A heart murmur is a biomarker for congenital heart disease (CHD) in puppies and kittens. Almost every dog or cat with this condition will have a murmur, but hearing what it’s telling you isn’t always easy.
Murmurs are either pathological or nonpathological. In puppies and kittens, pathological murmurs—which tend to be persistent, easy to localize, and harsh-sounding with mixed frequencies—are indicative of structural malformations of the heart. The most common congenital heart diseases in dogs are pulmonic stenosis, patent ductus arteriosus, and subaortic stenosis.
While young cats may have heart murmurs caused by CHD, the incidence is much less common than in dogs. Cats with congenital heart disease usually have ventricular septal defects (VSDs).
The presence of a murmur may or may not be an indication the animal has a serious problem. Nonpathological (i.e. innocent) murmurs are often physiological, meaning they are caused by a fever, stomach ache, fear, anxiety, or pain. Anemia, systemic diseases affecting the cardiovascular system such as hyperthyroidism or hypertension, and renal disease are other non-heart-related conditions that may result in development of a murmur.
True innocent murmurs have no apparent cause, says Simon Swift, MA, VetMB, CertSAC, Dip ECVIM-CA (cardiology), MRCVS, clinical associate professor of cardiology at University of Florida College of Veterinary Medicine. “There isn’t a physiological cause and there isn’t a pathological cause. There’s no reason why they still have a murmur,” he adds.
Murmur versus… murmur
In a puppy or kitten, any murmur is abnormal until it is shown not to be. Differential diagnosis begins with appropriate auscultation.
Both puppies and kittens can have innocent murmurs when they are young, usually less than 12 weeks old. Typically, innocent murmurs are located at the base of the heart. “They tend to be lower-grade, up to maybe two or three, and they occur when the heart is contracting during systole,” says Dr. Swift. Most innocent murmurs disappear by the time the animal is six months old, although some persist into adulthood.
Nonpathologic murmurs are usually heard in a single location on the left side. Some have a single high-pitched tone, often described as “musical.”
“Those go away over time, usually by 12 weeks of age,” says Mark Kittleson, DVM, PhD, DACVIM (cardiology). “That’s the big distinguishing characteristic. But certainly if it’s musical, that is always a benign murmur.”
Some soft murmurs are tricky, though. Dr. Kittleson gives the example of a dog with a large atrial septal defect (ASD). The patient could have a soft systolic murmur that sounds like an innocent murmur, but it doesn’t go away. Keep tabs on such murmurs to make sure they eventually disappear.
Subaortic stenosis and pulmonic stenosis present with a left basilar murmur that is systolic, Swift says. The patent ductus arteriosus (PDA) murmur is in the same region, but it has a continual machinery-like sound.
“The murmur of a PDA, although it’s very characteristic, is also very cranial. You’ve really got to push your stethoscope far forward to pick it up. It can be really localized and quite loud. Unless people listen in that cranial area, they often miss it, so we do see older dogs present with PDAs that have never had the murmur heard before. I encourage people to listen carefully in that basilar region.”
In cats, the presence of a gallop or arrhythmia in conjunction with a murmur may indicate significant heart disease, but a cat does not have to have a murmur or gallop to have this condition.
“Thoracic radiographs and an N-terminal (NT)-pro hormone BNP (NT-proBNP) may be useful screening tools in high-risk patients when no auscultation abnormalities exist and echocardiography is not readily available or feasible,” says Sarah Miller, DVM, DACVIM (cardiology).
Breeds that get innocent murmurs are often the ones that have congenital heart disease.
“If it’s a loud murmur or if there’s a thrill when we palpate the side of the chest, that is always going to be pathological,” Swift says. “The only way of diagnosing and assessing the severity is through an echocardiogram done by a boarded cardiologist.”
Auscultation should involve the left and right side of the chest, and should be done in a quiet location with the animal standing. Ask technicians and owners to limit talking and other noise in the room.
An assistant is often helpful to transiently close the mouth and occasionally occlude the nares temporarily. Auscultate young animals in the axillary area to avoid missing a continuous murmur, often heard best in that location, Dr. Miller says. In addition to murmurs, listen for extra heart sounds such as gallops, mid-systolic clicks, arrhythmias, and lung pathology (e.g. wheezes, crackles, and pops). Soft murmurs are usually heard best on the left side.
