Virtually all pulmonary air leak syndromes begin with some degree of PIE. When high airway pressures are applied to an infant’s lungs (e.g., during mechanical ventilation), the distal airways and alveoli often become overdistended—that is, they develop bleb or emphysema-like areas—and rupture (see Figure 35-1). In addition, gas trapping from an insufficient expiratory time can also cause alveolar overdistention and rupture. Once the gas escapes, it is forced into (1) the loose connective tissue sheaths that surround the airways and pulmonary capillaries, and (2) the interlobular septa containing pulmonary veins. In severe cases, the gas continues to spread peripherally by dissecting along the peribronchial and perivascular spaces to the hilum of the lung, producing the classic radiographic appearance of PIE that shows bubbles of air in the interstitial cuffs (Figure 35-2 and Figure 35-3).

The overdistention associated with PIE forces the lungs into a full inflation position and thereby decreases lung compliance (remember that static lung compliance is reduced at both very low and very high lung volumes). Moreover, air trapped within the interstitial cuffs compresses the airways and increases airway resistance. In addition, the trapped air in the interstitial spaces impairs lymphatic function, resulting in fluid accumulation in the interstitial cuffs and alveoli. Once the interstitial gas reaches the hilum of the lung, it either (1) coalesces to form large hilar blebs, or (2) tracks beneath the visceral pleura to form large subpleural pockets of air. In either case, the accumulation of gas can be large enough to significantly compress the lung or mediastinal structures.

A pneumomediastinum may occur when the excessive air associated with a PIE continues to track—and accumulate—through the perivascular and peribronchial cuffs and causes the gas in the hilum area to rupture into the mediastinum. In addition, the high gas pressures associated with a pneumomediastinum may also dissect into the pleural space and the fascial planes of the neck and skin, resulting in the condition known as subcutaneous emphysema.

A pneumothorax may occur because of the alveolar overdistention and subsequent rupture commonly associated with a PIE (Figure 35-4). A pneumopericardium can develop from direct tracking of interstitial air along the great vessels into the pericardial sac (Figure 35-5). Gas pressure in the pericardium restricts atrial and ventricular filling, resulting in a decreased stroke volume and, ultimately, a reduced cardiac output and systemic hypotension. During the late stages, inflammatory changes of the airways lead to increased capillary leakage and excessive bronchial secretions.

A pneumoperitoneum may develop from the tracking of gas along the sheaths of the aorta and vena cava and eventually may burst into the peritoneal cavity. Clinically, the infant with a pneumoperitoneum manifests a sudden onset of abdominal distention. The pneumoperitoneum may be large enough to block the descent of the diaphragm and may require drainage. Finally, the excessive gas accumulation associated with a pneumoperitoneum may end up in the scrotum in male babies or the labia in females.

In very rare cases, an intravascular air embolism may be seen. It is hypothesized that the air is actually pumped under high pressure through the pulmonary lymphatics into the systemic venous circulation. Intravascular air causes an abrupt cardiovascular collapse and is frequently diagnosed when air is observed in vessels on chest radiographs taken to establish the cause of cardiovascular collapse.

The major pathologic changes associated with pulmonary air leak syndromes are as follows:

Etiology and Epidemiology

Preterm infants who weigh less than 1000 g at birth have an increased risk for the early occurrence of pulmonary air leak syndromes (often within the first 24 to 48 hours of life), especially because of the weak noncartilaginous structures of their distal airways. The most frequent causative factor resulting in pulmonary air leak syndromes in preterm infants is the use of mechanical ventilation. Pulmonary air leak syndromes commonly result from the use of high levels of positive end-expiratory pressure (PEEP), high peak inspiratory pressures (PIPs), and prolonged inspiratory times (ITs). Occasionally, full-term babies will develop a spontaneous tension pneumothorax.

 OVERVIEW of the Cardiopulmonary Clinical Manifestations Associated with Pulmonary Air Leak Syndromes

The following clinical manifestations result from the pathologic mechanisms caused (or activated) by Atelectasis (see Figure 9-8)—the major anatomic alteration of the lungs associated with pulmonary air leak syndromes (see Figure 35-1).

