Pleural conditions of surgical interest in dogs and cats

PYOTHORAX: The aetiology of pyothorax in dogs and cats is in most cases unknown. In dogs, the aspiration of orally contaminated vegetable material (grass awns) is considered to be the most common route of infection. In feline pyothorax, the extension of pulmonary infections seems to be the most frequent cause but other suspected infection routes include: penetrating chest, neck or mediastinal wounds, oesophageal perforations, haematogenous or lymphatic spread from septic foci (discospondylitis), lung neoplasia or abscessation, iatrogenic causes from thoracocentesis or thoracotomies.

Thoracic radiographs mostly reveal bilateral effusion and ultrasonography is useful to detect septated fluid pockets and pleural masses. In the majority of cases cytological fluid evaluation shows the presence of cocci, rods and in some cases filamentous bacteria like Nocardia or Actinomyces. In the presence of Nocardia and Actinomyces contamination, culture results are often negative and non-degenerate neutrophils are present. Diagnosis is often based on cytological or histological evidence of filamentous bacteria.

Pasteurella spp.are frequently isolated in cats, whereas enteric organisms such as E. Coli are more frequently isolated in dogs. In 60% of positive cultures obligate anaerobes (Bacteroides, Fusobacterium) are identified and one-third of these cultures present a combined contamination of aerobes (E. coli, Pasteurella spp., Streptococcus and Staphylococcus) and anaerobes (Figs. 1 and 2)._{^{1, 2}}

CHYLOTHORAX

Chylothorax is a debilitating condition in dogs and cats characterized by the accumulation of chyle within the thoracic cavity (Fig. 3). Fossum et al. (2004) reported several different possible aetiologies, but generally chylothorax is considered idiopathic as the final cause cannot be found_³. In dogs and cats with idiopathic chylothorax (IC) the prognosis is guarded and non-invasive treatments are unsuccessful (Birchard et al. 1993, 1995, 1998). Results have shown that in dogs undergoing surgical management of IC the prognosis is fair._⁴

Patients with chronic chylothorax may develop fibrosing pleuritis, which is characterized by pleural thickening and pulmonary expansion restriction.

Chylous effusion associated with any of the aforementioned differential diagnosis for primary causes (primary cardiac disease, lung lobe torsion and lymphatic drainage obstructions) should be managed by elimination or treatment of the primary cause. In these cases the treatment should be supported by repeated thorachocentesis until resolution of the condition. In patients with thoracic duct trauma the chylothorax should resolve within one week when treated with thoracostomy drain placement.

Unfortunately, many different surgical treatments have been described for IC, suggesting that a curative therapy has not yet been found.

Conventional treatment of IC involves thoracic duct ligation combined with cisterna chyli ablation and subphrenic pericardiectomy. The addition of pleural omentalisation does not seem to improve outcomes_⁵.

Thoracic duct ligation leads to lymphatic hypertension in the lymphatics caudal to the ligature site, thus promoting the formation of collateral lymph vessels that may bypass the ligature itself. Ablation of the cisterna chyli both relieves lymphatic hypertension in the caudal thoracic duct and promotes new routes of abdominal lymphatic drainage (Hayashi et al. 2005). Furthermore, pericardiectomy decreases right heart and venous pressure, releasing the lymphatic venous junction obstruction. The single ligation of the thoracic duct has a low success rate, likely due to the existence of thoracic duct anatomical variability in the same species. Although appropriate ligature of the thoracic duct is performed, often one or two lymphatic vessels branch from the thoracic duct caudal to the ligature, leading to chyle effusion recurrence and surgical failure. Identification of the thoracic duct can be technically challenging as it is often collapsed and because of its thin wall; it may at times be impossible to identify against the mediastinum. Moreover, the mediastinum can be thickened and more opaque than normal, further precluding its visualization. For all these reasons, “en bloc” ligation of the caudal mediastinal tissue between the aorta and the thoracic vertebrae, preserving the parasympathetic trunk, has been reported and has become more widely used compared to thoracic duct dissection.

