Animals with heart disease may become easily tired or weak, and seem to generally have a limited tolerance for activity. Since there is no cure for most forms of heart disease in dogs and cats, the goal is to allow them to exercise enough to enjoy themselves, but no do so much exercise that they get into trouble. Short walks are tolerated by most dogs with heart disease, but excessive activity can worsen heart failure or trigger irregular heartbeats (see cardiac arrhythmias).
For dogs that usually pull on the leash or walk in front of their owners, if they slow down and are now walking along side you then this might be enough (or too much) activity. If dogs fall behind on the leash when going for a walk, or if they sit down and need to rest, then this was clearly too much activity and a shorter walk should be taken in the future. Animals with significant heart disease should not be pushed to exercise beyond their limits. In dogs with previously well controlled heart failure, the onset of reduced ability to exercise should trigger a recheck visit to your veterinarian to see whether changes in medication or diet can improve control of heart failure.