CHAPTER 56 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 is, the higher the incidence of airway obstruction. The carbon dioxide response curve is shifted farther 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 twice that of an adult, and oxygen consumption is two to three times greater. Consequently opioids, barbiturates, and volatile agents have a more profound effect on oxygenation and ventilation in neonates than in adults.
Cardiac factors. Newborn infants have relatively stiff ventricles that function at close to maximal contractility. Cardiac output is heart rate dependent, and neonates are highly sensitive to the myocardial depressant effects of many anesthetic agents, especially those that may produce bradycardia. Inhalational agents and barbiturates should be used cautiously.
Temperature. Infants have poor central thermoregulation, thin insulating fat, increased body surface area–to-mass ratio, and high minute ventilation. These factors make them highly susceptible to hypothermia in the operating room. 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.
Pharmacologic 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 and a larger distribution of their cardiac output to the vessel-rich tissues. Neonates often require a larger initial dose of medication but are less able to eliminate the medication. Uptake of inhalation agents is more rapid, and minimum alveolar concentration is lower.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.
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, limpness, temperature instability, and sweating.
4 What are the differences in the gastrointestinal or hepatic function of neonates?
Gastric emptying is prolonged, and the lower esophageal sphincter is incompetent; thus the incidence of reflux may be increased. Elevated levels of bilirubin are common in neonates. Kernicterus, a complication of elevated levels of bilirubin, may lead to neurologic 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 in premature and occasionally full-term 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 and full-term 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.
6 How is volume status assessed in neonates?
Blood pressure is not a reliable measure of volume in neonates. If the anterior fontanelle is sunken, skin turgor is decreased, and the infant cries without visible tears, the diagnosis is dehydration. Capillary refill after blanching of the big toe should be less than 5 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?
TABLE 56-1 Common Problems in Premature Infants
| Problem | Significance |
|---|---|
| Respiratory distress syndrome | Surfactant, which is produced by alveolar epithelial cells, coats the inside of the alveolus and reduces surface tension. Surfactant deficiency causes alveolar collapse. BPD occurs in about 20% of cases. |
| Bronchopulmonary dysplasia (BPD) | Interstitial fibrosis, cysts, and collapsed lung impair ventilatory mechanics and gas exchange. |
| Apnea and bradycardia (A and B) | This is the most common cause of morbidity in postoperative period. Sensitivity of chemoreceptors to hypercarbia and hypoxia is decreased. Immaturity and poor coordination of upper airway musculature also contribute. If apnea persists >15 sec, bradycardia may result and worsen hypoxia. |
| Patent ductus arteriosus (PDA) | Incidence of hemodynamically significant PDA varies with degree of prematurity but is high. Left-to-right shunting through the PDA may lead to fluid overload, heart failure, and respiratory distress. |
| Intraventricular hemorrhage (IVH) | Hydrocephalus usually results from IVH. Avoiding fluctuations in blood pressure and intracranial pressure may reduce the risk of IVH. |
| Retinopathy of prematurity | See Question 5. |
| Necrotizing enterocolitis | Infants develop distended abdomen, bloody stools, and vomiting. They may go into shock and require surgery. |
8 What special preparations are needed before anesthetizing a neonate?
The room should be warmed at least 1 hour before the start of the procedure to minimize radiant heat loss. A warming blanket and warming lights also help to decrease heat loss. Covering the infant with plastic decreases evaporative losses. Forced-air warming blankets have been shown to be very efficient at keeping infants warm. They work equally well if the infant is placed on them or if the blanket is placed on the infant. Temperature should be monitored carefully because it is easy to overheat a small infant.
Routine monitors in a variety of appropriately small sizes should be available. At least two pulse oximeter probes are helpful in measuring preductal and postductal saturation.
Placing 25 to 50 ml of balanced salt solution in a buretrol prevents inadvertent administration of large amounts of fluid.
9 What intraoperative problems are common in small infants?
TABLE 56-2 Common Intraoperative Problems in Infants
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| Problem | Possible causes | Solution |
|---|---|---|
| Hypoxia |