What is a heart murmur, and what does it mean for your feline friend?
Cats are unique in many ways, ranging from their ability to spot something to play with from across the house to their unnerving cries when they are scared or upset. Cats also seem to be made of rubber. This is a joke that I proudly abuse daily in the clinic, likely to the chagrin of my technicians. However, cats can mesh their bodies into the most unusual shapes and postures. They can obtain positions that should be physically impossible. Furthermore, they have an extraordinary tolerance for pain and injury. A former professor of mine used to say that if you leave two cat bones in the same room they’ll heal together.
True to form, feline hearts retain the same level of uniqueness as the body in which they reside. They are small and often aren’t even as large as a chili pepper. They sit nestled inside the ribcage closer to the sternum than in dogs, and they beat much faster than ours or our pet dogs’ hearts. The heart rate can range from 140–220 beats/minute, and when stressed they can be even faster than this.
Like with other organs, the heart can be affected by a wide variety of diseases and other maladies. Heart murmurs are often the veterinarian’s first clue that something is amiss with the heart itself; though they don’t always indicate a problem.
First, what is a heart murmur? A heart murmur is simply a term used to describe the noise that is heard when blood does not flow perfectly smoothly through the chambers of the heart like it is supposed to. Murmurs are generally graded based on their intensity on a scale of I-VI with I being the quietest and VI being the loudest. The grade of the murmur does not necessarily correlate to the level of disease.
The first difficulty with auscultating (listening to) feline heart murmurs is that they can be induced by the simple act of listening. Recall that I mentioned earlier that cats are rubber. When listening to a feline heart it is possible to put slightly too much pressure on their chest during the process. Since they have very compliant ribcages this pressure can gently compress the right side of the heart and create turbulent blood flow. This will be heard as a heart murmur by the listener.
Additionally, cats can fairly easily develop a heart murmur secondary to the stress of veterinary visits. Significant sympathetic nervous system stimulation (think fight or flight response) can cause cats to develop a heart murmur due to the increased heart rate and blood flow through the heart. So, when listening to a very anxious or stressed out kitty we may hear a repeated murmur. This is called a physiologic heart murmur. Physiologic and iatrogenic (caused by the doctor listening) heart murmurs would almost never be associated with overt heart disease.
Generally speaking, when we hear a murmur in a cat it is secondary to turbulent flow between one of the four valves of the heart. Two valves separate the major chambers of the heart from one another. There is one on the right and one on the left.
Two additional valves serve as a separation between the heart and major blood vessels that provide blood to the body and lungs. Blood that doesn’t flow smoothly through any one of these valves will create a murmur that your veterinarian can hear.
Technically, cats can develop problems at any of these valves; however, the most common heart disease in cats does not directly affect the valves. Hypertrophic Cardiomyopathy (HCM) is the most common heart disease of cats and occurs through hypertrophy (excessive growth) of the heart muscle. If a murmur is heard with this condition, it is actually from changes to pressures within chambers of the heart secondary to this muscle growth. The higher relative pressure causes blood to have a more difficult time flowing as smoothly as it should. This creates the turbulence required to make a heart murmur that your veterinarian hears.
While there is a known genetic component in some cats, like Maine Coons, often the disease develops for reasons unknown. Less than 50% of cats that are affected by HCM will have a heart murmur. Furthermore, less than 30% of cats with heart murmurs actually have true cardiac disease.
One important cause of HCM worth mentioning specifically is hyperthyroidism. This disease occurs when the body produces too much thyroid hormone, and the excessive stimulation from the thyroid causes the heart to hypertrophy. Hyperthyroidism is a common illness in cats and there are many treatments for it. The good thing about this particular form of HCM is that it can often be improved once the thyroid disease is treated.
Ultimately, it is always a little nerve-racking to hear from your veterinarian that your cat has a heart murmur. The reality is that hearing a murmur often does not equate with disease, especially in cats. So, if it is the first time that you’ve heard a heart murmur in your cat, there is no reason to panic. Your veterinarian will talk you through the next steps to try to figure out what is best for you and your feline friend.
There are a variety of ways to work up heart murmurs. Generally, a bloodwork panel that includes thyroid testing, X-rays of the chest, and an echocardiogram are the diagnostics your veterinarian will discuss with you. An echocardiogram, or ultrasound of the heart, is the gold standard (recognized best) to evaluate where the murmur is coming from and what, if anything, needs to be done about it.
A final aspect of felinity that makes listening to the heart and lungs interesting is purring. This is often a humorous interference with the physical exam as the purr is far too loud to confidently hear the heart sounds. As such, we have developed several tricks to try to interrupt purring. Sometimes, simply touching the cat on the nose will stop the behavior. For other cats, a gentle grab of their scruff may disrupt things; though, sometimes they purr more. The scent of alcohol will often disrupt purring too, so placing a piece of tissue or gauze nearby with some alcohol on it can help. Finally, the most reliable technique that I’ve used is turning on a faucet. For some reason, the running water interrupts most cats from purring allowing me to finally listen to the heart!
I hope this article helps provide a little insight into feline heart murmurs and the difficulty with identifying the cause of them. I hope too, that it provides some comfort to you, as the owner because a heart murmur in your cat does not directly translate to them having any form of heart disease. Thanks for reading!