Aspiration pneumonia

What it is

Aspiration pneumonia develops when material from the oropharynx, oesophagus or stomach enters the lower airways. Acidic gastric content causes a chemical pneumonitis with damage to the airway epithelium and alveoli, oedema and haemorrhage; bacteria carried with the material or arriving later cause a secondary bacterial bronchopneumonia. Large particles can also obstruct airways. The protective reflexes (swallowing, laryngeal closure, cough) and mucociliary clearance normally prevent it, so it almost always has an underlying cause.

Who it affects

Dogs with a predisposing condition: megaesophagus and other oesophageal disease, persistent vomiting, laryngeal paralysis (and after surgery for it), brachycephalic obstructive airway syndrome, neuromuscular disease (myasthenia gravis, polyneuropathy), reduced consciousness (seizures, sedation, anaesthesia), and force-feeding or feeding tubes. Puppies with cleft palate are also at risk.

Anatomy involved

• Lungs: gravity determines the distribution; with the dog standing or in sternal recumbency, the right middle lobe and the cranial lobes are most often affected. The right principal bronchus is wider and more in line with the trachea.
• Oesophagus and pharynx: the usual source of the aspirated material.

Clinical signs

Acute or progressive tachypnoea, dyspnoea, moist cough, fever, lethargy and anorexia, often after an episode of vomiting, regurgitation or anaesthesia. Auscultation reveals crackles or reduced sounds over the ventral lung fields. Severe cases show hypoxaemia and cyanosis. Some dogs with chronic regurgitation have recurrent, milder episodes.

Diagnosis

• Thoracic radiographs (three views): interstitial to alveolar pattern with air bronchograms in the cranioventral lobes, most often the right middle lobe; radiographic changes can lag behind clinical signs.
• Pulse oximetry or arterial blood gas: to assess oxygenation and guide oxygen therapy.
• Blood work: inflammatory leucogram in many cases.
• Airway lavage (tracheal wash or bronchoalveolar lavage) with culture and sensitivity: in severe, recurrent or refractory cases.
• Search for the cause: oesophageal imaging, neurological and laryngeal examination.

Treatment

Oxygen supplementation when hypoxaemic, intravenous fluids to keep the patient hydrated (which helps clear secretions), nebulisation and coupage, and broad-spectrum antibiotics, adjusted to culture results when available. Mild cases can be treated at home. The predisposing condition must be managed to prevent recurrence, for example upright feeding in megaesophagus or antiemetics and airway protection during anaesthesia.

In the cat

Aspiration pneumonia is less common in cats than in dogs, partly because megaesophagus and laryngeal paralysis are rare in this species. The most frequent causes are anaesthesia and sedation, severe vomiting, force-feeding by mouth or a misplaced feeding tube, swallowing disorders and oral administration of liquid paraffin or other oily laxatives for constipation or hairballs, which can cause lipid pneumonia. Cough is less consistent than in the dog; the most frequent signs are tachypnoea, dyspnoea, lethargy, anorexia and fever. Diagnosis and treatment follow the same principles as in the dog (thoracic radiographs, oxygenation, airway lavage with culture in severe cases, oxygen, fluids and antibiotics), bearing in mind that the stress of handling can decompensate a dyspnoeic cat.