The trachea divides into two principle bronchi which in turn subdivide into lobar bronchi that supply each lung lobe. Within each lung lobe, lobar bronchi divide into segmental bronchi that supply bronchopulmonary segments within each lobe. Dichotomous branching of the airway continues through subsegmental bronchi, terminal bronchioles, and respiratory bronchioles. Respiratory bronchioles give rise to alveolar ducts, alveolar sacs, and pulmonary alveoli. The pulmonary arteries follow a lobar distribution in close proximity to the cranial dorsal aspect of each bronchi. Bronchial branches of the bronchoesophageal arteries provide oxygenated blood to the airways down to the level of the respiratory bronchioles where they terminate in capillary beds continuous with the pulmonary arteries. Pulmonary veins course on the caudal and ventral aspect of each bronchi collecting blood from both the pulmonary and bronchial arteries.
The left lung of dogs and cats is divided into cranial and caudal lobes. The left cranial lung lobe is divided into a cranial and caudal portion, but shares a common lobar bronchus. The right lung is divided into four distinct lobes: cranial, middle, caudal and accessory. The accessory lobe passes dorsal to the caudal vena cava and is located medial to the plica vena cava.
PULMONARY RESECTION TECHNIQUES
Possible indications for pulmonary resection include pulmonary neoplasia, pulmonary trauma, pulmonary abscess, lung lobe torsion, bronchoesophageal fistula, and spontaneous pneumothorax. Normal animals can tolerate resection of as much as 50% of their lung capacity and still survive. However, generalized pulmonary disease substantially decreases the amount of lung resection that can be tolerated. Chronic obstructive lung disease and pulmonary hypertension in particular limit the extent of pulmonary resection that can be undertaken.
Lung lobectomy is indicated for severe traumatic injury, neoplasia, lobe torsion, or abscesses that are primarily confined to a single lung lobe. Lung lobes that can undergo separate lobectomy in small animals include the left cranial, left caudal, right cranial, right middle, and right caudal lobes. The accessory lobe divides incompletely from the right caudal lobe and generally is resected with the caudal lobe. The standard surgical approach for lung lobectomy is a fifth intercostal thoracotomy in dogs and a sixth intercostal thoracotomy in cats. The procedure can be performed one intercostal space cranial or caudal to the ideal intercostal space, if necessary. Lung lobectomy also can be accomplished from a median sternotomy, if this approach is indicated for other reasons. Lung lobectomy is performed by dividing the pulmonary vessels and oversewing the lobar bronchus. Lung lobes should be manipulated carefully during resection to avoid embolization of neoplastic cells or extrusion of purulent material into adjacent airways. The bronchus should be checked for leaks after closure by flooding the chest with saline and applying a positive pressure breath. Placement of a thoracostomy tube prior to closure of the thoracotomy is absolutely indicated.
Partial Lung Resection
Partial lung resection is indicated for lung biopsy or excision of localized pulmonary lesions that do not require complete lung lobectomy. Partial lung resection can be performed by a standard suturing technique or with a surgical stapling device. Standard partial lung resection can be accomplished with readily available materials, but some leakage of air from the surgery site can be anticipated after surgery. Stapling devices, when available, are fast and less likely to leak after surgery. Partial lung resection can be performed using either the TA or GIA surgical stapling device. The 3.5 mm (blue) staples are most appropriate for stapling lung tissue. Any leakage of air after surgery is readily evacuated by a thoracostomy tube and usually will be self limiting after several hours.
Primary neoplasia of lung is the most common indication for pulmonary surgery in small animals. Presumptive diagnosis of primary lung neoplasia is based on characteristic radiographic appearance of a solitary lung mass. Fine needle aspiration can be undertaken prior to surgery. Pulmonary lobectomy is indicated for suspected primary lung tumors without prior biopsy. Excision biopsy of hilar lymph nodes is indicated for staging if they are visualized. Diagnosis is confirmed by histopathologic examination of the excised specimen. Adenocarcinoma is the most common primary lung neoplasia in dogs, representing approximately 75% of the cases. Alveolar carcinoma and squamous cell carcinoma also occur. Pulmonary carcinomas are classified as differentiated or undifferentiated. Prognostic indicators for primary lung neoplasia include involvement of hilar lymph nodes at surgery, histologic type, tumor size, and presence of pleural effusion. The prospect for cure or long term remission with surgery alone is good for small differentiated adenocarcinomas. Undifferentiated carcinomas have over a 50% incidence of metastasis. The prognosis for squamous cell carcinoma is poor with a metastasis rate of over 90%.
Metastatic lung neoplasia is treatable by surgical excision under certain circumstances. Guidelines for surgical treatment of metastatic pulmonary disease include: control of the primary site for at least several months, no evidence of metastasis to sites other than lung, favorable histology (i.e., sarcomas are better than carcinomas), slow growth of metastatic tumors (i.e., size doubling time > 30 days), and less than 5 metastatic nodules present. Metastatic lung tumors should be excised by partial lung resection whenever possible to preserve lung volume.
LUNG LOBE TORSION
Lung lobe torsion is a rare condition that occurs most often in large deep-chested dogs. The condition may occur secondary to one of several predisposing factors including thoracic trauma, pleural effusion, diaphragmatic hernia, pneumothorax, or thoracic surgery. Lung lobe torsion occurs most often in the right middle lung lobe and less often in the left cranial lung lobe. Clinical findings associated lung lobe torsion include acute depression, weakness, dyspnea, tachypnea, cyanosis, nonproductive cough, hemoptysis, and tachyarrhythmias. Radiographically, lung lobe torsion appears as an isolated atelectasis of the right middle or left cranial lung lobes. Air bronchograms may be present early, but disappear over time. Pleural effusion is often apparent on radiographs. Evacuation of sanguinous effusion fails to expand the collapsed lobe. Confirmation of the diagnosis can be made by contrast bronchography, bronchoscopy, or exploratory surgery. At surgery, the involved lung lobe appears as a solid liver-like mass due to engorgement of the lung with blood. Surgical treatment consists of complete lung lobectomy of the affected lung, preferably without derotation of the lung lobe.
Pulmonary abscesses occur secondary to severe pulmonary infections or pulmonary foreign bodies. Foreign bodies can enter by inhalation or impalement through the thoracic wall. Pyothorax can occur concurrently with pulmonary abscesses. Persistent pulmonary atelectasis and signs of pneumonia despite appropriate antibiotic therapy suggests the presence of a pulmonary abscess. Pulmonary lobectomy of the affected lung lobe is indicated for suspected pulmonary abscesses. Caution during surgical manipulation of an abscessed lung lobe is necessary to prevent expulsion of purulent material into adjacent lung lobes. Morbidity and mortality associated with this procedure is high in animals that have active diffuse pneumonia at the time of surgery.
Pneumothorax results from the accumulation of air in the pleural space. The air can come from the respiratory tract, the esophagus and through the skin. Spontaneous pneumothorax are classified as primary or secondary. Primary pneumothorax are usually resulting from the rupture of a bleb or a bullae. Primary spontaneous pneumothorax are more common in large breed dog with deep chest. Secondary spontaneous pneumothorax results from a lung pathology that eroded through a bronchioles (pneumonia, abscess0 or from chronic obstructive lung disease (emphysema). Primary spontaneous pneumothorax are treated with drainage of the pleural space by thoracocentesis or thoracostomy tube. If a bullae is visible on thoracic radiographs, a lung lobectomy is then recommended. A pleurodesis can also be performed at the time of surgery to prevent recurrence.