Helio Autran de Morais, DVM, PhD, DACVIM and Maria de Lourdes E. Faria, DVM, PhD
Dyspnea is the sensation of difficult breathing. Because dyspnea is a subjective phenomenon it is not easy to evaluate and it has to be inferred by the clinician in companion animals. Dyspnea, tachypnea (rapid breathing), and hyperpnea (increased ventilation) are not synonymous but are frequently grouped together in veterinary patients. Dyspnea occurs when the demand for ventilation is out of proportion to the animal's ability to respond to this demand. Thus, breathing becomes difficult, uncomfortable, or labored. Dyspnea may result from alterations in any portion of the respiratory system or be due to abnormal mechanics of the lung and chest wall (Figure 1).
When does the Dyspnea occur?
The timing and pattern of respiration helps to determine the structure most likely responsible for the dyspnea. Dyspnea may occur during inspiration, expiration or both (mixed). Clinically, pure inspiratory dyspnea implies a lesion in the respiratory tract outside the thorax, whereas expiratory and mixed dyspnea occur in patients with thoracic or metabolic disease.
What is the pattern of respiration?
Mixed or expiratory dyspnea should be further classified as obstructive or restrictive. Obstructive diseases are associated with increased airway resistance in the tracheobronchial tree. Obstructive diseases may also occur in the upper respiratory tract, but those patients have inspiratory dyspnea. Patients with obstructive disease have increased total lung capacity (viewed as hyperinflation in chest radiographs) and increased residual volume (volume in the lungs at the end of the expiration) with decreased vital capacity (maximum amount of air that can be taken in and out in one respiration). As a result patients with obstructive disease have decreased expiratory flows and hyperinflated lungs. Hyperinflation further increases breathing labor, because at high lung volumes a greater change in pressure is required to attain a change in volume. Due to the tethering effect of lung parenchyma on the airways higher lung volumes decrease airway resistance. Restrictive diseases are those in which expansion of the lungs is restricted. Patients have decreased total lung capacity, residual volume and vital capacity with no increase in airway resistance (related to the lung volume). Lungs of patients with restrictive disease operate at smaller volumes and the patient has a rapid shallow breath. In patients with a non-pulmonary cause of restrictive, residual volume is normal or increased.
Can you hear Respiratory Sounds?
The physical examination assists in identifying the cause for the dyspnea. In patients with restrictive pulmonary disease, absence of respiratory sounds indicates a pleural cavity disease, whereas presence of pulmonary sounds occurs in patients with parenchymal pulmonary disease, metabolic diseases or abdominal distention.
Inspiratory dyspnea occurs with extrathoracic lesions in the respiratory tract (table 1). Patients with isolated pure nasal problems are able to breath normally when the mouth is open. Presence of abnormal respiratory sounds may help to localize the problem. Stridor is a loud musical inspiratory sound of constant pitch associated with laryngeal (and occasionally tracheal) alterations. Rhoncus is a rattling in the throat associated with pharyngeal/proximal tracheal diseases. Cough may occur in patients with inspiratory dyspnea. Cough receptors are located in the larynx, pharynx and large airways and diseases in any of those locations may be associated with cough. In patients with extrathoracic disease, cough is usually paroxystic and loud. Direct visual inspection or bronchoscopy is necessary determine the cause.
Table 1. Common Causes of Inspiratory Dyspnea
Elongated soft palate
Everted laryngeal saccules
Obstructive Expiratory Dyspnea
Obstructive expiratory dyspnea occurs in patients with intrathoracic airway diseases (table 2). Patients may have wheezes and cough. Wheezes are continuous musical sounds generated by air forced to pass through a narrow region abruptly into a wider region in the larger airways. Good quality chest radiographs and tracheal wash are necessary to rule in or rule out specific diagnosis.
Table 2. Common Causes of Obstructive Expiratory Dyspnea
Enlarged left atrium
Silent Restrictive Expiratory Dyspnea
Silent restrictive expiratory dyspnea occurs in patients with pleural cavity disease (table 3). Pulmonary sounds are absent or may be heard at specific locations (e.g., dorsal lung fields in patients with pleural effusion). Cough is usually absent. Chest radiographs and thoracocentesis are necessary to rule in or rule out the differentials.
Table 3. Common Causes of Silent Restrictive Expiratory Dyspnea
Noisy Restrictive Expiratory Dyspnea
Noisy restrictive expiratory dyspnea occurs in patients with parenchymal pulmonary diseases (table 4). Pulmonary sounds are audible and abnormal sounds like crackles might be heard. Crackles are short, explosive, non-musical sounds that are a non-specific sign of small airway disease. Cough may occur if small airways are also involved and is usually not loud. Chest radiographs and tracheal wash or bronchoalveolar lavage are necessary to rule in or rule out the differentials.
Table 4. Common Causes of Noisy Restrictive Dyspnea
Careful interpretation of the information obtained in the history and physical examination determining the timing and pattern of the dyspnea allow the veterinary practitioner to anatomically locate the origin of the dyspnea in most cases. Direct inspection visually or endoscopically, radiographs, and cytology are necessary to determine the cause of the dyspnea.
References are available on request.