Overview

Dogs and cats with respiratory tract disorders can present to veterinarians for a variety of clinical signs including nasal discharge, sneeze, reverse sneeze, noisy breathing (snoring/stertor, stridor, wheeze), cough, alterations in respiratory rate or effort, and respiratory distress. The most urgent of these clinical signs is respiratory distress. This finding mandates rapid assessment to streamline the diagnostic and therapeutic approach. This lecture will highlight a clinically useful approach to respiratory distress.

Classification System

Understanding the causes of respiratory distress in dogs and cats is critical for prompt and effective diagnosis and treatment. Respiratory distress can be broadly classified into one of eight causes: upper airway obstruction, lower airway obstruction, flail chest, abdominal enlargement, pulmonary parenchymal disease, pleural cavity disorders, pulmonary thromboembolism, and “look-alike” syndromes (i.e., causing labored breathing but not being a true disorder of the respiratory system). This scheme allows for prompt recognition of the first four causes listed above after initial auditory and visual examination of the patient. In particular, emphasis should be placed on the pattern of respiration (inspiratory distress, expiratory distress, or both; paradoxical respiration), audible noises (stridor or wheezing) or the absence thereof, obvious physical abnormalities (e.g,, trauma associated with flail chest, distended abdomen) and other findings on thoracic auscultation (adventitial lung sounds, diminished heart or lung sounds, arrhythmias, murmurs).

Upper airway obstruction can be due to mechanical or functional obstruction of the large airways (pharynx, larynx, or trachea) at or above the thoracic inlet.

Examples of mechanical obstruction include intra-or extraluminal masses (neoplasia, granuloma, abscess, and blood clots), foreign bodies, tracheal stenosis or tracheal stricture. Functional obstruction is often caused by laryngeal paralysis or tracheal collapse. Inspiratory stridor is an important discriminating feature.

Lower airway obstruction traditionally arises from narrowing of the bronchial lumen due to bronchospasm, accumulation of mucus or other exudate, bronchial wall edema, or external compression of the airways by a thoracic mass or other structure (e.g., lymph nodes). The classic example of a lower airway disease is feline asthma, which causes bronchospasm, mucus accumulation, and bronchial wall edema. Asthma in dogs is an exceedingly rare diagnosis, but lower airway obstruction in dogs can be seen with severe bronchomalacia that allows for passive collapse of the airways on expiration. Additionally, disorders of the intrathoracic trachea can also lead to lower airway obstruction. The aforementioned disorders of the extrathoracic trachea are included in the differentials for intrathoracic tracheal obstruction along with hilar lymphadenopathy. Expiratory wheezes are an important discriminating feature.

Flail chest results from trauma to the thoracic cavity that allows destabilization of a portion of the rib cage (i.e., the ribs are fractured at two different locations leaving a segment that is detached from the rest of the rib cage). Focal paradoxical respiration is seen so that, as an animal inspires, the injured chest wall segment is sucked inward, and as the animal exhales, the segment is pushed outwards in opposition to the remainder of the chest wall.

Severe abdominal enlargement exerts pressure on the diaphragm and makes it more difficult for the thoracic cavity to expand on inspiration. Examples of diseases associated with abdominal enlargement include ascites, gastric dilatation, hepatosplenomegaly, neoplastic abdominal masses and pregnancy or pyometra in females. Note that abdominal enlargement must be pronounced in order to affect respiration. Silent inspiratory distress in the presence of a large abdomen is a discriminating feature.

Pulmonary parenchymal diseases are diseases affecting the terminal and respiratory bronchioles, interstitium, alveoli or vasculature of the lung. They may be associated with infiltration by infectious microorganisms, inflammatory or neoplastic cells; the airspaces may be filled with edema fluid or foreign material; or lung tissue may be replaced with fibrotic tissue. Examples of diseases affecting the pulmonary parenchyma include infectious pneumonia (bacterial, fungal, viral, protozoal, and parasitic), aspiration pneumonia, interstitial lung diseases such as pulmonary fibrosis, pulmonary edema (cardiogenic or non-cardiogenic), and neoplasia.

