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Anatomy - Tube thoracostomy, open - general and visceral surgery
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Layers and structure of the chest wall
The soft tissues of the chest between the skin and rib cage comprise several fascias and muscle layers.: The pectoral fascia is a fascia overlaying the pectoralis major muscle, while the clavipectoral fascia invests the pectoralis minor muscle. The rib cage itself also has its own fascias: On the outside, the thoracic fascia covers the periosteum of the ribs as well as the muscles in the intercostal spaces. The endothoracic fascia is its inner counterpart on the periosteum of the ribs and the inner and innermost intercostal muscles. Across the interpleural space, it is invested by the parietal pleura (costal pleura). Insertion of a chest tube will perforate all these fasciae together with the intercostal muscles (see below).
The intercostal space
Since the blood vessels (intercostal vessels) and nerves (intercostal nerves) of the chest wall follow the original segments of the body, they course on the inside of the rib cage in annular fashion. Segment by segment, these intercostal blood vessels connect the posterior aorta and azygos/hemiazygos vein with the internal thoracic arteries and internal thoracic veins)anteriad. The former arises from the subclavian artery, courses caudad together with the homonymous veins paralleling the sternum on both sides immediately outside the endothoracic fascia and then divides at the level of the diaphragm (Larrey cleft or sternocostal trigonum) into the superior epigastric artery (anastomoses with the inferior epigastric artery from the external iliac artery) and an artery to the diaphragm (musculophrenic artery). Along its course, on the inner surface of the chest, it gives rise primarily to the anterior segments of the intercostal arteries and to mediastinal branches.
The intercostal arteries thus are fed anteriad from the internal thoracic artery and posteriad either directly from the aorta (branches 3-11) or the subclavian artery (via the costocervical trunk). The subcostal artery coursing inferior to the 12th rib is its counterpart. One aspect in the overall course of the intercostal arteries is particularly important in thoracentesis: Up to the level of the anterior axillary line the posterior segment of the intercostal artery (together with the corresponding intercostal vein and nerve) run along the inferior aspect of the rib immediately superior to the artery and then typically divides into a superior and inferior branch (at the inferior and superior aspect respectively of the adjacent ribs). In the chest wall region, typical collections of lymph nodes are found in the parasternal and axillary areas as well as just superior and inferior to the clavicle.
The intercostal veins parallel the homonymous arteries and empty anteriorly via the internal thoracic vein into the brachiocephalic vein and posteriorly into the azygos/hemiazygos veins or, in the first and second segments, also into the brachiocephalic vein.
After they leave the intervertebral foramen, the intercostal nerves, give off a posterior branch and course segmentally in the intercostal space along the inferior aspect of the corresponding rib and supply the skin of the lateral and anterior chest. The superior aspects of the chest are still supplied from the cervical plexus; as part of the brachial plexus (C5 to T1) segment T1 runs into the arm; starting at T2, these segments again innervate the skin, as described, and the intercostal muscles and superior aspects of the abdominal musculature (along with the lumbar plexus). This layout of the dermatomes results in a big discontinuity between segment C4 (level of the clavicle) and then immediately segment T2 for the superior chest wall (segment T5 runs at roughly the level of the nipples).
The intercostal musculature (external, internal, and innermost intercostal muscles) tightly encircle the lateral chest wall. Inhalation activates the external and anterior internal intercostal muscles, while exhalation triggers the posterior and lateral internal intercostal muscles.
The following factors should be considered when determining the ideal site in tube thoracostomy:
1. In bedridden patients, there may be fluid in the posterior basal region and air in the anterior apex of the pleural space. In the absence of any adhesions between the parietal and visceral pleura, the anterior and posterior pleural spaces communicate with each other. A chest tube in the posterior pleural space therefore drains both fluid and a pneumothorax. In case of any adhesions, the encapsulated space requires targeted drainage.
2. At the lateral chest wall the ribs directly protect the intercostal vessels and nerves in their annular course. This course changes in the anterior chest wall, where the structures reside in the intercostal space and thus may easily be injured.
3. Lateral tube thoracostomy is ideal when managing bedridden patients.
4. Complete pneumothorax may displace the diaphragm and intraabdominal organs far craniad.
As a result:
Chest tubes should ideally be placed in the “safe triangle”. The landmarks for this are the inferior aspect of the pectoralis major muscle (anterior axillary line), the anterior aspect of the latissimus dorsi muscle (posterior axillary line), and the mammillary line. In pneumothorax the intrathoracic part of the chest tube should follow a posterosuperior direction, while in pleural effusion the direction should be posteroinferior.