KEY POINTS
- During the first few weeks of life, neonates are vulnerable to the so-called
flip-flop circulation, that is, going from the adult-type circulation to a fetal-type
circulation. Factors such as hypoxia, hypercapnia, acidosis, infection, hypothermia,
and prematurity increase the potential for a sudden rise in pulmonary artery pressure
resulting in shunting of blood past the lungs through a patient foramen ovale or
the ductus arteriosus, which may reopen, particularly during the first 10 days of
life.
- The neonatal heart has reduced cellular mass devoted to contractility,
so the ventricles are less compliant. This reduced compliancy leads to sensitivity
to volume loading, poor tolerance to afterload (i.e., the development of biventricular
failure), and rate-dependent cardiac output. In addition, the reduced cardiac calcium
stores in neonates results in increased susceptibility to myocardial depression by
potent anesthetic agents. This also makes the neonate dependent on exogenous calcium
(blood ionized calcium) and vulnerable to the negative inotropic effects of ionized
hypocalcemia, particularly that caused by infusion of citrated blood products such
as fresh frozen plasma.
- The neonatal airway differs from the adult airway in five ways: the tongue
is larger, the larynx is located higher in the neck, the glottis is shaped differently
and angled over the laryngeal inlet, the vocal cords are angled, and the narrowest
portion is the subglottic region at the level of the cricoid cartilage. It is for
this reason that straight blades are more useful than curved blades in neonates and
that uncuffed endotracheal tubes are used.
- Glomerular filtration and tubular function are immature but develop rapidly
in the newborn period; adult levels are achieved at approximately 2 years of age.
The frequency with which medications excreted by the kidneys (e.g., antibiotics)
can be administered during the first month of life in particular changes rapidly.
Special attention is required in this period to avoid drug-induced toxicity (e.g.,
ototoxicity) caused by excessive plasma drug levels.
- Hepatic metabolic capacity is immature at birth. Some cytochrome P450
enzymes (phase I reactions) are fully developed, whereas others are approximately
50% of adult values. Phase II reactions, or reactions that make a drug more water
soluble, are usually impaired in neonates. Some of these reactions do not achieve
maturity until 1 year of age. This hepatic immaturity can have important clinical
implications regarding the neonate's ability to excrete some medications, for example,
benzodiazepines.
- Anesthetic drug overdose is a particular danger for neonates and infants
because of a combination of factors, including immaturity of the myocardium, decreased
calcium stores of the neonatal myocardium, and the "systems issue" of the vaporizers
for halothane and sevoflurane, which have different maximum MAC delivery capability;
that is, nearly 6 MACs of halothane may be delivered to a newborn whereas only 2.5
MAC multiples can be delivered with sevoflurane. Thus, anesthetic drug-induced cardiac
arrest is a function of the vaporizer design and vulnerability of the neonatal myocardium
to anesthetic-induced cardiac depression.
- Remifentanil is a unique potent opioid in neonates. Unlike virtually every
other medication used in newborns, the half-life is shorter rather than longer in
newborns than in older children. This shorter half-life makes the ability to rapidly
induce or stop an intense opioid effect even easier to achieve in neonates than older
patients. Careful attention to opioid-induced bradycardia and chest wall rigidity
is obviously very important.
- Former preterm infants younger than 60 weeks' postconceptual age are potentially
at risk for post-operative apnea, and those who are anemic (i.e., hemoglobin <10)
are at particular risk. Both gestational age and postconceptual age at the time
of surgery are independent risk factors. The use of regional anesthesia in these
patients may reduce, but does not eliminate the incidence of postanesthesia apnea.
- Preoperative laboratory testing is minimal for most pediatric patients.
The only group of children who require routine hemoglobin measurement is infants
younger than 6 months to assess the severity of the physiologic hemoglobin nadir
(especially former preterm infants with a potential risk for apnea) and older patients
expected to experience significant blood loss. Preoperative chest radiographs are
not generally indicated. Children who have undergone chemotherapy with anthracyclines,
children with congenital heart disease, and neonates at risk for associated cardiac
anomalies may require a preoperative echocardiogram. Children receiving chemotherapy
should generally have a complete recent hematology profile, including platelet counts.
Patients taking seizure medications will generally benefit from preoperative assessment
to ensure therapeutic levels.
- Temperature regulation is a particular issue for neonates and infants.
Because of the large body surface-to-weight ratio, they are especially vulnerable
to intraoperative hypothermia. Efforts to maintain a warm operating room, the use
of warming devices such as hot air mattresses, warm surgical skin preparation solutions,
and transport in an appropriate transport device, as well as keeping the patient
covered during transport, all help prevent dangerous hypothermia.
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