Basic Principles of Anaesthesia for Neonates and Infants
Highlights
- The diaphragm is flatter
and less domed than the adult with limited and
less efficient movement. It also contains a lower
percentage of fatigue-resistant type-I muscle
fibres (View Highlight)
- Tidal volume is relatively fixed and
increasing respiratory rate (not tidal volume)
increases alveolar minute ventilation. (View Highlight)
- Chest
wall compliance is high and the total compli-
ance of the chest approximates to that of the
lungs (View Highlight)
- The lungs are relatively stiff at birth, but
compliance increases dramatically over the first
few hours as residual lung fluid is removed. (View Highlight)
- During early fetal life, the bronchial tree grows
by a process of dichomatous branching to pro-
duce 16–25 generations that will form the
future airways. (View Highlight)
- Structures then develop, by a
process of branching and budding, to form res-
piratory bronchioles by 24 weeks’gestation and
extra-uterine survival of the fetus is possible at
this stage. (View Highlight)
- Functional residual capacity (FRC) is low in
the neonate due to the elastic recoil of the lungs
drawing in the compliant chest wall during
expiration. Closing volume is high and often
encroaches on FRC, a situation seen during
anaesthesia. This results in airway closure dur-
ing ventilation leading to intrapulmonary shunt-
ing and a reduction in oxygen tension. This
effect can be reversed with the use of continu-
ous positive end-expiratory pressure. (View Highlight)
- Lamellar bodies, which contain
mature components of the surfactant system,
increase in number and achieve a critical mass
at 30–32 weeks’ gestation, thereby increasing
surfactant production. (View Highlight)
- Oxygen
consumption is relatively high in the newborn
(6–8 ml kg–1 min–1), approximately twice that
of adult levels. (View Highlight)
- As the ratio of alveolar ventila-
tion to FRC is high in early life (5:1 compared
with 1.5:1 in older children and adults), FRC is
a less efficient buffer to changes in the partial
pressures of the inspired gas mixture, these
changes being more rapidly reflected in alveo-
lar and arterial pressures. One result of this is
that hypoxaemia develops rapidly when venti-
lation is interrupted (View Highlight)
- At birth, the lung contains relatively few
alveoli (10% of adult). The region for gas
exchange is made up of a few generations of
wide and smooth walled transitory ducts that
open into saccules with rounded contours.
Alveolar growth accelerates following birth and
continues until 8 years of age (View Highlight)
- The combination of a high
work of breathing, increased oxygen consump-
tion, high alveolar ventilation compared with
FRC, higher closing volume and susceptibility
to diaphragmatic muscle fatigue indicates that
anaesthesia in the very young is best achieved
with a technique based on intubation and con-
trolled ventilation. (View Highlight)
- In the newborn and during infancy, the ribs
extend horizontally from the vertebral column
and their configuration is circular, similar to
the position of deep inspiration in adults. (View Highlight)
- Infants have a large head and a short neck, the neck muscles
being insufficiently developed to maintain the head in position
without support (View Highlight)
- Neonates and infants are obligatory nasal
breathers, so that any obstruction of the nasal passages can
significantly increase the work of breathing. (View Highlight)
- Sarcomeres comprise about 60% of
the adult myocardium compared with only 30% in the newborn.
This results in a neonatal myocardium that is less compliant
with a fixed stroke volume. Thus, cardiac output is rate depen-
dant with normal heart rates of 130–160 beats min–1 at birth. (View Highlight)
- A large tongue in
relation to the oropharynx increases the likelihood of airway
obstruction and difficulties with laryngoscopy during anaes-
thesia (View Highlight)
- Sympathetic innervation of the myocardium is incomplete
at birth and parasympathetic tone predominates. Bradycardia
is easily induced by hypoxaemia and manoeuvres that
increase vagal tone. (View Highlight)
- The larynx is at a higher level (C3/4) compared with
the adult airway; the narrowest part is at the level of the
cricoid (View Highlight)
- Vagal stimulation initially leads to a
reduction in ventricular output with only small changes in
heart rate. However, when bradycardia develops, profound
decreases in cardiac output occur. (View Highlight)
- The epiglottis is long, U-shaped and angled at 45°.
