pediatric anatomy and physiology gerard t. hogan, jr., crna, msn clinical assistant professor...
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Pediatric Anatomy and Physiology
Gerard T. Hogan, Jr., CRNA, MSNClinical Assistant Professor
Anesthesiology Nursing ProgramFlorida 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 childYou must understand these differences and appreciate them to properly assess, plan, and deliver an anesthetic
Pediatric Anatomy/Physiology
Physical appearanceMost dramatic difference is physical sizeBSA can be computed using nomogramHead is large compared to the adult
Often in newborns it exceeds the circumference of the chest
Arms and legs are shorted and underdeveloped at birthMidpoint in length on child is umbilicusMidpoint 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 systemBone growth occurs at different rates throughout the body
This affects anatomical landmarks
In the neonate, the imaginary line joining the iliac crests occurs at S1Sacrum is not fused normally at birthAt birth spinal column has only the anterior curvatureCervical and lumbar curvature begin with holding head up and walking
Pediatric Anatomy/Physiology
Central Nervous SystemThe brain at birth is 1/10 the body weightOnly ¼ of the neuronal cells that exist in adults are present in the newbornNeuronal development finishes as age 12Myelination is not complete until age 3
Primitive reflexes (Moro, grasp) disappear with myelination
Pediatric Anatomy/Physiology
Central Nervous SystemAutonomic nervous system is developed at birth, though immatureParasympathetic system is intact and fully functionalLower 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 SystemMany profound changes after birth
SVR doubles after first breathPulmonary vasculature dilates, decreasing PVRForamen ovale closes as left atrial pressure becomes higher than right atrial pressureFlow reverses in the ductus arteriosis, preventing flow between the pulmonary artery and the aorta
Pediatric Anatomy/Physiology
Cardiovascular systemThe reason for closure is not fully understoodUmbilical vein flow ceases at birthMuscular 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 systemPersistent fetal circulation
Hypercarbia, hypoxia, and acidosis can precipitate pulmonary vasoconstrictionIf RA pressure exceeds LA pressure, the foramen ovale can open, and exacerbate the shuntIf the ductus arteriosus fails to close, a right to left shunt may continue
Pediatric Anatomy/Physiology
Pediatric Anatomy/Physiology
MyocardiumStroke volume of an infant is relatively fixed
“they live for (or better yet, by) heart rate”Myocardium is relatively stiffIncreasing preload will not increase COCardiac reserve is limitedSmall changes in end diastolic volume yield large changes in end diastolic pressure
Pediatric Anatomy/Physiology
MyocardiumTo increase CO, you must increase HRInfants (and prepubescent children, for that matter) are predisposed to bradycardia (“Vagus with legs”)
Parasympathetic cardiac innervation is completely developed (and ready for stress) at birthSympathetic innervation is sparse, but functional
Pediatric Anatomy/Physiology
Unbalanced parasympathetic tone can manifest in negative inotropy, predisposing them to CHFHeart 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 SystemPediatric airway
Head is large and neck is shortOcciput predominatesSupine, the chin meets the chestTongue is large and occupies entire oropharynxAbsence of teeth further predisposes the infant to airway obstruction
Pediatric Anatomy/Physiology
Respiratory SystemPediatric airway
Obligate nose breathers because of the close proximity of the epiglottis to the soft palateMouth breathing occurs only during cryingObligate nose breathing is vital for respiration during feeding
Pediatric Anatomy/Physiology
Respiratory SystemPediatric airway
The pharynx is almost completely soft tissueIt is easily collapsed by posterior displacement of the mandible, or external compression of the hyoidThe 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 SystemPediatric airway
LarynxFunnel shaped, as opposed to adult cylindrical shapeMore cephalad in location as compared to an adultIn adults, the larynx lies at the level of C 4-6, but in infants, it is 2 vertebral levels higherCricoid ring is complete, and is the narrowest point of the pediatric airway
Pediatric Anatomy/Physiology
Respiratory SystemPediatric airway
LarynxBecause the cricoid ring is the narrowest part of the airway, traumatizing it with multiple intubation attempts may lead to swelling and obstructionEpiglottis is short and narrow, and cords are angled
Pediatric Anatomy/Physiology
Respiratory SystemPediatric airway
Anatomical differences in the thoraxChest wall is very compliantRibs are horizontally located, limiting inspirationDiaphragm is deficient in type 1 muscle cells
These cells are required for continuous, repeated exercise activities
Pediatric Anatomy/Physiology
Respiratory SystemPediatric airway
LungsMaturation not complete until age 8Alveoli grow and increase in number to age 8Surfactant production begins at 20 weeks, but really increases between 30-34 weeksBreathing movements begin in utero, to prepare for the big eventBu 36 weeks, regular breathing movements of 70/min are noted
