Anesthetic risks associated with prematurity

Published on 07/02/2015 by admin

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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.

Box 197-2 Organ System Pathology in Premature Neonates

Intraventricular hemorrhage *

Delayed development

Congenital malformation

Persistent patent ductus arteriosus

Respiratory distress syndrome *

Bronchopulmonary dysplasia *

Gastrointestinal

Necrotizing enterocolitis *

Hyperbilirubinemia

Disordered swallowing/sucking

Gastroesophageal reflux

Bowel obstruction

Hepatic failure

Hyperalimentation hepatitis

Vitamin K deficiency

Chronic renal failure

Renal tubular acidosis

Electrolyte abnormalities

Retinopathy of prematurity

*Major causes of morbidity.

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.

Respiratory distress syndrome

Many premature infants have respiratory distress syndrome (RDS), also known as hyaline membrane disease. RDS is rare after 34 weeks of gestation in otherwise normal neonates and occurs three times more often in infants born via cesarean section than in those born vaginally. Immediate treatment with artificial surfactant has reduced mortality risk associated with RDS. A pneumothorax should be considered in any neonate whose oxygenation deteriorates suddenly.

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.

Apnea

All premature infants have some degree of periodic breathing. Apnea, defined as cessation of breathing that lasts for 15 sec or longer or a shorter respiratory pause associated with pallor or bradycardia (heart rate <100), is more common in premature infants than in full-term infants. The more premature the infant, the greater the likelihood that he or she will develop apnea spells. The risk of apnea spells is increased postoperatively, especially in preterm infants less than 44 weeks postconception age (the risk decreases after 44 to 60 weeks postconception age). In-hospital respiratory monitoring is recommended for at least 12 h after a preterm infant has undergone any surgical procedure, and some authorities recommend monitoring overnight in the hospital.

Patent ductus arteriosus

PDA results in a left-to-right shunt, left ventricular hypertrophy, and increased pulmonary blood flow and can lead to congestive heart failure, which manifests as respiratory failure. The first line of treatment is usually an attempt to close the PDA medically with indomethacin, fluid restriction, and diuresis before surgical ligation.

Infection

Infection is a constant threat to premature infants because these infants have reduced cellular and tissue immunity; pneumonia, sepsis, and meningitis are common. Sepsis can develop in the absence of a positive blood culture, elevated white blood cell count, or fever. The first signs of infection may be apnea, bradycardia, or acidosis. Strict compliance with handwashing and maintenance of universal precautions is mandatory when providing care to these infants.

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.

Retinopathy of prematurity

Premature infants are at increased risk of developing retinopathy of prematurity (ROP). The risk of ROP is inversely related to birth weight, with the risk highest in infants weighing less than 1000 g. The cause of ROP is multifactorial. Although the role of O₂ as a risk factor is controversial, minimizing O₂ exposure is prudent in premature infants less than 44 weeks postconception age. There is little convincing evidence that brief exposure to 100% O₂ is a risk factor for the development of ROP in infants of any age. Nonetheless, it is recommended that the PO₂ be maintained between 60 and 80 mm Hg whenever possible to reduce the risk of ROP in susceptible infants. Fortunately, most retinal changes regress spontaneously.

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.

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