.

Heart murmurs are also a common reason for exclusion from clinical trials and health screening programs. In one study of apparently healthy mature adult and senior dogs, heart murmurs were the second most common exclusion factor, with murmurs greater than grade II/VI leading to exclusion in 5 of 32 dogs (16%) that passed initial screening [11].

Anatomy and Physiology of Heart Murmurs

To understand why heart murmurs occur, a basic grasp of cardiac anatomy and hemodynamics is essential.

Normal Cardiac Blood Flow

The canine heart has four chambers: right atrium, right ventricle, left atrium, and left ventricle. Blood flows through the heart in a coordinated sequence:

1. Deoxygenated blood enters the right atrium via the cranial and caudal vena cavae
2. Passes through the tricuspid valve into the right ventricle
3. Is pumped through the pulmonic valve into the pulmonary artery for oxygenation
4. Oxygenated blood returns to the left atrium via pulmonary veins
5. Passes through the mitral valve into the left ventricle
6. Is ejected through the aortic valve into the systemic circulation

Mechanisms of Turbulent Flow

Turbulence occurs when blood flow velocity exceeds a critical threshold or when flow encounters an obstruction, a narrow orifice, or an irregular surface. The primary mechanisms include:

• Valvular regurgitation: Incomplete closure of a valve allows blood to leak backward (e.g., mitral regurgitation)
• Valvular stenosis: Narrowing of a valve orifice increases flow velocity (e.g., pulmonic stenosis)
• Shunts: Abnormal connections between chambers or vessels (e.g., patent ductus arteriosus, ventricular septal defect)
• High-flow states: Increased cardiac output (e.g., anemia, hyperthyroidism) can produce physiologic murmurs
• Structural abnormalities: Thickened, prolapsed, or malformed valves create irregular surfaces that disturb flow

Causes of Heart Murmurs in Dogs

The differential diagnosis for a canine heart murmur is broad and includes both congenital and acquired conditions.

Acquired Heart Disease

Myxomatous Mitral Valve Disease (MMVD)

MMVD is the most common acquired heart disease in dogs and the leading cause of heart murmurs in small-breed dogs. It is characterized by progressive myxomatous degeneration of the mitral valve apparatus, leading to valve thickening, prolapse, and regurgitation [5][12]. The disease progresses through stages defined by the American College of Veterinary Internal Medicine (ACVIM) consensus guidelines [22]:

• Stage A: At risk but no identifiable disease
• Stage B1: Preclinical disease without cardiomegaly
• Stage B2: Preclinical disease with cardiomegaly
• Stage C: Current or past congestive heart failure
• Stage D: Refractory congestive heart failure

In a large registry study, 65% of dogs with MMVD were in Stage B1, 15% in Stage B2, and 20% in Stage C [12]. Murmur intensity, heart rate, and radiographic vertebral heart score were positively associated with disease stage.

Dilated Cardiomyopathy (DCM)

DCM is primarily seen in large and giant breed dogs. It causes systolic dysfunction and chamber dilation, often leading to a soft systolic murmur secondary to mitral regurgitation. The murmur may be less prominent than in MMVD, and arrhythmias are common.

Infectious Endocarditis

Bacterial infection of the heart valves can cause destructive lesions leading to regurgitation and murmurs. This condition is less common but carries a guarded prognosis. Dogs may present with fever, lethargy, and a new or changing murmur.

Congenital Heart Disease

Congenital defects account for heart murmurs in younger dogs, though some may not be detected until adulthood.

Patent Ductus Arteriosus (PDA)

PDA produces a characteristic continuous "machinery" murmur heard best at the left base. A large left-to-right shunting PDA causes severe left heart volume overload and can lead to unilateral pulmonary edema, as reported in a 4-month-old mixed-breed dog with a grade 6/6 continuous murmur [17].

Pulmonic Stenosis

This obstruction of the right ventricular outflow tract produces a systolic ejection murmur at the left base. It is common in brachycephalic breeds and can cause right ventricular hypertrophy.

Ventricular Septal Defect (VSD)

A VSD creates a holosystolic murmur at the right sternal border. Defects can be small and hemodynamically insignificant or large enough to cause left heart volume overload.

Atrial Septal Defect (ASD)

ASDs produce a systolic murmur at the left base due to increased flow across the pulmonic valve. In a case series of five dogs with ASD, three were initially evaluated for an asymptomatic murmur noted during routine preventive care visits [15].

Tetralogy of Fallot

This combination of pulmonic stenosis, VSD, overriding aorta, and right ventricular hypertrophy produces a systolic murmur and cyanosis.

Other Causes

Cardiac Neoplasia

Primary cardiac tumors such as myxoma are rare in dogs. A case report described a 7-month-old Miniature Dachshund with a cardiac myxoma causing left ventricular outflow tract obstruction and a VSD, presenting with growth insufficiency, lethargy, and a cardiac murmur [20].