PNEUMOTHORAX

Pneumothorax is characterized by the accumulation of free air within the thoracic cavity (Fig. 4). To understand the pathophysiology of pneumothorax the dynamic relationship existing between the lungs and the chest wall must be well known. As previously described, the pleural space is a “potential space” containing few millilitres of fluid with a subatmospheric intrapleural pressure of -5 cm H20 at rest. This pressure represents the difference between the forces that recoil the lungs and those that expand the thorax.  When air enters the pleural space, the negative pressure diminishes, the interaction between the lung and thoracic wall is lost and the lungs become atelectatic. Atelectasis leads to a ventilation/perfusion mismatch. This can lead to hypoxaemia, which in turn may result in myocardial dysfunction and ultimately death if not corrected. When the lungs collapse, the tidal volume is reduced and tachypnea is the first respiratory response in the attempt to maintain the minute ventilation. The cardiovascular system is also negatively affected by the presence of a pneumothorax. Loss of the negative intrapleural pressure results in a decreased venous return to the heart and hypoxaemia can lead to pulmonary vasoconstriction with increased vascular resistance, right-sided heart failure and reduced cardiac output. Progressive pneumothorax results in increasing hypoxaemia and diminishing cardiac output. Prompt identification and reversal of these alterations are essential for the animal’s survival. Thoracic auscultation will reveal decreased bronchovesicular lung sounds and muffled heart sounds.

Air gains access to the pleural cavity by pleuro-cutaneous (e.g. penetrating thoracic trauma), pleuro-pulmonary (e.g. tracheal, bronchial or pulmonary lesions) or pleuro-oesophageal (e.g. oesophageal lesions or perforations) pathways.

Pneumothorax is classified as traumatic, spontaneous or iatrogenic based on its aetiology or in open or closed according to its pathophysiology. Traumatic pneumothorax is the most frequent form of pneumothorax diagnosed in cats and dogs._⁶

TRAUMATIC PNEUMOTHORAX
Patients with traumatic pneumothorax present with an obvious history of trauma (road traffic accident or high-rise syndrome). These patients often have other injuries associated with the trauma, such as lacerations, rib, bone and vertebral fractures, diaphragmatic hernia, haemothorax and pulmonary contusion (Fig. 5); radiographs should therefore be evaluated carefully for occult or subtle lesions. In the presence of air, the lungs become atelectatic and appear more radiopaque compared to normal lungs. On recumbent lateral views the cardiac silhouette appears elevated from the sternum.

Traumatic pneumothorax can be open or closed, determined by either the presence or absence of a penetrating thoracic wound or of an open communication between the pleural space and the environment. Trauma can be the cause of lung parenchymal rupture and consequently of a closed pneumothorax; fractured ribs can also lacerate lung lobes, resulting in the accumulation of air into the pleural space originating from the respiratory system.

Penetrating thoracic wounds, resulting in an open pneumothorax, have been termed “sucking wounds”, as the air influx into the pleural space occurs when the thoracic cavity expands on inspiration. Traumatic open pneumothorax can also result in a tension pneumothorax. This is the most severe form of pneumothorax and it occurs when a flap of skin or pulmonary lesions act as one-way valves, allowing air into the chest cavity during inspiration and preventing expulsion during expiration. The continued accumulation of air rapidly results in supra-atmospheric pressure within the chest, causing pulmonary atelectasis. If the condition is not promptly treated the ventilation is compromised and the patient experiences cardiovascular shock and death. Radiographs may show a mediastinal shift to the contralateral hemithorax, widened rib spaces and flattening of the hemidiaphragm on the affected side. Traumatic pneumothorax may also be associated with flail chest. Flail chest is characterized by fracture of several consecutive ribs, which lose their continuity with the rest of the thoracic wall and result in paradoxical movements of the separated chest wall segment.

SPONTANEOUS PNEUMOTHORAX
Patients with spontaneous pneumothorax do not have a history of trauma. They are presented with a few days history of sudden shallow rapid breaths, anorexia, coughing and lethargy.

Spontaneous pneumothorax is generally closed and has several different aetiologies. The most common cause of spontaneous pneumothorax is the rupture of pulmonary blebs or bullae (Fig. 6). Pulmonary blebs result from air accumulation within the visceral pleura, whereas bullae are the result of disruption of intra-alveolar septa and fusion of contiguous alveoli and are intraparenchimal lesions. When the aetiology of the pneumothorax is unknown, the condition is classified as primary spontaneous pneumothorax; if an underlying disease is present, like for example pulmonary neoplasia, bacterial or viral pneumonia, migrating foreign body or pulmonary abscess the pneumothorax is classified as secondary. Feline asthma can predispose cats to closed spontaneous pneumothorax.

