Millers_Anesthesia_Pediatric_Chapter_76-77

Highlights
- Prenatal development is usually divided into
three stages: (1) the germinal, (2) the embryonic, and
(3) the fetal stage. (View Highlight)
- The germinal stage starts with con-
ception and ends approximately 2 weeks later with the
implantation of the embryo into the uterine wall. (View Highlight)
- The embryonic stage comprises the period
between the third and eighth weeks of pregnancy and
is characterized by intense cell proliferation, migration,
and differentiation leading to the establishment of all
major organs. Increased vulnerability to a wide variety
of substrates, commonly called teratogens, during this
period can induce major developmental defects, many
of them incompatible with life (View Highlight)
- The fetal stage lasts
from the ninth week of pregnancy to birth and is char-
acterized by the growth and functional differentiation
of organs formed during the embryonic period. Numer-
ous exogenous factors, such as environmental toxins,
ionizing radiation, and maternal infections as well as
a multitude of drugs can interfere with the physiologi-
cal patterns of organ development throughout the fetal
period which, in turn, will result in organ dysfunction of
variable severity (View Highlight)
- pregnancy is considered to reach full term between
the completion of the 37th and the 42nd weeks of gestation (View Highlight)
- Apgar scores between 7 and 10 are considered reassuring,
a score of 4 to 6 as moderately abnormal, while scores 3
and below are usually indicative of poor outcome (View Highlight)
- fetuses reach an age of viability that may be considered,
under tight medical support, as compatible with extrauter-
ine life between the 22nd and 26th weeks after conception (View Highlight)
- Prematurity is stratified into mild preterm (32-37 weeks),
very preterm (28-31 weeks) and extremely preterm (<28
weeks) periods with increasing neonatal morbidity and
mortality based on degree of prematurity (View Highlight)
- Infants
weighing less than these norms can be classified as low
birth weight (<2500 g), very low birth weight (<1500
g), and extremely low birth weight (<1000 g) (View Highlight)
- During this critical period, the infant can readily revert
from the adult type of circulation to a fetal type of circula-
tion; this state is called transitional circulation (View Highlight)
- In neonates and infants, cardiac calcium stores
are reduced because of the immaturity of the sarcoplasmic
reticulum; consequently, these populations have a greater
dependence on exogenous (blood-ionized) calcium and
probably increased susceptibility to myocardial depression
by volatile anesthetics that have calcium channel–block-
ing activity. (View Highlight)
- The lung bud septates from the foregut during
the first trimester and the gas exchanging portions of the
airway are formed during the second trimester (View Highlight)
- In most cases, simple hyperventilation with resul-
tant reduction in arterial partial pressure of carbon dioxide
(PaCO2) will cause the pulmonary artery pressure to return
to normal. (View Highlight)
- Alveo-
lar ductal development starts at gestation week 24 while
the septation of the air sacs begins around gestational
week 36 (View Highlight)
- Alveoli then increase in number and size until
a child is approximately 8 years old. Further growth is
manifested as an increase in size of the alveoli and air-
ways. (View Highlight)
- Risk factors increasing the likelihood of prolonged tran-
sitional circulation include prematurity, infection, acidosis,
pulmonary disease resulting in hypercapnia or hypoxemia
(aspiration of meconium), hypothermia, and congenital
heart disease. (View Highlight)
- The myocardial structure of the heart, particularly the
volume of cellular mass devoted to contractility, is signifi-
cantly less developed in neonates than in adults (View Highlight)
- Respiration is less efficient in infants than adults. The
airway of infants is highly compliant and poorly sup-
ported by the surrounding structures. The chest wall is
also highly compliant; therefore the ribs provide little sup-
port for the lungs; that is, negative intrathoracic pressure
is poorly maintained (View Highlight)
- The small diameter of the airways
increases resistance to airflow. Thus functional airway
closure accompanies each breath. (View Highlight)
- Dead space ventila-
tion is proportionally similar to that in adults; however,
oxygen consumption is two to three times higher (View Highlight)
- developmental changes in contractile
proteins, produce a leftward displacement of the cardiac
function curve and less compliant ventricles. As a result
of these differences, cardiac output is strongly dependent
on heart rate; bradycardia is poorly tolerated because the
infant cannot easily compensate for the decreased heart
rate by increasing stroke volume to maintain normal car-
diac output. (View Highlight)
- In pre-
term infants, the work of breathing is approximately three
times that of adults. This increased work of breathing can
increase significantly by cold stress (i.e., increased meta-
bolic demand for oxygen) or any degree of airway obstruc-
tion. (View Highlight)
- The most frequently encountered arrhythmia in
pediatric populations is hypoxia-induced bradycardia
that can lead to asystole, if not appropriately handled.
