Difficulty breathing—which is also called dyspnea—is one of the most common signs seen in animals with heart disease. Unlike humans, animals cannot complain that they are having difficulty breathing, and so recognizing this sign requires close observation of breathing rate and effort. Difficulty breathing is the result of the animal trying to get enough oxygen and, as a result, the owner will notice a more rapid or forceful nature of the breathing pattern. A dog with difficulty breathing may sit or stand with its legs in a wide stance, sometimes with mouth open, and the neck is frequently outstretched. Cats with difficulty breathing may act in a similar fashion or they may simply sit quietly or they may seek out quiet places like in a closet or under the bed.
Most dogs and cats with difficulty breathing will show the most effort when they are breathing air in (inspiration), while others may also have difficulty breathing out (expiration). Some animals will take rapid, deep breaths, and in dogs this must be differentiated from normal panting—which is rapid, shallow breaths. Exercise can cause difficulty breathing in an animal with heart disease but this must be differentiated from overexertion on a hot day, obesity or other issues such as lung disease. Occasionally, dogs will develop difficulty breathing (or coughing) when lying down or asleep. This is a common finding in dogs with severe heart disease as a result of fluid building up in or around the lungs (see Congestive Heart Failure). Many dogs with severe difficulty breathing will actually refuse to lie down because it is harder for them to breathe in this position, causing them to become distressed. Instead, they will prefer to sit or stand and an owner might even observe them trying to fall asleep in these positions.
Difficulty breathing is almost always an emergency situation and veterinary care should be sought immediately.