Surgical Approaches to the Thoracic Cavity
World Small Animal Veterinary Association World Congress Proceedings, 2005
Eric Monnet, DVM, PhD, DACVS, ECVS
College of Veterinary Medicine, Colorado State University
Fort Collins, CO, USA

Thoracic approaches provide access to the thoracic cavity and as such are fundamental techniques for thoracic surgery. Two thoracic approaches, intercostal thoracotomy and median sternotomy, are used most often in small animals. Other thoracic approaches such as rib resection or transsternal thoracotomy are used less often. The choice of a thoracic approach depends mostly on the type of access to the thoracic cavity that is needed.

INTERCOSTAL THORACOTOMY

Intercostal thoracotomy is employed when exposure to a specific region of the thoracic cavity is needed. This approach provides good access to thoracic structures in the immediate area of the thoracotomy, but access to structures not in the area of the thoracotomy is limited. As a general rule, intercostal thoracotomy allows access to about one-third of the ipsilateral thoracic cavity and mediastinal structures. Access to structures in the contralateral thorax is very limited when using this approach. The exposure gained by intercostal thoracotomy can be increased approximately 33% by performing two small osteotomies, one dorsal and one ventral, on the rib just cranial or caudal to the intercostal incision. It is not necessary to routinely perform osteotomies of a rib, but the technique is useful on occasion when increased exposure is necessary.

The site of an intercostal thoracotomy may be the third through the tenth intercostal space depending on the structures being exposed. The intercostal spaces most commonly used to approach selected thoracic structures in dogs and cats are listed in Table 1. A lateral thoracic radiograph also can help determine the intercostal space that best exposes a desired thoracic structure. A caudal intercostal thoracotomy may be combined with an incision in the diaphragm to provide exposure to the cranial abdominal cavity. Complications associated with intercostal thoracotomy are uncommon as long as air-tight closure of the intercostal space is achieved.


Table 1.Location of Thoracic Structures via Intercostal Thoracotomy

Thoracic Structure

Intercostal Space

 

Left

Right

Heart and pericardium

4, 5

4, 5

- Ductus arteriosus

4, 5

 

- Pulmonic Stenosis

4

 

- Pericardiectomy

5

5

- Cardiopulmonary Bypass

 

4, 5

Lungs

4-6

4-6

- Cranial lobe

4, 5

4, 5

- Intermediate lobe

 

5

- Caudal lobe

5, 6

5, 6

Esophagus

   

- Cranial

3-5

3-5

- Caudal

7-9

7-9

Caudal Vena Cava

 

7-9

Thoracic Duct

   

- Dog

 

8-10

- Cat

8-10

 

RIB RESECTION THORACOTOMY

A rib resection thoracotomy provides more exposure than a simple intercostal thoracotomy and about the same exposure as an intercostal thoracotomy with osteotomies of an adjacent rib. A potential advantage of this approach is that it results in fewer adhesions of the lung to the incision because large circumcostal sutures are not required for closure of the thoracic wall. A disadvantage of rib resection is that secure closure is more difficult to obtain, especially in large dogs that have undergone wide retraction of the ribs during surgery. Rib resection thoracotomy is used infrequently in small animals and is reserved for specific indications.

MEDIAN STERNOTOMY

Median sternotomy is the only thoracic approach that allows access to the entire thoracic cavity, and therefore is indicated when exploration of the thoracic cavity is necessary. Structures in the dorsal thoracic cavity such as the great vessels and bronchial hilus are more difficult, but not impossible, to reach when this approach is used. Median sternotomy can be combined with a ventral midline celiotomy or cervical incision if a combined approach to the abdomen or neck is desired.

An important technical consideration when performing a sternotomy is that either the manubrium or xiphoid, or both, should be left intact. Doing so greatly enhances the ability to achieve a stable closure of the sternotomy, and thereby decreases the risk of postoperative complications. Reluctance to use this approach because of a perception that it is associated with excessive postoperative pain or complications is not justified. Pain and complication rates with this approach are not different from intercostal thoracotomy. In fact, median sternotomy probably causes less postoperative discomfort than intercostal thoracotomy. Sternotomy and thoracotomy are comparable in the degree of cardiopulmonary impairment that they cause. Thus, the decision to employ a sternotomy or thoracotomy approach should be based on the kind of access to the thoracic cavity needed, and not on a perception that one approach is superior with regard to intraoperative or postoperative morbidity.

THORACOSCOPY

Thoracoscopy is examination of the pleural cavity and its organs with a rigid scope. With the development of high-resolution micro-cameras, video optics, and fiberoptic light delivery systems, clear magnified images of the surgical field can be transferred to a video screen. In combination with minimally invasive surgical instruments, the ability to perform diagnostic and advanced therapeutic procedure is possible.

Thoracoscopy is indicated for exploration of pleural space and biopsy of the pleural surface for a patient with a pleural effusion of unknown origin, partial or complete lung lobectomy, biopsy of the lung, pericardectomy and exploration for the right atrium. Thoracoscopy can also be used to perform biopsies for frozen section to establish a diagnosis prior to thoracotomy. Cranial mediastinal mass may represent a challenge to the clinician to establish a definitive diagnosis. Therefore, thoracoscopy and frozen section of a biopsy from a mass in the cranial mediastinum might help differentiating a lymphoma from a thymoma. Thoracoscopy is used commonly in human thoracic surgery for diagnosis of intra-thoracic neoplasia and staging of oncology patients

The basic equipment to perform a thoracoscopy includes a surgical telescope, trocar-cannulas, a basic set of thoracoscopic surgical instruments, a light source, a video camera and a video monitor. Thoracoscopy does not require CO2 insufflation since the rib cage is maintaining the thoracic cavity expanded. Telescope comes with different angles. The most commonly used are the 0°-degree and the 30°-degree angled telescopes. Laparoscope trocar-cannula can be used to perform a thoracoscopy. Since the laparoscope trocar-cannulas have a valve, the thorax is not opened and a control pneumothorax needs to be established. Disposable thoracic cannulas do not have a valve. After insertion of the cannula, the thoracic cavity is opened to room air. The lungs are collapsing and positive ventilation is then required. One-lung ventilation with selective intubation of either the left or the right lungs will allow a better exposure of specific area of the thoracic cavity. Cannulas are placed in a triangular position. The cannula for the thoracoscope can be placed in a sub-xyphoid position. The cannulas for the instruments are then placed in the left and right hemithorax at the level of the 7th intercostal space.