Ventricular fibrillation is extremely rare in infants and
children. (View Highlight)
- Another important factor is the composition of the
diaphragmatic and intercostal muscles. These muscles do
not achieve the adult configuration of type I muscle fibers
until the child is approximately 2 years old (View Highlight)
- Differences in airway anatomy explain the more likely
potential for technical airway difficulties in infants than
in teenagers or adults (View Highlight)
- Although neonates and infants are considered as obli-
gate nasal breathers, they can also utilize the oral airway to
maintain ventilation both spontaneously and in response to
complete nasal obstruction.11 Even in preterm infants, the
prevalence of spontaneous oral breathing has been reported
to be as high as 50% during sleep, and oral breathing could
be consistently initiated in this population upon nasal
obstruction (View Highlight)
- Typically, the airway of infants dif-
fers from adults in five ways7,8: (1) The relatively large
size of the infant’s tongue, in relation to the oropharynx,
suggests that the infant is more likely to sustain airway
obstruction and technical difficulties during induction of
anesthesia and laryngoscopy. Recently, however, mag-
netic resonance imaging (MRI) studies have called this
into question by showing that soft tissues surrounding the
upper airway grow proportionally to the skeletal structures
during childhood.9 (2) Other anatomic differences may
account for some of the airway management challenges
in children. The larynx is located higher (more cephalic)
in the neck, thus making straight blades more useful than
curved blades. (3) The epiglottis is shaped differently, being
short, stubby, omega shaped, and angled over the laryngeal
inlet. Control with the laryngoscope blade is therefore more
difficult. (4) The vocal cords are angled; consequently, a
blindly passed tracheal tube may easily lodge in the anterior
commissure rather than slide into the trachea. (5) Finally,
the infant larynx is funnel shaped, the narrowest portion
occurring at the cricoid cartilage (View Highlight)
- While clas-
sic teaching is that the adult larynx is cylindrical and the
infant larynx is funnel shaped, it is now recognized that the
narrowest portion of the airway in approximately 70% of
adults is also in the same subglottic region at the level of the
cricoid cartilage as it is in children. (View Highlight)
- The composition of the diaphragm and intercostal muscles
significantly changes during the first 2 years of life. The number of type
I muscle fibers is inversely related to age and may account, in part,
for the ease of inducing respiratory fatigue as the work of breathing
increases (View Highlight)
- The narrowest part of the adult larynx and the pediatric lar-
ynx is at the level of the cricoid cartilage. Traditionally, the adult larynx
was thought to be cylindrically shaped, but autopsy data suggest that
the narrowing in adults (A) is not as pronounced as it is in infants (B).
The narrowest part of the infant larynx occurs at the level of the cricoid
cartilage; the normal adult configuration of the larynx is not achieved
until the teenage years. This anatomic difference is one of the reasons
uncuffed tracheal tubes have been traditionally preferred for children
younger than 6 years of age (View Highlight)
- In general, since the epiglottis is more “U” shaped in
young children and it may lie across the glottic opening,
straight blades are routinely used in neonates and toddlers
to directly elevate the epiglottis and visualize the vocal
cords (View Highlight)
- mandibular protrusion, Mal-
lampati’s classification, movement of atlantooccipital joint,
reduced mandibular space, and increased tongue thickness
have all been shown to be good predictors of airway prob-
lems. (View Highlight)
- Historically, uncuffed endotracheal tubes were recom-
mended in children less than 8 years of age, because it was
thought that the narrowest part of the airway was the cri-
coid ring and to minimize potential cuff-induced damage
of the tracheal mucosa as well as to allow reduction of air
flow resistance by inserting a larger size tube.192 Airway
trauma can also occur when using an uncuffed tube with
acceptable leak pressures. Moreover, a higher incidence
of laryngospasm with the use of uncuffed tubes has also
been reported, and there is no data of increased subglot-
tic airway trauma when cuffed versus uncuffed tubes are
used (View Highlight)
- Anesthetized
children are particularly prone to upper airway collapse; it
can be easily relieved by a combination of moderate head
tilt, chin lift, jaw thrust, and the application of continuous
positive airway pressure (View Highlight)
- A cuffed tube with no leak may also allow a more
accurate estimate of end tidal carbon dioxide (CO2) con-
