Pediatric Anatomy and Physiology Gerard T. Hogan, Jr., CRNA, MSN Clinical Assistant Professor Anesthesiology Nursing Program Florida International University

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


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


Frequently because there is a large difference in the proportions of body parts, providers use a BSA chart for drug dosages


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


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


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


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


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


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



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


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


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



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



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



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



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



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



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



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


Respiratory System

Pediatric airwayHigh metabolic rate necessitates high respiratory ratePulmonary parameters vastly different



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



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



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



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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


Pharmacologic considerationsMuscle relaxants

Increased doses due to increased volume of distributionWhen using succinylcholine, expect bradycardia if you didn’t pretreat with an anticholinergic agent

Documents

Pediatric Anatomy and Physiology Gerard T. Hogan, Jr., CRNA, MSN Clinical Assistant Professor Anesthesiology Nursing Program Florida International University