In patients with spontaneous pneumothorax radiographs should be evaluated for potential underlying causes. The presence of bullae or blebs may be difficult to appreciate; however, these lesions are occasionally visible. Computer tomography (CT) is routinely used in human medicine for the diagnosis of bullae and blebs because CT is considered much more sensitive than radiography._⁷

IATROGENIC PNEUMOTHORAX
Pneumothorax can develop after thoracocentesis or thoracic fine needle aspiration. In cats, iatrogenic pneumothorax has been also associated with tracheal rupture due to intubation. Overinflation of the endotracheal tube cuff is the most common cause of tracheal rupture or perforation. Animals with tracheal tears are usually presented with subcutaneous emphysema and pneumomediastinum. Positive end-expiratory pressure mechanical ventilation can lead to barotrauma and pneumothorax, especially in patients with pulmonary parenchymal disease requiring high pressure._⁸

MANAGEMENT OF THE MAIN PLEURAL SPACE DISEASES

The degree of respiratory distress in animals affected by pleural space diseases depends on the severity and duration of the condition. Larger volumes of air and more acute presentation result in more severe respiratory distress.

THORACOCENTESIS

Thoracocentesis is a simple therapeutic and diagnostic procedure which allows the quick removal of fluid or air from the pleural space. In dyspnoeic patients it should be performed immediately, together with oxygen administration, as it greatly improves the patient’s ability to ventilate by increasing the space available for pulmonary expansion. The equipment needed for thoracocentesis is minimal (Fig. 7).  In most cats and dogs, butterfly needles are well suited for thoracocentesis because they are available in different lengths and sizes sufficient to reach the thoracic cavity (20-22 gauge for cats or small dogs and 18-20 gauge for medium to large dogs). Over the needle catheters may also be used. Once the inner stylet is removed the flexible tip is atraumatic for the pulmonary parenchyma; it can however kink during the procedure obstructing drainage.

If the condition of the patient allows it a dorsoventral radiograph is recommended prior to thoracocentesis. This may document the presence of pleuric effusion or pneumothorax and may indicate which side might be the most suitable to evacuate first. Radiographs are also taken after thoracocentesis, in order to verify adequate aspiration of the chest and to detect potential underlying causes which could have been obscured by the presence of the effusion or by the collapsed lung lobes.

A three-way stopcock is placed between the end of the butterfly needle and the syringe. It is worthwhile to infiltrate a local anaesthetic under the skin at the insertion site before inserting the needle. Thoracocentesis is usually performed on conscious patients. With the animal standing, or even better placed in sternal recumbence (Fig. 8), the midventral aspect of the thorax is bilaterally clipped and aseptically prepared for thoracocentesis. The preferred site for the centesis is between the seventh and the ninth rib spaces (Fig. 9).

If fluid and air is present in the pleural cavity, the needle is inserted approximately half way up the chest wall. If only fluid is present, the needle should be inserted in the ventral third of the chest wall or in the dorsal third when dealing with a pure pneumothorax. Use of sterile gloves and removal of as much fluid as possible is recommended. To avoid the intercostal vessels and nerve, lying on the caudal aspect of each rib, the needle should be introduced close to the cranial rib border. To avoid pulmonary trauma, the needle should be inserted with a 45° angle. Once the needle is carefully advanced in the thoracic cavity, with the bevel facing the lung, under ultrasonography control, an assistant should aspirate the fluid and collect it in a kidney dish. A sample of the fluid (5 to 10 ml) should be submitted aseptically for analysis. This should be preferably collected before beginning any antibiotic treatment. To ensure complete evacuation of the pleural space, bilateral thoracocentesis is recommended.

