Anatomic Alterations of the Lungs

During normal fetal development, the diaphragm first appears anteriorly between the heart and liver and then progressively grows posteriorly. Between the eighth and tenth week of gestation, the diaphragm normally completely closes at the left Bochdalek foramen, which is located posteriorly and laterally on the left diaphragm. At about the tenth week of gestation (close to the same time the Bochdalek foramen is closing), the intestines and stomach normally migrate from the yolk sac. If, however, the bowels reach this area before the Bochdalek foramen closes, a hernia results—a congenital diaphragmatic hernia (CDH) (also called a Bochdalek hernia or posterior-lateral diaphragmatic hernia). In other words, a Bochdalek hernia is an abnormal hole in the posterolateral corner of the left diaphragm that allows the intestines—and in some cases the stomach—to move directly into the chest cavity and compress the developing lungs.*

As shown in Figure 38-1, the effects of a diaphragmatic hernia are similar to the effects of a pneumothorax or hemothorax—the lungs are compressed. As the condition becomes more severe, atelectasis and complete lung collapse may occur. When this happens, the heart and mediastinum are pushed to the right side of the chest—called dextrocardia. In addition, long-term lung compression in utero causes pulmonary hypoplasia, which is most severe on the affected (ipsilateral) side but also occurs on the unaffected (contralateral) side.

This pathologic process causes a marked reduction in the number of bronchial generations and alveoli per acinus. The concomitant increased muscularity of the small pulmonary arteries may contribute to the increased pulmonary vascular resistance and pulmonary hypertension commonly seen in these patients. Respiratory distress usually develops soon after birth. As the infant struggles to inhale, the increased negative intrathoracic pressure generated during each inspiration causes more bowel to be sucked into the thorax. Further compression of the heart occurs as the infant cries and swallows air, causing the intestine and stomach to distend further.

Finally, as a consequence of the hypoxemia associated with a diaphragmatic hernia, these babies often develop hypoxia-induced pulmonary arterial vasoconstriction and vasospasm, which produces a state of pulmonary hypertension. As a general rule, however, these babies only have a transient state of pulmonary hypertension until the diaphragmatic hernia is repaired. This is different from persistent pulmonary hypertension of the newborn (PPHN) (see Chapter 31).

The major pathologic or structural changes associated with diaphragmatic hernia may include the following:

Etiology and Epidemiology

A CDH occurs in an overall incidence ranging from 1 in 2000 to 4000 live births. The baby is usually mature, and two thirds of affected infants are male. About 95% of CDHs occur on the left side through the Bochdalek foramen. The mortality rate is about 40%. The prognosis depends on (1) the size of the defect, (2) the degree of hypoplasia, (3) the condition of the lung on the unaffected side, and (4) the success of the surgical diaphragmatic closure.

 OVERVIEW of the Cardiopulmonary Clinical Manifestations Associated with Congenital Diaphragmatic Hernia

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 diaphragmatic hernia (see Figure 38-1).

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. When a diaphragmatic hernia is present, the respiratory rate is generally 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

Clinical Manifestations Associated with More Negative Intrapleural Pressures during Inspiration

• Intercostal retraction

• Substernal retraction

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

• Flaring nostrils

Chest Assessment Findings

• Diminished or absent breath sounds over the affected side

• Bowel sounds over the affected side

• Apical heartbeat heard over the unaffected side (usually right)

Expiratory Grunting

Cyanosis

Barrel Chest

When the intestines are in the chest and distended with gas, the baby often demonstrates a barrel chest.

Scaphoid Abdomen

Depending on the degree of intestinal displacement into the thorax, the infant’s abdomen often appears flat or concave.

