Diagnostic Approach to Respiratory Disease
World Small Animal Veterinary Association World Congress Proceedings, 2008
Brendan McKiernan, DVM, DACVIM-SAIM
Southern Oregon Veterinary Specialty Center
Medford, OR, USA

The clinical signs of respiratory disease (what an owner complains about, what you detect on examination) represent the animal's expression of the functional changes that have resulted secondary to the particular disease. The basic understanding of respiratory physiology is very helpful (critical?) in the recognition of an animal's problem when it is suffering from a respiratory disease. It is the deviation from normal or more aptly the dys-function that alerts an owner that there is a problem.

It is the veterinarian's ability to collate and interpret an accurate history with a careful physical examination using a problem solving approach which will enable accurate and timely diagnostics and treatment. As there are few quantitative pulmonary function tests in veterinary medicine, the objective evaluation/quantification of respiratory disease may be difficult.

Signalment

Signalment, or the animal's description--is classically the age, breed and sex; it is a useful reminder of common differential diagnoses that might be encountered in a particular animal; a good example might be the 11yr old, male Black Lab with inspiratory noise and exercise intolerance (a good description of bilateral laryngeal paralysis based on signalment and history).

History

Many cases of respiratory distress are associated with trauma; the history and observations that an owner can provide can be very helpful in establishing a brief list of differentials.

Upper airway disorders typically result in airway obstruction (inability to breathe easily with mouth closed, stridor, snoring), excessive sneezing or reverse sneezing, excess nasal discharge.

Coughing is the hallmark of lower airway disease. The typical tracheobronchitis cough is the dry, hacking, non-productive cough; post-tussive gagging is common and owners misinterpret this as 'vomiting'. Pneumonia is associated with the moist, productive cough. Another difference is that tracheobronchial disease usually has few if any systemic signs (lethargy, anorexia, fever, depression). Coughing may occur at any time during the day but is common following exertion (+/- exercise intolerance), at night (nocturnal coughing) as secretions accumulate, or when the trachea is irritated--for instance with 'leash pulling'. Wheezing, breathing with an expiratory effort, cyanosis and even syncope may be noted.

Attention to the animal's environment is important; looking for potential airway irritants can be time consuming but when found very rewarding. Some of the possible triggers of airway irritation include: smokers in the house, dusty and/or scented cat litter, use of room fresheners or deodorizers, dirty filters on furnaces, and house remodeling or recent moving.

Physical Examination

A quick assessment of an animal's condition upon presentation (triage) is essential. Many animals in respiratory distress have minimal reserves with which to handle the additional stress of transportation, physical examination and diagnostic testing used to determine the underlying cause of their distress. A calm, compassionate approach that deals with the animal's fear, anxiety and possible pain is important.

If significant distress (open mouth breathing, cyanosis, orthopnea) is present the veterinarian must determine if the cause is airway (e.g., laryngeal paralysis, bronchial), pleural (hernia, effusion) or primary lung (pneumonia, pulmonary edema) and act accordingly. I use a visual assessment of the respiratory rate, effort (inspiratory vs expiratory) and auscultation to make my initial evaluation. Be sure to listen to both right and left lung fields, the upper airways (nasal and laryngeal) as well as the chest (and of course the heart for possible cardiac involvement).

I test for nasal patency using the bell on my stethoscope to listen to airflow. At rest, a healthy animal should be able to breathe without distress through one nostril when its mouth is held closed. I look at the type of discharge and ask the owner whether it has changed over time (e.g., a response to previous treatments?). Facial deformity and/or periosteal pain may be indicative of bony involvement--often associated with tumor, fungal infections, or occasionally a tooth abscess. Signs of pharyngeal disease usually are associated with reverse sneezing, airflow interference and interrupted sleep.

With pre-existing tracheal, bronchial and with most parenchymal disease 'increased tracheal sensitivity' (a non-specific indicator of existing inflammation or irritation) may be noted. In cats with bronchial disease, lung sounds may be normal at rest but (post-tussive) crackles become prominent after coughing is induced as secretions are loosened. Tracheal sensitivity should be evaluated in all patients. Tachypnea is a more frequent primary complaint in cats than in dogs with bronchitis.

