Classic clinical signs of respiratory distress include tachypnea, head and neck extension, opened mouth breathing, anxiety, cyanosis, nares flaring, abducted elbows, orthopnea, and paradoxical movement of the chest and/or abdomen. Cats readily mask disease severity, and commonly the only evidence of respiratory dysfunction is tachypnea and prominent respiratory motions in sternal recumbency.

The first step in examining a patient with a respiratory emergency is to perform a primary survey. If the airway is not patent, it must be cleared of obstruction and orotracheal intubation performed. An emergency tracheostomy may be needed. A clinician should evaluate the patient's breathing pattern and work of breathing; particular attention should be given to the phase of breathing affected. Increased inspiratory effort is associated with extra-thoracic disorders while increased expiratory effort is often due to intrathoracic diseases.

An obstructive breathing pattern indicates inappropriate movement of air in/out of the lungs, and is associated with a slower respiratory rate and deeper breaths than normal. A restrictive breathing pattern is characterized by inadequate expansion of the chest wall and/or lungs, resulting in shallow tachypnea. The initial evaluation of a patient should ideally begin before touching a patient, as the breathing pattern may change with manipulation.

Upper Airway

The upper respiratory tract is simply a conduit from the nares to the glottis. Diseases of the upper respiratory tract that includes the nares, nasal cavity, pharynx, and larynx are relatively common in both dogs and cats. Upon presentation an owner may report a patient snores and/or has exercise intolerance, dysphagia, regurgitation, and/or post-tussive retching. They may also report these clinical signs are associated with and/or exacerbated by excitement, humidity, and/or elevated ambient temperatures. Physical examination may identify referred upper airway sounds, orthopnea, bilateral elbow abduction, restlessness, dyssynchronous breathing, and cyanosis. Affected patients commonly have abnormal respiratory sounds and pattern of breathing. The classic upper airway breathing pattern is characterized by increased inspiratory time and effort compared to expiration. Stertor and/or stridor may be present depending on the site of airflow turbulence. Pulmonary crackles that form due to non-cardiogenic pulmonary edema may be auscultated. Patients affected by upper airway disorders should always be handled with minimal restraint, as they may decompensate with minimal manipulation.

Lower Airway

The lower airway is comprised of the intra-thoracic trachea, primary and lobar bronchi, and bronchiolar arborization. Feline patients are most commonly diagnosed with asthma. Foreign body obstruction must be considered, and while neoplasia can affect the lower airway, they rarely induce acute dyspnea due to their chronic, insidious nature. Pet owners may report a chronic and/or progressive history of coughing that is frequently productive although the sputum is most often swallowed. Affected patients commonly have a respiratory pattern characterized by prolonged expiration despite normal inspiratory time and effort. Additionally, a clinician may observe abdominal effort during expiration and/or auscultate expiratory wheezes. Patients may display orthopnea, and feline patients may breathe with an opened mouth.

Pleural Space

The pleural cavity is the potential space between the parietal and visceral pleura. When this potential space is occupied by air, fluid, masses, and/or viscera, the lungs are unable to expand normally. As a result, intrapleural pressure increases and ultimately exceeds intrapulmonary pressure, effectively reducing tidal volume. An affected patient must breathe rapidly to maintain adequate minute ventilation. Classically patients manifest a restrictive breathing pattern characterized by shallow tachypnea. With reduced tidal volume, alveoli progressively collapse to cause hypoventilation and hypoxemia due to ventilation-perfusion mismatch. Thoracic auscultation varies depending on the nature of the pleural disease.

Pulmonary Parenchyma

The pulmonary parenchyma may be affected by a number of diseases, and a thorough history will help a clinician develop appropriate diagnostic and therapeutic interventions. While coughing is relatively common in dogs, cats rarely have a history of chronic coughing.

