An animal with difficulty breathing will breathe more rapidly, more forcefully, use abdominal muscles more to breathe, and or breathe with an open mouth. In pets with heart disease, this can be a sign of a worsening fluid accumulation in or around the lungs (a condition known as Congestive Heart Failure).
In most dogs and cats the breathing rate, or number of breaths per minute, is usually less than 35 to 40 breaths per minute at rest; thus rapid breathing at rest can also mean fluid accumulation in the lungs and a need for medication adjustment. These signs may be worse after exercise or at night, and when the shortness of breath is worse even at rest then more furosemide, a diuretic, may be needed to help eliminate the excess fluid from the body. Ask your veterinarian whether it is alright to give an additional dose of furosemide at times of difficulty breathing.
Progressive difficulty breathing often indicates that the heart disease has progressed. Rather than exclusively relying upon more furosemide, worsening difficulty breathing should trigger a visit to your veterinarian in order to determine whether adjustments should be made to the doses or types of medications, or to the diet, in order to better control heart failure signs. Severe respiratory distress is almost always an emergency situation and veterinary care should be sought immediately.