Preterm Infant

Births at less than 37 post conceptual wk (PCW) rose sharply from 1990 to 2006 in USA due to the increase in assisted reproductive technology and multiple gestation. It has been slowly declining in the ensuing years.

In 2013, 11.4% of all live births were <37 PCW; 3.4% were <34 PCW.

67% of all infant deaths occur among the premature. Babies born at less than 32 wk are 88 times more likely to perish compared to full-term babies.

Neonates have the highest periop morbidity and mortality among pediatric pts, with premature infants carrying the highest risk.

Laryngospasm

Hypothermia

Hypoglycemia

Massive blood loss

Rapid onset hypoxia

Bradycardia, poor cardiac output

Apnea

Occult congenital abnormalities

Difficult intubation, vascular access

Cardiac decompensation

Persistent or reversion to fetal circulation and high pressures in the pulmonary vascular tree

High airway pressures

Oxygen toxicity

Medication or dilution errors

Pain control

Transport disaster (extubation or hemodynamic compromise)

Immature organ systems present very specific challenges to the anesthesiologist.

Cardiac physiology is different in the premature and early neonate. The heart has fewer and disorganized contractile elements. Muscle cells contain fewer mitochondria. With low compliance, cardiac output is dependent upon heart rate. Yet, parasympathetics dominate predisposing to bradycardia. The premature heart is exquisitely sensitive to drops in serum calcium levels.

Fetal lungs have inadequate surfactant production up until about 34–36 wk. The lungs are stiffer, harder to ventilate, and prone to atelectasis. They are vulnerable to volutrauma and barotrauma, which can lead to chronic pulmonary compromise.

High pressure caused by an ill-fitting endotracheal tube against the trachea can lead to post extubation stridor or potentially subglottic stenosis.

Babies have a higher oxygen demand and a lower oxygen reserve (FRC), but it is important not to over-oxygenate. Premature infants produce fewer antioxidants against the oxygen free radicals produced during oxygen therapy. Oxygen therapy is associated with retinopathy of prematurity and bronchopulmonary dysplasia.

Hepatic metabolism of drugs is immature in the premature infant, which alters pharmacokinetics.

Glucose homeostasis is immature. Glycogen stores are low, predisposing to hypoglycemia. These babies also frequently have dextrose or total parenteral nutrition infusions, which puts them at risk of iatrogenic hyperglycemia. These pts are also relatively insulin resistant.

Hypothermia is common and can occur rapidly. Premature babies have immature mechanisms for heat homeostasis; they burn brown fat in “nonshivering thermogenesis.”

Premature kidneys have a lower glomerular filtration rate and a decreased ability to concentrate urine. Renal clearance of drugs is lower.

The coagulation cascade of healthy neonates is immature but “functionally balanced.” In sick babies, however, this immaturity may predispose them to coagulopathies leading to bleeding (intraventricular hemorrhage) or thrombotic events.

Compared to an adult, the larynx is more cephalad, the epiglottis is omega shaped, the glottis lies at an angle, and the narrowest part of the airway is subglottic.

The risk of postop apnea is high, especially with concomitant anemia (HCT <30%). It is usually mixed central and obstructive and made worse by anesthetics. The risk of apnea is greater than 1% in babies born before 35 wk who have not yet reached 54 PCW and in babies born before 32 wk who have not yet reached 56 PCW. It is important to monitor these babies postop. There is some evidence that spinal anesthesia without additional sedatives is somewhat protective against postop apnea.

Common problems in critically ill premature infants include congenital abnormalities causing cardiac, respiratory, gastrointestinal, renal or hepatic insufficiency, intraventricular hemorrhage, necrotizing enterocolitis, hernias, or retinopathy of prematurity. These can be the reasons these babies come to the OR, or can complicate surgery done for another reason.

Etiology for prematurity is multifactorial and incompletely understood. Risk factors include:

• Maternal factors including previous preterm birth, race, extremes of age, substance abuse, including smoking, multiple gestation, obesity, infection, and anemia

• Fetal factors including congenital anomalies, intrauterine growth restriction, and male sex