| Discussion: |
Pulmonary air leak describes air escaping from the lungs into extra-alveolar spaces. It occurs when one or more alveoli rupture. Air exits the ruptured alveoli and then follows the bronchial vasculature towards the hilum, dissecting the connective tissue sheath as it travels. Depending upon where the air finally accumulates, air leak may be termed pneumothorax, pneumomediastinum, pneumopericardium, pulmonary interstitial emphysema, pneumoperitoneum, or subcutaneous emphysema. The perivascular connective tissue sheath that the air dissects along is more adherent and abundant in preterm infants as compared to term infants. As a result, air leak is more likely to become trapped in the perivascular space created by a partial dissection, which results in the previously mentioned pulmonary interstitial emphysema.
Mature lungs contain “pores of Kohn” which are interalveolar pores that allow equilibration of alveolar pressures. Neonatal lungs do not contain these pores yet, which contributes to elevated alveolar pressures, rupture, and air leak.
Air leak occurs most frequently in newborns, and especially those with underlying lung disease. Risk factors for air leak include prematurity, mechanical ventilation, RDS, meconium aspiration, and other lung diseases in newborns.
Pneumothorax is the most common manifestation of air leak, and as a result most of the incidence data are based upon it. Some reports suggest a rate of 1-2% of live births in infants with otherwise normal lungs. A series investigated preterm infants of birthweights from 500-1500 grams and reported and rate of 6.3% of live births. Another series studied the incidence of pneumomediastinum and reported it to be 25 per 10,000 live births. Most often, pneumomediastinum is asymptomatic however, so this rate is likely to underestimate the true number.
Obviously, as advances in the NICU continue to extend the threshold of viability, and fertility treatments continue to result in increased rates of multiple gestations and prematurity, the incidence of air leak will continue to rise.
Pneumothorax results when air escapes into the pleural space. It may be asymptomatic, and may resolve spontaneously. Usually however, signs of respiratory distress are present. Presentation may include chest asymmetry, decreased breath sounds on affected side, obvious distress, or shifted cardiac PMI. Tension pneumothorax may increase thorax pressure to such an extent that it decreases venous return, resulting in hypotension. Also, the tension pneumothorax will compress lungs, further increasing the work of breathing and producing a V/Q mismatch.
Pneumothoraces are usually easily detectable on standard chest radiographs. Inspiratory views are recommended for initial diagnosis, but the sensitivity for the detection of pneumothoraces is equal for inspiratory and expiratory views. Obviously, in a neonate, the point is moot. On a chest x-ray, a pneumothorax may appear as a radiolucent air space that separates the parietal and visceral pleura. CT scanning may reveal occult pneumothoraces that are not able to be visualized with conventional radiography. Additionally, a supine AP radiograph may reveal an unusually prominent costophrenic sulcus, supporting the diagnosis of pneumothorax. In neonates, visualization of the anterior junction line in a supine AP radiograph also suggests a pneumothorax. When infants with pneumothorax are placed supine, the intrapleural air may be seen medial to the anterior aspect of the lung. Tension pneumothorax may be diagnosed by identifying signs such as mediastinal shift, diaphragmatic compression, and rib cage expansion.
Management in infants who are stable and not in respiratory distress may consist of close observation, as many pneumothoraces will spontaneously resolve in 1-3 days. For those in distress, supplemental oxygen should be provided as need, and if mechanical ventilation is required, it should be adjusted to minimize the mean airway pressure. Tension pneumothoraces usually require either thoracentesis or chest tube placement to provide an outlet for the trapped air and to restore normal lung anatomy.
Air in the mediastinal space is termed pneumomediastinum. Usually this is asymptomatic, but in rare cases the accumulation of air may become so great that it tamponades the heart, lungs, or great veins. Pneumomediastinum is usually easily diagnosed with plain radiography. Air usually accumulates in the anterior-superior mediastinum. Findings include laterally displaced mediastinal pleura and separation of the thymus from the cardiac silhouette. The “spinnaker” sign can be identified in infants on both lateral and frontal views when enough air is present to elevate the thymus. In the setting of a very large pneumomediastinum, it may be difficult to distinguish from a pneumothorax. A decubitus film will help in this situation, as the air will move in the case of a pneumothorax, but not for a pneumomediastinum. A small pneumomediastinum may be revealed on plain radiography by a “sliver of air” along the cardiac border. Again, CT may be used to more precisely locate a small pneumomediastinum, or to detect an occult lesion.
Management most often consists of close observation and vigilance for pneumothorax. Pneumomediastinum usually resolves spontaneously.
Pulmonary Interstitial Emphysema
Pulmonary interstitial emphysema (PIE) refers to air trapped in the perivascular tissues of the lung. PIE causes decreased compliance, overdistension, and increased airway resistance. PIE may be radiographically visualized by identification of many small black dots within the interstitium, along the vascular sheath, and eventually along the lymphatics. Management is supportive and is aimed at decreasing the mean airway pressure to prevent further air leak. If PIE is unilateral, ventilation of the unaffected lung may be attempted to allow resolution.
Pneumopericardium is the presence of air in the pericardial space. This is a rare condition and is nearly always associated with mechanical ventilation. When severe, it may cause cardiac tamponade. Plain chest radiography may reveal air surrounding the heart but not extending beyond the exit of the great vessels. When air is present underneath the heart, it supports the diagnosis of pneumopericardium over pneumomediastinum. Management consists of observation unless the patient is symptomatic. In such cases, the ventilator should be adjusted to minimize mean airway pressure. Severe cases may require pericardiocentesis to relieve tamponade.
Pneumoperitoneum is the presence of air in the peritoneal space. While this usually has little clinical significance, it must be differentiated from intraperitoneal air due to a perforated viscus.