Discussion

Whilst acute respiratory distress is an uncommon presentation in equine practice, it often requires rapid evaluation and treatment if a successful outcome is to be achieved. Typically respiratory distress will be used to describe a horse showing significant dyspnoea and/or tachypnoea. Signs of respiratory distress may therefore include an increased respiratory rate, an abnormal respiratory pattern and thoracic/abdominal movements and an abnormal respiratory noise. These may be accompanied by one or more of an abnormal respiratory discharge, coughing, flaring of the nostrils, soft tissue swelling adjacent to the respiratory tract and abnormal appearance to the mucous membranes.
The first assessment to make in the examination of a horse with acute respiratory distress is to determine if it is immediately life-threatening. Dramatic abdominal excursions with a loud respiratory noise are always a cause of concern, and may be an immediate prelude to agonal gasping. In these situations the horse may start to stagger and collapse and attending personnel should be warned as some horses will start to show violent, uncontrolled movements as cerebral hypoxia develops.
Initial examination should be made from a distance with visual inspection of the horse. One vital feature to ascertain is whether the respiratory distress is inspiratory or expiratory in nature, or a combination of both.
Inspiratory dyspnoea will usually be associated with an exaggerated and prolonged inspiratory phase with increased inspiratory effort. A markedly increased abdominal effort during inspiration will be present as the ventral abdomen drops. Dilation of the nostrils and extension of the head and neck will often be present.
Expiratory dyspnoea is usually associated with a prolonged and laboured expiratory phase with an exaggerated expiratory contraction of the abdominal muscles as the ventral abdomen lifts. Protrusion of the anal sphincter during expiration may be seen. Nostril dilation and extension of the head and neck may also be present.
In general terms inspiratory dyspnoea is a result of extra-thoracic (URT) airway obstruction whilst expiratory dyspnoea is a result of intra-thoracic (LRT) airway obstruction. Inspiratory dyspnoea may also be seen in cases with a space-occupying mass in the thorax or restrictive lung disease. Combined inspiratory and expiratory dyspnoea will usually be a result of a physical airway obstruction.
Auscultation of the respiratory tract may be helpful in determining the site and nature of any respiratory obstruction. It should be performed over both the larynx and trachea and the lung fields. However, care should be taken in the interpretation of any airway sounds as, particularly in the dyspnoeic horse, abnormal sounds may be referred from a distant site. The absence of respiratory sounds at a site over the thorax is significant and may represent the presence of a thoracic mass, pleural fluid or a pneumothorax.
The presence of flaring of the nostrils usually is an indicator of severe respiratory disease. Nostril airflow should also be assessed. Altered airflow may be present at one or both nostrils depending on the site of any airway obstruction.

URT disease
Causes of URT respiratory distress are varied and can be divided into nasal obstruction and laryngeal/pharyngeal obstruction.

Nasal obstruction
This must be bilateral to cause a significant obstruction to airflow. It is generally uncommon but causes include nasal trauma,

anaphylaxis, insect sting/snake bite, bilateral jugular vein thrombosis and oedema as a result of lowered head/neck resulting from sedation or recovery from anaesthesia.
Clinical examination, in particular external palpation and inspection, will usually allow recognition of the primary disease and is essential to allow effective treatment.

Laryngeal/pharyngeal obstruction
These would be the most common causes of URT obstruction and can be rapidly life-threatening. Oedema is often a significant component and the increased airflow turbulence and increased negative thoracic pressure will only exacerbate the oedema. In an emergency creation of a patent airway is essential. If time allows a surgical tracheotomy is preferred but in a cyanotic recumbent horse then passage of a nasotracheal tube is faster. Causes include laryngeal oedema, arytenoid chondropathy, laryngeal paralysis, retropharyngeal lymphadenopathy, guttural pouch empyema, pharyngeal trauma/foreign body and neoplasia. Diagnosis may be apparent from the clinical history and clinical examination but endoscopy is often required. Care should be taken that the restraint and manipulations required for endoscopy do not worsen the respiratory obstruction.

LRT disease
LRT diseases: include tracheal obstruction and intra-thoracic disease.
Tracheal obstruction may be caused by collapse of the trachea or a tracheal foreign body. Endoscopy is required to confirm the diagnosis.
Intra-thoracic conditions will usually cause respiratory distress without respiratory noise as a major feature. Potential causes include pneumothorax, pulmonary oedema, acute respiratory distress syndrome, pleuropneumonia, RAO and infiltrative pulmonary disease.
Pneumothorax is usually a result of physical trauma to the thoracic wall, the lung parenchyma or a combination of both. In most instances respiratory distress will only be apparent when a bilateral pneumothorax is present. Pulmonary oedema may result from direct lung injury, as in smoke inhalation, or systemic conditions such as left sided heart failure or endotoxaemia. A frothy (often blood tinged) airway discharge is often a feature. Acute Respiratory Distress Syndrome is often poorly defined but includes conditions which cause a rapid infiltrative lung disease such as aspiration pneumonia, smoke inhalation, and R. equi infection. Although usually a more chronic disease, pleuropneumonia can cause acute respiratory distress, especially with a large volume of pleural effusion and marked endotoxaemia. RAO is also a chronic disease but can present with acute signs related to marked small airway obstruction by bronchoconstriction and mucus. Affected horses invariably have a history of respiratory disease with a cough and exercise intolerance and there is a pronounced increased expiratory effort.