Heartworm Disease (Dirofilariasis)

Adult Dirofilaria immitis worms in the pulmonary arteries and right heart can cause murmurs. A 17-year-old dog with heartworm disease presented with a cardiac mitral murmur, respiratory distress, and echocardiographic evidence of adult filariae in the right ventricle [19].

Systemic Diseases

Canine Monocytic Ehrlichiosis (CME) can cause myocarditis and arrhythmias. In dogs with chronic CME, elevated cardiac biomarkers (cTnI, NT-proBNP) and arrhythmias have been documented, potentially contributing to murmur development [3].

High-Pathogenicity Avian Influenza

A 16-year-old dog infected with A/H5N1 avian influenza presented with a left apical heart murmur, though this was likely incidental to the primary respiratory infection [18].

Dog Heart Murmur Symptoms

The clinical signs associated with a heart murmur in dogs depend on the underlying cause, severity, and stage of disease. Many dogs with murmurs are asymptomatic, particularly in early stages.

Asymptomatic (Preclinical) Phase

Dogs with low-grade murmurs (I-II/VI) and no structural heart disease or mild MMVD often show no clinical signs. These dogs are identified during routine physical examination. In the LOOK-Mitral registry, dogs in Stage B1 were younger and had fewer clinical signs than dogs in more advanced stages [12].

Clinical Signs of Progressive Disease

As heart disease advances, dogs may develop:

• Cough: Often a dry, hacking cough that may be worse at night or after exercise. Coughing in MMVD can result from airway compression by an enlarged left atrium or concurrent respiratory disease.
• Exercise intolerance: Reluctance to walk, play, or keep up on walks. This was reported in 14% of dogs in the LOOK-Mitral registry [12].
• Tachypnea: Increased respiratory rate at rest or during sleep.
• Dyspnea: Labored breathing, often indicating pulmonary edema (congestive heart failure).
• Syncope: Fainting episodes due to arrhythmias, hypotension, or reduced cardiac output.
• Abdominal distension: Due to ascites from right-sided heart failure.
• Weight loss or poor body condition: In advanced or chronic disease.

In the LOOK-Mitral registry, dogs in Stage C were more likely to have tachypnea (p<0.001), dyspnea (p<0.001), cough (p<0.001), and syncopal episodes (p<0.001) compared to dogs in Stage B1 and B2 [12].

Emergency Red Flags

Owners should seek immediate veterinary care if their dog with a known heart murmur develops:

• Rapid or labored breathing at rest (respiratory rate >40 breaths per minute)
• Blue or pale gums (cyanosis)
• Collapse or loss of consciousness
• Inability to stand or walk
• Distended abdomen
• Coughing up frothy fluid

Dog Heart Murmur Stages: The ACVIM Classification System

The ACVIM consensus guidelines provide a standardized staging system for MMVD, which is the most common cause of heart murmurs in dogs [22]. This system helps veterinarians determine prognosis and guide treatment decisions.

Stage A: At Risk

Dogs with no identifiable heart disease but with breed predisposition for MMVD (e.g., Cavalier King Charles Spaniels, Dachshunds). No murmur is present, and no treatment is indicated.

Stage B: Preclinical Heart Disease

Stage B1: Dogs with a heart murmur but no radiographic or echocardiographic evidence of cardiomegaly. These dogs are asymptomatic and do not require medical therapy. Regular monitoring (every 6-12 months) is recommended.

Stage B2: Dogs with a heart murmur and evidence of cardiomegaly on echocardiography or radiography, but no clinical signs of heart failure. This is a critical decision point. The EPIC trial demonstrated that pimobendan therapy delays the onset of congestive heart failure and extends survival in dogs with Stage B2 MMVD [6].

A target trial emulation study using primary-care data from the VetCompass database found that dogs prescribed pimobendan had a 5-year CHF cumulative incidence of 34.1% compared to 56.3% in dogs not prescribed pimobendan. Dogs receiving pimobendan gained 311 additional days free of CHF within 5 years [6].

Stage C: Congestive Heart Failure

Dogs with current or past clinical signs of CHF (pulmonary edema, ascites). These dogs require acute and chronic medical management, including diuretics, pimobendan, and ACE inhibitors.

Stage D: Refractory Heart Failure

Dogs with end-stage disease that do not respond to standard therapy. Management involves advanced medical strategies and palliative care.

How Veterinarians Investigate a Heart Murmur

The investigation of a canine heart murmur follows a systematic approach to determine the cause, severity, and clinical significance of the finding.

Step 1: History and Physical Examination

A thorough history should include:

• Signalment (breed, age, sex)
• Onset and progression of any clinical signs
• Exercise tolerance
• Coughing, respiratory rate, syncopal episodes
• Previous medical history and medications
• Travel history (heartworm endemic areas)

Physical examination includes:

• Auscultation: Systematic evaluation of heart sounds, murmurs, and rhythm. The murmur is characterized by grade, timing, PMI, and radiation.
• Femoral pulse quality: Bounding pulses suggest aortic regurgitation or PDA; weak pulses suggest decreased cardiac output.
• Mucous membrane color and capillary refill time: Assess perfusion status.
• Jugular vein evaluation: Distension or pulsation indicates right heart disease.
• Respiratory assessment: Lung auscultation for crackles (pulmonary edema) or wheezes.

