Pediatric Fundamentals

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Pediatric Fundamentals
Drs. Greg and Joy Loy Gordon 26 January 2005
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McGraw-Hill 2002
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Pediatric Fundamentals
Objectives
Explain and apply to your anesthetic practice
selected elements of pediatric
Growth and development Cardiovascular
physiology Respiratory physiology
including Airway maintenance
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Pediatric Fundamentals Prenatal Growth and
Development
Prenatal
Embryonic period first 8 weeks Organogenesis 4th
8th weeks Ectoderm Mesoderm Endoderm
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Pediatric Fundamentals Prenatal Growth and
Development
Organogenesis 4th 8th weeks Mesoderm somites
myotomes -gt segmental muscles of
trunk dermatomes -gt dermis of scalp, neck,
trunk sclerotomes -gt vertebral bodies,
arches abnormal induction -gt spinal bifida
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Pediatric Fundamentals Prenatal Growth and
Development
Developmental Abnormalities congenital
diaphragmatic hernia (CDH) esophageal atresia
spina bifida Hirschsprungs disease omphalocel
e gastroschisis
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Pediatric Fundamentals Prenatal Developmental
Abnormalities
Congenital diaphragmatic hernia (CDH)
1 in 2,500 live births 85 left side of
diaphragm defect in closure of pleuroperitoneal
canal impaired lung growth prenatal
(intrauterine) repair possible
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Pediatric Fundamentals Prenatal Developmental
Abnormalities
Esophageal atresia failure of proliferation of
esophageal endoderm in 5th week 5 types some
with associated tracheoesophageal fistula
E H-type (7)
80
10
2
1
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Pediatric Fundamentals Prenatal Developmental
Abnormalities
Spina bifida failure of closure of posterior
neural tube during 3rd embryonic week mild spina
bifida occulta severe meningomyelocele 80
lumbosacral in utero repair described
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Pediatric Fundamentals Prenatal Developmental
Abnormalities
Hirschsprungs disease defect in neural crest
migration
leads to paralysis of that segment of colon with
subsequent proximal dilation
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Pediatric Fundamentals Prenatal Developmental
Abnormalities
Omphalocele
1 in 2,500 live births failure of return of
midgut from yolk sac to abdomen by 10
weeks often associated with other abnormalities
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Pediatric Fundamentals Prenatal Developmental
Abnormalities
Gastroschisis
1 in 10,000 live births abdominal wall defect
between developing rectus muscles just
lateral to umbilicus right side may be due to
abnormal involution of right umbilical
vein during 5th and 6th weeks usually not
associated with other defects
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Pediatric Fundamentals Prenatal Growth and
Development
Consequences of maternal disorders on
intrauterine development epilepsy history of
previous child with neural tube defect diabetes
mellitus substance abuse alcohol tobacco
cocaine benzodiazepines infectious
diseases rubella toxoplasmosis human
immunodeficiency virus (HIV) herpes simplex
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Pediatric Fundamentals Consequences of Maternal
Disorders
Epilepsy
Congenital anomalies 2 to 3 times more
frequent Appear to associated with increase risk
of malformation phenytoin valproic
acid multidrug therapy Neural tube defects (e.g.
