Why is this patient coughing? (Proceedings)
Cough is one of the body's defense mechanisms to prevent the entry of noxious materials into the respiratory system, and to clear potentially harmful debris from the lungs and respiratory tract. It is normally associated with other protective processes such as bronchoconstriction and mucus secretion, both of which may aid in the effectiveness of the cough. Cough is an indicator of airway or pulmonary disease, although it also may occur as a primary sign of other thoracic disorders. Cough receptors are located in the larynx, pharynx and large airways. They are absent in the alveoli and alveolar sacs. As a result, diseases of the lung parenchyma may not stimulate the cough reflex to the degree that disorders within the airways do. This is teleologically understandable because even a vigorous cough would not move gas fast enough to cause turbulence and shearing forces at the airway wall in the smaller airways. Cough at this level therefore, would be ineffective. Cough is not a localizing sign being associated mostly with diseases of the lower respiratory tract (trachea, bronchi and lungs). Patients with upper respiratory tract disease may, however, cough if secretions are being drained into the larynx and trachea.
What is causing this cough?
Cough in dogs is usually caused by tracheal collapse, chronic bronchitis, compression of the left main stem bronchus by a large left atrium, congestive heart failure or pneumonia. Other causes of cough include: tracheitis, traumatic or mechanical problems (e.g. foreign body, irritating gases), parasitic disorders (e.g. lungworms, heartworm), and neoplasia. As rule, loud cough originates in large airways, whereas soft, discrete cough is associated with small airway disease (Table 1). In cats, cough is usually a sign of pneumonia, heartworm disease, asthma or lungworms.Table 1. Common causes of cough in dogs
Left main stem bronchial compression
The history may help establish the etiology for the cough. In dogs, nocturnal coughing is generally associated with cardiac disease, psychogenic causes, or tracheal collapse. Patients with tracheal collapse also may start coughing after drinking water. Cough due to respiratory disease is more likely to occur during the day. Tracheal diseases usually have a goose-honk cough that can be initiated by excitement or by pulling on the collar and leash.
The physical examination assists in identifying the reason for the cough and is particularly helpful in differentiating if the cough is due to lung or heart disease. Patients with heart failure usually have fast heart rates with soft crackles, whereas patients with primary lung disease tend to have pronounced sinus arrhythmia with crackles that are usually louder and of higher-pitch than the ones resulting from heart failure. Wheezes also can be present, particularly in patients with respiratory disease. Crackles are a non-specific sign of small airway disease. Wheezes are continuous musical sounds generated by air forced to pass through a narrow region abruptly into a wider region. Wheezes usually indicate disease in the larger airways. Maneuvers that stimulate the patient to take deep breaths may help in lung auscultation because abnormal lung sounds (especially crackles) may be missed if the lung is not adequately expanded. Closing the nostrils for 10 to 15 seconds usually will force the patient to inspire deeply.
Good quality chest radiographs are of paramount importance in evaluating a patient with cough. In many occasions the diagnosis can be made by visualization of a tracheal collapse, large left atrium, pulmonary masses, or radiographic patterns suggestive of pulmonary edema, pneumonia, bronchial disease or heartworm disease. There is a limited number of ways that the heart and lungs can respond to injury and produce radiographic patterns. Even when radiographs do not reveal the diagnosis they help in narrowing the differentials, and determining the next diagnostic step (e.g. cytologic diagnosis in a patient with bronchial pattern is more likely to be obtained performing a tracheal wash, whereas in a patient with pulmonary masses a fine needle aspiration may be more helpful).