Step 2: Thoracic Radiography

Radiographs provide information about cardiac size, shape, and pulmonary vasculature. The vertebral heart score (VHS) is a quantitative measure of cardiac size. A VHS of 11.5 units or greater on a lateral view suggests clinically relevant cardiomegaly [9].

Radiographic findings in MMVD include:

• Left atrial enlargement (loss of the caudal cardiac waist)
• Left ventricular enlargement
• Elevation of the trachea
• Pulmonary venous congestion
• Interstitial or alveolar pulmonary edema (CHF)

Step 3: Echocardiography

Echocardiography is the gold standard for diagnosing and staging heart disease in dogs with murmurs. It provides real-time assessment of cardiac structure and function.

Key echocardiographic measurements in MMVD include:

• Left atrial diameter (LAmax) and left atrial-to-aortic ratio (LA:Ao)
• Left ventricular internal diameter in diastole and systole (LVIDd, LVIDs)
• Fractional shortening (FS)
• Mitral valve morphology (thickening, prolapse)
• Color Doppler assessment of mitral regurgitation severity

In a study of small-breed dogs with MMVD, significant differences were observed in LAmax, LA:Ao, LVIDd, LVIDs, and several Doppler variables between dogs with varying severity of mitral regurgitation and between normal and enlarged hearts [5].

Step 4: Electrocardiography (ECG)

A standard 6-lead ECG evaluates heart rate, rhythm, and conduction abnormalities. In MMVD, common findings include:

• Sinus rhythm (often with respiratory sinus arrhythmia)
• Atrial fibrillation (in advanced disease with severe atrial enlargement)
• Ventricular premature complexes

Ambulatory ECG (24-hour Holter monitoring) provides more comprehensive arrhythmia detection. In dogs with experimentally induced mild mitral regurgitation, heart rate variability analysis revealed decreased high-frequency power and increased low-frequency-to-high-frequency ratio, suggesting autonomic imbalance even in early disease [4].

Step 5: Cardiac Biomarkers

N-terminal pro-B-type natriuretic peptide (NT-proBNP): This biomarker is released from ventricular myocytes in response to wall stretch. Plasma NT-proBNP concentrations greater than 1,100 pmol/L suggest clinically relevant cardiomegaly and increased risk of CHF [9].

Cardiac troponin I (cTnI): A specific marker of myocardial injury. Elevated cTnI concentrations can indicate myocarditis, ischemia, or severe heart disease.

Neutrophil-to-lymphocyte ratio (NLR): A retrospective study found that NLR was significantly higher in dogs with CHF secondary to MMVD (6.41) compared to dogs with preclinical MMVD (4.66) and healthy controls (3.95). Higher NLR was associated with increased diuretic requirements and longer oxygen supplementation [7].

Step 6: Advanced Imaging

Computed Tomography Angiography (CTA): Used for evaluating complex congenital heart disease, such as ASD, VSD, and PDA. Three-dimensional reconstruction with stereoscopic viewing can help assess suitability for device occlusion [15].

Cardiac MRI: Less commonly used in veterinary medicine but provides detailed tissue characterization.

Step 7: Emerging Technologies

Computer-aided auscultation: Research teams are evaluating algorithms that analyse recordings from electronic stethoscopes to help detect and grade heart murmurs. A recurrent neural network trained on recordings detected murmurs with 87.9% sensitivity and 81.7% specificity, and differentiated between Stage B1 and B2 preclinical MMVD in its study population (AUC 0.861) [2].

A deep learning-based algorithm (CNN6) using phonocardiogram data from a digital wireless stethoscope achieved 89.9% sensitivity, 92.7% specificity, and 90.9% accuracy for detecting heart murmurs, demonstrating diagnostic performance comparable to experienced veterinarians [13].

Another study using expert-consensus labeling to reduce label noise found that classification accuracy for mild murmurs improved from 37.71% to 90.98% sensitivity and from 76.70% to 93.69% specificity after noise reduction [8].

MicroRNA Profiling: A preliminary study of serum and plasma microRNA expression profiles showed compelling ability to differentiate healthy dogs from those with MMVD (sensitivity 0.85, specificity 0.82, accuracy 0.83) and to distinguish preclinical from clinical stages [10].

Evidence-Based Management of Heart Murmurs in Dogs

Management depends on the underlying cause, stage of disease, and presence of clinical signs.

Preclinical Disease (Stage B1)

No medical therapy is indicated. Regular monitoring (every 6-12 months) with physical examination, radiography, and echocardiography is recommended. Owners should be educated about signs of CHF and the importance of maintaining a healthy body weight.

