As for diuretic-REMEMBER-you need to stave off the possibility of the cat becoming congested by giving a daily maintenance dose of a diuretic like lasix. All HCM cats should be on a lasix. Once CHF-congestive heart failure-begins to occur, even after just one episode, the cat needs to be on larger doses more often during the day.
WHY? Because fluid in the lungs leads to a type of internal drowning; the physical sensation is painful. It's a painful form of suffocation.
The lasix dose could be 5mg three times a day, then 5mg four times a day, then 10 three times a day, or 10/5/10 or 10/5/10/5mg or...whatever it takes to keep control, to stay on top of congestion. As the cat's heart grows, as the heart condition worsens, congestion becomes more probable and more constant and you must fight it. NEVER be afraid to give extra diuretic when needed. I always advise to give an extra dose or 1/2 dose in between regular doses if the breathing rate has increased. You will do this as often as necessary until you see the rate return to normal. Yes, it can take a couple hours for the diuretic to seem to kick in. Give an extra dose, wait an hour, if the rate is declining, wait another hour. If it's still high and not declining any further, give another extra 1/2 or full dose and wait. If the rate has returned to normal after one extra dose, then just keep an eye on the cat. If the rate increases despite the extra doses, especially if the rate is a dramatic increase, then the cat needs the vet or vet ER immediately for oxygen and IV or injectable lasix.
BREATHING RATE-a normal cat at rest is 24 per minute or less sleeping; up to 30 while sitting around. While dreaming, the rate may fluctuate 24-32 but should decrease again soon. TO COUNT-one up and down chest movement is one count. How many of those in 15 seconds? Times that number by four for a minute count. So, one up and down in 15-if it's six then times by four =24. But if the cat is breathing an 8 or higher which is 32 per minute or higher, then the cat needs extra diuretic.
NEVER let the cat not be able to breathe at all. Never wait around for hours ignoring the distress the cat's in. Never wait for the vet if the cat's rate is up and not responding to the diuretic. Go to the local ER if you have one.