what to look for with a slightly enlarged heart

After our last annual checkup, the vet mentioned that our cat's heart was slightly enlarged on the ultrasound but the echo looked okay. He wants to monitor it closely. What specific symptoms should I be watching for at home that would mean I need to rush him back immediately?

Oh no, that is so scary! I am so sorry you are dealing with this uncertainty. My Coco has hip issues but I worry about her heart too since she is a Persian. I think the biggest thing to look for is breathing. If you see his sides moving really fast when he is just lying there, not after playing, that is a bad sign. I wrote down everything my vet told me in My cat is breathing fast, should I go to the emergency vet? because I panicked the last time and needed to double check if it was actually an emergency or just normal panting.

Lethargy is a big one. If he stops wanting to chase toys or climb up to his spot. My Brutus is a big guy so I notice when he doesn't want to move. But also check his gums. If they look pale or bluish, that's urgent. I think hiding is also a thing, see Brutus hides a lot, is he sick or just shy? for how I tell if it's just his personality vs illness.

Weight loss is subtle but important. Daisy is a picky eater so I have to be careful. I use a scale that measures in ounces to track small changes. You can see how I do it in Best way to track my cat's weight without a scale?. If he loses weight without you changing his food, that is a sign his body is working harder to keep up. Also, has your vet mentioned blood pressure? I think it is related. I asked about it here: Anyone actually check their cat's blood pressure at home?

This is making me so nervous for Pepper. She is only 1 but she is scared of everything. How do I know if she is lethargic or just shy? She sleeps a lot. Is that a bad sign? I am overwhelmed by all the things I need to watch for. Is there a specific time of day it gets worse?

Thanks for the info. I think I will keep a log of his breathing rate when he is sleeping. Duke is energetic so if he stops playing that will be obvious. I worry that if I start watching for every little thing I will stress him out. Do you think I should get a home monitor? I saw a thread about Side effects from heart meds my vet prescribed and it seems like a lot of people are on meds. Are we there yet?

You do not need a home monitor unless the vet says so! That is too much stress for the cat. Just watch the breathing. I found that counting breaths is hard if you are anxious, so I just look at the rhythm. If it is fast and shallow, call the vet. Also, if he starts vomiting or has diarrhea, that could be related to poor circulation. I hope he stays okay! Let us know how the next checkup goes.