Porth's Essentials of Pathophysiology, 4e

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Disorders of Ventilation and Gas Exchange

C h a p t e r 2 3

in persons with underlying lung disease in whom sec- ondary spontaneous pneumothorax develops or in per- sons with underlying heart disease who are unable to compensate with an increase in heart rate and stroke volumes. Regardless of etiology, the hypoxemia caused by the partial or total loss of lung function can be life-threatening. Without immediate intervention, the increased thoracic pressure will further impair both car- diac and pulmonary function, resulting in severe hypox- emia and hypotension. Diagnosis of pneumothorax can be confirmed by chest radiograph, CT scan, or ultrasonography. 8,13,14 Pulse oximetry and blood gas analysis may be done to determine the effect on blood oxygen levels. Treatment of pneumothorax varies with the cause and extent of the disorder. In small spontaneous pneumothoraces, the air usually reabsorbs spontaneously, and only obser- vation and follow-up chest radiographs are required. Supplemental oxygen may be used to correct the hypoxemia until the air is reabsorbed. In larger pneu- mothoraces, the air is removed by needle aspiration or a closed drainage system used with or without suction. This type of drainage system uses a one-way valve to allow air to exit the pleural space and prevent it from reentering the chest. Emergency treatment of tension pneumothorax involves the prompt insertion of a large-bore needle or chest tube into the affected side of the chest along with one-way valve drainage or continuous chest suction to aid in lung reexpansion. 8,13 Sucking chest wounds, which allow air to pass in and out of the chest cavity, should be treated by promptly covering the area with an airtight covering. Chest tubes are inserted as soon as possible. Atelectasis Atelectasis refers to an incomplete expansion of a lung or portion of a lung. 15,16 It can be caused by air- way obstruction, lung compression such as occurs in pneumothorax or pleural effusion, or increased recoil of the lung due to loss of pulmonary surfactant (see Chapter 21). The disorder may be present at birth (i.e., primary atelectasis) or develop during the neo- natal period or later in life (i.e., acquired or secondary atelectasis). Primary atelectasis of the newborn implies that the lung has never been inflated. It is seen most frequently in premature and high-risk infants. A secondary form of atelectasis can occur in infants who established respira- tion and subsequently experienced impairment of lung expansion. Among the causes of secondary atelectasis in the newborn is the respiratory distress syndrome associ- ated with lack of surfactant and airway obstruction due to aspiration of amniotic fluid or blood. Acquired atelectasis occurs mainly in adults. It most commonly results from airway obstruction, for example, by a mucus plug in the airway or by external compres- sion of the airway from fluid, a tumor mass, exudate, or other matter in the pleural cavity or area surrounding the

Obstructed airway

Space- occupying lesion

Absorption

Compression

FIGURE 23-3. Atelectasis caused by airway obstruction and absorption of air from the involved lung area (left) and by compression of lung tissue (right).

airway (Fig. 23-3). Portions of alveoli, a small segment of lung, or an entire lung lobe may be involved. Complete obstruction of an airway is followed by the absorption of air from the dependent alveoli and collapse of that portion of the lung. Breathing high concentrations of oxygen increases the rate at which gases are absorbed from the alveoli and predisposes to atelectasis. The dan- ger of obstructive atelectasis increases after surgery. 15 Anesthesia, pain, administration of narcotics, and immobility tend to promote retention of viscid bron- chial secretions and airway obstruction. The encourage- ment of coughing and deep breathing, frequent change of position, adequate hydration, and early ambulation decrease the risk for atelectasis. The clinical manifestations of atelectasis include tachypnea, tachycardia, dyspnea, cyanosis, signs of hypoxemia, diminished chest expansion, absence of breath sounds, and intercostal retractions. Both chest expansion and breath sounds are decreased on the affected side. There may be intercostal retraction (pull- ing in of the intercostal spaces) over the involved area during inspiration. Signs of respiratory distress are pro- portional to the extent of lung collapse. If the collapsed area is large, the mediastinum and trachea shift to the affected side. In compression atelectasis, the mediasti- num shifts away from the affected lung. The diagnosis of atelectasis is based on signs and symptoms. Chest radiographs are used to confirm the diagnosis. Computed tomography scans may be used to show the exact location of the obstruction. Treatment depends on the cause and extent of lung involvement. It is directed at reducing the airway obstruction or lung compression and at reinflation of the collapsed area of the lung. Ambulation, deep breathing, and body posi- tions that favor increased lung expansion are used when appropriate. Administration of oxygen may be needed to correct the hypoxemia. Bronchoscopy may be used as both a diagnostic and treatment method.