Neonatal anesthesia

1

Why are neonates and preterm infants at increased anesthetic risk?

• Pulmonary factors. Differences in the neonatal airway, including large tongue and occiput, floppy epiglottis, small mouth, and short neck predispose infants to upper airway obstruction. The more premature the infant, the higher the incidence of airway obstruction. The carbon dioxide response curve is shifted further to the right in neonates than in adults (i.e., infants have a comparatively decreased ventilatory response to hypercarbia). Newborn vital capacity is about one-half of an adult's vital capacity, respiratory rate is at least twice that of an adult (respiratory rate in the 30s would be normal for a term neonate and even higher values are acceptable for preterm neonates), and oxygen consumption is 2 to 3 times greater. Consequently, when neonates receive medications that decrease their respiratory drive they will desaturate much faster than adults. Neonates will also fatigue more easily because their diaphragms have less type 1 muscle fibers than adults

• Cardiovascular factors. Newborn infants have noncompliant ventricles that function at close to maximal contractility. Cardiac output is dependent on heart rate. Neonates are highly sensitive to the myocardial depressant effects of many anesthetic agents, especially those that may produce bradycardia. Inhalational agents should be used cautiously

• Temperature regulation. Infants have poor central thermoregulation, thin insulating fat, increased body surface area-to-mass ratio, and high minute ventilation. These factors make them susceptible to hypothermia in the operating room (OR). Shivering is an ineffective mechanism for heat production because infants have limited muscle mass. Nonshivering thermogenesis uses brown fat to produce heat, but it is not an efficient method to restore body temperature and increases oxygen consumption significantly. Cold-stressed infants may develop cardiovascular depression and hypoperfusion acidosis

• Pharmacological factors. Neonates have a larger volume of distribution and less tissue and protein binding of drugs than older children and adults. They also have immature livers and kidneys, which are less efficiently able to eliminate medications. There is a larger distribution of their cardiac output to the vessel-rich tissues. The uptake and elimination of inhalational anesthetics is more rapid than in an adult.

2

Do neonates have normal renal function?

Glomerular function of the kidneys is immature, and the concentrating ability is impaired. Renal clearance of drugs may be delayed. Extra salt and water are not handled well. Neonates will not compensate for hypovolemia, as efficiently as adults, but will still become tachycardic and have decreased urine output.

3

Why is it important to provide infants with exogenous glucose?

Neonates have low stores of hepatic glucose, and mechanisms for gluconeogenesis are immature. Infants who have fasted may develop hypoglycemia. Symptoms of hypoglycemia include apnea, cyanosis, respiratory difficulties, seizures, high-pitched cry, lethargy, temperature instability, and sweating. Hypoglycemia may be associated with long-term neurological sequelae. For this reason, in neonates, it is typical to give glucose containing fluid (e.g., D10NS) during operative procedures.

4

What are the differences in the gastrointestinal or hepatic function of neonates?

Gastric emptying is prolonged, the lower esophageal sphincter is incompetent, and neonates spend much of their time on their backs; thus the incidence of reflux may be increased. Elevated levels of bilirubin are common in neonates. Kernicterus, a complication of severely elevated levels of bilirubin, will lead to neurological dysfunction and even death in extreme cases. Commonly used medications, such as furosemide and sulfonamide, may displace bilirubin from albumin and increase the risk of kernicterus. Diazepam contains the preservative benzyl alcohol, which also may displace bilirubin. Hepatic metabolism is immature, and hepatic blood flow is less than that in older children or adults. Drug metabolism and effect may be prolonged.

5

What is retinopathy of prematurity?

Retinopathy of prematurity is a disorder that occurs primarily in premature infants who have been exposed to high inspired concentrations of oxygen. Proliferation of the retinal vessels, retinal hemorrhage, fibroproliferation, scarring, and retinal detachment may occur, with decreased visual acuity and blindness. Premature infants should have limited exposure to high concentrations of inspired oxygen. Oxygen saturation should be maintained between 92% and 95%, except during times of greater risk for desaturation. Use of an oxygen blender to control the fraction of inspired oxygen is useful.

6

How is volume status assessed in neonates?

Findings in neonates that are hypovolemic are tachycardia, sunken fontanelles, decreased skin turgor, delayed capillary refill, weight loss, crying without tears, and decreased urine output. Hypotension is a late finding. Capillary refill after blanching of the big toe should be less than 3 seconds. The extremities should not be significantly cooler than the rest of the body. Finally, the skin should look pink and well perfused—not pale, mottled, or cyanotic.

7

What problems are common in premature infants?

See Table 51.1 .