Respiratory distress in small animals: Pathophysiology and clinical approach

Background: Respiratory distress is a common reason for animals to present to the emergency room. An understanding of respiratory physiology is helpful to facilitate accurate localization of the patient's source of respiratory distress. This knowledge will aid implementation of more appropriate therapies.

Physiology/pathophysiology summary: Respiratory distress can occur secondary to lesions at any location of the respiratory system. Diseases affecting different portions of the respiratory track (eg, upper airway, lower airways, parenchymal, or the pleural space) lead to distinguishing clinical signs as the patient's respiratory physiology is affected differently.

Clinical importance: Respiratory distress can be life-threatening if it is not recognized and addressed in a timely matter. Prompt recognition of unique clinical signs may aid the clinician's ability to localize the disease process and implement targeted therapies.

Key points: Diseases affecting different portions of the respiratory track (eg, upper airway, lower airways, parenchymal, or the pleural space) lead to distinguishing clinical signs as the patient's respiratory physiology is affected differently. Recognition of respiratory patterns allows clinicians to correlate clinical signs to the most likely affected region of the respiratory tract. Ideally, alveolar gas (ventilation) and blood flow (perfusion) are equally matched. However, even a normal lung does not have ideal ventilation/perfusion (V/Q) matching. There are a variety of conditions that lead to decreased lung compliance, from pathology of the pulmonary parenchyma (eg, pulmonary edema, alveolar infiltrates such as pneumonia or contusions, or pulmonary fibrosis), pleural space disease (eg, pneumothorax or pleural effusion), abdominal distension, or the inability to expand the rib cage (eg, pain, thoracic wall injury, constricting thoracic bandages). The five classic causes of hypoxemia include low FiO₂ , diffusion impairment, hypoventilation, shunt, and V/Q mismatch. Being prepared to rapidly induce anesthesia and intubate dyspneic animals with upper airway obstruction may be required alleviate the risk of respiratory arrest.