spina bifida) valproic acid carbamazepine low
dose folate may decrease risk
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Pediatric Fundamentals Consequences of Maternal
Disorders
History of previous neural tube defect Risk of
subsequent neural tube defect increased 10 times
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Pediatric Fundamentals Consequences of Maternal
Disorders
Diabetes mellitus
Increased incidence of stillbirth congenital
malformations risk of major malformation (8
times greater) increased rate of high birth
weight
hypertophic cardiomyopathy in IDM
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Pediatric Fundamentals Consequences of Maternal
Disorders
Substance abuse alcohol
Fetal alcohol syndrome intrauterine growth
retardation (IUGR) microcephaly characteristic
facies CNS abnormalities with intellectual
deficiency Increased incidence of other major
malformations
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Pediatric Fundamentals Consequences of Maternal
Disorders
Tobacco
Low birth weight
Cocaine
prematurity clinical seizures EEG
abnormalities neurobehavioral abnormalities cerebr
al hermorrhagic infarction
Benzodiazepines no clear teratogenic link
sedation and/or withdrawal symptoms reported
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Pediatric Fundamentals Consequences of Maternal
Disorders
Infectious disease Rubella
Chromosomal abnormalities IUGR Ocular
lesions Deafness Congenital cardiomyopathy Especia
lly with infections before week 11
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Pediatric Fundamentals Consequences of Maternal
Disorders
Infectious disease Toxoplasmosis
IUGR Nonimmune hydrops Hydrocephalus Microcephaly
Later neurologic damage
Prompt spiramycin Rx until after delivery
decreases risk 50
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Pediatric Fundamentals Consequences of Maternal
Disorders
Infectious disease Human immunodeficiency virus
(HIV)
Transmission to fetus 12 30 less if mother
taking Zidovudine (no teratogenesis
reported) First signs appear at 6 months of
age Median survival 38 months
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Pediatric Fundamentals Consequences of Maternal
Disorders
Infectious disease Herpes simplex
Neonatal infections Two-thirds caused by
asymptomatic genital infection High morbidity
and mortality Seizures Psychomotor
retardation Spasticity Blindness
Learning disabilities Death Maternal
active infection C-section indicated to decrease
risk
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Pediatric Fundamentals Consequences of Maternal
Disorders
IUGR
3-7 of all pregnancies Major cause of perinatal
morbidity and mortallity Prognosis depends on
specific cause Up to 8 have major
malformations Head growth important determinant
of neurodevelopmental outcome (IUGR HC lt
3rdile -gt abnormal neurodevelopment
likely) Hemodynamic changes and/or infectious
disease often involved
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Pediatric Fundamentals - Prematurity
Definitions premature gestational age less than
37 weeks or 259 days moderately premature 31-36
weeks severely premature 24-30 weeks postterm
greater than 41 weeks low birth weight (LBW) lt
2,500 Gm (only a bit over half of LBW infants
are premature) very low birth weight (VLBW) lt
1,500 Gm newborn first day of life neonate
first month of life infant first year of life
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Pediatric Fundamentals - Prematurity
5-10 of live births High morbidity and mortality
due to immature organ systems Responsible for 75
of perinatal deaths Immediate/early
complications hypoxia/ischemia intraventricular
hemorrhage sensorineural injury respiratory
failure necrotizing enterocolitis cholestatic
liver disease nutrient deficiency social stress
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Pediatric Fundamentals - Prematurity
Special considerations
Respiratory breathing may initially be
exclusively nasal spontaneous neck flexion may
cause airway obstruction and apnea diaphragm
is most important respiratory muscle fewer
diaphragmatic type I fibers (10 vs
25) maternal betamethasone or
dexamethasone 48 hours before
delivery increases surfactant production and
decreases mortality after 30 weeks gestation
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Pediatric Fundamentals - Prematurity
Special considerations
Respiratory
apneas 25 of all prematures alleviated with
caffeine or theophylline PEEP stimulation
may be exacerabated by general anesthesia especi
ally infants lt 50 weeks postconceptional age
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Pediatric Fundamentals - Prematurity
Special considerations
Cardiovascular
PDA - treatment fluid restriction diuretics ind
omethacin surgical ligation Cardiac output