Preclinical Disease with Cardiomegaly (Stage B2)

Pimobendan (Vetmedin) is the cornerstone of therapy. The EPIC trial demonstrated that pimobendan delays the onset of CHF and extends survival in dogs with Stage B2 MMVD [6]. The target trial emulation study using primary-care data confirmed these findings, showing a 5-year CHF cumulative incidence of 34.1% in dogs prescribed pimobendan versus 56.3% in those not prescribed the drug [6].

Congestive Heart Failure (Stage C)

Acute management includes:

• Diuretics: Furosemide is the first-line diuretic for pulmonary edema
• Pimobendan: Positive inotrope and vasodilator
• ACE inhibitors: Enalapril or benazepril for afterload reduction
• Oxygen therapy: For hypoxemic patients
• Thoracocentesis: If pleural effusion is present

Chronic management involves:

• Furosemide: Titrated to the lowest effective dose
• Pimobendan: Continued long-term
• ACE inhibitors: Continued for neurohormonal modulation
• Spironolactone: Adjunctive diuretic with aldosterone antagonist properties
• Dietary sodium restriction: Moderate restriction is recommended

Refractory Heart Failure (Stage D)

Management is palliative and may include:

• Higher doses or continuous rate infusion of furosemide
• Addition of other diuretics (hydrochlorothiazide, torsemide)
• Sildenafil for pulmonary hypertension
• Pimobendan continued
• Palliative thoracocentesis or abdominocentesis as needed

Surgical and Interventional Options

Patent Ductus Arteriosus: Transcatheter occlusion with an Amplatz Canine Ductal Occluder is the standard of care. A case report described successful occlusion in a 4-month-old dog with unilateral pulmonary edema secondary to PDA [17].

Atrial Septal Defect: Device occlusion can be performed in selected cases with adequate rim tissue. In a case series, one of five dogs with ASD underwent successful device occlusion with a hybrid procedure [15].

Cardiac Neoplasia: Surgical resection is possible in rare cases. A 7-month-old Miniature Dachshund underwent successful simultaneous surgical repair of a cardiac myxoma and VSD, surviving 11 years postoperatively [20].

Heartworm Disease

Treatment involves adulticide therapy with melarsomine, along with macrocyclic lactone preventives. Surgical extraction of adult worms may be considered in severe cases.

Unsafe Home Remedies and Misconceptions

Several unsafe practices should be avoided:

• Hawthorn berry supplements: While some evidence suggests potential benefit, dosing is unregulated and interactions with cardiac medications are poorly understood.
• Garlic supplements: Can cause hemolytic anemia in dogs and has no proven cardiac benefit.
• Excessive salt restriction: In dogs without CHF, severe sodium restriction can lead to electrolyte imbalances and is not recommended.
• Withholding water: Some owners mistakenly restrict water to reduce coughing, which can lead to dehydration and worsen cardiac function.
• Over-the-counter diuretics: Dandelion leaf or other herbal diuretics are ineffective for CHF and delay appropriate veterinary care.

Prevention and Screening

Breed-Specific Screening

Certain breeds have a high prevalence of MMVD and should undergo regular cardiac screening:

• Cavalier King Charles Spaniel
• Dachshund
• Miniature Poodle
• Chihuahua [14]
• Cocker Spaniel
• Beagle

Heartworm Prevention

Year-round heartworm prevention with macrocyclic lactones (ivermectin, milbemycin, selamectin, moxidectin) is essential in endemic areas. The Companion Animal Parasite Council (CAPC) recommends year-round prevention in all regions of the United States.

Regular Veterinary Examinations

Annual physical examinations allow early detection of heart murmurs. For dogs over 6-7 years of age, especially small-breed dogs, biannual examinations are recommended.

Prognosis

The prognosis for dogs with heart murmurs varies widely depending on the underlying cause and stage at diagnosis.

MMVD: Dogs diagnosed in Stage B1 have a median survival time of several years. Dogs in Stage B2 that receive pimobendan have prolonged time to CHF onset [6]. Dogs in Stage C have a median survival time of 6-14 months with appropriate therapy.

Congenital heart disease: Dogs with small VSDs or mild pulmonic stenosis can have normal lifespans. Dogs with PDA that undergo successful occlusion have an excellent prognosis.

Heartworm disease: Prognosis depends on the severity of pulmonary vascular disease and the presence of right heart failure.

Infectious endocarditis: Guarded prognosis, with survival rates of 50-70% with aggressive therapy.

Clinical Reasoning: What a Murmur Does and Does Not Tell You

A heart murmur is an auscultatory finding, not a diagnosis. The clinical reasoning process begins the moment the veterinarian hears an abnormal sound. The first question is whether the murmur is pathologic or innocent. Innocent murmurs are typically soft (grade I-II/VI), systolic, and localized, often disappearing when the dog changes position or when heart rate slows. They are common in puppies, in dogs with fever, anemia, or excitement, and in thin-chested breeds. However, the presence of any murmur in a middle-aged or older dog, especially a small-breed dog, should raise suspicion for myxomatous mitral valve disease (MMVD) until proven otherwise.

