Anesthetic Considerations in Prematurity

Anesthetic Considerations in Specific Pediatric Conditions

PREMATURITY

Pathophysiology

Prematurity is defined as birth before 37 weeksof gestation. This is in contrast to small for gesta-tional age, which describes an infant (full-term or premature) whose age-adjusted weight is less than the fifth percentile. The multiple medical problems of premature neonates are usually due to immaturity of major organ systems or to intrauterine asphyxia. Pulmonary complications include hyaline mem-brane disease, apneic spells, and bronchopulmo-nary dysplasia. Exogenous pulmonary surfactant has proved to be an effective treatment for respira-tory distress syndrome in premature infants. A pat-ent ductus arteriosus leads to shunting, and may possibly lead to pulmonary edema and conges-tive heart failure. Persistent hypoxia or shock may result in ischemic gut and necrotizing enterocolitis. Prematurity increases susceptibility to infection, hypothermia, intracranial hemorrhage, and kernic-terus. Premature neonates also have an increased incidence of congenital anomalies.

Anesthetic Considerations

The small size (often <1000 g) and fragile medical condition of premature neonates demand that spe-cial attention be paid to airway control, fluid man-agement, and temperature regulation. The problem of retinopathy of prematurity, a fibrovascular pro-liferation overlying the retina that may lead to pro-gressive visual loss, deserves special consideration. While hyperoxia is associated with this blinding disease, the presence of fetal hemoglobin and treat-ment with vitamin E may be protective. Recent evi-dence suggests that fluctuating oxygen levels may be more damaging than increased oxygen tensions. Moreover, other major risk factors, such as respira-tory distress, apnea, mechanical ventilation, hypoxia, hypercarbia, acidosis, heart disease, bradycardia, infection, parenteral nutrition, anemia, and multiple blood transfusions, must be present. Nonetheless, oxygenation should be continuously monitored with pulse oximetry or transcutaneous oxygen analysis, with particular attention given to infants younger than 44 weeks postconception. Normal Pao₂ is 60–80 mm Hg in neonates. Excessive inspired oxy-gen concentrations are avoided by blending oxygen with air. Excessive inspired oxygen tensions can also predispose to chronic lung disease.

Anesthetic requirements of premature neo-nates are reduced. Opioid-based anesthetics are often favored over pure volatile anesthetic-based

techniques because of the perceived tendency of the latter to cause myocardial depression.

Premature infants whose age is less than 50 (some authorities would say 60) weeks postconcep-tion at the time of surgery are prone to postoperative episodes of obstructive and central apnea for up to 24 h. In fact, even term infants can experience rare apneic spells following general anesthesia. Risk fac-tors for postanesthetic apnea include a low gesta-tional age at birth, anemia (<30%), hypothermia,sepsis, and neurological abnormalities. The riskof postanesthetic apnea may be decreased by intra-venous administration of caffeine (10 mg/kg) or aminophylline.

Thus, elective (particularly outpatient) proce-dures should be deferred until the preterm infant reaches the age of at least 50 weeks postconception. A 6-month symptom-free interval has been sug-gested for infants with a history of apneic episodes or bronchopulmonary dysplasia. If surgery must be performed earlier, monitoring with pulse oximetry for 12–24 h postoperatively is mandatory for infants less than 50 weeks postconception; infants between 50 and 60 weeks postconception should be closely observed in the postanesthesia recovery unit for at least 2 h.

Sick, premature neonates often receive multiple transfusions of blood during their stay in the inten-sive care nursery. Their immunocompromised sta-tus predisposes them to cytomegalovirus infection following transfusion. Signs of infection include generalized lymphadenopathy, fever, pneumonia, hepatitis, hemolytic anemia, and thrombocytope-nia. Preventive measures include using cytomegalo-virus-seronegative donor blood or, more commonly, leukocyte-reduced blood cells.