The Respiratory System

By | July 8, 2015

Evaluation of the Respiratory System

Respiratory diseases are relatively common in puppies and kittens and can quickly result in life-threatening emergencies if not treated early. The most severe manifestation of respiratory disease is hypoxemia. Hypoxemia can result in clinical signs characterized by changes in respiratory rate, character, and effort. The respiratory rate may be increased or decreased, the depth may be impeded or exaggerated, and effort is often multiplied to the point that the puppy or kitten is weak and exhausted from the increased work effort. Mucous membrane color often ranges from cyanotic to pale. Kittens in severe respiratory distress often do not show the same degree of obvious difficulty as puppies. Tachypnea and less apparent increases in respiratory effort herald respiratory distress in kittens.

Abnormal Breathing Patterns

Care should be taken when assessing breathing patterns in puppies and kittens; their rates are more rapid and depth is shallower than those of adults. Respiratory distress and abnormal breathing patterns typically are identified in obstructive or restrictive respiratory diseases (). Obstructive respiratory diseases are characterized by upper or lower airway obstruction in which intraluminal, mural, or extramural lesions narrow the airway lumen. An altered breathing pattern of increased depth and slower rate reduces airway resistance and is generally observed in mild obstructive disease. Severe obstructive disease generally results in increased depth and rate.

Upper airway obstruction generally produces an increase in inspiratory effort, often accompanied by stertor (snoring or snorting sounds) or stridor (high-pitched wheezing sound), whereas lower airway obstruction causes an increase in expiratory effort. Restrictive respiratory diseases are characterized by respiratory distress and restricted expansion of the lungs that may be compensated for by a rapid, shallow breathing pattern. Open-mouth breathing is usually a sign of distress in puppies and kittens. Animals with nasal obstructive disease will necessarily breath through an open mouth.

Diagnostic Approach

A thorough history and physical examination are important parts of the data used to evaluate young animals with respiratory signs. Specific evaluation of the upper or lower respiratory tract may require one or more of the following procedures — nasal cavity: rhinoscopy, nasal swab for cytology or culture, nasal lavage, aspiration biopsy, or exploratory rhinotomy; pharynx and larynx: pharyngoscopy and laryngoscopy; trachea and bronchi: tracheobronchoscopy, transtracheal wash, and/or bronchoalveolar lavage; pleural cavity: thoracocentesis for fluid analysis, cytology, culture, or ultrasonography; mediastinum: ultrasonography or fine-needle aspiration for cytology; and pulmonary parenchyma: bronchoalveolar lavage, fine-needle aspiration for cytology and culture, or open lung biopsy. These procedures performed in puppies or kittens differ little from those performed in adults, and they are described elsewhere ().

The Upper Airway: Nasal Cavity, Paranasal Sinus, Nasopharyngeal, Pharyngeal, and Laryngeal Diseases

The Trachea and Major Bronchi

The Lower Airway: Small Bronchi and Lungs

The Pleural Cavity


Trauma is the most common cause of pneumothorax in the young dog or cat. Penetrating thoracic wall injuries from animal bites or projectiles, laceration of the tracheobronchial airway, blunt trauma to the thorax, rib fractures, alveolar rupture due to overzealous assisted positive pressure ventilation, and/or ruptured cystic lesions can all cause pneumothorax. Signs of pneumothorax may vary from mild polypnea to severe respiratory distress and cyanosis. Radio-graphically, retraction of lung lobes toward the hilum is noted; in the lateral view, the heart is separated from the sternum (). Pneumothorax in the puppy or kitten is managed in a similar way as in the adult.

Pleural Effusion

Pleural effusions are generally characterized as being transudate, modified transudate, exudate, or hemorrhage ().

