Oxygenated blood is delivered to the fetus by the umbilical vein. Intracardiac (i.e., foramen ovale) and extracardiac (i.e., ductus arteriosus and venosus) shunts form a parallel circulatory system that bypasses high resistance of the pulmonary vessels until birth. Figure 23.4 shows a schematic representation of neonatal circulation.

This transition to a normal neonatal circulation occurs after the umbilical cord is clamped and spontaneous breathing begins. As the pulmonary vascular resistance decreases, systemic blood flow is altered. Changes in pressure, plasma oxygen concentration, and diminishing placental prostaglandins help to close the shunts. However, conditions such as sepsis and severe acidosis may cause these shunts to remain open, resulting in persistent fetal circulation.

The architecture of the major conducting airways is established by the 16th week of gestation. Alveoli mature after birth and increase in number until 8 years of age. The chest wall of infants is composed predominantly of cartilage and deforms easily. Accessory muscles are poorly developed and tire quickly. The diaphragm has only a fraction of the typical adult fatigue-resistant type I muscle fibers. These attributes result in paradoxical chest wall movement when increased inspiratory effort is attempted. The increased caloric work is unsustainable, and respiratory fatigue and failure may follow.

There is less contractile tissue than in the adult heart. The chambers are also less compliant, meaning that they cannot significantly increase stroke volume (SV) to compensate for elevated metabolic needs. Cardiac output (CO = HR × SV) is therefore dependent upon heart rate (HR), and bradycardia in young children is an ominous sign of cardiovascular depression. Factors that contribute to low heart rates (e.g., hypoxia, hypercarbia, surgical manipulation) should be avoided or treated quickly.

The kidneys are very active in utero and fetal urine output contributes to the volume of amniotic fluid. The glomerular filtration rate (GFR) is lower at birth but quickly matures by the end of 1 year. A low GFR may result in infants’ and some young children’s inability to remove large amounts of fluids or drug metabolites from their bodies.

Infants, especially those that are preterm or small-for-gestational age, have limited glycogen stores to provide themselves energy. They should be monitored to prevent hypoglycemia, and a maintenance dextrose infusion is often used to prevent this occurrence. Albumin levels are also lower than in adults, and this may alter the binding and activity of certain anesthetic drugs.

Meconium is a mixture of water, pancreatic secretions, and intestinal cells that is usually passed within hours after birth. Premature evacuation, or meconium staining of amniotic fluid, is evidence of a “stressed” fetus and may pose a hazard if this material is aspirated into the immature lungs. The lower esophageal sphincter may take several weeks to reach the tone normally found in adults. Projectile vomiting after feedings is considered a classic sign of pyloric stenosis .

The estimated blood volume (EBV) is 85–90 ml/kg at term and gradually declines with age. The hemoglobin species HbF is most prevalent after birth and has a greater binding capacity for oxygen than HbA (predominant in adults). As HbF is replaced over the first 2–3 months of life, a mild anemia transiently develops (the so-called “physiologic anemia of infancy”).

Developmental milestones represent the average rate of neurobehavioral maturation. Deviations from the norm do not necessarily suggest significant disease, and in fact premature infants typically display development delay that is considered “normal” for them. However, some diseases (malnutrition, intracranial trauma) may adversely affect future development.

Infants and small children have a large surface area-to-weight ratio, meaning that they lose body heat quickly. They also have limited subcutaneous insulating fat and adipose reserves for generating heat. Infants rely upon a special brown adipose tissue for nonshivering heat generation. This is a catecholamine response which is quickly exhausted and may cause a decrease in peripheral perfusion, increased oxygen consumption, hypoxia and acidemia. The best way to maintain appropriate body temperature is to use ambient warming lamps, adjust the room thermostat, and cover exposed body parts to limit heat loss.

Changes in the volume of fat, muscle, and organ mass are age-dependent and affect pharmacodynamics and kinetics of anesthetic drugs. Since infants and young children have a higher body water content, the volume of distribution is also increased. Enzyme complexes are immature and drugs may have delayed metabolism. Age-related differences in drug responses may be due in part to variations in receptor sensitivities. Most drugs used for pediatric anesthesia have not been formally approved for use in children by the FDA. A weight-based dosing methodology presumes similar clinical responses, but this may be inaccurate. Nevertheless, this paradigm continues to be observed based upon best practice guidelines.

Many factors influence how parents and the child will remember their perioperative experience. The preanesthetic interview should be used to gather pertinent information and identify specific causes of anxiety. Potential procedure risks and side-effects should be described using simple, clear language. Setting reasonable expectations for postoperative discomfort and the manner in which it will be alleviated will reassure both parents and patient.

Comfort objects may be brought into the room with the child to ease induction. Parental presence in the OR may help facilitate the acceptance of the induction mask. Some children, especially those with a prior poor surgical experience, may benefit from additional sedation medication, as shown in Table 23.1

The otherwise healthy child who presents for a brief, outpatient procedure rarely requires more than a focused history, pertinent review of systems and a targeted physical exam to assess acute heart or lung dysfunction. Blood tests are often unnecessary and add to the cost of care while providing little benefit. However, the child with a complex past medical history may require a more thorough evaluation. Labs and noninvasive testing (echocardiogram or ultrasound) may be needed. Table 23.2 provides a template for preoperative patient evaluation, and Table 23.3 shows normal vital signs based on patient’s age.