The second step in clinical reasoning is to determine the murmur's timing. Systolic murmurs are far more common in dogs and are typically caused by mitral regurgitation (MMVD), tricuspid regurgitation, or ventricular septal defect. Diastolic murmurs are rare and suggest aortic or pulmonic regurgitation. Continuous murmurs are classic for patent ductus arteriosus. The point of maximum intensity (PMI) further refines the differential: a left apical PMI points to the mitral valve, a left base PMI to the aortic or pulmonic valve, and a right-sided PMI to the tricuspid valve or a ventricular septal defect.

The murmur grade provides a rough estimate of severity, but it is not perfectly correlated with disease stage. A grade III/VI murmur can be associated with mild or moderate regurgitation, while a grade II/VI murmur can occasionally be heard with severe disease if cardiac output is low. Conversely, a very loud murmur (grade V-VI/VI) almost always indicates significant hemodynamic disturbance. The configuration and quality of the murmur also carry meaning: a plateau-shaped holosystolic murmur is typical of mitral regurgitation, while a crescendo-decrescendo ejection murmur suggests outflow tract obstruction.

Veterinarians must also consider that a murmur may be an incidental finding in a dog with a more pressing non-cardiac illness. For example, a dog presenting for respiratory distress may have both pneumonia and a pre-existing heart murmur. The clinical reasoning process integrates the murmur with the entire history and physical examination to determine whether the heart is the primary problem or a secondary contributor.

Diagnostic Workflow: From Auscultation to Definitive Diagnosis

The diagnostic investigation of a heart murmur in dogs follows a stepwise, cost-conscious workflow that balances diagnostic yield with financial and practical constraints. The goal is to answer three questions: (1) What is the cause of the murmur? (2) How severe is the underlying disease? (3) Is treatment indicated now or in the near future?

Step 1: Recheck Auscultation and Basic Triage

Not every murmur requires immediate advanced imaging. In a young, asymptomatic puppy with a soft systolic murmur, a recheck examination in 4-6 weeks is reasonable. Many innocent murmurs resolve by 4-6 months of age as the cardiovascular system matures. If the murmur persists or intensifies, echocardiography is indicated.

In an adult or senior dog, any newly detected murmur warrants a minimum database: a thorough physical examination, blood pressure measurement, and thoracic radiographs. Systemic hypertension can cause or exacerbate murmurs and is a treatable comorbidity. Radiographs provide immediate information about cardiac size, pulmonary vasculature, and the presence of pulmonary edema.

Step 2: Thoracic Radiography and Vertebral Heart Score

Thoracic radiographs are the first-line imaging modality for evaluating a dog with a heart murmur. The vertebral heart score (VHS) is a quantitative, reproducible measure of cardiac size. A VHS of 10.5 or less is normal for most dogs; values above 11.5 suggest clinically relevant cardiomegaly. In one study, a VHS of 11.5 or greater was used as a threshold for initiating pimobendan therapy in dogs with preclinical MMVD [9].

Radiographs also reveal left atrial enlargement, which is a key predictor of progression to congestive heart failure. The loss of the caudal cardiac waist on the lateral view and the appearance of a double-wall sign on the dorsoventral view are classic findings. Pulmonary venous distension and interstitial or alveolar patterns indicate pulmonary edema.

Step 3: Echocardiography

Echocardiography is the gold standard for definitive diagnosis. It provides real-time, two-dimensional, and Doppler assessment of cardiac structure and function. In MMVD, echocardiography reveals thickened, prolapsing mitral valve leaflets, a regurgitant jet on color Doppler, and quantifiable chamber enlargement.

Key measurements include the left atrial-to-aortic ratio (LA:Ao), left ventricular internal diameter in diastole (LVIDd), and fractional shortening. An LA:Ao greater than 1.6 is considered enlarged in most dogs. The EPIC trial used an LA:Ao of 1.6 or greater and an LVIDd normalized to body weight as entry criteria for Stage B2 disease [6].

Echocardiography also rules out other causes of murmurs, such as dilated cardiomyopathy, pulmonic stenosis, and ventricular septal defect. In dogs with congenital heart disease, echocardiography guides the decision for interventional or surgical correction.

Step 4: Electrocardiography

A standard 6-lead ECG is indicated in dogs with murmurs, especially if there is a history of syncope, collapse, or palpitations. Atrial fibrillation is a common arrhythmia in dogs with advanced MMVD and severe left atrial enlargement. Ventricular premature complexes can occur in any form of heart disease and may contribute to syncope.

Ambulatory ECG (24-hour Holter monitoring) is reserved for dogs with intermittent clinical signs or suspected arrhythmias that are not captured on a brief in-hospital recording. Heart rate variability analysis, though primarily a research tool, has shown that dogs with mild mitral regurgitation have autonomic imbalance even before clinical signs develop [4].