relatively dependent on heart rate Immature
sympathetic innervation
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Pediatric Fundamentals - Prematurity
Special considerations
Renal urine flow begins 10-12 weeks
gestation decreased in premature (compared to
full term) GFR renal tubular Na
threshold glucose threshold bicarbonate
threshold relative hypoaldosteronism with
increased risk of hyperkalemia tubular
function develops significantly after 34 weeks
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Pediatric Fundamentals - Prematurity
Special considerations
Nervous system
Brain has 2 growth spurts 1. neuronal cell
multiplication 15-20 weeks gestation 2. glial
cell multiplication 25 weeks to 2 years of
life Blood vessels more fragile increased risk
of intracerebral hemorrhage Periventricular
leukomalacia ischemic cerebral
complication 12-25 of LBW infants increase
risk of mental handicap Retinopathy of prematurity
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Pediatric Fundamentals - Prematurity
Special considerations
Thermal problems
Immature thermoregulation system Body heat loss
by evaporation conduction convection radiatio
n
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Pediatric Fundamentals - Growth and Development
Maturational change in form and function
Prenatal Growth Gestational age
(wks) Mean birth wt (Gm) 25
850 28 1000 30 1400 33 1900 37 2900 4
0 3500
Postnatal Growth Birth weight doubles by 5
months triples by 1 year Birth length doubles
by 4 years
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Pediatric Fundamentals - Growth and Development
Maturational change in form and function
Percent body water Term newborn 80 1 year
old 70 Adult 60
Surface areaWeight premature gt full term gt
infant gt child greater surface area greater
evaporative heat loss rapid hypothermia if
unprotected
Girls Boys Puberty onset 11 years 11½
years Peak growth Tanner stage 3 Tanner stage 4
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Pediatric Fundamentals - Growth and Development
Fluid requirements
Metabolism of one calorie of energy consumes one
ml of H2O, so fluid requirements thought to
reflect caloric requirement Body weight (kg)
Calories needed (kcal/kg/day) Fluid
requirement (ml/kg/day) 0-10
100 10-20 1000
50/(kggt10) gt 20 1500
20/(kggt20)
Dividing by 24 (hours/day) yields the famous
421 Rule for hourly maintenance fluid 4
ml/kg/hr 1st 10 kg 2 ml/kg/hr 2nd 10 kg 1
ml/kg/hr for each kg gt 20
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Pediatric Fundamentals - Growth and Development
Airway/respiratory system
Gas exchange first possible approximately 24
weeks gestation Surfactant production appears by
approximately 27 weeks produced of Type II
pneumocytes exogenous form available
Number (and size) of alveoli increase to age 8
years (size only after 8 years)
First breaths of air pneumothorax or
pneumomediastinum less than 1 several hours to
reach normal lower lung fluid levels some
expelled during birth canal compression transien
t tachypnea of newborn (TTN) increased
incidence after C-section
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Pediatric Fundamentals - Growth and Development
Respiratory rate/rhythm pauses up to 10 seconds
normal in prematures without cyanosis or
bradycardia Age (years) Normal Rate 1 - 2 20 -
40 2 - 3 20 30 7 - 8 15 - 25
Obligate nose breathing especially
prematures able to mouth breath if nares
occluded 80 of term neonates almost all term
infants by 5 months
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Pediatric Fundamentals - Growth and Development
Airway differences infant vs adult epiglottis
and tongue relatively larger glottis more
superior, at level of C3 (vs C4 or 5) cricoid
ring narrower than vocal cord aperture until
approx 8 years of age 4.5 mm in term
neonate 11 mm at 14 years
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Pediatric Fundamentals - Growth and Development
Cardiovascular system
In utero circulation placenta -gt umbilical vein
(UV)-gt ductus venosus (50) -gt IVC -gt RA
-gt foramen ovale (FO) -gt LA -gt Ascending Ao
-gt SVC -gt RA -gt tricuspid valve -gt RV
(2/3rds of CO) -gt main pulmonary artery (MPA)
-gt ductus arteriosus (DA) (90) -gt descending
Ao -gt umbilical arteries (UAs)-gt
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Pediatric Fundamentals - Growth and Development
Cardiovascular system
Transition to postnatal circulation
Loss of large low-resistance peripheral vascular
bed, the placenta (UV, UAs constrict over
several days) With first air breathing marked
drop in pulmonary vascular resistance
with greatly increased pulmonary blood flow LA
pressure gt RA pressure closes FO Elevated PaO2
constricts DA hours to days Hgb F impairs
postnatalO2 delivery Higher newborn resting