Straight blade laryngoscopes are more useful for intubation in
neonates and infants as the tip of the blade can be used to lift
the long and floppy epiglottis to expose the larynx. (View Highlight)
- In the
newborn, the length of the trachea is only 5 cm and the tip of
an endotracheal tube may easily be displaced into a bronchus
during intubation. (View Highlight)
- During anaesthesia, systolic arterial pressure offers an
excellent guide to the blood volume status in neonates and
infants; hypotension usually indicates hypovolaemia. (View Highlight)
- Infants born prematurely are prone to develop apnoea follow-
ing anaesthesia and surgery, with an incidence of 20–30% in
otherwise healthy preterm infants undergoing inguinal hernia
repair. (View Highlight)
- Fluid
requirements in term and preterm infants are high, reflecting
high metabolic demands. However, requirements decrease
during infancy (View Highlight)
- Apnoeic episodes can occur up to a postconceptual age
(PCA) of 60 weeks (PCA = gestational age at birth + postna-
tal age). However, the peak incidence occurs in infants
younger than a PCA of 44 weeks. (View Highlight)
- in term and preterm babies, maintenance fluid
should contain 5% glucose as these children have less carbo-
hydrate reserve and ability to prevent hypoglycaemia. (View Highlight)
- Before birth, the combined ventricular output of the fetal heart
is high (500 ml kg–1 min–1) and the left ventricle contributes
only one-third to this combined output. After birth, the output
from the left and right ventricles is the same (300–400
ml kg–1 min–1). Therefore, the output of the left ventricle
increases by > 200% and determinants of myocardial perfor-
mance are close to maximum (View Highlight)
- Over the next few months, car-
diac output decreases to 150–200 ml kg–1 min–1, suggesting
that the heart acquires a greater functional reserve with matu-
ration. (View Highlight)
- Ventricular thickness equilibrates at 3–6 months and
then becomes greater on the left (View Highlight)
- Being unable to shiver, the neonate
produces heat by non-shivering thermogenesis. This involves
the oxidation of triglycerides located in brown fat stores. This
heat production requires an increase in basal metabolic rate
and oxygen consumption that may worsen any pre-existing
hypoxaemia. (View Highlight)
- The brain of a neonate weighs 350 g (25% of adult weight) and
grows rapidly, doubling in size by 6 months. At birth, approxi-
mately 25% of neuronal cells are present and cellular develop-
ment of the cortex and brain stem is complete at one year (View Highlight)
- The range of temperatures over which heat pro-
duction is kept to a minimum is known as the neutral thermal
environment. During anaesthesia, babies can be kept warm by
heating the operating theatre to 25–27°C before the child
arrives. (View Highlight)
- The
central nervous system begins to show signs of myelination
prior to birth and this process continues into the third year of life.
Myelination begins in the peripheral nervous system and the
motor nerves myelinate before the sensory nerves. Incomplete
myelination does not imply lack of function but merely a slow-
er conduction time. This is offset by a shorter interneuron dis-
tance for the impulse to travel. There is now overwhelming evi-
dence indicating that the neurophysiological components
required for pain perception are present by mid-gestation. (View Highlight)
- CBF is constant over a wide range of mean arterial pres-
sures, i.e. 50–150 mmHg. However, the range over which CBF
remains constant in the very young has not been determined (View Highlight)
- In general, uptake and dis-
tribution of drugs are increased or unchanged while elimination
is reduced, leading to an increased risk of overdose and toxicity. (View Highlight)
- Increased cardiac output in the neonate leads to a faster circula-
tion time and, therefore, faster distribution of drugs to their site
of action (View Highlight)
- A large percentage of a neonate’s body water is con-
tained in the ECF compartment (Table 1). This large ECF vol-
ume influences the volume of distribution of many drugs, espe-
cially those which are highly ionised, e.g. muscle relaxants. (View Highlight)
- Increases in protein binding and protein concentration occur
with age and concentrations of α1-acid glycoprotein are signifi-
cantly lower in newborns than in adults. (View Highlight)
- The occurrence of IVH during
anaesthesia should largely be preventable. Abrupt fluctuations in
cerebral blood flow, cerebral blood volume and cerebral venous
pressure play a role in the development of IVH and values
should be maintained at normal levels during anaesthesia.
Therefore, arterial and venous pressures should remain constant
and rapid fluid administration, hypoxaemia and hypercarbia
should be avoided. (View Highlight)
- Hepatic enzymes involved with drug metabo-
lism mature at different times. There is decreased activity and
concentration of the microsomal enzymes responsible for phase
I (non-synthetic) reactions involved in the metabolism of syn-
thetic opioids. The activity of this system reaches adult levels
within a few days of birth. (View Highlight)
- (phase II, synthetic reactions), for example with glucuronide,
is also reduced and does not approach adult levels until 6
months of age. This effects the elimination of morphine and
doses should be reduced accordingly (View Highlight)
- Renal function, both
glomerular and tubular, is reduced in the newborn thereby
affecting drug excretion. Proximal tubular secretion is impor-
tant in the elimination of conjugated drugs and reaches adult
values by 7 months of age. (View Highlight)
- The potency of an inhaled anaesthetic is determined by its
minimum alveolar concentration (MAC). MAC is lower in
preterm infants than in term infants and increases with PCA.
MAC generally increases to a maximum level by 6 months of
age in a term infant and, thereafter, decreases with increasing
age (View Highlight)
- There is substantial evidence to suggest that the neuromuscu-
lar junction in neonates is 3 times more sensitive to non-depo-
larising muscle relaxants than that of adults. However, this
sensitivity is balanced by an almost identical increase in the
volume of distribution (because of a large ECF) so that the
required dose is unaffected. However, as a result of the
pronged elimination time, doses of additional relaxants should
be reduced and given less frequently. (View Highlight)
- Sevoflurane is somewhat unusual in that there does not
appear to be an age related difference in MAC during early
infancy. After 6 months, an abrupt step down in MAC has
been observed with sevoflurane, following which it remains
constant during childhood (View Highlight)
- The recommended dose
of succinylcholine is twice that of adults (approximately 2
mg kg–1). Its duration of action, terminated via the action of
plasma cholinesterase, is 5–8 min. The rate of succinylcholine
hydrolysis may be slower in the preterm infant (immature
liver) than in older child. Due to the activity of succinyl-
choline at muscarinic acetylcholine receptors, significant
side-effects may occur. For example, the action of succinyl-
choline on the sino-atrial node in neonates and infants may
produce severe bradycardia and asystole. This can be attenu-
ated by the prior administration of atropine. (View Highlight)
- There is an increased sensitivity to barbiturates and opioids in
the neonate and they also have prolonged effects. This has
been attributed to the immaturity of the blood-brain barrier,
allowing faster penetration and rise in concentration of these
drugs in the brain. (View Highlight)