Canis ISSN: 2398-2942

Lung: atelectasis

Contributor(s): Lesley G King

Introduction

  • Cause: reduction in lung volume due to collapse of alveoli.
  • Signs: hypoxemia, cough.
  • Diagnosis: radiography.
  • Treatment: resolve underlying cause, provide oxygen.
  • Prognosis: good to guarded depending on underlying cause.

Pathogenesis

Etiology

  • Ateclectasis is collapse of empty alveoli. Because the alveoli are collapsed there is a decrease in size of the affected lung lobe(s).
  • It differs from consolidation, which is filling of alveoli by cells or fluid. Consolidated lung lobes are normal or increased in size.
  • Lung lobe atelectasis is caused by:
    • Compression of alveoli: most frequent cause of atelactasis in clinical practice.
    • Loss of negative intrapleural pressure.
    • Concurrent lung disease, usually inflammatory, which leads to loss of surfactant.
    • Absorption atelectasis due to complete obstruction of a bronchus: least common cause of atelectasis in clinical practice.

Predisposing factors

General
  • Obesity.
  • Large breed dog.
  • Prolonged recumbency.

Pathophysiology

  • Compression atelectasis:
    • Collapse of normal alveoli.
    • Most severe in ventral airspaces, which normally have smaller alveoli.
    • Often caused by pressure on the chest wall from obesity or tight bandages; or pressure forward on the diaphragm from an enlarged liver, abdominal effusion or mass.
    • Predictable in large breed or obese dogs recumbent on one side for prolonged periods of time (more than one hour when breathing sponataneously).
    • Predictable following initiation of anesthesia with spontaneous breathing.
  • Loss of negative intrapleural pressure:
    • Pleural space disease such as pneumothorax Pneumothorax or pleural effusion Pleural: effusion results in loss of negative intrapleural pressure, which normally helps keep alveoli open. Atelectasis is then caused by passive recoil of the lungs.
    • Smaller and more ventral lung lobes, eg right middle and caudal part of the left cranial, are most severely affected.
    • Large volumes of fluid or air can result in positive intrapleural pressure (eg tension pneumothorax Tension pneumothorax ) which then causes compression atelectasis.
    • Thoracocentesis Thoracocentesis usually allows re-expansion of the collapsed lung lobes.
  • Concurrent lung disease:
    • Because of lack of effective surfactant, atelectasis is often a component of inflammatory lung diseases such as pneumonia Lung: bacterial pneumonia or acute respiratory distress syndrome Acute Respiratory Distress Syndrome (ARDS).
    • During lung inflammation, surfactant is often defective or produced in decreased amounts due to injury to Type 2 alveolar epithelial cells.
    • Consolidation is usually more obvious than atelectasis in these patients (ie there is usually no decrease in size of the affected lung lobes).
    • Management should focus on treatment of the inflammatory disease.
  • Obstruction of bronchus (absorption atelectasis):
    • Airway obstruction must be complete, usually due to inhaled foreign body Foreign body migration (particularly pebbles or seeds); mucus or inflammatory debris secondary to pneumonia or bronchial disease (common in right middle lung lobe of cats with feline asthma); external compression of the bronchus by an extraluminal mass; intraluminal mass (neoplasm Lung: pulmonary neoplasia ) completely obstructing the bronchus.
    • If the bronchus is completed obstructed, air in the alveoli distal to the obstruction is gradually absorbed by diffusion into the blood, resulting in collapse of the alveoli.
    • Nitrogen (room air is about 80% nitrogen) is the slowest gas to be absorbed from the alveoli. Because of its slow diffusion into tissues compared with oxygen and carbon dioxide, it is said to create a "nitrogen scaffold" that holds alveoli open.
    • If a bronchus is obstructed while the animal is breathing 100% oxygen (eg under anesthesia), atelectasis occurs very quickly because oxygen is musch nore diffusible into blood compare with nitrogen.

Timecourse

  • Minutes to weeks.

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Winegardner K, Scrivani P V, Gleed R D (2008) Lung expansion in the diagnosis of lung disease. Compend Contin Educ Vet 30 (9), 479-489 PubMed.
  • Staffieri F, Franchini D, Carella G L et al (2007) Computed tomographic analysis of the effects of two inspired oxygen concentrations on pulmonary aeration in anesthetized and mechanically ventilated dogs. Am J Vet Res 68 (9), 925-931 PubMed.
  • Corcoran B M, Abercromby R H (1989) Effects of general anaesthesia on static respiratory complicance in dogs. Vet Rec 125 (18), 450-453 PubMed.


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