| CHEST DRAINAGE AS A THERAPEUTIC INTERVENTION The clinical need for chest drainage arises anytime the negative pressure in the pleural cavity is disrupted by the presence of air and/or fluid resulting in pulmonary compromise. The purpose of a chest drainage unit is to evacuate the air and/or fluid from the chest cavity to help re-establish normal intrathoracic pressure. This facilitates the re-expansion of the lung to restore normal breathing dynamics. The need also arises following heart surgery to prevent the accumulation of fluid around the heart. Patients with continual air or fluid leaks have a chest tube, also called a thoracic catheter, inserted. The distal end, which will be inside the patient’s chest, has a number of drainage holes. The last eyelet can be detected on a chest x-ray as intermittent breaks in the radiopaque line. Once the chest tube has been properly positioned and secured, the x-ray should be checked to ensure that all drainage holes are inside the chest wall. The location of the chest tube depends on what is being drained. Free air in the pleural space rises, so the tube is placed above the second intercostal space at the mid-clavicular line. Pleural fluid gravitates to the most dependent point, so the tube is placed at the 4th to 5th intercostal space along the mid-axillary line (figure 1). Mediastinal tubes placed to drain the pericardium after open-heart surgery are positioned directly under the sternum (figure 2). Once the chest tube is in place, it is connected to a chest drainage unit. NORMAL ANATOMY AND PHYSIOLOGY Before we discuss the chest drainage unit in detail, it is important to briefly review normal anatomy and physiology of the thorax with emphasis on the physiology of respiration. This will help us understand what can go wrong in the structure and function of the chest and how these problems can be treated. CHEST WALL The chest wall is made up of bones and muscles. The bones, primarily ribs, sternum and vertebrae, form a protective cage for the internal structures of the thorax. The main muscles of the chest wall, the external and internal intercostals, extend from one rib to the rib below (figure 3). The external intercostals enlarge the thoracic cavity by drawing the ribs together and elevating the rib cage, while the internal intercostals decrease the dimensions of the thoracic cavity. MEDIASTINUM Within this musculoskeletal cage of the thorax are three subdivisions. The two lateral subdivisions hold the lungs. Between the lungs is the mediastinum, which contains the heart, the great vessels, parts of the trachea and esophagus, and other structures (figure 4). LUNGS The lungs consist of airways (trachea and bronchi) that divide into smaller and smaller branches until they reach the air sacs, called alveoli. The airways conduct air down to the alveoli where gas exchange takes place (figure 5). The lung itself is covered with a membrane called the visceral (or pulmonary) pleura. The visceral pleura is adjacent to the lining of the thoracic cavity which is called the parietal pleura. Between the two membranes is a thin, serous fluid which acts as a lubricant – reducing friction as the two membranes slide across one another when the lungs expand and contract with respiration. The surface tension of the pleural fluid also couples the visceral and parietal pleura to one another, thus preventing the lungs from collapsing. Since the potential exists for a space between the two membranes, this area is called the pleural cavity or pleural space (figure 6). RESPIRATION Respiration is a passive, involuntary activity. Air moves in and out of the thorax due to pressure changes. When the diaphragm, the major muscle of respiration, is stimulated, it contracts and moves downward. At the same time, the external intercostals move the rib cage up and out. The chest wall and parietal pleura move out, pulling the visceral pleura and the lung with it. As the volume within the thoracic cavity increases, the pressure within the lung decreases. Intrapulmonary pressure is now lower than atmospheric pressure; thus air flows into the lung — inhalation (figure 7a). When the diaphragm returns to its normal, relaxed state, the intercostal muscles also relax and the chest wall moves in. The lungs, with natural elastic recoil, pull inward as well and air flows out of the lungs — exhalation (figure 7b). The lungs should never completely collapse for there is always a small amount of air, called residual volume, in them. Under normal conditions, there is always negative pressure in the pleural cavity. This negative pressure between the two pleurae maintains partial lung expansion by keeping the lung pulled up against the chest wall. The degree of negativity, however, changes during respiration. During inhalation, the pressure is approximately –8 cm H2O; during exhalation, approximately –4 cm H2O. If a patient takes a deeper breath, the intrapleural pressure will be more negative. Under normal conditions, the mechanical attachment of the pleurae, plus the residual volume, keep the lungs from collapsing. |
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CHEST DRAINAGE AS A THERAPEUTIC INTERVENTION
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