Let's review possible causes of lethargy, when it is common and when it is not common and the cat should see the vet.
Lethargy can be due to the difficulty of HCM-difficulty getting oxygen to the body from the lungs into the blood stream when the heart cannot pump enough blood due to swelling and heart failure.
Lethargy is also due to a lack of electrolytes-a lack of potassium, sodium, glucose, magnesium, potassium, calcium, phosphate, chloride, and bicarb-caused by the necessary use of diuretics which, when pulling fluid from the body, takes the electrolytes with the fluid. Low electrolytes leads to low heart rate, low blood pressure, a lack of what makes the heart beat and the kidneys work, etc. This makes one feel weak and faint. A cat owner can supplement electrolytes by giving the cat a potassium supplement (Renal K powder or gel, or human potassium pills such as Nature Made or Sundown), or some mashed banana or sweet potato-or baby food banana or sweet potato-and by giving sweetened condensed milk or corn syrup 1/8 tsp cut with 2 oz of water, added to food or fed by mouth a couple times a day. Sodium should not be given unless a cat has been vomiting or had diarrhea a few times a day for more than two days.
Lethargy is due to dehydration, caused by the necessity of diuretics. Dehydration lowers the heart rate and blood pressure, making one feel weak and faint. A cat owner can give water by mouth using a 3ml syringe (gently give 9mls or more this way), by adding it to wet food, and by providing many bowls of fresh water to encourage intake of fluid.
Lethargy can be caused by the use of medications for heart disease. These meds lower the heart rate and blood pressure in order to help the heart and may make one feel weak and faint. The cat may need time to get used to the meds but a cat owner can discuss the possibility of adjusting meds with the vet.
Lethargy can be caused by any number of issues or complications of heart disease, especially CHF-congestive heart failure-or the buildup of fluid in or around the lungs, making breathing difficult and painful.
Lethargy can be caused by any procedure or vet visit that has worn the cat out. The cat may take a day or two to recover but should be eating enough and using the litter box.
Lethargy can be caused by a lack of intake of food-a lack of nutrients and electrolytes found in foods. If a cat is not eating enough, hand feeding may be necessary. Hills AD is a good choice. It's high in calories and blends well with little water, and can easily be syringed up into a 3ml syringe and fed by mouth to the cat. Stimulants can also be used. Cyproheptadine, an antihistamine, increases appetite but does not wind up a cat. 1/4 tab once a day may be enough to help the cat eat enough. Mirtazapine is another stimulant which I use when the cat is terribly ill and cypro is not strong enough to spur eating. 1/4 of a tab once or twice a day may be needed. Further hand feeding may be necessary if the cat is very run down. Try using baby sweet potato for potassium, corn syrup in water for glucose, protein from tuna juice or homemade beef/chicken broth (plain-no salt or spices) or peanut butter are good protein choices. These should be fed a few times a day, giving the cat about 9-15mls at each feeding.
Whenever your HCM cat is lethargic, note it in your notebook; track what the cat eats, if the cat is moving about; track when the cat had meds and if you hand fed. Then if the cat is not improved after a day, take the cat to the vet and review your notes with the vet. Have the cat examined and blood work drawn to test for electrolyte levels, dehydration, kidney and liver values, and overall health.