Pleural cavity disorders arise when the potential space between the parietal and visceral pleura, which normally just contains a small amount of fluid for lubrication, becomes filled. This may occur due to accumulation of fluid (pleural effusion), air (pneumothorax), a mass, or displacement of abdominal organs (i.e., diaphragmatic hernia).

Pulmonary thromboembolism (PTE) refers to obstruction of blood flow in the pulmonary vasculature by a thrombus or embolus formed in the systemic venous system or right side of the heart. Any condition causing an abnormality in blood flow, endothelial damage, or hypercoagulability can predispose to the formation of thromboemboli. Common causes in veterinary medicine include immune mediated hemolytic anemia, protein losing nephropathy or protein losing enteropathy, and in certain circumstances, hyperadrenocorticism and cancer.

Finally, look-alike syndromes are conditions which result in apparent difficulty in breathing due to non-respiratory causes. Essentially, these conditions mimic respiratory disease when the respiratory system actually remains functional. As such, arterial oxygenation should remain unchanged. Examples of such mimics include pain, severe anemia, hyperthermia, acidosis, drugs (e.g., opioids), and hypotension. For look-alike syndromes, both diagnostic and therapeutic efforts must be focused on the underlying condition. Oxygen supplementation is not beneficial to these animals.

Diagnostic Approach

The first four etiologies of respiratory distress (upper and lower airway obstruction, flail chest, and abdominal enlargement) can be discerned on initial examination of a patient. Patients with upper respiratory distress will have a characteristic stridorous or squeaking noise that is readily audible, even without a stethoscope. Similarly, patients with a lower airway obstruction should have an audible wheeze on auscultation. Visual examination will reveal if a flail chest or abdominal enlargement are present. Pleural space disease is sometimes readily apparent based on examination, but is sometimes more difficult to confirm without additional testing. Clues to pleural space disorders include shallow inspiratory effort with muffled lung and heart sounds, hyper-resonance on chest percussion, and paradoxical movement of the chest and abdomen (paradoxical movement can also be seen in some animals with obstructive airway disease).

To determine which of the remaining etiologies of respiratory distress are causing a patient difficulty breathing, further diagnostic testing will need to be performed. If mimics are a possibility, a simple PCV and measurement of oxygen saturation are in order. When true respiratory disease is suspected, thoracic (and sometimes cervical) radiographs are often indicated early in disease diagnosis. In addition to examination of extrathoracic structures, the pleural and mediastinal space, and heart size and shape, careful attention should be given to pulmonary pattern (e.g., alveolar, interstitial, bronchial, or vascular), distribution (e.g., affected lobes, predominantly ventral or perihilar distribution), and severity. Size of the lungs should also be assessed (e.g., atelectatic lung lobes are small, lung lobes with infiltrate are large).

Other useful diagnostics may include some of the following: complete blood counts, serology for infectious diseases, fecal examination, heartworm testing, advanced thoracic imaging (ultrasound, computed tomography, mucociliary scintigraphy), abdominal imaging (looking for related disease in the abdominal cavity), fundic examination, arterial blood gas, fine-needle aspiration for cytology/culture, transtracheal wash/endotracheal wash/bronchoalveolar lavage, bronchoscopic examination, bronchial mucosal or mass biopsies, and biopsies obtained by a key-hole procedure or thoracoscopy or thoracotomy. More details on the diagnostic workup of these conditions will be presented using clinical case examples.

Summary

Respiratory distress is unfortunately a relatively common clinical sign in dogs and cats indicative of severe and often life-threatening illness. It requires rapid recognition of the site of disease within the respiratory tract to help narrow the list of differential diagnoses and promptly select a rational diagnostic and therapeutic plan. The pattern of respiration and the presence or absence of audible noises is tremendously helpful in determining the anatomic location/possible cause of respiratory distress. In this lecture, a series of videos of animals in respiratory distress will be presented to illustrate how the pattern of respiration can be used quickly and efficiently to narrow the involved region of disease and narrow the list of differential diagnoses.