Structure and Function of the Respiratory Tract in Dogs
Dr. Bari Spielman
Pulmonary or lung disorders often produce coughing, increased respiratory rates, respiratory distress, and exercise intolerance. When the functional capacity of the lungs is severely compromised, the animal is incapable of taking in enough oxygen. In such cases the dog may collapse and stop breathing. Disorders of the lungs include the following:
Pneumonia is an infection within the lungs. It is most often caused by bacteria, but may also arise with viral, fungal, protozoal, and parasitic infections. Pneumonia may also occur if food, fluid, medications, or vomitus is aspirated into the lungs.
Edema or fluid in the lungs may occur with a variety of conditions, including heart disease, electric cord shock, trauma, snake bite and other exposure to other toxins, smoke inhalation, near drowning, overdosage of intravenous fluids, and numerous other systemic diseases.
Cancer of the lungs may be primary or secondary (metastatic). Primary lung tumors are usually malignant and arise from either cells of the lungs or airways. They include squamous cell carcinomas, the bronchoalveolar carcinomas, and pulmonary adenocarcinomas. Many cancerous tumors in the body can spread to the lungs via the blood vessels. Such tumors include cancers of the kidneys, spleen, bones, skin, mammary glands, and soft tissues of the body.
Pulmonary contusions or bruises may develop when trauma causes hemorrhaging into the lungs. They are common after automobile accidents, dog bite wounds, falling from heights, and other forms of blunt chest trauma.
Diseases of the pleural cavity (the potential space between the lungs and chest wall) can also affect the function of the respiratory system. Normally there is only a tiny amount of fluid in this space, and the fluid serves to lubricate the lungs and to decrease friction between the lungs, the rib cage, and diaphragm. When fluid accumulates in the space, the lungs are compressed and cannot expand to their full amount, thereby decreasing the oxygenation capacity of the lungs. Diseases of the pleural cavity include the following:
Pleural effusion is the accumulation of fluid within the chest cavity. Such fluid may be composed of blood (hemothorax), lymph chyle (chylothorax), pus or purulent material (pyothorax), fluid from heart failure, twisting of a lung (lung lobe torsion), other systemic diseases, or cancer.
Pneumothorax is the accumulation of air in the chest cavity. It is often secondary to trauma with rupture or laceration of the lungs, trachea or lower airways, or from penetrating injuries to the chest. Air is also allowed into the chest cavity when the chest is surgically opened or penetrated during various medical procedures.
A diaphragmatic hernia occurs when there is laceration or rupture in the diaphragm. The diaphragm is the muscle that separates the abdomen from the chest cavity. When the diaphragm is torn, abdominal organs may slide into the chest cavity and compromise normal respiration. A diaphragmatic hernia develops most often from blunt trauma to the chest and abdomen, such as being struck by an automobile or from a fall. In some rare instances, animals may be born with a diaphragmatic hernia if the membranes that separate the chest cavity from the abdominal cavity do not form correctly.
What Types of Diagnostic Tests Are Used to Evaluate the Respiratory Tract?
Depending on the history and clinical signs, the following tests may be indicated to help make a diagnosis and design a proper treatment protocol.
Auscultation of the chest, which is listening to the heart and lungs through a stethoscope, is performed in the exam room. Diseases of the lower airways and lungs frequently cause abnormal sounds within the chest.
Evaluation of mucous membrane color is also usually performed. The color of the gums of the mouth provides valuable information on the blood circulating to the body tissues. If oxygen levels are low in the blood, then the gums appear blue or cyanotic. If the blood is anemic, the gums may appear pale. If carbon monoxide levels are high in the blood, the gums are usually very bright red. These changes allow indirect assessment of the gaseous exchange functions of the lungs.
Thorough physical examination of all visible components of the airway system, including the nostrils, front of the nose, nasopharynx, and opening of the larynx is indicated. Palpation of the trachea is also useful. Close examination of the rest of the body for signs of trauma, heart disease, cancer, and other diseases is also beneficial.
Evaluation of a complete blood count (CBC), biochemical profile and urinalysis may reveal changes suggestive of infection, inflammation, electrolyte and acid-base imbalances, and/or other organ involvement.
Specialized testing for infectious diseases and heartworm disease may also be recommended. The measurement of oxygen and carbon dioxide (also known as blood gas analysis) may be performed in blood samples from both arteries and veins.
Thoracic radiographs (chest X-rays) are very useful to evaluate the lungs, thoracic trachea, lower airways, heart, diaphragm and pleural cavity. They may detect evidence of pneumonia, edema, free air, foreign bodies, airway deformities or obstruction, tumors with the lungs or chest cavity, fluid within the chest, intestines within the chest, enlargement of the heart, and abnormalities of the esophagus.
An echocardiogram (ultrasound of the heart) should be considered in those cases where heart disease is suspected. Your veterinarian may refer your dog to an internal medicine specialist, a cardiologist, or a radiologist to have this test performed.
When fluid is present within the chest, an ultrasound may help identify its source and detect a diaphragmatic hernia.
Skull and/or nasal X-rays may be very helpful in diagnosing several causes of rhinitis and sinusitis including tumors, foreign bodies, trauma, pneumonia and cancer.
Computed tomography (CT scan) or magnetic resonance imaging (MRI) are advanced techniques that are very sensitive in diagnosing nasal disorders, sinus and upper airway disorders.
Rhinoscopy is a procedure that allows direct examination of the nasal passageway through a small rigid or flexible scope. Biopsies and samples for culture may be taken during this procedure. It is always performed with the animal under general anesthesia.
Rhinotomy is surgical exploration of the nose and is usually done when a definitive diagnosis has not been achieved with other less invasive techniques.
Bacterial cultures of the airway may be helpful in determining if there is a bacterial infection present and to define appropriate antibiotic therapy.
Cytology (microscopic analysis of fluid or cells retrieved from the respiratory system) may confirm fungal, cancerous or parasitic diseases . Samples may be retrieved during rhinoscopy, tracheoscopy, bronchoscopy, transtracheal washings, and aspiration of masses.
Tracheoscopy is evaluation of the trachea using a rigid or flexible scope, and bronchoscopy is examination of the bronchi. Both procedures may be helpful in cases of suspected tracheal or bronchial inflammation, infection, foreign bodies, tumors, etc.
A transtracheal wash is a procedure that involves the passage of a small catheter into the trachea and lower airway. It can be done without the use of expensive endoscopes, often under local anesthesia and light sedation. It facilitates the retrieval of cells from the trachea and lungs for analysis and culture.
A lung aspirate is another way to sample the lung tissue or a lung mass. It is done through the body wall with a small needle.
At times surgical exploration of the chest is also needed to define and/or correct problems that are occurring within the chest.