THORACOSTOMY TUBE PLACEMENT

In presence of on-going accumulation of fluid or air in the pleural cavity and when frequent or repeated thoracocentesis are required, the placement of a chest tube is indicated.  This allows the repetition of less traumatic thoracic effusion or air evacuation and pleural cavity lavage. A chest drain may form part of the definitive treatment plan (like in the case of pyothorax) or may be used for stabilisation of a patient before the definitive surgical treatment (like in the case of spontaneous pneumothorax). Chest drains are also placed at the end of thoracotomies to allow evacuation of iatrogenic pneumothorax, re-expansion of the lungs and collection of the residual fluid in the postoperative period. Furthermore, thoracotomy tubes can also be used to deliver intrapleural local anaesthetic after thoracic surgery.

Commercially available chest drains are made of silicone or PVC and are flexible. The tip of thoracostomy tubes usually present multiple fenestrations. These disrupt the radiopaque strip running along the length of the tubes, so that the intrapleural location of the holes can be visualized radiographically. Chest tubes may be accompanied by blunt or sharp stylets that can add stiffness and facilitate thoracic wall perforation respectively (Fig. 10).

In emergency, a chest drain may be placed in conscious patients, using a combination of local nerve block and sedation. However, it is recommended to anaesthetize the animal as the procedure results less stressful for the animal and intubation enables direct provision of oxygen and manual positive pressure ventilation.

The material required for chest tube placement should be prepared previously in order to have everything ready when the drain is inserted. Materials needed for each drain include: chest drain connector, three-way stopcock, two bungs, gate clamp, 20-50 ml syringe and ideally a Heimlich valve in case of pneumothorax.

The chest drain is selected based on the size of the patient. The width must correspond to that of the main stem bronchus and it has to be smaller than the intercostal space where it is supposed to be inserted. Usually, 14-16 Fr drains are used for cats and very small dogs; 18-24 Fr for small and medium dogs; 26-36 Fr for large to giant dogs.

Based on the radiographic findings the thoracostomy tube may be placed mono-or bilaterally.

The entire hemithorax should be clipped, prepared and draped, including the 13th rib in the field. Ideally, clipping and surgical preparation should be performed whilst the patient is conscious in sternal recumbence; anaesthesia is then induced rapidly and the drain inserted with the patient lying on the side. An assistant pulls the skin cranially. A small skin incision is then made with a blade in the dorsal third of the 10_^th or 11_^th intercostal space (ICS). Using the stylet or vascular forceps a tunnel under the skin is created and the tip of the drain advanced in cranio-ventral direction to approximately the 8_^th ICS (Fig. 11).  Tunnelling provides a “flap valve” effect that limits entry of air into the chest along the tube’s surface. The drain is held firmly at its base aiming the stylet towards the contralateral elbow. It is then introduced into the chest by means of a controlled push at the distal end. Once the thoracic wall has been penetrated, the drain is advanced to approximately the level of the 2_^nd rib, whilst the trocar is removed. Before removing completely the trocar, the drain is temporarily clamped with forceps. The trocar can now be removed, a gate clamp and a connector are placed on the drain and a three-way stopcock attached. In case of pneumothorax, instead of using the three-way tap a Heimlich valve may be connected to the drain in order to allow a constant unidirectional evacuation of the air.

The forceps can now be removed and the pleural space can be evacuated using a syringe. After the syringe drainage, two bungs are placed on the three-way stopcock and the drain secured to the chest wall by means of a purse-string skin suture placed around the base of the tube and a Chinese finger-trap suture. In some cases the Chinese finger-trap suture can slip along the drain, which is no longer held in place. To avoid this, the application of a tape over the suture, allowing the proper adhesion between the suture material and the tube, is recommended. A dressing is placed to protect the chest drain and changed once daily. All the chest drain connections must be checked at least once daily. The patient is not allowed to interfere with the drain; an Elizabethan collar must therefore be used.

