Transition From Fetal to Pediatric Anesthesia

We begin this journey into the magical world of pediatric anesthesia by describing fetal cardiopulmonary physiology and the physiologic changes that occur during birth. Knowledge of these changes is important for understanding the pathophysiologic conditions that occur in neonates when these changes do not occur normally.

First, for clarity and consistency across the pediatric anesthesia literature, and to make sure we are all on the same page, there are a few universal definitions that you need to know.

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A neonate is a child in the first 28 days (or 1 month) of life, regardless of gestational age.
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An infant is a child from birth to 12 months of life.
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The gestational age is the number of weeks between conception and birth.

The following definitions are relatively new :

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Early term delivery refers to birth during the period between 37 and 38 6/7 weeks’ gestation.
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Full term refers to delivery between 39 and 40 6/7 weeks’ gestation.
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Late term refers to delivery between 41 and 41 6/7 weeks’ gestation.
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Postterm refers to delivery after 42 weeks’ gestation.
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Extremely preterm infants are born before 28 weeks’ gestation.
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Very preterm infants are born between 28 and 31 6/7 weeks’ gestation.
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Moderate to late preterm infants are born between 32 and 36 6/7 weeks’ gestation.

Sometimes, newborn infants are classified by their weight relative to their gestational age. For example:

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Appropriate for gestational age (AGA) describes an infant with a birth weight between the 10th and 90th percentiles.
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Small for gestational age (SGA) describes an infant with a birth weight below the 10th percentile.
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Large for gestational age (LGA) describes an infant with a birth weight above the 90th percentile.

Intrauterine growth restriction (IUGR) is an abnormal pattern of restricted fetal growth for gestational age. It is mainly an obstetric term that is used to describe a pattern of growth over a period of time in utero , whereas SGA is a term used by pediatricians to describe the infant at or shortly after birth.

Lung Growth and Development

During gestation, the fetal lungs grow and develop but are not responsible for oxygenation and ventilation. We normally rely on the lungs to provide oxygenation and ventilation, but in fetal life, that is the placenta’s responsibility. The intrauterine environment allows the alveoli and bronchial tree of the lungs to develop. The fetal lungs are filled with fluid and transition to their role as the organ of gas exchange shortly before, during, and after the birth process.

Fetal lung development is divided into four stages of progressive lower airway and alveolar growth ( Table 1.1 ). In the embryonic stage, primitive lung tissue is developed and vascular connections are made. In the pseudoglandular stage, the bronchial tree begins to form lumens, and in the canalicular stage at 16 to 26 weeks’ gestation, alveoli begin to form. It is also during this period that blood and lymphatic vessels begin to develop in parallel to the bronchial tree, and surfactant production begins. Finally, during the saccular stage, the fetal lung completes the physiologic processes that allow it to accomplish respiration in the extrauterine environment. This includes maturation of the alveolar-vascular interface and development of a full complement of surfactant, which will reduce surface tension within the alveoli and prevent their collapse after transition to the outside world. During gestation, the fetal airways and alveoli become distended by secreted lung fluid, which becomes a component of the amniotic fluid and allows for proper development of the lungs.

Table 1.1

Stages of Fetal Lung Development

Stage	Gestation	Events
Embryonic	4–17 weeks	Formation of primitive lung tissue and vascular connections.
Pseudoglandular	5–17 weeks	Development of a bronchial tree that begins to form lumens.
Canalicular	16–26 weeks	Alveoli begin to form. Vascular and lymphatic systems develop alongside the bronchial tree. Differentiation of type 1 and type 2 pneumocytes with beginning of surfactant production. Extrauterine life possible at later weeks.
Saccular	24 weeks–birth	Peripheral bronchiole branching. Maturation of surfactant system. Breathing efforts begin. 30–50 million alveoli at birth.
Alveolar	Birth–3+ years	Continued alveolar growth to adult level of approximately 500 million. Reduction of interstitial tissues.