A history of recent vomiting should raise concern for aspiration pneumonitis/pneumonia, and patients with trauma may develop pulmonary contusions and/or non-cardiogenic pulmonary edema. Recent generalized seizure activity, electrocution, traumatic brain injury, and/or strangulation should raise concern for non- cardiogenic pulmonary edema. Patients with primary cardiac disease may develop cardiogenic pulmonary edema. Affected patients do not have a specific respiratory pattern, but frequently manifest respirations that are rapid and deep. They may be presented with cyanosis and dyspnea, and increased abdominal effort is relatively common. Auscultation commonly reveals end- inspiratory crackles; alveolar fluid may be blood, purulent exudate, or transudate. Alveolar hemorrhage may occur secondary to trauma and/or coagulopathy. A patient with cardiogenic pulmonary edema may have a systolic murmur and hypothermia while a patient with pneumonia or aspiration pneumonitis/pneumonia may have pyrexia. Patients should be efficiently examined for evidence of ecchymoses, obvious soft tissue, and/or orthopedic injury and thermal injury.

Chest Wall

Common thoracic wall injuries include blunt trauma, penetrating trauma and flail chest. Blunt trauma may cause crush and shear injuries to both soft tissues and skeletal structures; the skeletal structures of the chest wall are relatively resistant to blunt force trauma. However, muscle is uniquely sensitive to crushing injury, and when damaged in low velocity accidents may become edematous, inelastic, and may lose the ability to contract. Thoracic wall compliance decreases and work of breathing subsequently increases. This may manifest as tachypnea, but breaths may be shallow or deep; subsequent hypoxemia and/or hypoventilation are common. High velocity accidents are associated with shearing injury. While soft tissue damage to the thoracic wall rarely contributes to patient morbidity, energy may be transmitted to intra-thoracic organs to induce crush and shearing injuries at these sites. A blunt force applied laterally to the thoracic wall may cause rib fractures; pressure applied dorsoventrally to the thoracic cage infrequently results in rib and sternebrae fractures but such injuries may contribute to reduced thoracic compliance.

Penetrating injuries to the thoracic wall are relatively common in domestic dogs and cats. These wounds induce stretching and crushing of tissues in the direct path of penetration. While the actual chest wall penetration is frequently a minor issue, one must be aware of the potential underlying pleural and intra-thoracic injuries. The type and severity of injury directly influences how a patient is presented, and the medical team must be prepared to triage a patient in respiratory distress that needs immediate orotracheal intubation and intra-thoracic stabilization.

Flail chest results from the segmental fracture and/or dislocation of two or more adjacent ribs. This type of injury is uncommon is dogs and cats because of the inherent compliance and anatomic shape of the thoracic cage. Blunt trauma and bite injuries are the most common causes of flail chest, and concurrent pulmonary damage is a major contributor to morbidity. These contusions may lead to decreased pulmonary compliance, hypoventilation, and shunting. The pathognomonic respiratory pattern is paradoxical movement of the unstable ribs (flail segment) during respiration. Splinting due to pain associated with respiration leads to a decreased cough reflex, hypoventilation, hypoxemia and atelectasis. Several techniques for stabilization of flail chest have been reported, but the primary focus should be on oxygen supplementation, underlying intrathoracic injuries, and multimodal analgesia.

Diaphragm

A myriad of disease may induce partial or complete diaphragmatic paralysis, including pleuroperitoneal hernias, botulism and phrenic nerve damage or degeneration. The classic respiratory pattern is tachypnea with marked inspiratory excursion of the cranial half of the thorax. Paradoxical abdominal movement is also common, and may be due either to thoracic displacement of abdominal viscera during vigorous inspiration or an inability to maintain abdominal girth during inspiration.

Non-respiratory Causes

There are multiple causes of tachypnea and dyspnea that do not directly arise from the respiratory tract.

A common example is non-pyogenic hyperthermia. Measurement of body temperature is essential in patients presented with respiratory signs, and appropriate cooling measures should be implemented when hyperthermia is documented. Intracranial diseases may stimulate the medullary respiratory center. Patients with intracranial diseases may have abnormal respiratory patterns, and neurological examination often identifies other deficits that aid in determining a definitive diagnosis. Pain and acid-base disorders also induce tachypnea without hypoxia in patients. Abdominal distension is common cause of tachypnea and dyspnea.

References

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5.  Olsen D, Renberg W, Perrett J, et al. Clinical management of flail chest in dogs and cats: a retrospective study of 24 cases (1989–1999). J Am Anim Hosp Assoc. 2002;38:315–320.