Feline Hypertrophic Cardiomyopathy
Until recently, cats presented with two types of cardiomyopathy:
Hypertrophic cardiomyopathy (HCM) is now the most common form of heart disease seen in cats. It used to be equal or second to Feline Dilated Cardiomyopathy which is now uncommon since the addition of Taurine to commercial cat foods a number of years ago.
Basically, the heart appears normal in size on x-rays, but it is actually thicker than normal. The chambers inside the heart get smaller and smaller, meaning less room to fill up fully and therefore, less blood gets pumped out of the heart.
Cats are very good at disguising respiratory and cardiac illness. It’s not until they are down to their last 25% of lungs that they start to show shortness of breath or other symptoms. A panting cat sets off alarm bells in a vet’s mind.
- Panting when stressed or on a hot day
- Sudden collapse and pain due to blood clots (thrombus) - see below
Blood clots (thrombus) formation
Unfortunately, some cats develop blood clots inside the dilated small chambers of the heart (atria) seen with this condition. These clots may go un-noticed until they suddenly leave the heart and cause a blockage (embolus) in one of several possible locations:
Main arteries to the hind legs
- Cats are suddenly unable to walk, have cold hind legs and are in a lot of pain
- Cats present in acute severe renal failure and are in a lot of pain
Right forelimb arteries
- Cats suddenly have a limp cool foreleg which sometimes never regains full function
Intestinal blood supply
- Cats are in extreme pain and part of their intestines is without a blood supply
If a cat is in heart failure, we treat it with drugs to remove fluid from the lungs (diuretics e.g. Lasix).
Because the heart is pumping rapidly and has only a small chamber to fill up in between beats, we give drugs to slow the heart down (beta-blockers or calcium channel blockers) which gives the following advantages:
- The smaller chambers have more time to fill up before being "ordered' to make the next contraction
- This means more oxygen rich blood leaves the heart with each beat to supply vital organs
- Also, more oxygen poor blood can get into the heart in between beats that is returning from the body
- The thick heart walls relax, and therefore the tiny oxygen carrying arteries supplying oxygen to the heart deep in their walls are not "pinched off" by tense muscular walls surrounding them (decreased wall tension-pre load).
All cats should be blood tested for hyperthyroidism and high blood pressure as these conditions can cause secondary hypertrophic cardiomyopathy and occur at any age. A normal result should be repeated at a later stage as it may be just "around the corner" so to speak. If diagnosed with hyperthyroidism or high blood pressure, we treat it.
Until recently it was thought that giving Aspirin to cats would prevent blood clots from forming. However, recent evidence has shown that Aspirin does not work in cats.
A newer medicaiton, Clopidogrel (Plavix) has been shown to have anti-clotting capabilites when combined with aspirin. Due to it's bitter tatse, it is usually made up in capsules with a low dose of Apsirin and given once a day.
Prof Mark Kittleson, in a recent paper, has also suggested that because cats sleep 75% of the time, they don't really put their diseased hearts "through the paces" except for the odd trip to the vet or being chased by a dog. He believes that the use of calcium channel and beta blockers makes no difference to the long term outcome, and has since stopped recommending it as a treatment for cats not showing signs of heart failure when resting at home.