RESPIRATORY DISTRESS IN HORSES
Emergency treatment Upper respiratory tract obstruction
Partial or complete upper airway tract obstruction can occur due to a variety of reasons ranging from severe nasal or pharyngeal oedema and soft tissue swelling, strangles, arytenoid chondritis, obstruction by a foreign body to bilateral laryngeal spasm or paralysis. The lat- ter can occur following general anaesthesia, with severe hepatic dis- ease, lead or other toxicities or for unknown reasons (Abrahamsen et al., 1990 ; Allen, 2010 ; Dixon et al., 2001 ; Hughes et al., 2009 ). Irrespective of the underlying cause, the clinical presentation is often similar as outlined above and an emergency tracheotomy is usually the only effective treatment option. If the horse is stable, it should be sedated. Starting with a low dose of a short acting seda- tive such as xylazine (0.2–0.4 mg/kg; 100–200 mg or 1.0–2.0 mL for a 500 kg horse) might be advisable to avoid possible collapse. If the horse is stable and not sufficiently sedated, higher doses or a longer acting drug, such as detomidine with or without butorphanol, can be administered. An area of skin is clipped, aseptically prepared and in- filtrated with a local anaesthetic between the upper and middle third of the neck where the trachea can be palpated. An approximately 10 cm long incision is made vertically on midline, through the skin, subcutaneous tissues and cutaneous colli muscle and then extended between the paired sternothyrohyoideous muscles (Figure 3a ). Once the trachea is exposed, a horizontal incision is made through the an- nular ligament between two adjacent tracheal rings, avoiding the tracheal cartilages and extended to the left and right (Figure 3b ). The incision should not exceed more than third of the tracheal cir- cumference. If available, a self- retaining metal tracheostomy tube or short cuffed silicon tube can then be inserted and secured in place (Figure 3c ). Alternatively, a piece of nasogastric tube or large plastic syringe casing can be used (Prange, 2019 ). The relief of res- piratory distress is almost instantaneous and respiratory rate and effort should improve within 1–2 h. Persistent or worsening distress
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despite resolution of an upper respiratory tract obstruction could indicate a secondary problem such as negative pressure pulmonary oedema (see below).
Pneumothorax
Pneumothorax in horses often arises from blunt or penetrating thoracic trauma or injury to air- containing structures (trachea, oe- sophagus, lung parenchyma). A recent case report describes hae- mopneumothorax after a bronchoalveolar lavage procedure (Kopec et al., 2024 ). It can also be secondary to pleuropneumonia causing pulmonary necrosis or formation of bronchopleural fistulas. Another cause is tracking of air from subcutaneous emphysema, after airway surgery or a pneumomediastinum into the pleural space as it might occur following a deep axillary chest wounds or rupture of the ret- ropharyngeal recess. Pneumothorax can be unilateral or bilateral in horses suggesting that some have a complete and others an incom- plete mediastinum (Boy & Sweeney, 2000 ). A tension pneumothorax describes a situation where air enters the pleural space with each inspiration but cannot escape with expiration, leading to progres- sive accumulation of air and increasing intrathoracic pressures. This, in turn, results in progressive pulmonary collapse, compression of the vena cava causing decreased venous return and decreasing car- diac output which ultimately results in cardiovascular collapse (Tran et al., 2021 ). Clinical signs are variable depending on the speed of onset, extent and concurrent disease processes. Auscultation can be helpful in identifying a pneumothorax, but has a low sensitiv- ity, being diagnostic in less than 40% of cases in one study (Boy & Sweeney, 2000 ). Ideally, ultrasonography or radiographs are used to confirm a diagnosis but obtaining radiographs might be difficult in an emergency situation and ultrasonography can be hampered by the presence of subcutaneous emphysema. If the combination of history and clinical findings strongly suggests a pneumothorax, aspiration of air from the thoracic cavity via a needle thoracocentesis can be
FIGURE 3 To perform a tracheotomy, the area is clipped, sterilely prepared and infiltrated with local anaesthetic before a longitudinal incision is made (a). A second incision is then made horizontally between the tracheal rings (b) to allow insertion of a tracheostomy tube (c).
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