Specific types of heart disease may be caused by a deficiency of taurine or carnitine. It is recommended by many veterinarians to test these levels. Although your cat's heart disease is not caused by a specific deficiency of taurine or carnitine, your cat may still benefit from supplementation.
Most veterinary prescription heart diets are supplemented with high level of taurine and carnitine to support healthy heart function.
If your cat is on diuretics (water pills), because of heart failure, certain electrolytes can become deficient. Very often it is necessary to replenish the loss of B vitamins and magnesium. Most veterinary prescription heart diets supplement these nutrients.
Supplements...
Giving supplemental omega 3 fatty acids (fish oils) to your cat with heart disease or heart failure can improve appetite and can help decrease production of negative inflammatory hormones.
The recommended daily dosage is around 65 mg per kilogram of body weight with an appropriate distribution of eicosapentanoic acid (EPA, 40mg/kg) and docosahexanoic acid (DHA, 25 mg/kg).
Other supplements frequently recommended for heart failure patients include Co-enzyme Q10 and vitamin E. Even if there is some logic as to why these supplements may be beneficial, there is no scientific proof.
Note: In clinical studies, dogs in heart failure taking omega 3 fatty acids have been proven to survive longer. Studies have also proven the benefits of additional taurine and carnitine in patients with heart failure.