CLINICAL DATA OBTAINED FROM LABORATORY TESTS AND SPECIAL PROCEDURES

Pulmonary Function Test Findings (Extrapolated Data for Instructional Purposes) (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

Arterial Blood Gases

MILD TO MODERATE DIAPHRAGMATIC HERNIA

Acute Alveolar Hyperventilation with Hypoxemia (Acute Respiratory Alkalosis)

pH	Paco2		Pao2
↑	↓	↓ (slightly)	↓

SEVERE DIAPHRAGMATIC HERNIA

Acute Ventilatory Failure with Hypoxemia (Acute Respiratory Acidosis)

pH*	Paco2	*	Pao2
↓	↑	↑ (slightly)	↓

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.

RADIOLOGIC FINDINGS

Chest Radiograph

• Increased opacity (ground-glass appearance of compressed lung)

A typical radiograph shows fluid- and air-filled loops of intestine in the chest and a shift of the heart and mediastinum to the unaffected side. Atelectasis and complete lung collapse may be present. The lungs may appear hyoplastic and may not expand to meet the chest wall. A nasogastric tube (in the patient’s stomach, it is hoped) may be seen on the chest radiograph. It is used to decompress the abdominal viscera. The presence of a diaphragmatic hernia on a chest x-ray film usually confirms the need for surgery (Figure 38-2).

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

^†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.

General Management of a Congenital Diaphragmatic Hernia

Severe diaphragmatic hernia is one of the most urgent neonatal surgical emergencies. Although prompt surgical repair is imperative, a number of therapeutic measures may be instituted until the baby is stabilized for surgery. The baby may not be stable enough for surgery for several days.

As soon as the diagnosis of a diaphragmatic hernia is made, a double-lumen oral gastric tube should be inserted with intermittent or low continuous suction. This reduces the amount of gas in the stomach and bowels and thereby reduces lung compression. Oxygen therapy should be started immediately. The infant also may be placed in the semi-Fowler’s position, which reduces the intrathoracic pressure and facilitates the downward positioning of the abdominal viscera. Placing the infant on the affected side aids expansion of the good lung. The infant must not be manually ventilated with a bag and mask, because of the danger of air swallowing.

The infant must, however, be intubated and ventilated. Mechanical ventilation should be applied with low peak airway pressures (<30 cm H₂O) and rapid respiratory rates. A typical set of ventilator parameters would be as follows: peak inspiratory pressure (PIP) +18 to +20 cm H₂O, respiratory rate (RR) 40, Fio₂ 100%, positive end-expiratory pressure (PEEP) +2 to +3 cm H₂O, and inspiratory time (Ti) 0.4. High-frequency oscillatory ventilation and jet ventilation are sometimes successful. Because the infant’s lungs are fragile and rupture easily, the incidence of pneumothorax is high. Therefore the physician may need to insert one or more chest tubes during mechanical ventilation. Paralysis with pancuronium and sedation with morphine are helpful at times. Paralysis eliminates the swallowing of air, which helps to keep the bowels compressed. These infants are usually treated with extracorporeal membrane oxygenation (ECMO) as long as the diaphragmatic hernia is present. While on ECMO, the infant is usually ventilated only three or four times per minute to keep the lung inflated.

The surgical procedure entails repositioning the abdominal contents into the abdomen and closing the diaphragmatic defect. In some infants the peritoneal cavity may be too small to contain the abdominal contents. In these cases the surgeon leaves the fascia open and closes only the skin. This results in a ventral hernia that is repaired several months after the initial surgery. After surgery the baby is placed back on the ventilator and weaned per ventilator protocol. Mechanical ventilation with PEEP and continuous positive airway pressure (CPAP) commonly are required to offset the atelectasis and hypoplasia associated with the disorder. Often, the lung on the affected side is hypoplastic, and days or weeks of therapy may be required for full expansion to occur.

Occasionally, certain pharmacologic agents may be administered to offset the infant’s pulmonary hypertension. Such drugs include tolazoline, digitalis agents, diuretics, nitroglycerin, and inhaled nitric oxide (iNO). The physiologic action of iNO is believed to be similar to that of the vasoactive substance endothelium-derived relaxing factor (ERF). The use of iNO has significantly reduced the need for ECMO therapy.