Most bronchitic animals are bright, alert and afebrile. Bronchovesicular lung sounds and end-inspiratory crackles are commonly heard. Wheezing may be noted, especially when airflow initially moves through airways obstructed by secretions. An end-expiratory 'snap' may be heard in dogs with decreased cartilage rigidity (malacia), as the increased intrathoracic pressure generated with an active expiratory effort often collapses central airways (normal airways will narrow but do not collapse) and the airway walls literally 'snap' together. An expiratory abdominal push (increased effort during quiet/resting breathing) and/or end-expiratory wheezing are characteristics encountered in patients with severe small airway disease.

A careful cardiac examination is important in order to differentiate heart disease from chronic bronchitis (CB); in many cases this can be difficult to do. Murmurs secondary to valvular insufficiency are common in older, small breed dogs (but not in cats); these cases must not be misinterpreted as being in congestive heart failure (CHF). A simple but fairly accurate method of determining whether CHF or CB is present in the dog (less so in the cat) is to examine the resting heart rate; CHF is associated with an elevated heart rate while CB usually results in a normal to slower heart rate due to vagal stimulation.

The patient's body condition score (BCS) should be determined as obesity is a common and significant morbidity factor in many diseases including chronic bronchitis.

Diagnostic Tests

Clinical Pathology

Includes the routine CBC, chemistry panel and full urinalysis. These are most useful in patients with systemic signs (pneumonia cases primarily). Less than 40% of confirmed allergic airway cases (those with eosinophils on airway cytology) have an absolute peripheral eosinophilia. Be careful--allergic airway disease or severe pneumonia can exist despite a normal CBC! Parasite evaluation is an important part of respiratory diagnostics (especially in endemic areas and in animals with a travel history).

Arterial Blood Gas Analysis

These are easy to do and provide the only functional assessment of overall lung function available in practice. They are useful to monitor daily progress to treatment; changes in blood gases happen sooner than changes on chest radiographs.

Radiography

General anaesthesia is required in order to properly position small animal patients for the best skull radiographs. Thoracic radiography provides one of the most widely available methods for evaluating the tracheobronchial tree and lung parenchyma. Bronchial disease normally demonstrates the thickened bronchi ('donuts', 'tram lines'). Parenchymal diseases usually cause an increase in interstitial density, which increases with severity to an alveolar pattern and eventually to lobar consolidation. Changes may be patchy, lobar or diffuse. Remember however that functional changes and visible structural changes do not always parallel each other. Thoracic radiography should include views made in at least two planes; lateral and either the VD or DV position. I prefer to obtain 3 views of the chest, both right and left laterals and the VD view for the evaluation of lung diseases. For optimal demonstration of parenchymal lesions, thoracic radiographs should be obtained at peak inspiration (for dynamic airway lesions both peak inspiratory and peak expiratory radiographs should be obtained). Advanced imaging (CT and MRI) have become more readily available in large private practices and universities, greatly extending the sensitivity and specificity of respiratory imaging.

Airway Sampling

This is necessary to determine an etiologic diagnosis and in order to recommend the most specific therapy. Upper airway samples should be examined cytologically; lower airway samples by both cytology and also culture/sensitivity determination (bacterial cultures from the upper airways are not useful due to contamination).

Lower respiratory tract cytology may be obtained using the transtracheal aspiration biopsy (TTA) technique, a fine needle lung aspirate (FNA) or actual bronchoscopy (including a full bronchoalveolar lavage (BAL). Macrophages are the normal cell to be seen on a good lower airway cytology sample. There is no question that bronchoscopy is the gold standard for the diagnosis of lower respiratory tract diseases in small animals. BAL fluid is evaluated with total and differential cell counts as well as Mycoplasma and quantitated aerobic bacterial cultures. Microbiological results are best interpreted in light of the cytology from the same site.

Endoscopy

Although an involved procedure (requiring general anesthesia, advanced equipment and training) airway endoscopy (rhinoscopy, laryngoscopy and bronchoscopy) is one of the very best diagnostic tests available to veterinarians. The visual assessment as well as ability to selectively sample the airways are second to none.

Electrocardiography (ECG) and Echocardiography (ECHO)

A major differential diagnosis for the coughing animal is heart disease and these tests help with this differentiation.

Thoracotomy

If other diagnostics have failed to establish a firm diagnosis, exploratory surgery should be considered; pulmonary fibrosis for instance requires a tissue sample to confirm this diagnosis. Thoracoscopy is beginning to be used to obtain lung biopsies for histopathologic confirmation of suspected lung disease.

Speaker Information
(click the speaker's name to view other papers and abstracts submitted by this speaker)

Brendan McKiernan, DVM, DACVIM-SAIM
Southern Oregon Veterinary Specialty Center
Medford, Oregon, USA


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