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

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Pediatric Anatomy and Physiology

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Pediatric anatomy and physiology l.jpg

Pediatric Anatomy and Physiology

Gerard T. Hogan, Jr., CRNA, MSN

Clinical Assistant Professor

Anesthesiology Nursing Program

Florida International University


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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


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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


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Pediatric Anatomy/Physiology

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


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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


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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


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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


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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


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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


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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


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Pediatric Anatomy/Physiology


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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


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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


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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


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Pediatric Anatomy/Physiology


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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


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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


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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


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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


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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


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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


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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


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Pediatric Anatomy/Physiology

  • Respiratory System

    • Pediatric airway

      • High metabolic rate necessitates high respiratory rate

      • Pulmonary parameters vastly different


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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 water-soluble agents are affected greatly by the larger Volume of Distribution


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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


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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


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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


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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


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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


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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


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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


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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


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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


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