CLINICAL DATA OBTAINED AT THE PATIENT’S BEDSIDE

The Physical Examination

Vital Signs

Increased Respiratory Rate (Tachypnea)

Normally, a newborn’s respiratory rate is about 40 to 60 breaths/min. During the early stages of pulmonary air leak syndromes, the respiratory rate generally is well over 60 breaths/min. Several pathophysiologic mechanisms operating simultaneously may lead to an increased ventilatory rate:

• Stimulation of peripheral chemoreceptors (hypoxemia)

• Decreased lung compliance–increased ventilatory rate relationship

• Stimulation of central chemoreceptors

Increased Heart Rate (Pulse) and Blood Pressure

Apnea (See Table 31-1)

Clinical Manifestations Associated with More Negative Intrapleural Pressures during Inspiration

• Intercostal retractions

• Substernal retraction and abdominal distention (seesaw movement)

• Cyanosis of the dependent portions of the thoracic and abdominal areas

• Flaring nostrils

Chest Assessment Findings

• Wheezes

• Rhonchi

• Crackles

• Change in point of maximum impulse (PMI)

The PMI is defined as the point at which the impulse of the left ventricle can be felt most strongly (normally over the fifth intercostal space).* When gas accumulates in the chest, the heart is pushed to the unaffected side, and the position of the PMI changes. Often the presence of a pneumothorax can be identified by PMI changes long before a chest x-ray can be obtained.

Expiratory Grunting

Cyanosis

CLINICAL DATA OBTAINED FROM LABORATORY TESTS AND SPECIAL PROCEDURES

Pulmonary Function Test Findings (Extrapolated Data for Instructional Purposes) (Primarily Restrictive Lung Pathophysiology)

FORCED EXPIRATORY FLOW RATE FINDINGS

FVC	FEVT	FEV1/FVC ratio	FEF25%-75%
↓	N or ↓	N or ↑	N or ↓
FEF50%	FEF200-1200	PEFR	MVV
N or ↓	N or ↓	N or ↓	N or ↓

LUNG VOLUME AND CAPACITY FINDINGS

VT	IRV	ERV	RV*
N or ↓	↓	↓	↓
VC	IC	FRC*	TLC	RV/TLC ratio
↓	↓	↓	↓	N

^*↑ When airways are partially obstructed.

Arterial Blood Gases

MILD TO MODERATE PULMONARY AIR LEAK SYNDROMES

Acute Alveolar Hyperventilation with Hypoxemia (Acute Respiratory Alkalosis)

pH	Paco2		Pao2
↑	↓	↓ (slightly)	↓

SEVERE PULMONARY AIR LEAK SYNDROMES

Acute Ventilatory Failure with Hypoxemia (Acute Respiratory Acidosis)

pH*	Paco2	*	Pao2
↓	↑	↑ (slightly)	↓

^*When tissue hypoxia is severe enough to produce lactic acid, the pH and values will be lower than expected for a particular Paco₂ level.

Oxygenation Indices*

	Do2†			O2ER
↑	↓	N	N	↑	↓

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^*, Arterial-venous oxygen difference; DO₂, total oxygen delivery; O₂ER, oxygen extraction ratio; , pulmonary shunt fraction; , mixed venous oxygen saturation; , oxygen consumption.

^†The Do₂ may be normal in patients who have compensated to the decreased oxygenation status with (1) an increased cardiac output, (2) an increased hemoglobin level, or (3) a combination of both. When the Do₂ is normal, the O₂ER is usually normal.

Increased Transillumination

Transillumination of the chest is performed by placing a high-intensity fiberoptic light source against the infant’s chest in a dark room. When air is present in the chest cavity, an increased illumination is seen on the affected side.

RADIOLOGIC FINDINGS

Chest Radiograph

The chest x-ray film may show focal or generalized problem areas. When significant partial airway obstruction, air trapping, and alveolar hyperinflation are present, the chest x-ray film appears hyperlucent and the diaphragms may be depressed. The diagnosis of pulmonary interstitial emphysema (PIE) is commonly confirmed when the x-ray film reveals lung hyperinflation and a fine, bubbly appearance (emphysema-like blebs) extending from the hilum to the pleura (Figures 35-2 and 35-3).

