Heart patients are often on lasix which can lead to dehydration. Being dehydrated can lead to a myriad of symptoms and other very serious issues and affect kidneys, liver, balance, ability to eat and hold down food, energy levels, etc. For pets, blood work would show dehydration because all the blood chemistry values, especially for electrolytes, would be off (often too low.) The skin tent test is often a good indicator-pull skin up at scruff and if it falls immediately, the cat is hydrated. If it takes its time to fall, the cat is dehydrated.
Lasix leads to dehydration but it is a necessary medication. But that means that HCM cats need more water in their system but at levels that help them with electrolyte balance. Give your cat fresh water if your cat doesn't drink a lot of tap water. Purchase bottled water (not Evian but purified gallon jug water from the store) or use filtered water from home. Add about 1ml or about 1/4 or so of water to wet food and mix in. Feed the cat mostly wet food and less dry. You can also give water by mouth 1 to three times a day (whatever you determine is the need) with either an eye dropper (typically measuring at 1 ml) or a 3 ml syringe from the vet (and give however much you feel comfortable dosing.) Give it slowly by the side of the mouth so that the cat does not inhale or choke on the water.
Another sign of dehydration is constipation in cats. Cats need water in their colon to move stool. If your cat is often gassy or has trouble with bowel movements, the use of a fiber source (pumpkin or powdered sources such as Miralax or inulin) as well as water by mouth and in the food will help solve the issue.
Aside from lasix, other heart meds can decrease the body's electrolytes through various means (depends on the drug.) Electrolytes: sodium, potassium, magnesium, chloride, glucose, bicarb (and more)-need to be near normal or if low, need to be boosted. They are as important for our bodies as they are for our HCM cats. Any one could be low and cause weakness, fainting, vomiting, etc. You must make sure your cat has a low electrolyte before you try to boost it and make sure the cat is not also suffering from diabetes or kidney issues.
You can give a potassium supplement either in tablets (we use a human form in Sundown potassium gluconate and cut in 1/4 and give twice a day), a potassium made for cats (Tumil-k, Renal-k), or feed it a small part of a mashed banana (natural source of potassium) but NEVER the peel. Potassium helps regulate heart beat and helps with kidney function. If glucose is low, you could add a drop of corn syrup to a few ounces of water for the cat to drink. Club soda made of sodium bicarb could also be added if bicarb levels in the blood are low. Bicarb boosts oxygen levels in cells, helping with energy and kidney function. A good balanced vitamin made for cats will boost the body's system and replenish what is lost via heart meds.
Finally, the meds they take can upset systems. Certain cats might have upset stomachs and might need an antacid (but cannot take if on Plavix per FDA warning.) If you're not giving food to the cat before or after meds, give more food to help the stomach digest the meds. Perhaps amounts and timing of dosing could be changed which requires a discussion with the vet.
The body-theirs and ours-is one large functioning system. Any part that isn't functioning well throws off all other functioning parts. Just because our cats have a dangerous and ultimately fatal disease does not mean that we are to rely only on heart meds to help the heart. We've the entire rest of the body to keep in balance as well. A balanced, healthy body can help support heart function. For whatever reason, vets are often not focused on the rest of the body's function when there is a major disease involved. So, we must do that work. We must be aware of the other needs and be proactive and informed as best we can and give our cats what they need.