Introduction

Airway obstruction can be a life-threatening condition in the dog and the cat. The respiratory tract can be divided into upper and lower regions. Typically the upper respiratory tract includes nasopharynx, oropharynx and larynx whilst the lower respiratory tract includes the trachea, mainstem bronchi and smaller airways. Obstruction of the lower respiratory tract is an uncommon problem. In most instances of respiratory obstruction the upper airway is affected. It is important to be able to differentiate between the two locations as the management and treatment of the obstructions will be different depending on the region affected. Different causes of lower airway obstruction are outlined in Figure 1.

Figure 1. Causes of lower airway obstruction.

 Tracheal foreign bodies

 Bronchial foreign bodies/abscesses

 Tracheal/bronchial neoplasia

 Tracheal stenosis (post-traumatic)

 Fluid--blood, mucus, oedema

 Tracheal collapse

 Lung lobe torsion

History and Clinical Features

Patients with lower airway obstruction can be presented with a variety of clinical features. Commonly, affected patients have a history of mild non-specific respiratory signs that do not affect them greatly clinically. Acute, severe respiratory distress can be seen but is uncommon. Sudden onset coughing, choking or gagging possibly with associated dyspnoea may be described by the owners of affected animals. Occasionally, the owner may have witnessed inhalation of a foreign body. Exercise intolerance, respiratory distress and cyanosis may be seen in severe cases.

On physical examination tachypnoea and dyspnoea may be noted. Forced expiratory effort can be a feature of lower airway obstruction which is contrary to the inspiratory dyspnoea commonly seen in patients with upper airway obstruction. Increased respiratory noise is often a feature of lower airway obstruction and in some cases this may be audible without the aid of a stethoscope.

Initial Management

Because of the life-threatening nature of tracheal obstruction in particular, all animals suspected to have airway obstruction should be treated as critical emergency patients. Even apparently stable patients can deteriorate very quickly and it is vital to avoid causing unnecessary stress. All dyspnoeic patients should be given supplemental oxygen prior to examination, provided by the least stressful method available. Oxygen cages and flow-by oxygen are the most useful in the dyspnoeic patient for this reason.

Thoracic auscultation is extremely useful in assessing the dyspnoeic patient. Patients with lower airway obstruction may have normal lung sounds. In some instances, abnormal noises such as whistling, wheezing or harsh lung sounds may be heard in affected animals.

In stable patients an intravenous catheter should be placed and blood taken for at least a minimum database (packed cell volume (PCV), total solids, electrolytes and venous blood gas analysis if available).

Patients in respiratory distress may benefit from light sedation. Urgent anaesthesia is only rarely necessary. Once the patient is anaesthetised, however, the airway must be secured by means of an endotracheal tube. Although this will not bypass a lower airway obstruction it will allow the provision of 100% oxygen and enable assisted ventilation which may improve the patient's oxygenation. Once the patient is anaesthetised further investigations should be performed prior to definitive treatment.

Investigation

The aim of the investigation is to determine the cause of the respiratory distress. Because of the myriad of potential causes of dyspnoea (upper or lower respiratory obstruction; pleural space disease (pneumothorax, pleural effusion and diaphragmatic rupture); pulmonary parenchymal disease (feline asthma and pulmonary thromboembolism)) a logical approach is required. Occasionally, dyspnoea in affected animals is so severe, additional diagnostics are contraindicated. In these situations the clinician must rely on physical examination findings to guide the next therapeutic step as in tension pneumothorax, massive pleural effusion and some cats with asthma.

Plain radiographs should be made of the neck and thorax of the stable patient. In the dyspnoeic patient thoracic radiographs may be made under conscious restraint with extreme care so as to avoid precipitating respiratory arrest and only in sternal recumbency. Again, anaesthesia and a secured airway may be safer than sedation for some respiratory patients. Plain radiographs may show evidence of tracheal or bronchial obstruction due to a foreign body or mass. Inspiratory and expiratory radiographs or a fluoroscopic study may allow a diagnosis of tracheal collapse. Other abnormalities such as a pulmonary abscess due to a foreign body may also be seen.

