Gestational Age Assessment

The gestational age of an infant may be assessed in one of three ways. The most accurate means of assessing gestational age is by measuring the crown-rump length of the fetus during a first-trimester ultrasonographic examination. Another method involves calculating gestational age from the first day of the mother’s last menstrual period, but this is commonly inaccurate, leading to errors in estimation. Finally, the Dubowitz scoring system is a well-accepted method combining neurologic and physical criteria of the infant to provide an accurate assessment of gestational age.^1,2 A summary of the more significant neurologic and physical signs of maturity is presented in Table 2-3.

TABLE 2-3 Neurologic and External Physical Criteria to Assess Gestational Age

Weight and Length

Assessment of growth is measured by changes in weight, length, and head circumference. Percentile charts are valuable for monitoring the child’s growth and development. Deviation from growth within the same percentile for a child of any age is of greater significance than any single measurement (Figs. 2-1 and 2-2). Weight is a more sensitive index of well-being, illness, or poor nutrition than length or head circumference and is the most commonly used measurement of growth. Change in weight reflects changes in muscle mass, adipose tissue, skeleton, and body water and thus is a nonspecific measure of growth. Measurement of length provides the best indicator of skeletal growth because it is not affected by changes in adipose tissue or water content.

FIGURE 2-1 Postnatal growth curve (weight) for term male infants. This figure represents normal growth curves. Triangles indicate a normal child. Circles demonstrate failure to thrive in a child with severe renal failure.

FIGURE 2-2 Postnatal growth curve (weight) for preterm infants. This figure represents normal growth curves for preterm infants. Triangles indicate a normal preterm infant. Circles demonstrate failure to thrive in an infant with bronchopulmonary dysplasia.

Term infants may lose 5% to 10% of their body weight during the first 24 to 72 hours of life from loss of body water. Birth weight is usually regained in 7 to 10 days. A daily increase of 30 g (210 g/week) is satisfactory for the first 3 months. Thereafter, weight gain slows so that at 10 to 12 months of age it is 70 g each week (Table 2-4).

TABLE 2-4 Approximate Relationship of Age to Weight

When plotting the weight of a preterm infant on a growth chart, it is common to use the infant’s corrected gestational age (postmenstrual age; postconceptual age is taken from conception and is approximately 2 weeks shorter) instead of his or her chronologic age (postnatal age, i.e., from birth) during the first 2 years of the infant’s life in order to correct for prematurity.

Weight and length are important but changes affect the composition of the body itself, especially total body water, which decreases at the expense of the extracellular compartment, with adult levels attained at 1 year of age.^3,4 This finding has implications for drug dosing and distribution in the infant. Males have a greater percentage of water, whereas females have a slightly greater percentage of fat. The percentage decrease in extracellular water is greater than the decrease in total body water because of the simultaneous increase in intracellular water (Table 2-5).⁵

Another, more precise way to assess development is to calculate the body surface area (BSA).⁶

BSA can also be described using an allometric equation with an exponent of  (see Chapter 6):

Head Circumference

Head size reflects growth of the brain and correlates with intracranial volume and brain weight. Changing head circumference reflects head growth and is a part of the total body growth process; it may or may not indicate underlying involvement of the brain. An abnormally large or small head may indicate abnormal brain development, which must alert the anesthesiologist to possible underlying neurologic problems. A large head may indicate a normal variation, familial feature, or pathologic condition (e.g., hydrocephalus or increased intracranial pressure), whereas a small head may indicate a normal variant, familial feature, or pathologic condition such as craniosynostosis or abnormal brain development.

During the first year of life, head circumference normally increases 10 cm, and it increases 2.5 cm in the second year. By 9 months of age, head circumference reaches 50% of adult size, and by 2 years it is 75%. Head circumference is closely followed on standard percentile growth curves. As with weight, deviations of growth of the head within the same percentile are more significant than a single measurement.

