Learning that your dog or cat has a heart murmur can be surprising and frightening at the same time. Some of the heart murmurs are innocent and are not connected to the cardiac disease at all. Many of them, on the other hand, are present because of an actual heart problem. Early diagnosis and accurate treatment give your pet best chance to live a long and happy life, even with the cardiac disease.
What is a heart murmur?
Normal heartbeat makes two sounds like "lub-dup" when heart valves are opening and closing. Veterinarian uses stethoscope in order to evaluate normal heartbeat and possible abnormalities. Examination is called heart auscultation. Additional swishing/whooshing noises produced by turbulent blood flow within (or near) the heart are called heart murmurs.
Normally blood flows through the heart in a quiet, smooth manner (laminar flow). However, narrowing or other obstacles create turbulence, which is heard as a murmur:
- innocent murmurs are produced when structures within the heart vibrate as blood flows past them, when the blood is too thin (anemia) or even with excitement (when heart pumps faster and harder than normally). It is very common for young puppies, especially large breed, to develop an innocent heart murmur while they are growing rapidly. Puppy with an innocent heart murmur will usually outgrow it by about 4-6 months of age.
- malfunctioning heart valve (e.g., it doesn’t open or close properly). The most common murmurs in dogs are associated with leaky mitral valves.
- hole in the heart between two chambers or two arteries that are not normally connected
- narrowing (stenosis) within a chamber or vessel.
Heart murmurs can be present at birth (congenital) or develop later in life. Some of the heart murmurs are harmless (innocent), other are abnormal. An innocent heart murmur is not a sign of heart disease and doesn't need treatment. Abnormal heart murmurs require follow-up testing to determine the cause.
Types of heart murmurs
Special terms are used by veterinarians to describe results of a heart auscultation. Below you can find short explanation of what you can usually see in your pet’s chart after a vet visit and what it actually means.
Most of the times pathologic heart sounds are described by three separate features: timing, localization and grade. Type of the heart murmur can help differentiate between pathologies and direct diagnostic process.
First of all, heart murmurs are classified based on the timing of the murmur during cardiac cycle (cardiac cycle is a sequence of contraction and relaxation of the heart in order to pump blood throughout the body). There are three main types of murmurs:
- Systolic murmurs happen when the heart muscle contract.
- Diastolic murmurs happen when the heart muscle is relaxing in between beats.
- Continuous murmurs happen throughout your dog’s regular heartbeat cycle.
Second feature is a localization of the heart murmur called point of maximal intensity (PMI). PMI describes an area over the chest where a heart murmur is heard the loudest, eg. right/left and base/apex. Sometimes more precise descriptions can be found: MV (mitral valve), TV (tricuspid valve), AV (aortic valve) or PV (pulmonic valve), but it is vital to remember that localization of the murmur is approximate and accurate diagnosis of the cardiac pathology based on auscultation only is usually not possible.
Echocardiographic examination (cardiac ultrasound) is the main test used to determine the cause of a heart murmur. An echocardiogram uses ultrasound waves to create detailed images of the heart's structure and function. It can help identify abnormal heart valves and detect defects. Doppler ultrasound examination allows to see how blood flows through the heart, blood vessels and reveals the origin of the turbulence within the heart.
Heart Murmur Grades
Veterinarians use grading to help describe the severity of the abnormal heart sounds.
A scale of one to six is the one most commonly used (murmurs 1-6/6)
Grade I (1/6) - silent murmur, barely audible. Can be heard only in quiet environment after careful auscultation over a localised cardiac area. Clearly softer than heart sounds
Grade II (2/6) - low-intensity murmur, approximately equal to heart sounds. Can be heard consistently, but only when auscultating over the point of maximum intensity (PMI).
Grade III (3/6) - moderate-intensity murmur, clearly louder than heart sounds. Can be heard immediately when applying stethoscope. Most pathological murmurs are at least grade III.
Grade IV (4/6) - high-intensity murmur, very loud. Heard over several areas of auscultation. Murmur that radiates widely, often including opposite side of the chest without palpable precordial thrill.
Grade V (5/6) - high-intensity murmur with precordial thrill (meaning can be felt by holding a hand against the animal’s chest). Heard over several areas of auscultation. Murmur that radiates widely including opposite side of the chest
Grade VI (6/6) - most severe of the heart murmurs. High-intensity murmur with precordial thrill (meaning can be felt by holding a hand against animal’s chest). Heard over several areas of auscultation. Murmur that can be heard even with stethoscope lifted of the chest or even without using stethoscope.
How often pets have heart murmurs and what does it actually mean?
According to scientific studies physiologic (innocent) heart murmurs account for approx. 5-15% in dogs. These murmurs are mostly silent: low grade (I-II) systolic murmurs. Similar functional murmurs can occur in cats. Even up to 25% of healthy cats can have a silent, low grade (I-II/III) murmur.
Loud murmurs (grade III-VI) are very unlikely to be physiologic and should always be investigated. Also, persistent (heard more than on one or two occasions) low-grade murmurs should be examined by veterinary cardiologist, as they can be associated with underlying cardiac disease.
However loud murmurs are more likely to be abnormal, loudness of a heart murmur does not necessarily correlate with the severity of the problem.
More importantly: some of the heart diseases are silent. Lack of the heart murmur does not rule out cardiac disease completely.
Read more about cardiac diseases in dogs and cats here.
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If you suspect your pet may suffer from the heart disease it is best to get an opinion from veterinary cardiologist.
Frequent signs of the cardiac disease in dogs and cats are: increased respiratory rate, breathing difficulties, loss of consciousness, lowered activity level and cough.
It is not unusual that heart disease progresses silently, without any symptoms, until the disease is very advanced and difficult to control.