Tracheobronchoscopy is invaluable in investigating airway obstruction. This can allow a good view of the lumen of the trachea and the bronchi and can enable retrieval of a biopsy sample if a mass lesion is seen. In very small patients this is not possible and the scope may compromise the airway. Most of the larger endoscopes allow oxygen to be infused through the scope. In these patients total intravenous anaesthesia using propofol may be necessary. Additional oxygen supplementation for patients undergoing tracheobronchoscopy may be provided by oxygen insufflation via a dog urinary catheter passed into the airway adjacent to the endoscope.

Treatment

The treatment of lower airway obstruction varies depending on the cause. On the whole it is preferable to avoid surgery where possible as this invariably involves a thoracotomy and hence considerable patient morbidity.

Tracheal foreign bodies are most commonly seen in cats and are best treated by forceps removal under fluoroscopic or endoscopic guidance. The author favours fluoroscopy as it is quick and safe. Recovery is normally rapid and complication free and no specific post-removal treatment is necessary. Bronchial foreign bodies are best removed with the aid of bronchoscopy. Bronchial foreign bodies are most commonly seen in active dogs and are frequently grass seeds or grass awns. If it is not possible to remove the foreign body in this way then surgical removal via a thoracotomy is indicated. Foreign bodies in the trachea or mainstem bronchus are removed via a tracheotomy or bronchotomy. Foreign bodies located in the small airways and those which have formed an abscess are best treated by removal of the affected lung lobe.

Neoplasia of the lower airway is very uncommon. Biopsy of a tracheal or bronchial mass can be achieved under endoscopic guidance but surgery is necessary to attempt removal after definitive diagnosis in those tumours that are surgically resectable.

Tracheal stenosis can be caused by trauma to the trachea. This can be related to traumatic rupture or associated with the use of low-volume, high-pressure cuffed endotracheal tubes in cats. Over-inflation of these cuffs can cause damage to the tracheal mucosa and subsequent scarring and narrowing can occur. Tracheal stenosis can be treated medically with corticosteroids and balloon dilatation or surgically by resection of the stenotic area and reanastomosis of the trachea.

Post-Treatment Care

Post-treatment or postoperative care varies according to the cause of the obstruction and the interventions that were performed. Patients treated non-surgically for tracheal or bronchial foreign bodies rarely require any specific treatment. Relief of the obstruction results in an immediate post-anaesthetic improvement in clinical signs. Antibiotics may be indicated in patients with bronchial foreign bodies to manage the post-obstructional pneumonia that commonly accompanies these. Corticosteroids (at an antiinflammatory dose) may be given in patients with inflammation of the airway or if there was excessive manipulation during removal.

Patients may benefit from oxygen supplementation following anaesthesia until they are alert and able to maintain their own airway. Arterial blood gas analysis provides the best method of assessing oxygenation and can be used to help decide on the provision of oxygen. When this is not available pulse oximetry provides some information.

Cats have particularly sensitive airways and excessive manipulation may result in severe bronchoconstriction, hypoxaemia and bradycardia. This should be treated with oxygen therapy, bronchodilators and, potentially, anti-inflammatory doses of corticosteroids.

Non-cardiogenic (or neurogenic) pulmonary oedema is a rare complication that may cause continued deterioration in the patient's dyspnoea and oxygen saturation in the hours following removal. There is no specific therapy for this form of pulmonary oedema other than oxygen supplementation. Diagnosis is based upon radiographs which reveal bilateral alveolar infiltrates particularly in the dorsocaudal lung lobes.

Patients that have undergone a thoracotomy or median sternotomy require the most intensive nursing. These patients will invariably have a thoracostomy tube in place. The tube should be drained in an aseptic manner as frequently as necessary. Multimodal analgesia should be provided with opioids (morphine or methadone), local anaesthetic agents instilled down the thoracostomy tube (ropivacaine or bupivacaine) and non-steroidal anti-inflammatory drugs (provided that corticosteroids have not been administered). The patient should be kept comfortable, preferably on a padded bed or mattress. Routine monitoring of the patient's parameters including temperature, pulse rate and quality, respiratory rate, invasive or non-invasive blood pressure, arterial blood gas analysis, serum electrolytes and urine output should be performed as necessary and used to help adjust postoperative care. Oxygen supplementation and intravenous fluid therapy should be provided as necessary.