Physiology of the Infant and the Transition Period

The newborn period has been defined as the first 24 hours of life and the neonatal period as the first month. The first 72 hours are especially significant for the cardiovascular, pulmonary, and renal systems.

The Cardiovascular System (Fetal Circulation) (Fig. 41-1: Schematic Representation of Fetal and Neonatal CirculationA)
1. Changes at Birth (Fig. 41-1: Schematic Representation of Fetal and Neonatal CirculationB)
2. Myocardial function is different in neonates because the cardiac myocytes have less organized contractile elements than in children and adults. The neonatal myocardium cannot generate as much force as in older children and is relatively noncompliant. Consequently, there is limited functional reserve in the neonatal period, and afterload increases are particularly poorly tolerated.
3. Especially in the first 3 months of life, the influence of the parasympathetic nervous system on the heart is more mature than the influence of the sympathetic system, and the myocardium does not respond to inotropic support as well as in older children and adults.
4. Even in the absence of stress, the neonatal heart has limited ability to increase cardiac output compared with the mature heart (Fig. 41-2: Schematic of reduced cardiac reserve in fetal and newborn animal hearts compared with adult hearts).
The Pulmonary System
1. The airways and alveoli continue to grow after birth, with the alveoli increasing in number until about 8 years of age.
2. With the initiation of ventilation, the alveoli transition from a fluid-filled to an air-filled state, and a normal ventilatory pattern with normal volumes develops in the first 5 to 10 minutes of life. Blood gases stabilize with the establishment of increased pulmonary blood flow (Table 41-1: Normal Blood Gas Values in Neonates).
3. Tidal volume is about the same in neonates as in children and adults on a volume per kilogram body weight measure, but the respiratory rate is increased in neonates (Table 41-2: Comparison of Normal Respiratory Values in Infants and Adults and Fig. 41-3: Static lung volumes of infants and adults).
  1. Increased minute ventilation mirrors the higher oxygen consumption in neonates (about double that seen in adults).
  2. The ratio of minute ventilation to functional residual capacity (FRC) is two to three times higher in newborns. As a result, anesthetic induction and emergence with a volatile anesthetic agent should be faster. In addition, the decrease in FRC relative to minute ventilation and oxygen consumption means that there is less “oxygen reserve” in the FRC than in older children and adults. There is a more rapid decrease in arterial oxygen levels in newborns in the presence of apnea or hypoventilation.
4. Decreased surfactant production caused by prematurity or other conditions such as maternal diabetes can cause respiratory distress syndrome (RDS). Commercially available surfactant is extraordinarily useful in treating and preventing RDS in susceptible patients. In addition, surfactant can improve gas exchange in preterm infants who may not have RDS but are stressed by sepsis, heart failure, or other systemic problems.
5. Neonates do not respond as well to hypercapnia as older children.
Persistent Pulmonary Hypertension of Newborns (Fig. 41-4: Correlation of mean pulmonary arterial pressure with age in 85 normal-term infants studied during the first 3 days of life)
1. Hypoxia and acidosis, along with inflammatory mediators, may cause pulmonary artery pressure to persist at a high level or after initially decreasing to increase to pathologic levels (persistent fetal circulation).
2. The elevated pulmonary vascular resistance causes both the ductus arteriosus and foramen ovale to remain open, with subsequent right-to-left (bypassing the pulmonary circulation) shunting.
3. Treatment goals are to achieve a PaO₂ of 50 to 70 mm Hg and a PaCO₂ of 50 to 55 mm Hg.
Meconium Aspiration
1. Meconium aspiration may be a marker of chronic fetal hypoxia in the third trimester. This condition is different from the meconium aspiration that occurs during delivery, which is thick and mechanically obstructs the tracheobronchial system.
2. Current recommendations for intubation and suctioning for newborns at delivery with frank meconium aspiration or meconium staining (approximately 10% of newborns) emphasize a conservative approach. Routine oropharyngeal suctioning of meconium is recommended immediately at the time of delivery, but tracheal intubation and suctioning should be performed selectively.
