I'm sorry to read about your Agnes. She's a beautiful girl.
I've had a few cats with heart disease over the years, HCM ( hypertrophic cardiomyopathy) is the most common heart disease in cats. It can cause sudden death.
Monty, a lovely ginger boy, I had up until Sept of 2010.. he was diagnosed with a murmur at 3. I had brought him into work for a pre dental exam back in 2003. (I've worked in vet clinics off and on for 17 years.)
He was ultra-sounded and put on Fortekor for his early stage HCM. He was re ultra-sounded a few months later and showed improvement. He was on that drug for the next 7 years, he went longer than any other cats I had seen at various clinics with same disease. It's generally progressive over time.
Eventually I switched him to Lotensin, which is the human version of the vet product Fortekor, made by the same company, much less expensive. Your vet could call it into a pharmacy for you if this is the drug they are going to put Agnes on. I'm not sure if in the US, Lotensin is under a different name.
Eventually, one night, Monty had a blood clot. He was 100% paralyzed in his hind quarters and was screaming in agony.
I knew from other cats I had seen, that had this happen, not one had survived much time with any real quality of life.. they were heavily sedated, in a lot of pain and paralysed. I knew if he ever threw a clot, I would euthanise him ASAP and that's what I did that night at the emergency clinic.
The medication definitely extended his years, as shown by his ultrasounds. He had great life with no limitations over that time. He took his pill every night, no problems. And loved the Temptations he got afterwards as a reward. :)
I'm so sorry your kitty was given this diagnosis :(