If your cat or dog has heart disease, keeping them at a healthy weight is extremely important. The more weight your pet is carrying, the harder your pet's heart has to work. Even though your pet has heart disease, regular mild to moderate exercise is helpful to keep them healthy and fit. Be sure to take it slow and don't let them overexert themselves. Taking water and treats along during a walk, especially during the summer can really help. Intense exercise can aggravate the heart and lungs and can be dangerous for your pet. Also with the summer heat, walk in the early morning and evening only.