Principles of Neonatal Anesthesia
The neonate has unique requirements for equipment, intravenous
access, fluid and drug therapy, anesthetic dosage, and environmental control. The
many possible neonatal procedures are too numerous to describe in this limited space.
However, basic anesthetic management is the same for all neonates. An understanding
of the basic differences in physiology, pharmacologic and pharmacodynamic responses,
and the underlying pathology of the
Figure 60-15
Eight types of anesthesia circuits were compared regarding
ventilatory losses (circuit efficiency) as a result of compression of anesthetic
gases (compression volume) and stretching of the circuit (compliance volume). Compression
volume and compliance volume losses vary by as much as a factor of 5, depending on
the volume of the circuit and the material from which it is constructed. Low-volume,
low-compliance circuits (Mapleson D systems) are the most efficient and give the
anesthesiologist the greatest amount of tactile feedback regarding tidal volume and
lung compliance in small patients. Although large-volume, high-compliance circuits
(adult circle systems) can be used in small patients, it is very difficult to estimate
tidal volume and lung compliance because circuit-related losses in compression and
compliance volumes are so large. (Redrawn from Coté CJ, Petkau AJ,
Ryan JF, et al: Wasted ventilation measured in vitro with eight anesthetic circuits
with and without inline humidification. Anesthesiology 59:442–446, 1983.)
surgical problem is essential for development of a safe anesthesia plan. Most of
the complications that arise are attributable to a lack of understanding of these
special considerations before induction of anesthesia. The care of neonates is fraught
with danger, sudden changes, unexpected responses, and unknown congenital problems.
If anesthesiologists are to deliver optimal pediatric anesthesia care, they must
always be prepared for the unexpected, have the proper size and variety of equipment
available, and obtain the highest level of support, both in the operating room and
in the intensive care unit. If the anesthesiologist only occasionally cares for
infants, the likelihood for a problem dramatically increases.[321]
Children younger than 1 year have a higher incidence of complications
than older children do.[80]
[304]
[305]
[306]
[307]
[322]
[323]
These
complications relate to oxygenation, ventilation, airway management, and response
to anesthetic agents and medications; they occur more frequently in ASA physical
status 3 and 4 patients. The neonate—particularly a premature infant—functions
on a marginal basis, so any type of stress is usually poorly tolerated. This vulnerability
may relate to the technical difficulty of taking care of small patients, the immaturity
of their organ systems (especially the cardiovascular, pulmonary, renal, hepatic,
and nervous systems), their high metabolic rate, their large ratio of body surface
area to weight, and the ease of miscalculating a drug dose.
When caring for infants and neonates, special attention must be
paid to all aspects of anesthesia and surgical management. The anesthesiologist
must devote particular care to the calculation of drug dosage and the dilution of
drugs. Prevention of paradoxical air emboli requires that all air be vented from
intravenous devices and syringes before use (aspirate each intravenous injection
port to remove air trapped at these junctions and eject some drug before intravenous
administration to clear the air from the dead space of the needle). A volume of
air that is clinically unimportant to an adult may prove catastrophic to an infant.
Warming of skin preparation solutions and irrigation fluids before application minimizes
heat loss. Intravenous fluids should be administered with volume-limiting devices;
infusion pumps are particularly helpful in preventing overadministration of intravenous
fluid. The composition and infusion rate of flush solutions should be noted and
calculated into maintenance fluid therapy.
Every effort must be made to maintain the infant's temperature
to minimize thermal stress. The operating room environment should be warmed so that
the whole operating room constitutes a giant incubator. Heated air mattress devices
are particularly useful for maintaining temperature.[324]
Monitoring of expired concentrations of carbon dioxide can be
less accurate in small infants, but it is still very useful for diagnosing changes
over time and issues such as bronchospasm and endobronchial intubation. Besides
the usual data from a pulse oximeter, this device can be used to prevent extreme
hyperoxia. Maintaining oxygen saturation at 93% to 95% keeps a preterm infant on
the steep side of the oxygen-hemoglobin dissociation curve. This consideration may
be important to infants still susceptible to retinopathy of prematurity, that is,
those less than 44 weeks' postconceptual age. However, because these infants have
the highest oxygen consumption, oxygen saturation in the 93% to 95% range can change
to severe hypoxemia within seconds. When managing such a delicate balance—while
at the same time bearing in mind the slight inaccuracies of these monitors[325]
—the
anesthesiologist must be extremely vigilant and prepared to respond rapidly to changes
in oxygen saturation. Keeping arterial carbon dioxide values within the normal range
(35 to 45 mm Hg) may also be important in preventing retinopathy of prematurity.
Many other factors beyond the control of the anesthesiologist often contribute to
the development of this condition.[326]
The Stress Response
There is little doubt that a neonate, even an extremely premature
infant, is capable of feeling pain and responding to painful stimuli; however, what
actually constitutes a state of "anesthesia" has yet to be proved.[327]
[328]
No child should be denied analgesia or anesthesia
because of size or age. Clearly, the cardiovascular system of a premature infant
rarely tolerates the cardiovascular depressant effects of potent inhaled anesthetics.
However, the synthetic narcotics (e.g., fentanyl, sufentanil, alfentanil, remifentanil)
are usually well tolerated by even critically ill infants. These potent narcotics
must be carefully titrated to response, and the anesthesiologist must always be wary
of narcotic-induced bradycardia and its consequences on cardiac output. Low concentrations
of potent inhaled anesthetics can be used with narcotics to provide a way of controlling
hemodynamic responses without significantly depressing the myocardium. The relative
merits of one anesthetic technique over another are not clear, and the few studies
examining this issue are poorly controlled.[329]
Narcotics and inhaled anesthetics suppress the hormonal responses to pain.[330]
The important result of the provocative studies undertaken to clarify this issue
is that they have heightened everyone's awareness of the need to provide adequate
anesthesia and analgesia to neonates. As described earlier, it is not so much the
drugs but how they are used that make the difference in terms of outcomes. In the
hands of skilled pediatric anesthesiologists, no differences were found in adverse
events or recovery from anesthesia for pyloromyotomy managed with halothane or remifentanil.
[203]