Step 5: Cardiac Biomarkers

Cardiac biomarkers provide adjunctive information and can help guide the urgency of echocardiography. N-terminal pro-B-type natriuretic peptide (NT-proBNP) is released from ventricular myocytes in response to wall stretch. Concentrations greater than 1,100 pmol/L suggest clinically relevant cardiomegaly and increased risk of CHF [9]. Cardiac troponin I (cTnI) is a marker of myocardial injury and is elevated in myocarditis, ischemia, and severe heart disease.

The neutrophil-to-lymphocyte ratio (NLR) is an inexpensive, readily available biomarker that has been associated with disease severity. In one study, dogs with CHF secondary to MMVD had a significantly higher NLR (6.41) compared to dogs with preclinical MMVD (4.66) and healthy controls (3.95). Higher NLR was also associated with increased diuretic requirements and longer oxygen supplementation [7].

Step 6: Advanced Imaging and Emerging Technologies

Computed tomography angiography (CTA) is used for evaluating complex congenital heart disease, such as atrial septal defects, ventricular septal defects, and patent ductus arteriosus. Three-dimensional reconstruction with stereoscopic viewing can help assess suitability for device occlusion [15].

Machine learning algorithms for automated heart murmur detection and grading are an emerging technology. A recurrent neural network algorithm trained on electronic stethoscope recordings detected murmurs with 87.9% sensitivity and 81.7% specificity, and effectively differentiated between Stage B1 and B2 preclinical MMVD [2]. A deep learning-based algorithm using phonocardiogram data achieved 89.9% sensitivity, 92.7% specificity, and 90.9% accuracy for detecting heart murmurs [13]. These tools may eventually allow for remote screening and earlier detection of heart disease.

MicroRNA profiling is another promising area. A preliminary study showed that serum and plasma microRNA expression patterns could differentiate healthy dogs from those with MMVD (sensitivity 0.85, specificity 0.82, accuracy 0.83) and distinguish preclinical from clinical stages [10].

Evidence Limitations and Diagnostic Uncertainty

Veterinary cardiology, like all fields of medicine, operates under inherent uncertainty. The evidence base for managing heart murmurs in dogs has important limitations that clinicians and owners should understand.

Limitations of the Murmur Grade

The murmur grade is a subjective assessment that varies between observers. Even experienced veterinarians may disagree on the grade of a murmur, especially for soft murmurs (grade I-II/VI). The grade can also change with heart rate, body position, and the dog's level of excitement. A murmur that is grade II/VI at a routine wellness visit may be grade III/VI during a stressful examination.

Limitations of Radiography

Thoracic radiographs are operator-dependent and can be technically challenging in dyspneic or uncooperative dogs. The vertebral heart score has breed-specific variations; for example, deep-chested breeds like the Doberman Pinscher may have a higher normal VHS. Radiographs also cannot definitively distinguish between cardiogenic and non-cardiogenic pulmonary edema.

Limitations of Echocardiography

Echocardiography requires specialized training and equipment. Measurements such as LA:Ao and LVIDd have intra- and inter-observer variability. The presence of a regurgitant jet on color Doppler does not always correlate with the hemodynamic severity of the lesion. In dogs with tachycardia or arrhythmias, accurate measurements can be challenging.

Limitations of Clinical Trials

The EPIC trial, which established the benefit of pimobendan in Stage B2 MMVD, enrolled a specific population of dogs with defined echocardiographic criteria [6]. The results may not be generalizable to all dogs with MMVD, particularly those with atypical presentations or concurrent diseases. The target trial emulation study using VetCompass data confirmed the EPIC findings in a real-world population, but observational studies are subject to confounding by indication [6].

The Problem of Overdiagnosis

Not all heart murmurs require treatment. The detection of a soft murmur in an asymptomatic dog can lead to unnecessary anxiety, repeated testing, and even inappropriate medication. The ACVIM staging system is designed to prevent overtreatment by reserving therapy for dogs with documented cardiomegaly (Stage B2). However, the decision to initiate therapy is not always straightforward, and some dogs with borderline measurements may be treated or monitored based on clinician judgment.

Owner Observation and Preparation for a Veterinary Visit

Owners play a critical role in the early detection and monitoring of heart disease in dogs. Knowing what to observe and how to prepare for a veterinary visit can improve outcomes.

What Owners Should Observe at Home

Resting Respiratory Rate: This is the single most important parameter for owners to monitor. The normal resting respiratory rate in dogs is 15-30 breaths per minute. An increase above 40 breaths per minute at rest is a red flag for pulmonary edema. Owners should count breaths when the dog is sleeping or resting quietly, not after exercise or excitement.

Coughing: A cough associated with heart disease is often dry, hacking, and worse at night or after exercise. However, coughing can also be caused by respiratory disease, collapsing trachea, or kennel cough. Owners should note the timing, frequency, and character of the cough.

Exercise Tolerance: A dog that used to enjoy long walks but now tires easily or stops frequently may have reduced cardiac output. Owners should track any changes in activity level, willingness to play, or ability to climb stairs.

Syncope and Collapse: Fainting episodes are alarming and require immediate veterinary evaluation. Owners should note the circumstances (during exercise, after coughing, or at rest), duration, and recovery time.

