Pneumothorax

Pneumothorax

Elizabeth A. Rozanski, North Grafton, Massachusetts

Erin Mooney, Melbourne, Australia

Pneumothorax is defined as free air in the pleural space. Normal intrapleural pressure is approximately −5 cm H₂O, which means that, in the healthy dog or cat, intrapleural pressure is negative compared with the atmosphere. This negative intrathoracic pressure helps to maintain lung expansion and promote venous return. Normal intrapleural pressure varies depending on the phase of respiration, with inspiration associated with more negative pressure during spontaneous breathing. Pneumothorax develops when there is either leakage of air from damaged or diseased pulmonary parenchyma or external damage to the thoracic cavity resulting in the entrance of air into the chest cavity from the atmosphere. Clinical signs of pneumothorax include a short and shallow (“restrictive”) breathing pattern and absent lung sounds on thoracic auscultation. Diagnosis of pneumothorax may be confirmed with thoracic radiographs (Figure 165-1) or by a positive thoracocentesis. Computed tomography (CT) will also highlight pneumothorax (Figure 165-2). In emergency practice, limited ultrasonography of the thoracic cavity (thoracic focused assessment with sonography for trauma [T-FAST]) also has been used to detect pneumothorax in dogs and cats with suspected traumatic pneumothorax (Lisciandro, 2008). Pneumothorax can be characterized as traumatic, spontaneous, or iatrogenic, and as open or closed (Pawloski and Broaddus, 2010). A tension pneumothorax can result from any type of pneumothorax, although it is usually closed. It is characterized by a barrel-shaped (hyperinflated) appearance to the chest cavity with limited thoracic motion despite vigorous movements of the head and neck in an attempt to ventilate, and by hypoxemia and cardiovascular collapse caused by continued entrapment of air within the pleural space due to lack of an escape pathway.

Figure 165-1 Lateral thoracic radiograph documenting pneumothorax in a dog. Note apparent “lifting” of the heart off of the sternum and retraction of the dorsal lung fields.

Figure 165-2 Computed tomographic image of spontaneous pneumothorax in a dog showing retraction of the lung from the rib cage on the left and pleural air accumulation.

Therapeutic Options for Treatment of Pneumothorax

The recommendations for management of pneumothorax differ based on the cause. Simple supportive care is adequate in some cases, whereas other cases benefit from more aggressive medical or surgical therapy including placement of a thoracostomy (chest) tube. However, it is important to remember that clinically significant pneumothorax may develop even if a chest tube is in place, and if there is any doubt as to the patency of the chest tube, a new tube should be placed or needle thoracocentesis performed in a patient showing new or progressive respiratory distress.

Observation

Observation is appropriate for the animal with a small-volume pneumothorax of known cause detected radiographically that is not showing clinical signs or evidence of progression, because small volumes of air will be reabsorbed across the pleural space over a period of a few days to a week. However, ideally, animals with pneumothorax should be observed continuously for the first 24 to 36 hours.

Supplemental Oxygen

Oxygen therapy has been proposed as a method to hasten the resolution of pneumothorax. For animals breathing room air, the major component of pleural air is nitrogen gas (78%) and a much smaller proportion is oxygen (21%). Thus, if supplemental oxygen is provided to a patient, the relative partial pressure of nitrogen decreases in the surrounding tissues, and nitrogen is reabsorbed more quickly. Regardless, if pneumothorax is causing clinical signs, the clinician should evacuate the pleural space rather than relying solely on oxygen therapy.

Needle Thoracocentesis

Pneumothorax can be treated by evacuation using a syringe, extension set, and needle or butterfly catheter. Intravenous catheters also may be used in place of a needle and have the advantage of limiting the potential for iatrogenic trauma, but slightly more technical acumen is required to use them effectively. Needle thoracocentesis is a good first-line approach for most cases of pneumothorax. Recall that the mediastinum in small animals typically is not intact, and although bilateral chest tap may be performed, there is not a clinical concern regarding the volume of air withdrawn from each hemithorax. Caveats include the need to make sure the needle is long enough to reach the pleural space as well as the need to recognize that, if there is a large volume of intrapleural air, needle thoracocentesis may be insufficient to allow rapid recovery of ventilation. Classically, needle thoracocentesis for removal of air is performed more dorsally on the chest wall; however, in severe cases air can be retrieved from any location.

Small-Bore Chest Tube

A small-bore chest tube has been introduced in recent years as a modification of a jugular catheter (MILA chest tube). These catheters (14 gauge [6.3 Fr]) are placed readily using a modified Seldinger technique and can be secured easily in an awake patient. They are proving particularly popular in patients with moderately large-volume pneumothoraces, and their use appears to be more comfortable than placement of standard large-bore chest tubes or intermittent thoracocentesis. Small-bore chest tubes are less effective when continuous suction is required because generation of continuous negative pressure (–15 to –25 cm H₂O) appears to collapse the chest tube. Sterile red rubber feeding catheters also are recruited occasionally for use as chest tubes. These are an acceptable alternative, although they typically require general anesthesia or heavy sedation combined with local analgesia for placement because of the need to tunnel the catheter under the skin. A thoracic radiograph should be taken after placement of a small-bore chest tube to ensure adequate location in the thorax.

Large-Bore Chest Tube

Large-bore chest tubes (20 to 36 Fr) are placed when larger-volume pneumothoraces are present or when continuous suction is chosen as a therapeutic modality. Large-bore chest tubes typically are inserted under general anesthesia in all but the most severely compromised patients, although occasionally clinicians prefer placement using local anesthetics. As with small-bore chest tubes, thoracic radiographs should be obtained after placement of a large-bore chest tube to confirm proper positioning.

Heimlich Valves

Heimlich valves are one-way valve devices that attach to large-bore thoracostomy tubes. They are used very commonly in people and large animals such as horses. In cats and in dogs weighing less than 20 kg Heimlich valves are potentially less effective because of limited intrathoracic pressure changes. However, a recent report describes successful use of Heimlich valves in 34 dogs (Salci et al, 2009), and their use should be considered, particularly when continuous-suction units are not available. Each Heimlich valve costs approximately $30 USD. A potential problem with valves is that blood or other fluid sometimes can collect in the valve. This can lead to clogging and malfunction of the device. Patients with Heimlich values should be closely monitored.

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Tags: Kirks Current Veterinary Therapy XV

Jul 18, 2016 | Posted by admin in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off

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