One horse in our yard kept getting colic ,condition was poor no matter what we fed vet gave him a full check up and found a heart mummer the owner retired him .
Sorry your horse has not been too well, my 28yo tb had a stroke 5 yrs ago and has a heart murmur, her legs fill if she is in for any length of time, its the circulation thats the problem. I put some thermatex leg wraps on when she is in and it stops all filling. I try and keep her out 24/7 as much as possible and keep her moving about.
I very sadly lost my 29 year old pony 3 years ago through heart failure. If you me to pm you let me know. But I understand if you don't. Mine was however very sudden.