Appetite and Weight: Dogs with advanced heart disease may lose weight due to reduced appetite or increased metabolic demands. Conversely, fluid retention can cause weight gain.

Abdominal Distension: A distended belly can indicate ascites from right-sided heart failure. Owners should look for a pear-shaped abdomen and note any difficulty breathing when lying down.

How to Prepare for a Veterinary Visit

Bring a Timeline: Write down when the murmur was first detected, any changes in clinical signs, and the results of previous tests. If possible, bring copies of prior radiographs, echocardiograms, and lab work.

Bring a Medication List: Include all current medications, supplements, and dosages. Note any recent changes or missed doses.

Bring a Video: A video of the dog coughing, breathing rapidly, or collapsing can be invaluable for the veterinarian. Smartphone videos are acceptable.

Prepare Questions: Owners should ask about the cause of the murmur, the stage of disease, the recommended treatment, the prognosis, and the signs of worsening disease. Understanding the ACVIM staging system can help owners participate in decision-making.

Plan for Follow-Up: Ask about the recommended recheck interval and what tests will be performed at each visit. Some dogs require echocardiography every 6-12 months, while others can be monitored with physical examination and radiographs.

Prevention and Early Detection

While not all heart murmurs can be prevented, several strategies can reduce the risk of progression and improve outcomes.

Breed-Specific Screening

Certain breeds have a high prevalence of MMVD and should undergo regular cardiac screening starting at 2-3 years of age. The Cavalier King Charles Spaniel is the most well-known example, with nearly all individuals developing MMVD by 10 years of age. Other at-risk breeds include the Dachshund, Miniature Poodle, Chihuahua [14], Cocker Spaniel, and Beagle.

Heartworm Prevention

Heartworm disease can cause murmurs and right heart failure. Year-round prevention with macrocyclic lactones is essential in endemic areas. The Companion Animal Parasite Council recommends year-round prevention in all regions of the United States.

Regular Veterinary Examinations

Annual physical examinations allow early detection of heart murmurs. For dogs over 6-7 years of age, especially small-breed dogs, biannual examinations are recommended. Early detection of a murmur allows for timely echocardiography and staging, which can delay the onset of CHF.

Lifestyle Modifications

Maintaining a healthy body weight reduces the hemodynamic burden on the heart. Obesity increases cardiac output and can exacerbate valvular regurgitation. Regular, moderate exercise is beneficial, but strenuous activity should be avoided in dogs with significant heart disease.

Dental Health

Periodontal disease is a source of chronic inflammation and bacteremia, which can contribute to infectious endocarditis. Regular dental cleanings and home dental care reduce this risk.

Prognosis and Long-Term Management

The prognosis for dogs with heart murmurs varies widely depending on the underlying cause and stage at diagnosis.

MMVD Prognosis

Dogs diagnosed in Stage B1 have a median survival time of several years, and many die of non-cardiac causes. Dogs in Stage B2 that receive pimobendan have a prolonged time to CHF onset. The EPIC trial showed that pimobendan reduced the risk of CHF or cardiac death by 33% [6]. Dogs in Stage C have a median survival time of 6-14 months with appropriate therapy, though some live longer.

Congenital Heart Disease Prognosis

Dogs with small ventricular septal defects or mild pulmonic stenosis can have normal lifespans. Dogs with patent ductus arteriosus that undergo successful occlusion have an excellent prognosis, with resolution of the murmur and normalization of cardiac size.

Heartworm Disease Prognosis

Prognosis depends on the severity of pulmonary vascular disease and the presence of right heart failure. Dogs with mild disease and no pulmonary hypertension have a good prognosis with adulticide therapy.

Infectious Endocarditis Prognosis

Infectious endocarditis carries a guarded prognosis, with survival rates of 50-70% with aggressive therapy. The prognosis is worse for dogs with aortic valve involvement, large vegetations, or congestive heart failure.

Long-Term Monitoring

Dogs with heart murmurs require lifelong monitoring. The frequency of recheck examinations depends on the stage of disease. Stage B1 dogs should be rechecked every 6-12 months. Stage B2 dogs should be rechecked every 3-6 months. Stage C and D dogs require more frequent monitoring, often every 1-3 months.

At each recheck, the veterinarian will perform a physical examination, assess clinical signs, and may repeat thoracic radiographs or echocardiography. Owners should continue to monitor resting respiratory rate and report any changes.

Special-Population Considerations

Puppies and Juvenile Dogs

Heart murmurs in puppies are common and often innocent. However, a loud or persistent murmur warrants investigation. Congenital heart disease is the primary concern. Patent ductus arteriosus, pulmonic stenosis, and ventricular septal defect are the most common congenital defects. Early diagnosis and intervention, when indicated, can prevent irreversible cardiac remodeling.