The respiratory care practitioner should always be alert for the sudden development of a pneumomediastinum or pneumothorax in infants with pulmonary air leak syndromes. When the excessive air associated with a PIE continues to track—and accumulate—through the perivascular and peribronchial cuffs, it may cause the gas in the hilum area to rupture into the mediastinum, resulting in a pneumomediastinum. A pneumothorax may occur because of the alveolar overdistention and subsequent rupture commonly associated with PIE (Figure 35-4). Figure 35-5 shows an infant with a pneumopericardium, which develops from the direct tracking of interstitial gas along the great vessels of the pericardial sac.

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^*Some practitioners use the stethoscope to determine the point at which the heart sounds are most audible.

General Management of Pulmonary Air Leak Syndromes

Prevention is the best treatment for pulmonary air leak syndromes. These syndromes may be prevented by the use of low mechanical ventilator pressures and the maintenance of good ventilation and oxygenation. Selective intubation of the unaffected or less affected lung may allow the injured lung time to heal. High-frequency ventilation has been successful in treating infants with pulmonary air leak syndromes. Survivors of pulmonary air leak syndromes often develop bronchopulmonary dysplasia (see Chapter 37) as a result of overly vigorous mechanical ventilation. Finally, the respiratory care practitioner must always be on alert for signs and symptoms of subcutaneous emphysema, pneumothorax, pneumopericardium, pneumoperitoneum, and intravascular air embolism. Mechanical removal of free air from the intrathoracic cavity, pericardium, or mediastinum may sometime be necessary, especially if vascular collapse is present.

A 32-week-gestation, preterm female infant was delivered by emergency cesarean section to a healthy 25-year-old mother. The infant weighed 2750 g. The mother’s admitting history showed her to be a primigravida with normal prenatal care and no history of illness during her pregnancy. The cesarean section was performed because of repeated and prolonged fetal heart rate decelerations that did not improve with maternal positioning or oxygen administration. At delivery, the infant was found to have the umbilical cord twice wrapped tightly around her neck. She was limp, appeared pale and cyanotic, and was apneic. She showed no response to tactile stimuli, and her heart rate was 65 bpm. Her 1-minute Apgar score was 1 (color 0, pulse 1, grimace 0, reflex irritability 0, muscle tone 0, respiratory 0).

Immediately the neonatologist, nurse, and respiratory therapist started cardiopulmonary resuscitation (CPR) procedures, including ventilation with a bag-valve-mask at an Fio₂ of 1.0 and vigorous chest compressions. The 5-minute Apgar was 5 (color 1, pulse 2, grimace 0, reflex irritability 0, muscle tone 1, respiratory 1). Despite the fact that the baby’s condition had started to improve, she suddenly took a turn for the worse. Her heart rate started to drop; she again appeared cyanotic, and her muscle tone decreased.

At this time, the respiratory therapist noted that the baby’s breath sounds were absent over the right lung and severely decreased over the left upper and middle lobes. Her heart sounds were muffled and faint, and the point of maximum impulse (PMI) was displaced to the left. Transillumination showed a large right pneumothorax. This was later confirmed by chest x-ray examination as a right tension pneumothorax. The neonatologist inserted a chest tube, and the baby was placed on a mechanical ventilator with the following settings: intermittent mandatory ventilation (IMV) 30, Fio₂ 1.0, positive inspiratory pressure (PIP) +25 cm H₂O, positive end-expiratory pressure (PEEP) +5 cm H₂O, inspiratory time (Ti) 0.35, and flow 6 L/min.

Moments later, an umbilical artery catheter (UAC) was inserted; it showed the following values: pH 7.19, Paco₂ 77, 19, Pao₂ 31, and Sao₂ 47%. The ventilator rate was increased immediately to 40 breaths/min. ABG values 20 minutes later were as follows: pH 7.33, Paco₂ 43, 21, Pao₂ 47, and Sao₂ 83%.

A second chest x-ray examination an hour later showed that the right lung had reexpanded, with segmental atelectasis throughout. At this time, the infant appeared pink and her vital signs were stable, with a heart rate of 155 bpm and blood pressure of 68/35. Breath sounds revealed bilateral crackles and rhonchi. ABG values were as follows: pH 7.34, Paco₂ 42, 22, and Pao₂ 53. The Sao₂ was 89%. The respiratory therapist recorded the following in the baby’s chart: