Pediatric Anatomy and
Physiology
Gerard T. Hogan, Jr., CRNA, MSN
Clinical Assistant Professor
Anesthesiology Nursing Program
Florida International University
Pediatric Anatomy/Physiology
The physiologic appearance of a
newborn contrasts sharply with that
of a toddler and even more so with
that of a school-age child
You must understand these
differences and appreciate them to
properly assess, plan, and deliver an
anesthetic
Pediatric Anatomy/Physiology
Physical appearance
Most dramatic difference is physical size
BSA can be computed using nomogram
Head is large compared to the adult
Often in newborns it exceeds the circumference of
the chest
Arms and legs are shorted and underdeveloped
at birth
Midpoint in length on child is umbilicus
Midpoint in length on an adult is the symphysis
pubis
Pediatric Anatomy/Physiology
Frequently
because there is a
large difference in
the proportions of
body parts,
providers use a
BSA chart for
drug dosages
Pediatric Anatomy/Physiology
Musculoskeletal system
Bone growth occurs at different rates
throughout the body
This affects anatomical landmarks
In the neonate, the imaginary line joining the
iliac crests occurs at S1
Sacrum is not fused normally at birth
At birth spinal column has only the anterior
curvature
Cervical and lumbar curvature begin with
holding head up and walking
Pediatric Anatomy/Physiology
Central Nervous System
The brain at birth is 1/10 the body
weight
Only ¼ of the neuronal cells that exist in
adults are present in the newborn
Neuronal development finishes as age 12
Myelination is not complete until age 3
Primitive reflexes (Moro, grasp) disappear
with myelination
Pediatric Anatomy/Physiology
Central Nervous System
Autonomic nervous system is developed
at birth, though immature
Parasympathetic system is intact and
fully functional
Lower end of the cord is at L3 at birth
Receeds to L1 by 1 year of age
Dural sac shortens from S3 to S1 by 1
y/o
Pediatric Anatomy/Physiology
Cardiovascular System
Many profound changes after birth
SVR doubles after first breath
Pulmonary vasculature dilates, decreasing
PVR
Foramen ovale closes as left atrial pressure
becomes higher than right atrial pressure
Flow reverses in the ductus arteriosis,
preventing flow between the pulmonary
artery and the aorta
Pediatric Anatomy/Physiology
Cardiovascular system
The reason for closure is not fully
understood
Umbilical vein flow ceases at birth
Muscular contraction shuts off the ductus
venosus, and portal venous pressure rises,
directing flow through the liver
Persistent fetal circulation may require
surgical intervention
Pediatric Anatomy/Physiology
Cardiovascular system
Persistent fetal circulation
Hypercarbia, hypoxia, and acidosis can
precipitate pulmonary vasoconstriction
If RA pressure exceeds LA pressure, the
foramen ovale can open, and exacerbate the
shunt
If the ductus arteriosus fails to close, a
right to left shunt may continue
Pediatric Anatomy/Physiology
Pediatric Anatomy/Physiology
Myocardium
Stroke volume of an infant is relatively
fixed
“they live for (or better yet, by) heart rate”
Myocardium is relatively stiff
Increasing preload will not increase CO
Cardiac reserve is limited
Small changes in end diastolic volume yield
large changes in end diastolic pressure
Pediatric Anatomy/Physiology
Myocardium
To increase CO, you must increase HR
Infants (and prepubescent children, for
that matter) are predisposed to
bradycardia (“Vagus with legs”)
Parasympathetic cardiac innervation is
completely developed (and ready for stress)
at birth
Sympathetic innervation is sparse, but
functional
Pediatric Anatomy/Physiology
Unbalanced parasympathetic tone can
manifest in negative inotropy,
predisposing them to CHF
Heart rate in infants is higher and
decreases gradually over the first 5
years of life to near adult levels
Pediatric Anatomy/Physiology
Pediatric Anatomy/Physiology
Respiratory System
Pediatric airway
Head is large and neck is short
Occiput predominates
Supine, the chin meets the chest
Tongue is large and occupies entire
oropharynx
Absence of teeth further predisposes the
infant to airway obstruction
Pediatric Anatomy/Physiology
Respiratory System
Pediatric airway
Obligate nose breathers because of the
close proximity of the epiglottis to the soft
palate
Mouth breathing occurs only during crying
Obligate nose breathing is vital for
respiration during feeding
Pediatric Anatomy/Physiology
Respiratory System
Pediatric airway
The pharynx is almost completely soft tissue
It is easily collapsed by posterior displacement of
the mandible, or external compression of the
hyoid
The pharyngeal lumen may collapse with negative
pressure generated through inspiratory effort,
particularly when the muscles that maintain
airway structure are depressed or paralyzed
Pediatric Anatomy/Physiology
Respiratory System
Pediatric airway
Larynx
Funnel shaped, as opposed to adult cylindrical
shape
More cephalad in location as compared to an adult
In adults, the larynx lies at the level of C 4-6, but
in infants, it is 2 vertebral levels higher
Cricoid ring is complete, and is the narrowest
point of the pediatric airway
Pediatric Anatomy/Physiology
Respiratory System
Pediatric airway
Larynx
Because the cricoid ring is the narrowest part of
the airway, traumatizing it with multiple
intubation attempts may lead to swelling and
obstruction
Epiglottis is short and narrow, and cords are
angled
Pediatric Anatomy/Physiology
Respiratory System
Pediatric airway
Anatomical differences in the thorax
Chest wall is very compliant
Ribs are horizontally located, limiting inspiration
Diaphragm is deficient in type 1 muscle cells
These cells are required for continuous,
repeated exercise activities
Pediatric Anatomy/Physiology
Respiratory System
Pediatric airway
Lungs
Maturation not complete until age 8
Alveoli grow and increase in number to age 8
Surfactant production begins at 20 weeks, but
really increases between 30-34 weeks
Breathing movements begin in utero, to prepare
for the big event
Bu 36 weeks, regular breathing movements of
70/min are noted
Pediatric Anatomy/Physiology
Respiratory
System
Pediatric airway
High metabolic
rate
necessitates
high respiratory
rate
Pulmonary
parameters
vastly different
Pediatric Anatomy/Physiology
Respiratory System
Pediatric airway
FRC is relatively close to adult
No where near as effective based on
metabolic rate, O2 consumption, and high
degree of alveolar ventilation
Infants initially hyperventilate in response
to hypoxia, but will not sustain and begin to
slow down their breathing
Pediatric Anatomy/Physiology
Respiratory System
Pediatric airway
Infants increase their respiratory rate in
the presence of hypercarbia
Not as much as adults because chemoreceptors
are immature
Periodic breathing occurs in 78% of infants,
usually during quiet sleep
Hemoglobin level is around 19g/dl, most is
HbF, which has a greater affinity for O2
Pediatric Anatomy/Physiology
Respiratory System
Pediatric airway
Oxygen is bound more tightly to HbF, so
cyanosis occurs at a lower PO2 than in the
adult
O2 tissue delivery is not as good as adult due
to HbF’s poor reactivity to 2,3-DPG
Normal PO2 in the newborn is 60-90 mmHg
HbF rapidly disappears in the first few
weeks of life
Pediatric Anatomy/Physiology
Respiratory System
Pediatric airway
Physiologic anemia peaks at 3 months of age
Hgb remains relatively low until teenage
years (10-11g/dl)
Children have a lower oxygen affinity for
hemoglobin; therefore tissue unloading is
higher, that is why they can have lower HGB
levels and not be affected
Pediatric Anatomy/Physiology
Renal System
Full term infants have the same number
of nephrons as adults
Glomeruli are much smaller than in adults
GFR in the newborn is 30% that of the
adult
Tubular immaturity leads to a relative
inability to concentrate urine
Pediatric Anatomy/Physiology
Renal System
Fluid turnover is 7 times greater than
that of an adult
Altered fluid balance can have
catastrophic consequences
Organ perfusion and metabolism count on
adequate hydration
Infants and children are at a much
higher risk for developing dehydration
Pediatric Anatomy/Physiology
Hepatic System
Neonatal liver is large
Enzyme systems exist but have not been
sensitized or induced
Neonates rely on limited supply of
stored fats
Gluconeogensis is deficient
Plasma proteins are lower, greater levels
of free drug exist
Pediatric Anatomy/Physiology
GI System
Gastroesophageal reflux is common until
5 months of age
Due to inability to coordinate breathing and
swallowing until then
Gastric pH and volume are close to adult
range by 2nd day of life
Gastric pH is alkalotic at delivery
Pediatric Anatomy/Physiology
Pharmacologic considerations
Uptake
Route of administration affects uptake
IV – fastest
Oral and rectal routes slowest
Transdermal faster than adults, due to realtively
thin skin layers
Pathological conditions of the liver and heart can
significantly effect uptake
Pediatric Anatomy/Physiology
Pharmacologic considerations
Distribution
55-70% of body weight is water in infants
and children
Large ECF leads to large Vol. of distribution
In adults, ECF accounts for 20% of body weight
In children, ECF accounts for up to 40% of body
weight
The concentration and effects of watersoluble agents are affected greatly by the
larger Volume of Distribution
Pediatric Anatomy/Physiology
Pharmacologic considerations
Plasma protein binding
Lower levels of serum albumin yield higher
levels of free drug
Plasma protein levels are even lower in
certain disease states, like nephrotic
syndrome or malnutrition
Endogenous molecules, like bilirubin, can be
displaced by protein bound drugs
Pediatric Anatomy/Physiology
Pharmacologic considerations
Metabolism
Soundness and maturity of the liver affect
metabolism
Glucuronidation is underdeveloped in
neonates
Maternal use of drugs may affect enzyme
induction
Medications, like phenobarbital, induce
enzymes rapidly
Pediatric Anatomy/Physiology
Pharmacologic considerations
Excretion
Renal excretion is dependent on glomerular
filtration, active tubular secretion, and
passive tubular reabsorption
Drugs dependent on renal excretion, like
Pancuronium and Digoxin, can be markedly
affected by immature kidney function
Kidneys receive a lower percentage of CO
than in adults
GFR does not reach adult level until age 3-5
Pediatric Anatomy/Physiology
Pharmacologic considerations
ONLY body weight or BSA should be
used to calculate and determine correct
pediatric drug dosages
Body weight is used in premature infants
As always, titrate to effect
Pediatric Anatomy/Physiology
Routes of administration
Oral
Sometimes it is difficult to gain cooperation
Liquid forms have greater absorption
Place in back corner of mouth in infants
Intramuscular
Gluteus medius muscle over age 2
Vastus lataralus under 2
Pediatric Anatomy/Physiology
Pharmacologic considerations
Intravenous
Good luck starting it!
May necessitate mask induction
Use of EMLA or other anesthethetic cream
Usually better luck the more peripheral you
are
Well protected and secured
Pediatric Anatomy/Physiology
Pharmacologic considerations
Intravenous agents
Typically pediatric patients require a
larger kg dose than adults
Example – Thiopental
Adult 3-5mg/kg
Neonate 3-4mg/kg
Infant 5-7mg/kg
Children 5-6mg/kg
Pediatric Anatomy/Physiology
Pharmacologic considerations
Pediatric patients can be very sensitive
to the repiratory depressant effects of
narcotics
Careful titration is vital
Morphine 0.05-0.2mg/kg up front is
commonly used in peds
Fentanyl and demerol cause more
respiratory depression
Pediatric Anatomy/Physiology
Pharmacologic considerations
Muscle relaxants
Increased doses due to increased volume of
distribution
When using succinylcholine, expect
bradycardia if you didn’t pretreat with an
anticholinergic agent