cardiac index with decreased ability to further
increase
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Pediatric Fundamentals - Growth and Development
Cardiovascular system
Normal murmurs up to 80 of normal
children vibratory Stills murmur basal
systolic ejection murmur physiologic peripheral
pulmonic stenosis (PSS) venous hum carotid
bruit S3
Murmur only in diastole abnormal
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Pediatric Fundamentals - Growth and Development
Gastrointestinal notes
Gastric pH higher at birth decreases over
several weeks Young infants diminished lower
esophageal sphincter tone 50 have daily emesis
(usually remits by 18 months) more show reflux
if esophageal pH monitored only 1 in 600 develop
complications of reflux Physiologic
jaundice Colic lt 3 months Umbilical
hernia common frequently resolve
spontaneously Teeth primary 7 months to 2 or 3
years permanent 6 years to 20 years
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Pediatric Fundamentals - Growth and Development
Renal system
Urine production begins first trimester Newborn
GFR low (correlates with gestational
age/size in prematures) rises sharply first 2
weeks adult values by age 2 years limited
concentrating ability (600 vs adult 1200
mOsm/kg) ability to dilute urine relatively
intact
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Pediatric Fundamentals - Growth and Development
Hematologic system
Infant Hgb F higher O2 affinity Hgb A
production largely replaces Hgb F by 4
months Hgb/Hct decrease to nadir at about age 2
months exaggerated in prematures (low total body
Fe stores) Blood volume (ml/kg) Prematures 105
Term newborn 85 Adult 65
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Pediatric Fundamentals - Growth and Development
Neuro notes
Nervous system anatomically complete at birth
except Myelination rapid for 2 years complete
by 7 years Posterior fontanelle closed by 6
weeks Anterior fontanelle closed by 18
months Primitive reflexes disappear in few months
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Pediatric Fundamentals - Growth and Development
Developmental pediatrics
Approach to patient depends on stage of
development Stranger anxiety 7 months 25
9 50 12 75 Toddlers magical thinking
(belief that own thought or deed causes external
events) temper tantrums (aggravated if tired,
ill, uncomfortable) Toilet training ability
develops by 18 months usually complete by 2 to 3
years (day before night) bedwetting 15 - 20
at 5 years with gradual decrease to 1 at 15
years 6 -11 years - concrete operations
phase can consider different points of
view develop explanation based on
observation beginning logical reasoning but
still tend to dogmatic 11 and older - development
of abstract thinking Adolescent - increasing need
for autonomy, participation in care http//metrohe
althanesthesia.com/edu/ped/pedspreop3.htm
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Pediatric Fundamentals - Growth and Development
Developmental pediatrics
History and physical notes
Newborn pregnancy and delivery Infancy
developmental milestones Toddler poor
localization of symptoms and very
suggestible (e.g., pharyngitis or pneumonia
presenting as abdominal pain or distress) Older
child involve in discussion/decision Preadolesce
nt and older consider interview without
parents Exam opportunistic approach in infants
and young children observation
essential distraction useful
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Pediatric Fundamentals Heart and Circulation
Embryology 1. Cardiovascular system begins
forming at 3 weeks (diffusion no longer
adequate) 2. Angiogenetic cell cluster and blood
islands -gt intraamniotic blood vessels 3. Heart
tube 4. Heart begins to beat 22 23 days 5.
Heart looping -gt 4 chambers, 27 37 days 6.
Valves 6 9 weeks
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Pediatric Fundamentals Heart and Circulation
Transitional circulation Placenta Out and Lungs
In PVR drops dramatically (endothelial-derived
NO and prostacyclin) FO closes DA closes 10-12
hours to 3 days to few weeks prematures closes
in 4-12 months PFO potential route for systemic
emboli DA and PFO routes for R -gt L shunt in PPHN
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Pediatric Fundamentals Heart and Circulation
Persistent pulmonary hypertension of the newborn
(PPHN)
Old PFC misnomer Primary Secondary meconium
aspiration sepsis birth asphyxia Treatment card
iopulmonary support inhaled NO ECMO
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Pediatric Fundamentals Heart and Circulation
Nitric oxide (NO) cGMP transduction pathway
l-arginine
?
eNOS (endothelial NO synthetase)
oxidation of quanidine N moiety
NO
activates
?
GTP
?
sGC (soluble guanylate cyclase)
cGMP (cyclic-3,5-guanosine monophosphate)
activates
?
?
PDE (phosphodiesterase)
protein kinase
GMP
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Pediatric Fundamentals Heart and Circulation
Neonatal myocardial function
Contractile elements comprise 30 (vs 60 adult)
of newborn myocardium Alpha isoform of
tropomyosin predominates more efficient binding
for faster relaxation at faster heart
rates Relatively disorganized myocytes and
myofibrils Most of postnatal increase in
myocardial mass due to hypertrophy of existing
myocytes Diminished role of relatively
disorganized sarcomplasmic reticulum (SR) and
greater role of Na-Ca channels in Ca flux
so greater dependence on extracellular Ca may
explain
Increased sensitivity to calcium channel
blockers (e.g. verapamil) hypocalcemia digitalis
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Pediatric Fundamentals Heart and Circulation
Myocardial energy metabolism
Young infant heart lactate primary
metabolite later glucose oxidation and amino
acids (aas) metabolize glucose and aas under
hypoxic conditions (may lead to greater
tolerance of ischemic insults) Gradual transition
to adult fatty acid primary metabolite by 1-2
years
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Pediatric Fundamentals Heart and Circulation
Normal aortic pressures
Wt (Gm) Sys/Dias mean 1000 50/25
35 2000 55/30 40 3000
60/35 50 4000 70/40 50
Age (months) Sys/Dias mean 1 85/65
50 3 90/65 50 6 90/65
50 9 90/65 55 12 90/65
55
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Pediatric Fundamentals Heart and Circulation
Adrenergic receptors
Sympathetic receptor system Tachycardic
response to isoproterenol and epinephrine by 6
weeks gestation Myocyte ß-adrenergic receptor
density peaks at birth then decreases
postnatally but coupling mechanism is
immature Parasympathetic, vagally-mediated
responses are mature at birth (e.g. to hypoxia)
Babies are vagotonic
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Pediatric Fundamentals Heart and Circulation
Normal heart rate
Age (days) Rate 1-3 100-140 4-7 80-145
8-15 110-165
Age (months) Rate 0-1 100-180
1-3 110-180 3-12 100-180
Age (years) Rate 1-3 100-180 3-5
60-150 5-9 60-130 9-12 50-110 12-16
50-100
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Pediatric Fundamentals Heart and Circulation
Newborn myocardial physiology
Type I collagen (relatively rigid) predominates
(vs type III in adult)
Neonate Adult Cardiac output HR
dependent SV HR dependent Starling
response limited normal Compliance less norm
al Afterload compensation limited effective Ve
ntricular high relatively low
interdependence
So
Avoid (excessive) vasoconstriction Maintain heart
rate Avoid rapid (excessive) fluid administration
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Pediatric Respiratory Physiology
Drs. Greg and Joy Loy Gordon January 2005
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Pediatric Respiratory Physiology
Prenatal Embryo
Ventral pouch in primitive foregut becomes lung
buds projecting into pleuroperitoneal
cavity Endodermal part develops
into airway alveolar membranes glands Mesenchym
al elements develop into smooth
muscle cartilage connective tissue vessels
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Pediatric Respiratory Physiology
Prenatal Development
Pseudoglandular period starting 17th week of
gestation Branching of airways down to terminal
bronchioles Canalicular period Branching in to
future respiratory bronchioles Increased
secretary gland and capillary formation Terminal
sac (alveolar) period 24th week of
gestation Clusters of terminal air sacs with
flattened epithelia
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Pediatric Respiratory Physiology
Surfactant
Produced by type II pneumocytes appear 24-26
weeks (as early as 20 weeks) Maternal
glucocorticoid treatment 24-48 hours before
delivery accelerates lung maturation and
surfactant production Premature birth
immature lungs -gt IRDS (HMD) due to
insufficient surfactant production
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Pediatric Respiratory Physiology
Prenatal Development
Proliferation of capillaries around saccules
sufficient for gas exchange 26-28th week (as
early as 24th week) Formation of alveoli 32-36
weeks saccules still predominate at birth
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Pediatric Respiratory Physiology
Prenatal Development
Lung Fluid expands airways -gt helps stimulate
lung growth contributes ? of total amniotic
fluid prenatal ligation of trachea in congenital
diaphragmatic hernia results in accelerated
growth of otherwise hypoplastic lung (J
Pediatr Surg 281411, 1993)
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Pediatric Respiratory Physiology
Perinatal adaptation
First breath(s) up to 40 (to 80) cmH2O needed
to overcome high surface forces to introduce
air into liquid-filled lungs adequate surfactant
essential for smooth transition Elevated PaO2
Markedly increased pulmonary blood flow
-gt increased left atrial pressure with closure
of foramen ovale
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Pediatric Respiratory Physiology
Postnatal development
Lung development continues for 10 years most
rapidly during first year At birth 20-50x107
terminal air sacs (mostly saccules) only one
tenth of adult number Development of alveoli from
saccules essentially complete by 18 months of
age
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Pediatric Respiratory Physiology
Infant lung volume disproportionately small in
relation to body size VO2/kg 2 x adult value
gt ventilatory requirement per unit lung volume
is increased less reserve more rapid drop in
SpO2 with hypoventilation
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Pediatric Respiratory Physiology
Neonate Lung compliance high elastic fiber
development occurs postnatally static elastic
recoil pressure is low Chest wall compliance is
high cartilaginous ribs limited thoracic muscle
mass More prone to atalectasis and respiratory
insufficiency especially under general
anesthesia Infancy and childhood static recoil
pressure steadily increases compliance,
normalized for size, decreases
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Pediatric Respiratory Physiology
Infant and toddler more prone to severe
obstruction of upper and lower airways absolute
airway diameter much smaller that
adult relatively mild inflammation, edema,
secretions lead to greater degrees of
obstruction
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Pediatric Respiratory Physiology
Control of breathing prenatal
development fetal breathing during REM
sleep depressed by hypoxia (severe hypoxia
-gt gasping) may enhance lung growth and
development
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Pediatric Respiratory Physiology
Control of breathing perinatal adaptation
Neonatal breathing is a continuation of fetal
breathing Clamping umbilical cord is important
stimulus to rhythmic breathing Relative hyperoxia
of air augments and maintains rhythmicity Independ
ent of PaCO2 unaffected by carotid
denervation Hypoxia depresses or abolishes
coninuous breathing
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Pediatric Respiratory Physiology
Control of breathing infants Ventilatory
response to hypoxemia first weeks
(neonates) transient increase -gt sustained
decrease (cold abolishes the transient increase
in 32-37 week premaures by 3 weeks sustained
increase
Ventilatory response to CO2 slope of
CO2-response curve decreases in
prematures increases with postnatal
age neonates hypoxia shifts CO2-response
curve and decreases slope (opposite to adult
response)
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Pediatric Respiratory Physiology
Periodic breathing apneic spells lt 10
seconds without cyanosis or bradycardia (mostl
y during quiet sleep) 80 of term neonates
100 of preterms 30 of infants 10-12 months
of age may be abolished by adding 3 CO2 to
inspired gas
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Pediatric Respiratory Physiology
Central apnea apnea gt 15 seconds or briefer
but associated with bradycardia
(HRlt100) cyanosis or pallor rare in full
term majority of prematures
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Pediatric Respiratory Physiology
Postop apnea in preterms
Preterms lt 44 weeks postconceptional age (PCA)
risk of apnea 20-40 most within 12 hours
postop (Liu, 1983)
Postop apnea reported in prematures as old as 56
weeks PCA (Kurth, 1987)
Associated factors extent of surgery anesthesia
technique anemia postop hypoxia (Wellborn,
1991)
44-60 weeks PCA risk of postop apnea lt 5 (Cote,
1995) Except Hct lt 30 risk remains HIGH
independent of PCA
Role for caffeine (10 mg/kg IV) in prevention of
postop apnea in prematures?
(Wellborn, 1988)
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Pediatric Respiratory Physiology Pulmonary and
Thoracic Receptors
Upper airway Pharyngeal receptors -gt inhibition
of breathing closure of larynx contraction of
pharyngeal swallowing muscles
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Pediatric Respiratory Physiology Pulmonary and
Thoracic Receptors
Upper airway - Larynx three receptor
types pressure drive (irritant) flow (or
cold) response to stimulus apnea coughing c
losure of glottis laryngospasm changes in
ventilatory pattern newborn increased
sensitivity to superior laryngeal nerve stimulus
-gt ventilatory depression or apnea H2O more
potent stimulus than normal saline (Cl-)
76
Pediatric Respiratory Physiology Pulmonary and
Thoracic Receptors
Infant (especially preterm) reflex response to
fluid at entrance to larynx
Normal protective swallowing central apnea (H2O
gt NS) sneezing laryngeal closure coughing or
awakening (less frequent)
During inhalation induction pharyngeal
swallowing reflex abolished laryngeal reflex
intact -gt breath holding or central
apnea positive pressure ventilation may -gt
push secretions into larynx -gt laryngospasm
77
Pediatric Respiratory Physiology Pulmonary and
Thoracic Receptors
Laryngospasm
Sustained tight closure of vocal cords by
contraction of adductor (cricothyroid)
muscles persisting after removal of initial
stimulus More likely (decreased threshold)
with light anesthesia hyperventilation with
hypocapnia Less likely (increased threshold)
with hypoventilation with hypercapnia positive
intrathoracic pressure deep anesthesia maybe
positive upper airway pressure Hypoxia (paO2 lt
50) increases threshold (fail-safe mechanism?)
So suction before extubation while patient
relatively deep and inflate lungs and maybe a
bit of PEEP at time of extubation
78
Pediatric Respiratory Physiology Pulmonary and
Thoracic Receptors
Slowly adapting (pulmonary stretch) receptors
(SARs)
Posterior wall of trachea and major
bronchi Stimulus distension of airway during
inspiration hypocapnia Response inhibit
inspiratory activity (Hering-Breuer inflation
reflex) May be related to adult apnea with ETT
cuff inflated during emergence from anesthesia
and rhythmic breathing promptly on cuff
deflation
79
Pediatric Respiratory Physiology Pulmonary and
Thoracic Receptors
Rapidly adapting (irritant) receptors (RARs)
Especially carina and large bronchi Stimulus lung
distortion smoke inhaled anesthetics histamine
Response coughing bronchospasm tracheal mucus
secretion Likely mediate the paradoxical reflex
of Head with vagal afferents partially blocked
by cold, inflation of lungs -gt sustained
contraction of diaphragm with prolonged
inflation may be related to sigh mechanism
(triggered by collapse of parts of lung
during quiet breathing and increasing surface
force) neonatal response to mechanical lung
inflation with deep gasping breath
80
Pediatric Respiratory Physiology Pulmonary and
Thoracic Receptors
C-fiber endings (J-receptors)
Juxta-pulmonary receptors Stimulus pulmonary
congestion edema micro-emboli inhaled
anesthetic agents Response apnea followed by
rapid, shallow breathing bronchospasm hypersec
retion hypotension bradycardia maybe
laryngospasm
81
Pediatric Respiratory Physiology Chemical
Control of Breathing
Central Chemoreceptors
Near surface of ventrolateral medulla Stimulus H
(pH of CSF and interstitial fluid
readily altered by changes in
paCO2) Response increased ventilation,
hyperventilation
82
Pediatric Respiratory Physiology Chemical
Control of Breathing
Peripheral Chemoreceptors
Carotid bodies 3 types of neural
components type I (glomus) cells type II
(sheath) cells sensory nerve fiber
endings carotid nerve -gt C.N. IX,
glossopharyngeal nerve Stimulus paCO2 and
pH paO2 (especially lt 60 mmHg) Response
increased ventilation Contribute 15 of resting
ventilatory drive Neonate hypoxia depresses
ventilation by direct suppression of medullary
centers
83
Pediatric Respiratory Physiology Chemical
Control of Breathing
84
Pediatric Respiratory Physiology Chemical
Control of Breathing
Chronic hypoxemia (for years)
Carotid bodies lose hypoxemic response E.g.,
cyanotic congenital heart disease (but hypoxic
response does return after correction and
restoration of normoxia)
85
Pediatric Respiratory Physiology Chemical
Control of Breathing
Chronic respiratory insufficiency with hypercarbia
Hypoxemic stimulus of carotid chemoreceptors
becomes primary stimulus of respiratory
centers Administration of oxygen may
-gt hypoventilation with markedly elevated
paCO2
86
Pediatric Respiratory Physiology Assessment of
Respiratory Control
CO2 response curve
87
Pediatric Respiratory Physiology Assessment of
Respiratory Control
Effects of anesthesia on respiratory control
Shift CO2 response curve to right Depress
genioglossus, geniohyoid, other phayrngeal
dilator muscles -gt upper airway obstruction
(infants gt adults) work of breathing decreased
with jaw lift CPAP 5 cmH2O oropharyngeal
airway LMA Active expiration (halothane)
88
Pediatric Respiratory Physiology Lung Volumes
and Mechanics of Breathing
60 ml/kg infant
after 18 months increases to adult 90 ml/kg by
age 5
50 of TLC

may be only 15 of TLC in young infants under
GA plus muscle relaxants
25 TLC
89
Pediatric Respiratory Physiology Lung Volumes
and Mechanics of Breathing
Elastic properties, compliance and FRC
Neonate chest wall compliance, CW 3-6 x CL,
lung compliance tending to decrease FRC,
functional residual capacity By 9-12 months CW
CL
Dynamic FRC in awake, spontaneously ventilating
infants is maintained near values seen in older
children and adults because of 1. continued
diaphragmatic activity in early expiratory
phase 2. intrinsic PEEP (relative tachypnea with
start of inspiration before end of preceding
expiration) 3. sustained tonic activity of
inspiratory muscles (probably most
important) By 1 year of age, relaxed
end-expiratory volume predominates
90
Pediatric Respiratory Physiology Lung Volumes
and Mechanics of Breathing
Under general anesthesia, FRC declines by
10-25 in healthy adults with or without muscle
relaxants and 35-45 in 6 to 18 year-olds
In young infants under general anesthesia
especially with muscle relaxants FRC may
only 0.1 - 0.15 TLC FRC may be lt closing
capacity leading to small airway
closure atalectasis V/Q mismatch declinin
g SpO2
91
Pediatric Respiratory Physiology Lung Volumes
and Mechanics of Breathing
General anesthesia, FRC and PEEP
Mean PEEP to resore FRC to normal infants lt 6
months 6 cm H2O children 6-12 cm H2O
PEEP important in children lt 3 years essential
in infants lt 9 months under GA muscle
relaxants (increases total compliance by
75) (Motoyama)
92
Pediatric Respiratory Physiology Dynamic
Properties
Poiseuilles law for laminar flow
where R resistance l length ? viscosity
R 8l?/pr4
For turbulent flow R a 1/r5
Upper airway resistance adults nasal passages
65 of total resistance Infants nasal
resistance 30-50 of total upper airway ? of
total resistance NG tube increases total
resistance up to 50
93
Pediatric Respiratory Physiology
Anesthetic effects on respiratory mechanics
Relaxation of respiratory muscles -gt decreased
FRC cephalad displacement of diaphragm contribu
tes to decreased FRC much less if patient not
paralyzed airway closure atalectasis minimized
by PEEP 5 cm H2O in children process slowed by
30-40 O2 in N2 (vs 100 O2) V/Q
mismatch Endotracheal tube adds the most
significant resistance
94
Pediatric Respiratory Physiology
Anesthetic effects on respiratory mechanics
Endotracheal tube adds the most significant
resistance
95
Pediatric Respiratory Physiology
Ventilation and pulmonary circulation
Infants VA per unit of lung volume gt adult
because of relatively higher metabolic rate,
VO2 relatively smaller lung volume
Infants and toddlers to age 2 years VT
preferentially distributed to uppermost part of
lung
96
Pediatric Respiratory Physiology
Oxygen transport
(Bohr effect)
27, normal adult (19, fetus/newborn)
97
Pediatric Respiratory Physiology
Oxygen transport
Bohr effect increasing pH (alkalosis) decreases
P50 beware hyperventilation decreases tissue
oxygen delivery
Hgb F reacts poorly with 2,3-DPG P50 19
By age 3 months P50 27 (adult level) 9
months P50 peaks at 29-30
98
Pediatric Respiratory Physiology
Oxygen transport
If SpO2 91 then PaO2
Adult 60 6 months 66 6 weeks 55 6
hours 41
99
Pediatric Respiratory Physiology
Oxygen transport
P50 Hgb for equivalent tissue oxygen
delivery Adult 27 8 10 12 gt 3 months 30
6.5 8.2 9.8 lt 2 months 24 11.7 14.7 17.6
Implications for blood transfusion older infants
may tolerate somewhat lower Hgb levels at
which neonates ought certainly be transfused
100
Pediatric Respiratory Physiology
Surfactant
Essential phospholipid protein complex Regulates
surface tension Stabilizing alveolar pressure
LaPlace equation P nT/r where P ressure r
adius of small sphere T ension n 2 for
alveolus
Surface tension 65 of elastic recoil pressure
101
Pediatric Respiratory Physiology
Surfactant
Produced by cuboidal type II alveolar pneumocytes
(27th week) Lecithin (phosphatidylcholine,
PC)/sphingomyelin (L/S) ratio in amniotic fluid
correlates with lung maturity
102
Pediatric Respiratory Physiology
Surfactant
Synthesis increased by glucocorticoids thyroxine
heroin cyclic adenosine monophosphate
(cAMP) epidermal growth factor tumor necrosis
factor alpha transforming growth factor beta
Synthetic surfactant used in treatment
of premature infants with surfactant deficiency
PPHN CDH meconium aspiration syndrome ARDS
(adults and children)
103
Pediatric Respiratory Physiology Selected Points
Basic postnatal adaptation lasts until 44 weeks
postconception, especially in terms of
respiratory control Postanesthetic apnea is
likely in prematures, especially anemic Formation
of alveoli essentially complete by 18 months Lung
elastic and collagen fiber development continues
through age 10 years Young infant chest wall is
very compliant and incapable of sustaining FRC
against lung elastic recoil when under general
anesthesia, especially with muscle
relaxants leading to airway closure and
progressive atalectasis of anesthesia Mild
moderate PEEP (5 cmH2O) alleviates Hemoglobin
oxygen affinity changes dramatically first months
of life Hgb F low P50 (19) P50 increases,
peaks in later infancy (30) implications for
blood transfusion
104
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