Canine Pneumothorax Study Guide

Overview and Clinical Importance

Pneumothorax is defined as the abnormal accumulation of free air within the pleural space. This condition disrupts the normal negative intrapleural pressure (approximately -5 cmH2O), causing partial or complete lung collapse and significant respiratory compromise. Pneumothorax is a common emergency presentation in dogs and represents a high-yield topic on the NAVLE, requiring understanding of classification, diagnosis, emergency management, and definitive treatment.

The condition can be life-threatening if not promptly recognized and treated. Traumatic pneumothorax is the most common form, occurring in approximately 50% of significant thoracic trauma cases in dogs. Spontaneous pneumothorax, while less common, requires thorough understanding due to its high recurrence rate without surgical intervention.

Classification of Pneumothorax

Etiological Classification

1. Traumatic Pneumothorax

Traumatic pneumothorax is the most common cause of pneumothorax in dogs. It results from injury to the chest wall, airways, or lung parenchyma. Common causes include motor vehicle accidents, bite wounds, blunt force trauma, rib fractures, and penetrating injuries.

2. Spontaneous Pneumothorax

Spontaneous pneumothorax occurs without antecedent trauma and is further classified as primary (no underlying lung disease) or secondary (associated with underlying pulmonary pathology). In dogs, the most common cause is rupture of pulmonary blebs or bullae.

3. Iatrogenic Pneumothorax

Iatrogenic pneumothorax results from veterinary procedures including thoracocentesis, lung biopsy, thoracotomy, tracheal intubation injury, or positive pressure ventilation. Patients with chronic pleural effusion and thickened pleura are at increased risk during thoracocentesis.

Pathophysiological Classification

Causes of Spontaneous Pneumothorax

Breed Predispositions

Spontaneous pneumothorax occurs primarily in large, deep-chested dogs with no sex predilection. There is significant overrepresentation of certain breeds:

• Siberian Husky - Most commonly affected breed; typically develop bullae
• Other Northern/sled dog breeds (Alaskan Malamute)
• Giant breeds: Great Danes, Irish Wolfhounds, Scottish Deerhounds
• Medium to large breed dogs generally

Board Tip - Memory Aid: 'HUSKY LUNGS LEAK' - Huskies and other large, deep-chested dogs are predisposed to spontaneous pneumothorax from pulmonary blebs/bullae. Think of the negative pressure gradient being greater at the lung apices in tall, deep-chested dogs, similar to tall, thin humans who develop spontaneous pneumothorax.

Pathophysiology

Normal intrapleural pressure is approximately -5 cmH2O, which maintains lung inflation. When air enters the pleural space, this negative pressure is lost, causing:

• Lung collapse: Partial or complete atelectasis depending on air volume
• Decreased tidal volume: Reduced lung expansion capacity
• V/Q mismatch: Ventilation-perfusion abnormalities leading to hypoxemia
• Cardiovascular compromise: In severe cases (especially tension), decreased venous return and cardiac output

Pulmonary Blebs vs. Bullae:

• Blebs: Small (less than 1 cm) air collections trapped between internal and external layers of visceral pleura; typically at lung apices
• Bullae: Larger (greater than 1 cm) air-filled spaces within lung parenchyma from destruction and confluence of alveoli; blister-like lesions at lung lobe margins

Clinical Presentation

Clinical Signs

Physical Examination Findings

• Auscultation: Muffled or absent lung sounds DORSALLY (air rises); muffled heart sounds in severe cases
• Percussion: Hyperresonance dorsally (tympanic sound)
• Barrel-shaped chest: Fixed in maximal extension with tension pneumothorax
• Subcutaneous emphysema: Crepitus over neck, thorax - indicates pneumomediastinum or chest wall injury

Diagnostic Approach

CRITICAL: In a dyspneic patient with clinical suspicion of pneumothorax, perform therapeutic thoracocentesis BEFORE radiographs. Stabilization takes priority over imaging.

Radiography

Radiographic Findings:

• Cardiac silhouette elevation: Heart appears lifted away from sternum on lateral view - classic finding
• Lung lobe retraction: Lungs collapse and retract from chest wall; visible lung margins
• Lack of pulmonary vessels peripherally: Vessels do not extend to thoracic wall
• Increased thoracic lucency: Radiolucent area between lung and chest wall
• Tension pneumothorax signs: Small collapsed lungs, mediastinal shift, diaphragmatic tenting, small heart/vessels

Best Views: Left lateral recumbent view is most sensitive for detecting pneumothorax (heart elevation best seen). Horizontal beam VD views improve detection of small pneumothorax. VD/DV views have LOWEST sensitivity.

TFAST Ultrasound

TFAST (Thoracic Focused Assessment with Sonography for Trauma) is a rapid point-of-care diagnostic that can be performed faster than radiography in emergency patients.

Computed Tomography (CT)

CT is more sensitive than radiography for detecting pulmonary blebs and bullae and is recommended for surgical planning in spontaneous pneumothorax. However, blebs/bullae are detected on radiographs in only ~25% of affected dogs, and even CT has sensitivity of only 42-58% for identifying the causative lesion. CT better localizes affected lung lobes than radiography.

Treatment

Emergency Stabilization

• Oxygen supplementation: Flow-by, mask, or oxygen cage; helps resolve pneumothorax faster
• Thoracocentesis: Immediate needle decompression in dyspneic patients - both diagnostic and therapeutic
• IV access: For fluid support if concurrent trauma/shock
• Analgesia: Opioids for pain (rib fractures, surgical patients)

Thoracocentesis Technique

Chest Tube (Thoracostomy Tube) Placement

Indications for chest tube:

• Requires greater than 2 thoracocenteses in 24 hours
• Tension pneumothorax
• Spontaneous pneumothorax (typically requires tube)
• Post-thoracotomy
• Ongoing air leak or persistent pneumothorax

Tube Types: Argyle (PVC with trocar), Axiom (silicone), or MILA small-bore wire-guided tubes (Seldinger technique). For pneumothorax, tube should be placed DORSALLY (apical position). Tube diameter should approximate mainstem bronchus size on radiograph.

Tube Removal: For pneumothorax, tubes are typically removed 24 hours after no air has been aspirated and radiographs confirm lung re-expansion.

Surgical Treatment

Indications for Surgery:

• Spontaneous pneumothorax (conservative management has ~50% recurrence rate)
• Persistent air leak not resolving with chest tube drainage (greater than 3-5 days)
• Identifiable pulmonary lesion (bulla, abscess, neoplasia)
• Open pneumothorax with large chest wall defect

Surgical Approaches:

Surgical Procedure: Lung lobectomy of affected lobe(s) using stapling devices or vessel sealing. Pleural abrasion (mechanical pleurodesis) may be performed to prevent recurrence by creating adhesions between visceral and parietal pleura. Normal dogs can tolerate resection of up to 50% of lung capacity.

Prognosis and Outcomes

Board Tip - Memory Aid: 'PNEUMO-3-50' For spontaneous pneumothorax: Surgery = ~3% recurrence (success!), Medical = ~50% recurrence (failure!). The numbers tell the story - always recommend surgery for spontaneous pneumothorax.