Heart Murmur in Dogs: What It Is, Grades, Causes and Which Tests to Run

A heart murmur is an extra sound the veterinarian hears through the stethoscope when blood flow inside the heart stops being silent and becomes turbulent. It is not a disease in itself but a sign that something in the heart deserves investigation. In dogs the most common finding behind a murmur is a change in the heart valves, and the right course of action is always defined by the veterinarian based on the physical exam and complementary tests.

How a murmur is graded: from I to VI

Murmurs are classified by intensity on a scale from I to VI, adapted from human cardiology and used in veterinary medicine for decades. Put simply: grade I is very faint, audible only in a quiet room and after careful listening; grade II is soft but already audible as soon as the stethoscope touches the chest; grade III is of moderate intensity and clearly audible; grade IV is loud with no palpable thrill; and grades V and VI are loud and accompanied by a precordial thrill, meaning the vibration can be felt with a hand on the chest.

It is worth keeping one idea in mind: the grade describes how loud the murmur sounds, not necessarily how severe the disease is. A loud murmur can exist in a still-compensated heart, and meaningful disease can present with a discreet murmur. That is why grading guides but does not replace imaging investigation. In the consensus guidelines, a murmur of intensity equal to or greater than III/VI is one of the criteria that reinforce the suspicion of cardiac remodeling, to be confirmed by tests.

Common causes by age and size

The most frequent cause of a murmur in adult and senior dogs is myxomatous mitral valve disease, also called chronic degenerative valve disease. It is the most common acquired heart disease in dogs: it is estimated that about 10% of dogs seen in routine practice have heart disease, and mitral valve disease accounts for roughly 75% of cardiac cases in dogs in North America. It mainly affects small-breed dogs under 20 kg, and prevalence rises sharply with age, with evidence of the valve lesion in up to 85% of small dogs by 13 years of age.

Breed carries significant weight. The Cavalier King Charles Spaniel is classically predisposed and develops the disease earlier, with progression that is at times faster than in other breeds. Dachshunds, Miniature and Toy Poodles and other small breeds also appear frequently. In puppies and young dogs a murmur may have another origin, such as congenital heart disease, and that changes the investigation. Reading age, size and breed helps raise hypotheses, but confirmation is up to the veterinarian.

Associated signs that deserve attention

Many dogs with a murmur remain asymptomatic for years, still in the preclinical phase of the disease. When the heart starts to lose its ability to compensate, signs appear that the owner can notice at home: cough, especially linked to exertion or to changes in the breathing pattern, tiring easily and reduced exercise tolerance, and fainting or collapse episodes, called syncope, which often happen during activity or while coughing.

An early and very useful sign is an increase in the resting or sleeping respiratory rate, which can indicate fluid building up in the lungs, the so-called pulmonary edema, the hallmark of congestive heart failure. Counting the breaths of a sleeping dog at home is a simple tool the owner can bring to the veterinarian. Any of these signs warrants evaluation, and the final interpretation is always clinical.

Which tests the veterinarian may request

Echocardiography, performed by an experienced operator, is the recommended test to definitively identify the cause of the murmur and measure the enlargement of the cardiac chambers. The consensus guidelines use normalized echocardiographic measurements to define disease stage, such as the left atrium to aorta ratio equal to or greater than 1.6 and the normalized left ventricular internal diameter in diastole equal to or greater than 1.7, signs of remodeling that guide the timing for starting treatment.

Thoracic radiography is recommended to assess the hemodynamic relevance of the disease, obtain a baseline study and help confirm pulmonary edema. Blood pressure measurement is recommended in all patients to identify or rule out concurrent systemic hypertension. The electrocardiogram, or ECG, does not diagnose the murmur itself, but it is essential when an arrhythmia is heard on auscultation: it characterizes rhythm and rate and helps investigate arrhythmias such as atrial fibrillation, more common in hearts with left atrial enlargement. In some cases blood tests such as NT-proBNP can add information. The choice and sequence of tests, as well as the diagnosis and management, are always the veterinarian's decision.

Connected technology supporting the diagnosis

When facing a murmur with suspected arrhythmia, a well-recorded ECG is a key part of the investigation. The INpulse ecosystem connects the INcardio X electrocardiograph to the INcloud cloud platform, allowing the tracing to be recorded at the clinic and, when the veterinarian finds it useful, forwarded for cardiology review. The diagnostic and therapeutic decision always remains with the responsible veterinarian; the technology exists to make the signal more accessible and follow-up more consistent over time.