Valtolina (2009) described the use of small-bore wire-guided chest drains as effective alternative to larger gauge drains. These are 14 gauge polyurethane, 20 cm long multi-fenestrated chest drains ( Mila International) placed using a modified Seldinger technique._⁹

An introducer catheter, provided in the Mila Chest Drain Kit (Fig. 12) is tunnelled subcutaneously to the 8_^th intercostal space and entered the pleural space at the cranial edge of the rib to minimize the risk of injuring the neurovascular bundle, situated at the caudal aspect of the rib. A J-wire is introduced into the introducer catheter and advanced into the thoracic cavity. The introducer catheter can then be removed over the guide wire, leaving the latter in place. The small chest tube is advanced into the thoracic cavity over the guide wire which can at this stage be removed. Accurate placement of the drain can be assessed by aspiration of either fluid or air depending on the underlying disease. The chest drain is finally secured to the skin through the suture holes. The use of the Mila chest tube is replacing the use of the trocar thoracostomy tubes in veterinary medicine. This is based on the experience with humans where the use of this type of chest tube is recommended as it is associated with fewer insertional and infectious complications and it is also considered more comfortable for the patients._⁹ Placement of small-bore catheters using the Sendlinger technique in veterinary patients is most of the times possible in conscious patients or with local anaesthesia (Fig. 13).

Post chest tube placement radiographs are taken to verify the correct positioning. The tube should run along the lateral thoracic wall to the level of the second, third rib; all fenestrations must be in the pleural space; the tip must not extend in the cranial mediastinum and finally the tube must not be kinked. If chest tube positioning is adjusted, radiographs are repeated.

Animals with chest drains should undergo continuous supervision in order to observe any changes in respiratory rate and effort, to prevent any interference and to ensure security of the connections.

In order to minimize the risk of nosocomial infections chest drainage should always be performed using gloves and sterile syringes. All staff involved in these procedures must wash their hands before and after dealing with these patients.

COMPLICATIONS ASSOCIATED WITH CHEST DRAINS

Complications include: leaks due to a wide skin incision and subcutaneous tunnel, leaks at improperly secured connectors, accidental displacement of the tube due to a poorly placed chinese finger-trap suture, accidental removal, blockage and subcutaneous emphysema. In some cases, inadequate drainage can require tube repositioning.

MANAGEMENT OF PYOTHORAX

Treatment of pyothorax involves chest drainage, fluid culture, lavage, pain management and antibiotics. These patients usually need immediate cardiovascular support such as fluid therapy to correct shock, dehydration, acid-base and electrolytes imbalances. Placement of bilateral thoracostomy tubes is often required to remove purulent effusion and to flush the thoracic cavity.

Drainage of the pleural space is initially performed every 2-4 hours for the first 24 to 48 hours. Within the first few days the amount of pleural effusion should decrease and the frequency of drainage can be decreased to three times a day. After maximal drainage of the purulent effusion, lavage of each hemithorax should follow. 10 to 20 ml/kg of warmed isotonic, 0,9% saline or lactated ringer’s solution is aseptically slowly instilled into one hemithorax, left for 3 to 5 minutes and then retrieved. Rolling the patient from side to side may maximise the cleaning effect of the injected solution. Seventy-five per cent of the infused volume should be collected. If respiratory distress occurs, a smaller volume of lavage solution should be used. Furthermore, the lavage process should be repeated two-three times on each side until the retrieved fluid appearance is clear. Infusion of Heparin (1500 IU/100 ml of lavage solution) may be beneficial in reducing clotting and fibrin deposition.

Repeat thoracic radiographs and cytological examination of aspirated fluid every 48 hours may be useful for monitoring the success of lavage treatment (Figs. 14 a-d). Decreasing cell counts, loss of degenerative changes of the neutrophils and absence of bacteria at cytological examination are indicative of improvement. With time, the presence of a chest drainage causes a foreign body reaction, with resultant serosanguineous pleural effusion. The pleural effusion produced by the simple presence of a chest tube is of around 2ml/kg/day pleural. If the volume of fluid removed decreases to this value and cytology shows a satisfying improvement, the chest tube/s can be removed.

Serum protein, albumin and electrolytes levels should be monitored during lavage therapy because these patients tend to rapidly develop hypoalbuminaemia and hyperchloraemia. Therefore, nutritional support is very important and above all cats may benefit from the placement of an oesophagostomy tube.

No single antibiotic is effective against the mixture of bacteria isolated from patients with pyothorax, thus a combination of antibiotics targeting the different bacterial populations is necessary.

Initially, antibiotics should be given IV until clinical improvement is noted. At this point oral treatment can begin for at least 4 to 6 weeks after chest tube removal.