When it comes to stethoscopes, size matters. Using a huge stethoscope on a tiny kitten results in picking up extraneous noises that can interfere with listening for heart sounds. Pressing the stethoscope hard against the chest can initiate a murmur by causing blood flow to become turbulent. Hold it gently to the chest wall, says Stephen Ettinger, DVM, DACVIM (SAIM and cardiology).
A purring kitten can be difficult to auscultate. Try turning on a water faucet, Dr. Ettinger suggests, adding the sound of running water can make a cat stop purring.
In addition to murmur location, breed can be useful in formulating a differential diagnosis before obtaining an echocardiogram—the gold standard for disease diagnosis. Breed-associated congenital diseases are rarely seen in cats, but certain dog breeds may be predisposed to specific types of CHD.
Boxers, Newfoundlands, golden retrievers, and Rottweilers get aortic stenosis, Swift says, whereas bulldogs and French bulldogs, as well as smaller breeds such as Chihuahuas, frequently have pulmonic stenosis. Poodles are the poster children for PDA, which is also seen in Cavalier King Charles Spaniels, a breed better known for acquired early-onset mitral valve disease.
When to refer
Congenital heart disease is complex, and signs can be subtle.
“For example, an atrial septal defect is oftentimes quite difficult to diagnose, with false positives and false negatives,” Kittleson says. “It can look like there’s a hole there and there’s not. Or there can be a hole there and you can’t find it.”
When a murmur is determined through auscultation to be pathological, the next step is to identify and address its cause through an echocardiogram performed by a boarded veterinary cardiologist.
Besides auscultation abnormalities, other situations that warrant referral to a cardiologist are clinical signs of heart disease, such as coughing, labored breathing, syncope, cardiomegaly diagnosed on thoracic radiographs, elevated NT-proBNP, breed predisposition or familial history of heart disease or sudden death, or a positive cardiac genetic mutation test.
In locations where a cardiologist is unavailable, a mobile sonographer may perform an initial echocardiogram, with images viewed remotely by a board-certified cardiologist. That serves as a screening test to determine how urgent it is for the client to travel to see the specialist.
For clients with limited funds, referral to a cardiologist early on may help save them money. In most cases, for a puppy with a murmur, an electrocardiogram (ECG) and radiographs are not useful diagnostics, but an echocardiogram is, Swift says.
“That’s the only thing that will give them a diagnosis and give me the detail of how severe the disease is and whether we need to intervene.”
Depending on the condition, medication or surgical repair may be options to treat congenital heart disease.
Where medication is needed, avoid prescribing prematurely without an echocardiogram, Miller says. Without the necessary information about systolic function an echocardiogram provides, prescribing medications can be dangerous to the patient.
Cats also can be challenging to medicate consistently, she says. “They are sensitive to the multiple medications used to treat heart disease and can frequently experience side effects from treatment. Cats tend to be more fragile with their heart disease, and stress can push them over the edge into life-threatening situations.”
Minimally invasive procedures are available for congenital heart defects, such as patent ductus arteriosus, pulmonic and other types of valvular stenoses, as well as for cor triatriatum dexter, a rare congenital heart anomaly in which the right atrium is divided into two chambers by a membrane.
Animals with a PDA murmur should be treated as soon as possible, Swift says. “The sooner we close that patent ductus, the better the long-term outcome.”
A surgeon can tie off the PDA via a thoracotomy, or a cardiologist can insert a catheter through a small incision in the groin and block it with a device called an Amplatz canine duct occluder.
For pulmonic stenosis, a balloon valvuloplasty is generally successful in opening up the pulmonary valve. “For those who don’t respond well to ballooning, we can stent the pulmonary artery,” Swift says. “Sadly, for aortic stenosis we don’t have great treatment options, but at least we’ll know how severe the disease is and can give owners a better idea of what long-term outcomes are likely to be. For other congenital diseases, if we can intervene early, maybe we can get them to live a normal life.”