Diagnosis of Pneumothorax
Pneumothorax occurs whenever air enters the pleural cavity and compromises the negative pressure that keeps the lungs expanded and allows the horse to breathe. Pneumothorax can be secondary to blunt trauma, penetrating wounds to the thorax from sharp objects, gunshot entry tracts, or barotrauma, or it may arise spontaneously. Open pneumothorax is a breach in the integrity of the thoracic wall, resulting in air influx and subsequent lung collapse. Closed pneumothorax is the leakage of air into the pleural space from a pulmonary source such as a bronchopleural fistula. Tension pneumothorax results from a tissue valvelike lesion that allows air to enter but not leave the pleural cavity. In all instances, air accumulates within the pleural space, leading to a loss of negative pleural pressure and possibly the development of positive pressure in the thorax. Spontaneous pneumothorax as described in humans has not been documented in horses. Therefore in the absence of coexisting lung disease presumably pneumothorax is due to trauma.
Penetrating wounds are usually obvious and may even be iatrogenic, for example in horses with indwelling thoracic tubes for the treatment of pleuropneumonia. In these cases, when fluid drainage from the tube wanes or ceases, the one-way valve at the end of the tube can become incompetent and allow air to be sucked into the pleural cavity. Rib fractures can be obvious when accompanied by external thorax wounds. However, they can be internal as a result of blunt trauma. The latter most commonly are observed as a result of birth trauma in neonates. In these cases a sharp rib fragment may lacerate a lung and allow for bronchopleural fistulation and subsequent lung collapse.
Hemothorax is the most commonly encountered consequence of lung laceration, and pneumothorax as a result of the injury is rare. Although rare, both pneumothorax and hemothorax may occur concurrently. Bronchopleural fistulas also may develop in association with gangrenous pneumonia or pleuropneumonia, but pneumothorax as a complication of this problem is relatively uncommon because the process of insidious necrosis promotes adherence or scarring to the thoracic wall.
The clinical compromise created by pneumothorax coincides with the collapse of one or both lungs secondary to the positive air pressure within the thoracic cavity. Pneumothorax may tend to be bilateral because horses commonly have an incomplete mediastinum. In cases of pleuropneumonia in which indwelling thoracic tubes have created a tension pneumothorax, pneumothorax is usually unilateral because the pleural effusion and inflamed mediastinum tend to prevent air movement between the pleural cavities.
Clinical Signs of Pneumothorax
The clinical signs of pneumothorax include an acute increase in respiratory rate, sweating, cyanosis, and in profound cases of bilateral lung collapse, severe dyspnea. These clinical signs should provide for the primary suspicion of pneumothorax whenever an obvious thoracic wound, injury, or indwelling thoracic tube is present.
The diagnosis of pneumothorax secondary to thoracic trauma often can be made from the history but not by auscultation alone. Thoracic radiography can reveal a horizontal shadow beneath the thoracic transverse processes, which is consistent with a “line” representing a collapsed lung(s). Thoracic radiographs also can reveal primary lesions such as rib fractures or penetrating foreign bodies.
Ultrasonographic evaluation of the thorax reveals horizontal air artifacts in the midthoracic to dorsal regions, without the defining pattern of the pleura. In these cases, although the parallel lines of air reverberation artifact is present as is the case in horses with normal lungs, the examiner is not able to identify the sliding motion of the visceral pleura against the parietal pleura as is apparent in a normal examination. This finding, in conjunction with the clinical examination, should be considered pathognomonic for pneumothorax and is an indication for suction as an immediate treatment for the respiratory distress.
When neither radiography nor ultrasonography is available, the clinician should not hesitate to use suction as a means of diagnosis. The relief provided by treatment should be considered consistent with the diagnosis of pneumothorax.
Treatment of Pneumothorax
The preferred treatment is the prompt removal of the free air by suction of the dorsal reaches of the thorax. This procedure rapidly reexpands the lung and relieves respiratory distress. In horses with an open thoracic wound, surgical closure of the wound is necessary regardless of whether the pneumothorax is unilateral or bilateral. If both lungs are collapsed, surgical closure may require intubation under anesthesia and positive pressure ventilation during the thoracic repair. When an indwelling thoracic tube is the cause of the pneumothorax, the tube should be clamped and the Heimlich valve or condom replaced.
Thoracic cavity air suction is performed in patients with a competent external thorax by surgically preparing a penetration site in the upper midthorax between the ribs. The site is anesthetized locally followed by a stab incision made full thickness through the skin using a number-15 scalpel blade. Next a 4-inch blunt teat cannula with an optional three-way stopcock is inserted into the incision and advanced through the intercostal musculature until the popping sensation denoting penetration into the pleural cavity is appreciated. If a suction apparatus is available, the cannula is attached to vacuum tubing and a vacuum is applied. If a suction machine is not available, air can be repeatedly aspirated by use of a 60-ml syringe. The lung should be reinflated sufficiently to begin rubbing or causing friction against the tip of the teat cannula. Ultrasound can be used to detect the return of the sliding visceral pleura once reinflation has occurred. The cannula is then removed and a single interrupted suture is placed aseptically in the skin for closure of the incision site. The patient should show an obvious decrease in respiratory effort and dissipation of respiratory distress. Thoracic radiographs can be obtained to further document the expansion of the lung, but in most cases, ultrasound imaging along with the patient’s relief, are sufficient to document the success of the procedure.