Characteristics of the various types of pleural effusions are presented in Table Causes and Characteristics of Pleural Effusion. The most common causes of pleural effusion in puppies and kittens are pyothorax, cardiomyopathy, and the effusive form of feline infectious peritonitis. Thoracocentesis of pleural effusion is always required to characterize the fluid. Cytologic findings of the different types of pleural effusions are presented in Table Causes and Characteristics of Pleural Effusion. Specimens of pleural fluid that have the cytologic characteristics of an exudate should be cultured for aerobic and anaerobic bacteria and tested for antimicrobial sensitivity. Management of pleural effusion in puppies and kittens is similar to that in adults.

Causes and Characteristics of Pleural Effusion


  • Causes: Hypoalbuminemia, right heart failure
  • Characteristics: Colorless, clear. Protein content <2.5 g/dl. Nucleated cells <500 cells/µl

Modified Transudate

  • Causes: Persistent transudation with added cells and protein content, lymphatic obstruction, chylous and pseudochylous effusions
  • Characteristics: Appearance variable according to the cause. Protein content generally >2.5 g/dl. Nucleated cells generally >500 celis/µl. In chylous effusions, lymphocytes and/or neutrophils are the predominant cell types; the triglyceride content of the effusion is greater than that of serum


  • Causes: Increased vascular permeability and inflammation
  • Characteristics: Turbid. Protein content generally >3 g/dl. Nucleated cells >3000 cells/µl and predominandy neutrophils. Neutrophils may be nondegenerate in nonseptic inflammatory processes or degenerate in septic processes


  • Causes: Usually trauma, bleeding disorders
  • Characteristics: Recent hemorrhage: bright red. Protein content and nucleated cell numbers are less than those of peripheral blood. Cytologically, intact red blood cells and white blood cells are in morphologic condition and distribution to those of peripheral blood. Longstanding hemorrhage: buff red. Protein content and nucleated cell numbers are less than those of peripheral blood. Cytologically, red blood cells and white blood cells are distorted, and hypersegmented neutrophils and phagocytized red blood cells are seen

The Diaphragm

Congenital Diaphragmatic Hemia

Two types of congenital diaphragmatic hernias are seen in young dogs and cats — peritoneopericardial and pleuroperitoneal diaphragmatic hernias.

Congenital Peritoneopericardial Hemia

Congenital peritoneopericardial hernias are much more common in the dog and cat than are congenital pleuroperitoneal hernias (). A ventral diaphragmatic defect allows abdominal viscera into the pericardial sac. The defect is developmental but does not appear to be inherited. The defect has been reported in the collie and cocker spaniel to affect multiple animals from a single litter. Other congenital defects, including sternal defects, cranial midline abdominal wall hernia, umbilical hernia, abnormal swirling hair along the ventrum, cardiac defects, congenital portosystemic vascular shunt, and pulmonary vascular disease may be seen in conjunction with peritoneopericardial hernia. Signs depend somewhat on the organ(s) that protrude through the diaphragmatic defect and the amount of displaced tissue contained in the hernia. Gastrointestinal, cardiovascular, or respiratory signs may be noted (). Liver incarceration or a concurrent congenital portosystemic vascular shunt may cause encephalopathic signs. In addition, there may be a displacement of the apex beat of the heart, and intestinal sounds may be auscultated in the thorax. Gastrointestinal signs of vomiting, anorexia, and/or diarrhea are seen most commonly (). Diagnosis of peritoneopericardial hernia can usually be made radiographically or ultrasonographically (). The cardiac silhouette is usually enlarged and appears round or ovoid. The density of the cardiac silhouette may be nonhomogeneous owing to the presence of soft tissue, fat, and/or gas densities. Pericardial ultrasonography is useful in differentiating a peritoneopericardial hernia from pericardial effusion or cardiomegaly when it is not obvious from the radiographs (). Treatment for congenital peritoneopericardial diaphragmatic hernia is surgical, especially if the animal is showing signs of the hernia ().

Congenital Pleuroperitoneal Hemia

Congenital defects in the diaphragm resulting in pleuroperitoneal hernia are uncommon in dogs and cats. When present, the defect is usually in the dorsolateral part of the diaphragm and may result in stillbirth or death shortly after birth, especially if the defect is on the left side. This form of hernia is likely to be inherited as an autosomal recessive trait ().