Pediatric Anatomy/Physiology
Respiratory System
Pediatric airwayHigh metabolic rate necessitates high respiratory ratePulmonary parameters vastly different
Pediatric Anatomy/Physiology
Respiratory SystemPediatric airway
FRC is relatively close to adultNo where near as effective based on metabolic rate, O2 consumption, and high degree of alveolar ventilationInfants initially hyperventilate in response to hypoxia, but will not sustain and begin to slow down their breathing
Pediatric Anatomy/Physiology
Respiratory SystemPediatric 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 sleepHemoglobin level is around 19g/dl, most is HbF, which has a greater affinity for O2
Pediatric Anatomy/Physiology
Respiratory SystemPediatric airway
Oxygen is bound more tightly to HbF, so cyanosis occurs at a lower PO2 than in the adultO2 tissue delivery is not as good as adult due to HbF’s poor reactivity to 2,3-DPGNormal PO2 in the newborn is 60-90 mmHgHbF rapidly disappears in the first few weeks of life
Pediatric Anatomy/Physiology
Respiratory SystemPediatric airway
Physiologic anemia peaks at 3 months of ageHgb 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 SystemFull term infants have the same number of nephrons as adultsGlomeruli are much smaller than in adultsGFR in the newborn is 30% that of the adult Tubular immaturity leads to a relative inability to concentrate urine
Pediatric Anatomy/Physiology
Renal SystemFluid turnover is 7 times greater than that of an adultAltered fluid balance can have catastrophic consequencesOrgan perfusion and metabolism count on adequate hydrationInfants and children are at a much higher risk for developing dehydration
Pediatric Anatomy/Physiology
Hepatic SystemNeonatal liver is largeEnzyme systems exist but have not been sensitized or inducedNeonates rely on limited supply of stored fatsGluconeogensis is deficientPlasma proteins are lower, greater levels of free drug exist
Pediatric Anatomy/Physiology
GI SystemGastroesophageal 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 lifeGastric pH is alkalotic at delivery
Pediatric Anatomy/Physiology
Pharmacologic considerationsUptake
Route of administration affects uptakeIV – fastestOral and rectal routes slowestTransdermal faster than adults, due to realtively thin skin layersPathological conditions of the liver and heart can significantly effect uptake
Pediatric Anatomy/Physiology
Pharmacologic considerationsDistribution
55-70% of body weight is water in infants and childrenLarge ECF leads to large Vol. of distribution
In adults, ECF accounts for 20% of body weightIn children, ECF accounts for up to 40% of body weight
The concentration and effects of water-soluble agents are affected greatly by the larger Volume of Distribution
Pediatric Anatomy/Physiology
Pharmacologic considerationsPlasma protein binding
Lower levels of serum albumin yield higher levels of free drugPlasma protein levels are even lower in certain disease states, like nephrotic syndrome or malnutritionEndogenous molecules, like bilirubin, can be displaced by protein bound drugs
Pediatric Anatomy/Physiology
Pharmacologic considerationsMetabolism
Soundness and maturity of the liver affect metabolismGlucuronidation is underdeveloped in neonatesMaternal use of drugs may affect enzyme inductionMedications, like phenobarbital, induce enzymes rapidly
Pediatric Anatomy/Physiology
Pharmacologic considerationsExcretion
Renal excretion is dependent on glomerular filtration, active tubular secretion, and passive tubular reabsorptionDrugs dependent on renal excretion, like Pancuronium and Digoxin, can be markedly affected by immature kidney functionKidneys receive a lower percentage of CO than in adultsGFR does not reach adult level until age 3-5
Pediatric Anatomy/Physiology
Pharmacologic considerationsONLY body weight or BSA should be used to calculate and determine correct pediatric drug dosagesBody weight is used in premature infantsAs always, titrate to effect
Pediatric Anatomy/Physiology
Routes of administrationOral
Sometimes it is difficult to gain cooperationLiquid forms have greater absorptionPlace in back corner of mouth in infants
IntramuscularGluteus medius muscle over age 2Vastus lataralus under 2
Pediatric Anatomy/Physiology
Pharmacologic considerationsIntravenous
Good luck starting it!May necessitate mask induction
Use of EMLA or other anesthethetic creamUsually better luck the more peripheral you areWell protected and secured
Pediatric Anatomy/Physiology
Pharmacologic considerationsIntravenous agents
Typically pediatric patients require a larger kg dose than adults
Example – ThiopentalAdult 3-5mg/kgNeonate 3-4mg/kgInfant 5-7mg/kgChildren 5-6mg/kg
Pediatric Anatomy/Physiology
Pharmacologic considerationsPediatric patients can be very sensitive to the repiratory depressant effects of narcoticsCareful titration is vitalMorphine 0.05-0.2mg/kg up front is commonly used in pedsFentanyl and demerol cause more respiratory depression
Pediatric Anatomy/Physiology
Pharmacologic considerationsMuscle relaxants
Increased doses due to increased volume of distributionWhen using succinylcholine, expect bradycardia if you didn’t pretreat with an anticholinergic agent