This week, we’ll jump into feline cardiology. I want to talk about one particular type of heart disease that is common in cats. We’ll do a quick review of the anatomy and then get to the specifics.
Last week, we looked at how the heart is divided into chambers, the atria and ventricles. The ventricles are the larger chambers on the ‘bottom’ of the heart. Ventricles are asymmetrical, with the right being skinnier and thinner-walled. The left is much bigger and has greater thickness to its walls. Between the left and right ventricle is a wall called the interventricular septum. The easiest way I know to illustrate the relationship between the two ventricles is to make a loose fist with your left hand, then wrap your right hand around your fist. The left fist is the thicker left ventricle, and the right hand is the thinner right ventricle. The image below also illustrates the two chambers cut in a different view.
When cats get heart disease, they usually get a type called Hypertrophic Cardiomyopathy. This means that the muscle walls of the ventricles thicken. They thicken so much that they aren’t able to stretch much at all, which means less blood can fill the chamber. The chamber also gets so small that not much blood can get into the ventricles, and the ventricles then can’t pump much blood out of the heart to the lungs and body. The analogy I like to use is to think about the ventricle like a small room in your house. If you started to place bricks on all 4 walls from floor to ceiling, and kept adding another layer over and over, the room would gradually get smaller and smaller. That’s how hypertrophic cardiomyopathy sets in. The images below shows how this looks.
HCM (hypertrophic cardiomyopathy) is a problem because it changes the flow of blood through the heart and body. When we talk about how much blood the heart pumps, we’re talking about “cardiac output”, or CO. The CO is determined by how much blood can fill the heart, and how fast the heart is beating. It’s just like the old formula for figuring out time and distance in a moving object. Rate times time equals distance, remember? Well, heart rate (HR) times stroke volume (SV) equals cardiac output (CO).
HR x SV = CO
Why is that important? Well, think about the ventricles having thicker and thicker walls, so less blood can get into them. The body needs a certain level of CO to function normally. The only way a heart with HCM can keep up the cardiac output is to beat faster. Another analogy that I hope is helpful: You have to empty a bathtub in ten minutes, using either a bucket or a teacup. The big bucket would move more water with each pass, so you wouldn’t have to move very fast to get the tub empty in the given time. You could also get the tub empty with the teacup, but you’d have to move really fast and throw tons of little teacups of water out to do it. An HCM heart is the teacup.
The other big problem with an HCM heart is that all of these changes in stroke volume and heart rate and cardiac output screw up the plumbing. Remember that the right side of the heart goes to the lungs, and it’s a low-pressure system. The left is a high-pressure system sending blood to the arteries and the body. When the left side of the heart isn’t able to accept as much blood with each stroke, pressure builds up in the structure just before the left ventricle — the left atrium. And when the left atrium is backed up with blood, the structure before it also gets backed up — the lungs. When the vessels in the lungs are too high in pressure/volume, some of the fluid in the blood leaks out. This same concept can be applied to the right side of the heart, with the right atrium getting flooded, and then the vessels in the body getting overfilled and becoming higher-pressure.
We call this backup of blood and pressure congestive heart failure. HCM hearts in cats generally have more trouble on the left side, which means the lungs are the structure that suffers the consequences. The parts of the lung normally filled with air can fill with blood fluid – pulmonary edema; or the space around the lungs and heart fills up with fluid – pulmonary effusion. The second is more common, with the cat’s chest filling up with fluid.
This first x-ray shows a normal cat chest. Air-filled areas look darker black on the x-ray. I’ve outlined the structures for you in the second image. Notice how the lungs fully stretch out to the full extent of the chest cavity.
Now have a look at these images. You can see that the heart is not visible, and the most of the chest looks white. The lungs are the little bunch of leaves in the upper right. That white stuff is all fluid, and it has caused the lungs to be compressed. The lungs can’t expand well enough to do their job.
Cats that come in with a chest full of fluid are in life-threatening trouble. They often pant or open-mouth breathe. They may start to turn blue, too, from lack of oxygen. We usually have to insert a needle into the chest cavity to suck that fluid out so the lungs can expand again. If the fluid is in the lungs themselves, we have to use medications like diuretics to try to get it to clear up.
The only way to really, truly diagnose HCM in a cat is with an ultrasound. We might hear an abnormal heart when we listen with a stethoscope, or a cat might show up already in heart failure with a chest full of fluid, but we need to see those ventricle walls to know if they’re too thick or not. Ultrasounds allow us to take some very specific measurements, and they tell us how badly a particular cat is affected.
Medications can be given to help treat HCM. We use drugs to make the ventricle walls relax a little more so they can stretch and the ventricle can move more blood with each heartbeat. Diuretics are used to keep fluid out of the lungs/chest. Some cats also have high blood pressure tied to the HCM, so we treat that with drugs to lower blood pressure. Some cats also get beta blockers to help with high heart rate.
Some cats with HCM can live a very long time. Others go into heart failure and don’t survive very long at all, even with aggressive medication use. We never know with a given patient how things are going to go, so we usually advise at least trying to get some improvement.
The signs that you can watch for at home include: less activity than normal, faster breathing rate than normal, hiding, not eating well, and vomiting. Cats usually do NOT cough when they have heart trouble. On a physical exam, if we hear a murmur, that is a big flag that the cat may have heart disease and should get an ultrasound of the heart done.
Well, I think I’ve rambled on more than enough for this week! Next week we’ll talk about the major kind of heart disease that dogs get. Thanks for reading!