Anesthetic risks associated with prematurity

Published on 07/02/2015 by admin

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Last modified 22/04/2025

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Anesthetic risks associated with prematurity

Wayne H. Wallender, DO

Approximately 12% of infants (one of every eight) born in the United States are born prematurely. Prematurity was previously defined as infants born before 38 weeks of gestation or with a weight of less than 2500 g at birth, but current definitions take into account that morbidity and mortality risks are more closely related to gestational age than to birth weight. The World Health Organization currently defines prematurity as a gestational age of less than 37 completed weeks regardless of birth weight. More than 90% of premature infants who weigh at least 800 g survive. Preterm birth is often due to a combination of fetal, placental, uterine, and maternal factors (Box 197-1).

Premature infants rarely present for surgery unless they are severely ill, and most will have multiorgan disorders. The underlying medical conditions are usually life-threatening respiratory, cardiovascular, or bowel crises (Box 197-2). Premature infants have a higher-than-normal perioperative complication rate following even minor operations. Anesthetic morbidity rate increases directly with the degree of prematurity.

Anesthetic considerations in premature infants

Temperature instability

Impaired temperature regulation in premature infants can be due to a variety of factors, including increased surface-to-volume ratio; lack of adipose tissue, which insulates neonates from their environment; fewer brown fat cells (cells stimulated by norepinephrine to increase heat production) than in full-term infants; and thin skin, resulting in heat and water loss. (The epidermis is not mature until after 32 weeks of gestation.)

Hypothermia is associated with hypoglycemia, respiratory distress, jaundice, and sepsis. Outcome is improved if premature (and sick) infants are cared for in a neutral thermal environment. Therefore, when anesthetizing such patients, the anesthesia provider must maintain patient temperature at or about 37° C through the use of radiant warmers, maintenance of warmer-than-normal room temperature, etc.

Bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD), defined as O2 dependence at 36 weeks postconception age (>28 days of total O2 therapy), is a chronic disorder, usually occurring in infants with a history of RDS. The more severe the RDS, the greater the degree of BPD. The incidence of BPD has not decreased in recent times despite improved neonatal care.

Although the cause of BPD is unknown, risk factors associated with BPD include increased inspired fraction of O2, the use of positive-pressure ventilation, infection, patent ductus arteriosus (PDA), and fluid overload in the first 5 to 6 days of life. BPD is characterized by increased airway resistance, decreased pulmonary compliance, ventilation/perfusion mismatch, decreased PO2, tachypnea, increased O2 consumption, and an increased number of pulmonary infections.

Necrotizing enterocolitis

Primarily a disease of small preterm infants, necrotizing enterocolitis has a multifactorial cause, but hypoperfusion of the gastrointestinal tract, resulting in ischemia, seems to be a primary factor. Small preterm infants (<32 weeks of gestation and 1500 g) are at greatest risk for developing necrotizing enterocolitis. Initial signs are abdominal distention and bloody feces; shock may develop from multiple bowel perforations. Patients are often hypovolemic and require fluid resuscitation before anesthesia is induced. Rapid fluid administration to preterm neonates may cause intracranial hemorrhage or reopening of the ductus arteriosus. Necrotizing enterocolitis is often associated with disseminated intravascular coagulopathy and thrombocytopenia. The use of N2O should be avoided, and perioperative blood pressure should be maintained.

Intracranial hemorrhage

The four types of intracranial hemorrhage that may occur in premature neonates are subdural, primary subarachnoid, periventricular-intraventricular (the most common), and intracerebral. Newborn immaturity is the single most important risk factor for the development of intracranial hemorrhage, which rarely occurs after 10 days after birth, with the vast majority of bleeds occurring in the first 3 days.

A variety of mechanisms are involved in intracranial hemorrhage. Impaired autoregulation of cerebral blood flow (CBF) occurs in stressed neonates. An elevation in blood pressure causes increased CBF, which may, in turn, result in hemorrhage. Arterial hypoxemia and hypercapnia that occur during asphyxia can also precipitate hemorrhage. The effects of anesthesia on CBF in neonates are not known, but it is prudent to prevent hypoxemia and hypercapnia and to avoid cerebral hyperperfusion by maintaining blood pressure in the normal range. Hyperosmolarity is also a contributing factor. Therefore, the use of hyperosmolar fluids should be avoided (e.g., dilute bicarbonate and give slowly).

An inverse correlation between IQ and the frequency of general anesthesia in infants has been shown. However, many clinicians maintain that the association between IQ and exposure to general anesthesia is secondary to the fact that many premature infants also have multiple organ problems and are, therefore, frequently anesthetized for operations and that chronic disease is the real cause of the decrease in IQ. Independent of concerns about general anesthesia on the developing brain, other issues also have an impact on the effects of drugs on preterm infants. Hepatic metabolism is decreased and renal clearance of almost all drugs secreted in the urine is also decreased in premature infants. Many drugs and drug doses safe for adults may be harmful in premature infants.