The anterior fontanel should be palpated to assess whether it is sunken (dehydration) or bulging abnormally (suggesting increased intracranial pressure as in hydrocephalus, infection, hemorrhage, or increased partial pressure of carbon dioxide in the arterial blood [PaCO₂]). If it is bulging, the sutures should be palpated for abnormal separation as a result of increased intracranial pressure. The anterior fontanel closes between 9 and 18 months of age; the posterior fontanel closes by 2 to 4 months of age (Fig. 2-3). Cranial molding occurs particularly in LBW infants and is usually of no clinical importance.

FIGURE 2-3 Cranial development. A and B depict the skull of a neonate with wide-open suture. C and D depict the skull of a 7-year-old boy with fused sutures.

Face

Although the cranial vault increases rapidly in size, the face and base of the skull develop at a slower rate. At birth, the mandible is small; but as a child develops, forward growth occurs, reducing the obliquity of the mandibular angle. Failure of prenatal development of the mandible may be associated with severe congenital defects (e.g., Pierre Robin, Treacher Collins, or Goldenhar syndromes). These syndromes often have other associated anomalies. After 2 years of age, the cranial vault increases only marginally in size, whereas the facial configuration undergoes substantive changes. The upper jaw grows rapidly to accommodate the developing teeth. In addition, the frontal sinuses develop by 2 to 6 years of age, and the maxillary, ethmoidal, and sphenoidal sinuses appear after 6 years of age.

Teeth

The first tooth, usually a lower incisor, erupts at approximately 6 months after birth (deciduous dentition). Eruption of all deciduous teeth is usually complete by 28 months of age. Permanent teeth appear at 6 years, with the shedding of the deciduous teeth; this process takes place during the next 6 to 8 years. Abnormally developed teeth occur with hereditary disorders, Down syndrome, cerebral palsy, medications (eg., tetracycline) and nutritional defects. Preterm infants may show severe enamel hypoplasia in their primary dentition.^7,8

Comportment and Behavior

The neurologic status and social acquisition of the child are part of the development assessment and will be described later in this chapter.

Upper Airway Development

During the course of development, the infant upper airways undergo deep anatomic modifications that include changes in size, shape, and interrelationship; this is particularly prominent during the first few years of life.

The face and the nasal chamber, the oropharynx with the tongue, and the laryngotracheal lumen are the three main parts of the upper airway involved. The development of the neurocranium will lead to the maturation of the cranial vault and skull base, and the development of the viscerocranium to the skeletal part of the face. The primordial areas involved in forming the covering of the tongue appear early in the second month of development.

The larynx is developed embryologically from ectodermal, endodermal, and mesodermal tissues that are derived from the third, fourth, and sixth branchial arch and pouch apparatus. The development of the larynx and airway in the neonate is outlined in detail in Chapter 12. The laryngeal opening (epiglottis and vocal cords) in a neonate and 2-year-old boy are shown in Figure 2-4. Note the omega-shaped long epiglottis and the pearly white vocal cords in the neonate.

FIGURE 2-4 Larynx development from neonate to 2 years old. The larynx in the neonate (A and B), with the long epiglottis (A) and the vocal cords (B, close-up). The larynx in a 2-year-old (C and D), with a shorter epiglottis (C) and the vocal cords (D, close up).

The skull base grows rapidly until age 6 years, with relatively slower growth thereafter. The cranial base flexes postnatally in a rapid growth trajectory that is complete by 2 years of age.

The depth of the nasopharynx increases due to remodeling of the palate as well as changes in the angulation of the skull base. During childhood, the soft tissues of the pharyngeal structures surrounding the upper airway grow proportionally to the skeletal structures. After birth, the dimensions of the nasal cavity increase very rapidly. During the first year of life, the total minimal cross-sectional area is increased by 67%, and the volume of the anterior 4 cm of the nasal airways by 36%.⁹

The volume of the oral cavity in the neonate is proportionally less than that in the adult, owing to a significantly shorter mandibular ramus. The volume of the oral cavity significantly increases during the first 12 months because of rapid growth in the height of the mandibular ramus.

Compared with the adult, the tongue in the neonate contains considerably less fat and soft tissue, but overall is large in size relative to the dimensions of the mouth, with relatively larger extrinsic musculature and a less developed superior longitudinal muscle resulting in a flat dorsal surface with poor lateral mobility (see also Chapter 12).