ECMO may be indicated to treat circulatory and respiratory complications after surgery for infants who do not respond favorably to conventional medical therapy. Pulmonary surfactant usually is administered because the lungs are immature and hypoplastic. The administration of pulmonary surfactant may not only offset the infant’s surfactant deficiency and improve compliance but may also lower pulmonary vascular resistance and improve pulmonary blood flow.

A full-term baby boy was delivered at 2:25 am with no remarkable problems to a mother who had received no prenatal care. After delivery, however, the baby made one cry and quickly became blue and limp, started to have bradycardia, and became apneic. The baby’s 1-minute Apgar score was 3 (heart rate 1, respiration 0, tone 1, reflex irritability 1, color 0). The nurse handed the baby to a student intern, who immediately began to ventilate the baby manually. Both the respiratory therapist and the nurse noted that the baby’s abdomen was scaphoid; the therapist stated that the baby might have a diaphragmatic hernia and that bagging should be stopped immediately. Moments later, the neonatologist entered the room, confirmed the scaphoid abdomen, noted that the lungs were very stiff in response to the bagging, and ordered a stat intubation with a 3.5-mm tube and a chest x-ray examination.

The infant was then transferred to the neonatal intensive care unit (NICU). The chest x-ray film confirmed a left diaphragmatic hernia and hypoplastic left lung. At this time, a nasogastric tube was inserted, and suction was begun. The baby was sedated and placed on a pressured cycled mechanical ventilator. An intravenous (IV) line and umbilical artery catheter were then secured. The initial ventilatory settings were respiratory rate (RR) 30/min, inspiratory time (Ti) 0.6, positive end-expiratory pressure (PEEP) +4, positive inspiratory pressure (PIP) + 25, and Fio₂ 1.0. Initial arterial blood gases were pH 7.19, Paco₂ 63, 21, Pao₂ 24, and Sao₂ 38%. No breath sounds could be heard over the infant’s left lung. The neonatologist diagnosed pulmonary hypertension of the neonate. The respiratory therapist then adjusted the ventilatory settings as follows: RR 35/min, Ti 0.6 second, PEEP +5, PIP +28 cm H₂O, and Fio₂ 1.0. A second set of arterial blood gases taken 15 minutes later showed pH 7.29, Paco₂ 49, 23, Pao₂ 44, and Sao₂ 74%.

The baby was placed on extracorporeal membrane oxygenation (ECMO), with the ventilator set to minimal settings. Even though the ECMO was doing all the oxygenation, the baby’s lungs were expanded by the ventilator about four times a minute. Four days later, the baby’s pulmonary artery pressure was determined to be low enough for surgery. The diaphragmatic hernia was repaired, and the baby was returned to the NICU with a chest tube in the left side of the chest. The baby was again placed on a ventilator.

The ventilator settings 3 days later were RR 8/min, Ti 0.6, PIP +20, PEEP and CPAP +4, and Fio₂ 0.45. His vital signs were heart rate 145 bpm, blood pressure 70/45, RR 65 (between ventilator breaths), and temperature 37° C (96.8° F). His skin was pink and normal. Good breath sounds were auscultated over the right lung, and rhonchi and crackles could be heard over the left lung.

Arterial blood gas values at this time were as follows: pH 7.36, Paco₂ 44, 23, Pao₂ 73, and Sao₂ 94%. The baby’s chest x-ray film showed good lung expansion on the right side. Although the upper half of the left lung was well expanded, atelectasis and hypoplasia were still seen over the lower half of the left lung. Bubbles were no longer coming from the left-sided chest tube. A small amount of thin, clear secretions was suctioned from the baby’s endotracheal tube three or four times an hour. At that time the respiratory therapist wrote the following assessment in the infant’s chart.