The Thoracic Wall

Congenital Disorders


Deformities in the number, position, and shape of the ribs do occur but usually are of little if any significance to health (). If the deformity does cause a restriction in ventilation, surgical correction may be indicated. Surgery is also occasionally performed for cosmetic reasons.


Pectus excavatum results from intrusion of the sternum into the thorax. This disorder has been identified in both puppies and kittens (). Other signs that may be present in addition to the sternal malformations are uncommon; they include poor growth rate, respiratory distress, and exercise intolerance. Radiographically, the ventral ends of the ribs are turned medially to join the dorsally displaced sternebrae. Mild cases require no treatment. The reader is referred elsewhere for treatment of more severe cases ().


Rib and sternebra fractures are usually the result of blunt trauma. The flexibility of the thoracic wall in young animals is highly protective against such fractures; nevertheless, these fractures, with or without intrathoracic injuries, do occasionally occur. Accompanying injuries may include pulmonary contusion, intercostal artery laceration, pulmonary parenchymal and diaphragm laceration, and possibly concurrent laceration of abdominal structures (). Intrathoracic injuries may also occur in the absence of rib or sternebra fractures. Signs vary depending on the degree of respiratory distress, severity of the fracture(s), and accompanying intrathoracic lesions. Rib and sternebra fractures can usually be identified by thorough palpation and confirmed radiographically. Rib and sternebra fractures that do not interfere with the overall integrity of the thoracic wall are usually left to heal undisturbed. If shock and/or intrathoracic injuries are present, appropriate medical and surgical treatment is indicated. A grossly unstable thoracic wall that is causing severe respiratory compromise is best managed surgically ().

The Mediastinum


Mediastinitis can result from a tracheal or esophageal rupture or from extension of an infection from an adjacent area (). If a mediastinal infection is present, it may extend into the fascial planes of the neck. Fever, thoracic pain, restricted breathing pattern, and, less commonly, regurgitation and/or coughing may be present, depending somewhat on the origin of the mediastinitis. Radiographically, mediastinitis is usually identified by a diffuse widening of the mediastinum (). Pneumomediastinum and/or pneumothorax may also be evident. Treatment of mediastinitis is directed toward correction of the underlying cause and supportive care.

Mediastinal Masses

Mediastinal masses are characterized by their location in the mediastinum, that is, involving the cranial, middle, or caudal portion of the mediastinum. Craniodorsal mediastinal masses tend to depress the trachea ventrally and include esophageal dilations such as those caused by megaesophagus, vascular ring anomalies, and esophageal foreign bodies. Thymic lymphosarcoma is the most common mediastinal mass in the young cat and tends to elevate the trachea. Signs associated with mediastinal masses depend on the region of the mediastinum involved and tissue of origin. Signs may include coughing, respiratory distress, restricted or obstructed breathing pattern, exercise intolerance, regurgitation, neck and forelimb edema, Horner’s syndrome, changes in phonation, and fever (). Inability to compress the thoracic wall cranial to the heart in a young cat is suggestive of thymic lymphosarcoma. Granulomatous masses of the cranial mediastinum in cats younger than 6 months have also been reported ().

Survey thoracic radiographs usually reveal the presence of mediastinal masses and their location (). To identify the cellular or tissue origin of a mediastinal mass, a transthoracic aspiration for cytology or ultrasound-guided percutaneous biopsy is required. If pleural fluid is present, removal of the fluid may be required before thoracic radiographs can identify a mediastinal mass. Cytologic analysis of the fluid may yield a definitive diagnosis, particularly if the mass is caused by lymphosarcoma. Treatment of mediastinal masses is directed at the underlying cause and supportive care.


Selections from the book: “Veterinary pediatrics: dogs and cats from birth to six months”. Johnny D. Hoskins. (2001)