Respiratory System Development

The development of the respiratory system begins during week 4 of gestation. Three “laws” that describe the temporal development of the conducting airways, alveoli, and pulmonary vessels govern normal lung growth.

Transition to Air Breathing

Fetal breathing movements have been detected as early as 11 weeks of gestational age; they are interspersed with long periods of apnea and produce little tidal movement of lung fluid.^10,11 The critical event in the change from placental to pulmonary gas exchange is the first inspiration, which initiates pulmonary ventilation, promotes the clearance of lung fluid, and triggers the change from the fetal to the neonatal pattern of circulation.

The first breath is a gasp that generates a transpulmonary distending pressure of 40 to 80 cm H₂O.¹² This moves the tracheal fluid (100 times more viscous than air), overcomes surface forces that develop as the air−fluid interface reaches the small airways, and overcomes tissue resistance. In some children, the removal of lung fluid may be delayed, producing the syndrome called transient tachypnea of the newborn.¹³ Tachypnea lasts for 24 to 72 hours and is associated with a characteristic chest radiographic appearance consisting of increased perihilar markings, fluid in the interlobar fissures, and streaky linear opacities in the parenchyma.

With the onset of pulmonary ventilation, pulmonary blood flow sharply increases. Decreased pulmonary vascular resistance (PVR) and increased peripheral systemic vascular resistance (loss of the umbilical circulation) are the two crucial events involved in the immediate transition from the fetal circulation to the normal postnatal pattern. The increase in systemic afterload causes an immediate closure of the flap valve mechanism of the foramen ovale and reverses the direction of shunt through the ductus arteriosus. Until these fetal shunt pathways close anatomically, the pattern of circulation is unstable. Increased pulmonary vascular reactivity in response to hypoxia and acidosis may precipitate a reversal to right-to-left shunting (“flip-flop” circulation).

In the first few minutes of life, a state of “normal” asphyxia exists as a result of impairment of placental blood flow during labor. The partial pressure of oxygen in arterial blood (PaO₂) and pH are low, whereas the PaCO₂ is increased immediately after birth, but these parameters change rapidly in the first hour of life. Extrapulmonary shunting through fetal channels and intrapulmonary shunting, probably through unexpanded regions of the lung, persist for some time after birth, so that in neonates the physiologic right-to-left shunt is about three times that in adults.¹⁴

Mechanics of Breathing

Total Lung Capacity

Adults have a markedly greater total lung capacity (TLC) than infants (Fig. 2-5). This difference reflects the fact that TLC is an effort-dependent parameter, depending on the strength and efficiency of the inspiratory muscles, which can be estimated by the maximum inspiratory pressure at functional residual capacity (FRC). An adult can generate negative pressures in excess of 100 cm H₂O; negative inspiratory pressures as high as 70 cm H₂O have been recorded for neonates, a surprisingly high value in view of their underdeveloped musculature and highly compliant chest wall. This may be a consequence of the small radius of curvature of an infant’s rib cage, which by the Laplace relationship converts a small tension into a large pressure difference.²¹

FIGURE 2-5 Lung volumes in infants and adults. Note that, in infants, tidal volume breathing occurs at the same volume as closing volume. CC, Closing capacity; FRC, functional residual capacity; VC, vital capacity.

(Modified from Nelson NM. Respiration and circulation after birth. In: Smith CA, Nelson NM editors. The physiology of the newborn infant. Springfield, Ill.: Charles C Thomas; 1976. p. 207.)

Functional Residual Capacity

FRC is similar on a per-kilogram basis at all ages, but the mechanical factors on which it is based are different in infants and adults.²² In adults, FRC is the same as the volume at which the elastic forces generated by the passive recoil of the chest wall are balanced by the recoil of the lung (Fig. 2-6); this is the volume attained at end-expiration with an open glottis.

FIGURE 2-6 Compliance curves for the chest wall, lungs, and thorax (combination of chest wall and lungs) in infants and adults.

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