centrations and avoid pollution of the operating room (View Highlight)
- the relatively frequent need for changing the
endotracheal tubes due to significant leak associated with
insertion of an uncuffed tube is also virtually eliminated by
using cuffed tubes. Repeat laryngoscopy is avoided since
inflating the cuff may allow insertion of a smaller tube and
using the cuff to occlude the airway without the need for
replacing the tube with a larger tube. (View Highlight)
New highlights added August 13, 2023 at 5:15 PM
- Routine requests for preoperative electrocardiograms (ECG) are not recommended for healthy children. There is, nevertheless, an ongoing controversy about the need for routine neonatal screening for long QT syndrome (LQTS) (View Highlight)
- Premedication with an
aerosol of salbutamol has been shown to be effective in both
the prevention and treatment of perioperative broncho-
spasm in children with bronchial hyperreactivity (View Highlight)
- Since both perioperative stress and a number of anesthetics can lengthen QT interval, performing ECG in neonates and infants under 6 months of age may be an option. (View Highlight)
- Bronchial hyperreac-
tivity, resulting primarily from the impact of the viral infec-
tion on the autonomic nervous system, may persist for up to
6 weeks or longer, well beyond the disappearance of clini-
cal symptoms (View Highlight)
- Infants up to 9 months of age are less prone to separation anxiety, and will most probably accept parental surrogates (including soothing voices, gentle rocking, and being held) (View Highlight)
- Separation anxiety is the greatest problem in children between 1 and 3 years of age. (View Highlight)
- Most
national guidelines recommend the “6-4-2 rule” meaning
a minimum of 6-hour-long fasting for solid foods, 4-hour-
long fasting for breast milk, and a 2-hour-long fasting for
clear fluids. (View Highlight)
- the incidence of preoperative hypoglycemia is less than
2.5% and is usually associated with fasting periods well
exceeding currently recommended fasting guidelines (View Highlight)
- the use of 5% dextrose-containing fluid
during surgery in pediatric patients has led to hypergly-
cemia which, in turn, can also lead to increased morbid-
ity and mortality (View Highlight)
- Neonates and infants
also need dextrose supplementation during anesthesia sur-
gery albeit at a lower rate than their normal maintenance
requirements.228,229 The use of 1% to 2.5% dextrose-con-
taining isotonic solutions for intraoperative maintenance
in neonates and infants appears to be the best option while
serial blood glucose measurements are needed to maintain
blood glucose levels in the optimal range (View Highlight)
- Prevention of paradoxical air emboli is critical for the
pediatric patient. A volume of air that is clinically unim-
portant in an adult may prove catastrophic in an infant (View Highlight)
- Monitoring expired concentrations of carbon dioxide
can be less accurate in small infants, but is still extremely
useful for diagnosing changes over time and for issues (View Highlight)
- Propofol and inhaled anesthetics can result in profound
cardiovascular depression in the neonate. In contrast,
the synthetic opioids (e.g., fentanyl, sufentanil, alfentanil,
remifentanil) are usually well tolerated, even in critically
ill infants (View Highlight)
- During stable situations it is reasonable to aim for
an oxygen saturation between 93% and 95% in preterm
infants. However, because these infants have the highest
oxygen consumption, oxygen saturation in the 93% to 95%
range can change to severe hypoxemia within seconds. (View Highlight)
- When used, these potent opioids must be care-
fully titrated to a defined response. Anesthesiologists must
be particularly cautious about opioid-induced bradycardia
and its consequences on cardiac output (View Highlight)
- The neonatal lung is fragile and particularly prone to
injury from excessive tidal volumes. In contrast, careful
attention to ventilation is required to maintain functional
residual capacity and avoid atelectasis (View Highlight)
- Positive end expi-
ratory pressures should be used. Even brief disconnection
of the airway circuit or mechanical ventilator can lead to
significant alveolar collapse and should thus be avoided if
possible (View Highlight)
- The optimal blood pressure during neonatal anesthesia is
unknown. Traditionally an acceptable mean arterial pres-
sure in mm Hg was judged to be roughly the same as the
postmenstrual age of the child in weeks; however there is
little if any evidence to support this. (View Highlight)
- The major anesthesia concern around infant inguinal
hernia repair is the risk of postoperative apnea. The major-
ity of infants who develop postanesthesia apnea are former
preterm infants (gestational age <37 weeks) and younger
than 44 weeks postmenstrual age (PMA) (View Highlight)