Heavy breathing may be caused by respiratory system problems
Upper airway obstruction can make deep breathing difficult. Causes may include narrowing or blockage of the nasal cavity or nostrils, functional abnormalities in the throat, polyps or tumors in the nasopharyngeal area, tracheal collapse or compression, or conditions such as acromegaly.
Diseases of the lower respiratory tract can also reduce the efficiency of gas exchange in the lungs. These include pulmonary edema, pulmonary hemorrhage, pneumonia, interstitial fibrosis, bronchiectasis, emphysema, lung lobe torsion, primary or metastatic lung tumors, and asthma-like disease. Pleural cavity disorders such as pleural effusion, pneumothorax, mediastinal tumors, or diaphragmatic hernia may also lead to heavy breathing.
Heavy breathing may also be caused by heart dysfunction
Poor heart function is another possible cause of labored breathing. This may include heartworm disease, pulmonary embolism, or abnormal development of the heart and pulmonary vessels. Left-sided congestive heart failure, such as mitral valve insufficiency, congenital shunts from left to right, cardiomyopathy, or atrial fibrillation, may all contribute. Problems with inadequate cardiac output, such as arrhythmia, subaortic or pulmonary stenosis, and pericardial tamponade, may also play a role. Right-to-left shunting conditions, such as patent ductus arteriosus or tetralogy of Fallot, can also affect breathing.
Heavy breathing may also be caused by blood disorders
If the blood's ability to carry oxygen is reduced, heavy breathing may follow. Examples include anemia, methemoglobinemia-like problems, or carbon monoxide poisoning. Because the blood carries too little oxygen, the dog must breathe more forcefully in order to obtain enough oxygen.
Heavy breathing after exercise or in hot weather can be normal
Not all heavy breathing is caused by disease. Under normal conditions, a dog that has just finished exercise will naturally breathe faster and more heavily. During summer, hot and stuffy weather also causes dogs to breathe harder in order to cool themselves. These situations are generally normal and breathing should return to normal once the dog calms down. However, if the condition has been present for a long time and gradually worsens over time, disease or poor heart function becomes much more likely, and the dog should be taken to a veterinarian as soon as possible.