Trachea separates from foregut and developing esophagus

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  Failure to separate results in tracheoesophageal fistula

Lungs go through several stages of development from primitive bronchi to alveoli capable of respiration

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  Until alveoli develop (~ 25 weeks gestational age) fetus would not be able to breath if delivered

  
  
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  Adequate intrathoracic volume required for normal pulmonary development

  
  
  
  
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    Intrathoracic masses (especially diaphragmatic hernia) and chest wall abnormalities (e.g., skeletal dysplasias) restrict space for lung growth

  
  
  
  
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  Amniotic fluid and fetal breathing also required for normal lung development; oligohydramnios has severe adverse effect on lung development

Diaphragm development is complex with 4 embryologic structures which must fuse; failure of fusion results in a congenital diaphragmatic hernia (CDH)

Required chest views include: 4-chamber heart, outflow tracts, and diaphragm

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  Evaluation of lungs is not specified but are generally well seen when obtaining required views

Heart occupies ~ 1/3 of intrathoracic area

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  Cardiothoracic ratio is calculated by dividing cardiac circumference by thoracic circumference (TC); normal being ~ 50%

  
  
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  Increased ratio usually indicates cardiomegaly but may also occur when chest is small

Lungs are initially similar to liver in echogenicity but become more echogenic with advancing gestation

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  Any aberration in this homogeneous echotexture raises suspicion for mass

Diaphragm appears as a thin, arched, hypoechoic band

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  Best imaged in sagittal plane; need to assess both sides completely moving from chest wall to chest wall during real-time scanning

  
  
  
  
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    Most CDHs occur posteriorly; if only viewed in anterior coronal plane, a CDH may be missed

  
  

Thymus may be prominent in 3rd trimester

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  Above the heart and is hypoechoic compared to lungs with a slightly reticular appearance

  
  
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  Look for thy-box to confirm it is not a chest mass

  
  
  
  
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    Color Doppler of superior mediastinum, at level of 3-vessel view, shows thymus between internal mammary arteries creating box appearance

  
  

Fetal breathing is essential for normal lung development and is one component of biophysical profile

The following questions form a framework for evaluation of the fetal chest; specific diagnoses will be discussed in other chapters

Is the chest normal in size?

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  A TC is not generally performed unless there is concern that the chest is small (e.g., skeletal dysplasia)

  
  
  
  
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    Performed at level of 4-chamber view with soft tissues excluded

  
  
  
  
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  Can compare to expected value for gestational age or as a ratio with the abdominal circumference (AC)

  
  
  
  
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    TC:AC ratio is stable throughout gestation with normal being > 0.8

    
    
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    Enlarged chest size is unusual but is a prominent feature of congenital high airway obstruction sequence (CHAOS)

  
  

Is the axis of the heart deviated?

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  Any shift in cardiac axis is highly suspicious for a thoracic mass or, alternatively, a cardiac defect

  
  
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  While a normal axis rules out most significant chest masses, small masses may not deviate axis

Where is the stomach?

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  Absence of the normal abdominal stomach bubble is a cardinal sign of a left-sided CDH (stomach in chest)

  
  
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  Important to note, however, that a left-sided hernia may contain only bowel &/or liver, with stomach remaining below diaphragm

  
  
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  With a right-sided CDH, stomach remains in the abdomen but is often more midline than normal

Is the mass cystic or solid?

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  Although there is overlap in the 2 differentials [congenital pulmonary airway malformation (CPAM) and CDH may look either cystic or solid], this is the starting point for forming a differential diagnosis

If cystic, is it a simple cyst or a complex cystic mass?

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  Simple cyst is more likely to be a foregut duplication cyst, while a complex cystic mass is more likely to be a CDH, macrocystic CPAM, or lymphangioma

  
  
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  Do not confuse an effusion with a cystic mass; lung will float within an effusion and have a wing-like appearance, while a cystic mass will displace and compress lung

If solid, what does the Doppler show?

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  CPAM: Vascular supply from pulmonary circulation

  
  
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  Sequestration: Prominent feeding vessel from aorta

  
  
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  CDH containing liver will show hepatic and portal veins

Where is the mass?

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  Sequestrations are almost always at left lung base (or below the diaphragm), while CPAMs are more variable in location, occurring equally on both sides

  
  
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  Bilateral chest masses are less common but include bilateral CPAMs, bilateral CDHs, or CHAOS

Does the mass extend beyond the chest wall?

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  Lymphangiomas are primarily in subcutaneous tissues, with secondary intrathoracic involvement

  
  
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  Teratomas can be locally aggressive and erode through chest wall

Is there hydrops?

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  Development of hydrops is a poor prognostic sign and may warrant clinical intervention (e.g., cyst drainage, in utero resection, early delivery); all chest masses should be monitored carefully for developing hydrops

Are there other anomalies?

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  CDH has a high association with chromosomal anomalies, other structural anomalies, and syndromes

  
  
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  Especially important to carefully evaluate the heart

  
  
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  Because the cardiac axis is often distorted, it may be more difficult to adequately evaluate, and a dedicated fetal echo may be warranted