Thoracoscopy can be performed with a trans-xyphoid or an intercostal approach. Trans-xyphoid approach gives access to the left and right hemithorax after partial resection of the mediastinum. Trans-xyphoid approach is the approach of choice for exploration of the thoracic cavity since the right and the left sides of the thoracic cavity can be visualized. Pericardectomy can be performed with this approach. Intercostal approach requires positioning of the animal in lateral recumbency. Only one side of the thoracic cavity can be explored. However, visualization of the hilus of the lung is better for lung resection, and lymph nodes evaluation. In human thoracic surgery, approach through the thoracic inlet has been described for exploration of the cranial mediastinum.

INTRAOPERATIVE CONSIDERATIONS

Thoracic surgery is always associated with some degree of cardiopulmonary compromise during surgery. Thoracotomy and sternotomy differ somewhat in the type of cardiopulmonary compromise they are most often associated with. Because animals undergoing thoracotomy are laterally recumbent, they are more likely to develop congestion of the dependent lung and impairment of pulmonary gas exchange. This tendency is countered by maintaining good ventilation to the dependent lung and by avoiding overzealous administration of crystalloid fluids during surgery. On occasion, positive end expiratory pressure (PEEP), applied by immersing the expiratory portion of the ventilator into 5 to 7 cm of water, may be necessary to maintain adequate gas exchange during thoracotomy. On the other hand, animals undergoing median sternotomy in dorsal recumbency are more likely to develop some degree of cardiovascular compromise during surgery because the weight of non-suspended thoracic structures compresses the base of the heart and impedes venous return. This tendency is compensated for somewhat by volume loading the patient to increase cardiac filling pressures. In the end, while it is useful to be aware of these tendencies, it is most important to closely monitor each animal during surgery and provide supportive therapies that best address specific problems and needs.

Placement of a thoracostomy tube prior to closure of the thoracic cavity is essential to ensure proper and timely evacuation of the pleural space during and after surgery. Failure to place a thoracostomy tube invites substantial risk of developing life threatening complications. The period of transition from an open to closed thoracic cavity is especially critical. During this period, inadvertent tension pneumothorax can be avoided by keeping the thoracostomy tube open to atmosphere during initial closure of the thoracotomy. Then, after the closure is airtight, the pleural space can be evacuated and the thoracostomy tube closed. It is also important to be aware that pulmonary compliance can decrease suddenly when the thoracic cavity is closed, and that inspiratory pressures may have to be increased to maintain an adequate tidal volume.

POSTOPERATIVE CARE

The period immediately after surgery is critical for animals undergoing thoracic surgery. Hypoventilation, hypoxemia, hypothermia, acid base disorders, hypotension, shock, and oliguria are among the problems that may arise after surgery. Ventilation may be depressed by anesthetic drugs, pneumothorax, or somatic pain. Hypoventilation causes hypoxemia and respiratory acidosis; neither is well tolerated postoperatively. A tidal volume less than 10 ml/kg measured with a Wright's respirometer suggests that ventilation may be inadequate. Suspected hypoventilation in the recovery period is confirmed by a PaCO2 > 50 mm Hg. Impaired pulmonary gas exchange resulting from pulmonary shunt or VA/Q mismatch also can cause hypoxemia after surgery. The most common cause of pulmonary shunt or VA/Q mismatch after thoracic surgery is pulmonary congestion. For this reason, it is very important that animals recovering from surgery be turned over at least every two hours as long as they remain laterally recumbent. A PaO2 < 60 mm Hg or PA-aO2 > 25 mm Hg while the patient is breathing room air suggests that serious gas exchange impairment is present. Serious hypoxemia due to impaired gas exchange may require treatment with supplemental oxygen or PEEP therapy.

Animals that are hypothermic after surgery should be slowly surface warmed with warm water bottles or circulating water blankets. Hypothermia, hypovolemia, anesthetic drugs, or pain may cause varying degrees of hypotension during the recovery period. Appropriate treatment of circulatory compromise with fluids is essential. Titrating fluid therapy to central venous or pulmonary wedge pressure is the best way to ensure adequate volume loading and avoiding overhydration. Ideally, central venous pressure should be maintained between 5 and 10 cm of water in animals with cardiovascular compromise. Animals also should be evaluated for acid base and electrolyte disorders. Urine production should be monitored to assure adequate renal function.

Analgesia is indicated in all animals after thoracic surgery. Selective intercostal nerve block, intrapleural anesthetics, epidural morphine, or parenteral opiate drugs are all viable methods for postoperative analgesia. Chest bandages aid in sealing and protecting a thoracotomy incision, however bandages should be wrapped loosely after thoracic surgery as they can impair ventilation. Placement of a thoracostomy tube during thoracic surgery is always indicated since it allows close monitoring the pleural space for the presence of air or blood during the recovery period. The thoracostomy tube should be hand-aspirated every hour for four hours, and then every two to four hours until it is removed.

Speaker Information
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Eric Monnet, DVM, PhD, DACVS, ECVS
College of Veterinary Medicine, Colorado State University
Fort Collins, CO


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