Senior Dogs

Heart murmurs in senior dogs are most often due to MMVD. However, concurrent diseases such as chronic kidney disease, hypertension, and hyperthyroidism can complicate management. Senior dogs may also have reduced tolerance for medications and anesthesia. A thorough geriatric assessment is essential before initiating cardiac therapy.

Brachycephalic Breeds

Brachycephalic breeds (e.g., Bulldogs, Pugs, French Bulldogs) have a high prevalence of congenital heart disease, particularly pulmonic stenosis. They are also prone to respiratory disease, which can mimic or exacerbate signs of heart failure. Differentiating between cardiac and respiratory causes of dyspnea can be challenging.

Large and Giant Breeds

Large and giant breeds (e.g., Doberman Pinschers, Great Danes, Irish Wolfhounds) are predisposed to dilated cardiomyopathy. The murmur in DCM is often soft and systolic, secondary to mitral regurgitation from annular dilation. Echocardiography is essential for diagnosis, as the murmur grade does not correlate well with disease severity.

Dogs with Concurrent Systemic Disease

Dogs with chronic kidney disease, hyperthyroidism, or hypertension may have murmurs that are exacerbated by their systemic condition. Managing the underlying disease can reduce the murmur intensity and improve cardiac function. For example, treating hypertension with amlodipine can reduce afterload and decrease mitral regurgitation severity.

Athletic and Working Dogs

Athletic and working dogs (e.g., agility dogs, hunting dogs, police dogs) have higher cardiac demands. A soft murmur that is asymptomatic in a sedentary dog may cause exercise intolerance in a working dog. A thorough cardiac evaluation is essential before clearing a working dog for duty.

Dogs Undergoing Anesthesia or Surgery

Dogs with heart murmurs are at increased risk for anesthetic complications. Preoperative cardiac evaluation, including echocardiography and ECG, is essential to assess the severity of heart disease and optimize anesthetic protocols. Dogs with well-controlled Stage B1 or B2 disease can safely undergo elective procedures with appropriate monitoring. Dogs with CHF should have elective procedures postponed until heart failure is stabilized.

Frequently Asked Questions

1. Can a heart murmur in dogs go away on its own?

Innocent or physiologic murmurs in puppies often resolve by 4-6 months of age as the cardiovascular system matures. However, murmurs caused by structural heart disease, such as MMVD or congenital defects, do not resolve spontaneously and typically progress over time.

2. What does a grade 4 heart murmur mean in dogs?

A grade 4/VI heart murmur is loud and radiates widely across the chest. It indicates significant turbulent blood flow, often associated with moderate to severe valvular regurgitation or a congenital shunt. Dogs with grade 4 murmurs require thorough cardiac evaluation, including echocardiography, to determine the underlying cause and stage of disease.

3. How long can a dog live with a heart murmur?

Survival depends on the cause and stage. Dogs with innocent murmurs or Stage B1 MMVD can live normal lifespans. Dogs with Stage B2 MMVD receiving pimobendan have a median survival of approximately 3-4 years from diagnosis. Dogs with CHF (Stage C) have a median survival of 6-14 months with appropriate therapy.

4. Is a heart murmur in dogs painful?

A heart murmur itself is not painful. However, the underlying heart disease can cause discomfort through symptoms such as coughing, difficulty breathing, and exercise intolerance. Dogs with CHF may experience respiratory distress, which is distressing and requires immediate veterinary attention.

5. Can a dog with a heart murmur have surgery?

Yes, but the anesthetic risk is increased. Preoperative cardiac evaluation, including echocardiography and ECG, is essential to assess the severity of heart disease and optimize anesthetic protocols. Dogs with well-controlled Stage B1 or B2 disease can safely undergo elective procedures with appropriate monitoring.

6. What is the difference between a heart murmur and an arrhythmia in dogs?

A heart murmur is an abnormal sound caused by turbulent blood flow, while an arrhythmia is an abnormal heart rhythm. They can occur independently or together. For example, dogs with advanced MMVD may develop atrial fibrillation (arrhythmia) in addition to a systolic murmur.

7. How often should a dog with a heart murmur be checked?

Dogs with Stage B1 MMVD should be rechecked every 6-12 months. Dogs with Stage B2 or C disease should be rechecked every 3-6 months, or more frequently if clinical signs change. Regular monitoring includes physical examination, thoracic radiography, and echocardiography.

8. Can diet help manage a heart murmur in dogs?

A balanced, high-quality diet is important for overall health, but no specific diet can reverse valvular disease. For dogs with CHF, moderate sodium restriction is recommended. Omega-3 fatty acid supplementation may have anti-inflammatory benefits. Always consult a veterinarian before making dietary changes.

Related Veterinary Guides

• Canine Congestive Heart Failure: Diagnosis and Management
• Myxomatous Mitral Valve Disease in Dogs: A Complete Guide
• Veterinary Cardiac Auscultation: A Practical Guide
• Canine Dilated Cardiomyopathy: Current Concepts
• Thoracic Radiography in Dogs: Vertebral Heart Score
• Heartworm Disease in Dogs: Prevention and Treatment

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