  1. If the newborn is vigorous and crying, no further suctioning is needed.
  2. If meconium is present and the newborn is depressed, the trachea should be intubated and meconium and other aspirated material suctioned from beneath the glottis.
The Renal System
1. At birth, the glomerular filtration rate (GFR) is low but increases significantly in the first few days and doubles in the first 2 weeks; however, it does not reach adult levels until about 2 years of age. The limited ability of newborns' kidneys to concentrate or dilute urine results from this low GFR and decreased tubular function.
2. The half-life of medications excreted by means of glomerular filtration is prolonged in newborns. The relative inability to conserve water means that neonates, especially in the first week of life, tolerate fluid restriction poorly. In addition, the inability to excrete large amounts of water means that newborns tolerate fluid overload poorly.
Fluid and Electrolyte Therapy in the Neonate
1. Total body water (TBW) decreases to about 75% of body weight for term infants at birth. Preterm infants have higher TBWs than term infants, often in the 80% to 85% range. TBW decreases during the first 12 months of life to about 60% to 65% of body weight and stays at this level through childhood.
2. TBW is distributed between two compartments, intracellular fluid (ICF) and extracellular fluid (ECF). The ECF volume is larger than the ICF volume in fetuses and newborns, usually in the 40% (ECF) and 20% (ICF) of body weight ranges. This is the opposite of the situation in infants and children.
  1. The ECF and ICF volumes (20% and 40% of body weight, respectively) approach adult values by about 1 year of age. This dramatic shift is beneficial to the child, especially in increasing the mobility of reserves in the face of dehydration.
  2. Fluid can be easily mobilized from ICF volumes to replenish intravascular volume that is lost from fasting, fever, diarrhea, or other causes. This means that nonneonates are better situated to maintain intravascular volume in these situations than neonates.
3. The blood volume in normal full-term newborns is approximately 85 mL/kg and approximately 90 to 100 mL/kg in preterm newborns.
4. Maintenance fluid requirements have been estimated to be 60, 80, 100, and 120 mL/kg/24 hours for the first 4 days of life, respectively. For the rest of the neonatal period, a maintenance rate of 150 mL/kg/24 hours is appropriate.
  1. Because of ongoing sodium loss secondary to the inability of the neonatal distal tubule to respond fully to aldosterone, intravenous (IV) fluids in neonates must contain some sodium (balanced salt solution such as lactated Ringer's solution or Plasmalyte).
  2. The other issues for fluid choice in neonates center on appropriate glucose administration. Neonates who are scheduled for surgery and have been receiving hyperalimentation fluids or supplementary glucose must continue to receive that fluid during surgery or must have their glucose levels monitored because of concerns of hypoglycemia.
Blood Component Therapy in the Neonate
1. The indications in the perioperative period for red blood cells are similar to those in adults, but the target values in available guidelines are higher. Transfusion is indicated for hemoglobin <10 g/dL for major surgery. The hemoglobin in transfused blood is hemoglobin A as opposed to hemoglobin F in neonates at birth. An advantage of the transfused blood is better release of oxygen at the tissue level from hemoglobin A.
2. It is recommended that platelets be kept above 50,000/mL³ for invasive procedures. These recommendations are based on expert consensus, not prospective studies.
The Hepatic System
1. The functional capacity of the liver is immature in newborns, especially synthetic and metabolic functions. Because of this immaturity, some drugs that undergo hepatic biotransformation, such as morphine, have prolonged elimination half-lives in newborns. Up to 85% of unmetabolized caffeine may be found in the urine in newborns compared with 1% in adults.
2. Decreased metabolism of a drug may actually increase its safety profile. Acetaminophen undergoes less biotransformation by the cytochrome P450 system in newborns, producing less reactive metabolites that are toxic.

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