Most obligate anaerobes are sensitive to amoxicillin-clavulanic acid and metronidazole, whereas several aerobic gram-negative and gram-positive bacteria are resistant to amoxicillin and first-generation cephalosporins. Non-enteric gram-negative bacteria are usually susceptible to enrofloxacin.

If clinical and radiographic signs have not improved with intensive medical treatment in 3 to 7 days, exploratory thoracic surgery is recommended. Approaches include intercostal thoracotomy if the disease can be localized to one hemithorax or median sternotomy if a full exploration is needed. Aim of the surgery is usually the breakdown of adhesions and pockets, identification and removal of foreign bodies, resection of unhealthy tissue and lavage of the thoracic cavity. Removal of necrotic tissue may help to reduce bacterial contamination and also allows better antimicrobial penetration; excised tissue should be checked for foreign bodies and submitted for histopathologic examination (Figs. 15a,b).

MANAGEMENT OF PNEUMOTHORAX

Thoracic auscultation will reveal decreased bronchovesicular pulmonary and muffled heart sounds.

The treatment of pneumothorax varies depending on the aetiology, severity and clinical presentation of the animal. Initial treatment should focus on stabilization and oxygen supplementation. Many patients with traumatic or iatrogenic pneumothorax can be successfully treated with conservative management. This include strict cage rest, oxygen and in some cases thoracocentesis. Cage rest is important to prevent continuous friction between parietal and visceral pleura and to promote sealing of any pulmonary or brochopleural lesion. In humans, the absorption rate of free pleural air is 1.25% daily (Kramek 1987). This percentage increases when supplemental oxygen is provided. Air absorption is dependent on the diffusion gradient of nitrogen; oxygen supplementation lowers blood nitrogen concentration and thus increases nitrogen absorption from the pleural space into the blood, hastening the resolution of the pneumothorax.

Thoracocentesis should be performed on both sides of the thorax, in the 7_^th-9_^th intercostal space, in the dorsal third of the lateral thoracic wall if the animal is sternal. If the patient is in lateral recumbence, thoracocentesis is performed at the level of the costochondral junction. Once the negative pressure is achieved it is important to take three-view radiographs to evaluate for evidence of lung disease that could have resulted in the pneumothorax in absence of trauma.

Open chest wounds should be covered immediately with sterile, occlusive dressing to prevent further deterioration of the pneumothorax and allow effective ventilation.

If air re-accumulation is too rapid after having restored the negative pressure a thoracocentesis needs to be repeated more than three times in a 24-hour period; if tension pneumothorax is present, placement of a thoracostomy tube is indicated. Thoracostomy tubes allow air to be removed intermittently or continuously. Intermittent drainage with syringe is in most cases sufficient and the chest is drained at regular intervals, usually every 1 to 4 hours. Intermittent drainage allows quantification of the air collected from the chest.

Continuous pleural space drainage is indicated when air accumulation is very rapid and intermittent suction is not effective in achieving negative pressure. It is also used when continuous sub-atmospheric pressure is required within the pleural space to enable sealing of small airway leaks. Continuous suction is based on underwater three-chamber suction systems. Continuous monitoring of the patient is mandatory when using a continuous drainage system because unnoticed disconnection of the system could rapidly lead to a life threatening pneumothorax.

An alternative to continuous suction systems is the Heimlich valve, designed to exploit the pressure generated by expiration to expel pleural air while preventing air from entering the pleural cavity (Fig. 16). The Heimlich valve should be used only in medium-to large dogs because small animals may not produce enough pressure on expiration to expel the air. Moreover, it should not be used in patients with pleural effusion because the valve will fill and seal, preventing air evacuation.

Thoracostomy tubes can be removed when air production is absent for 24/48 hours. The tube is retrieved with steady traction and the entry site is covered with an occlusive dressing for 6-24 hours. The incision is left healing by secondary intention. In the presence of spontaneous pneumothorax treatment with only thoracocentesis and thoracostomy tubes is usually insufficient, as recurrence is very likely. Spontaneous pneumothorax is therefore considered a surgical disease. Median sternotomy is the procedure of choice for exploration of the thorax when the site of air leak is unknown. Prognosis for animals with traumatic pneumothorax is usually good if no other life-threatening injuries are present. For spontaneous pneumothorax the prognosis depends on the method of treatment used.

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