Pediatric Anatomy and Physiology

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

2. 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

3. 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

4. Pediatric Anatomy/Physiology • Frequently because there is a large difference in the proportions of body parts, providers use a BSA chart for drug dosages

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. Pediatric Anatomy/Physiology

12. 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

13. 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

14. 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

15. Pediatric Anatomy/Physiology

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. Pediatric Anatomy/Physiology • Respiratory System • Pediatric airway • High metabolic rate necessitates high respiratory rate • Pulmonary parameters vastly different

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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

36. 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

37. 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

38. 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

39. 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

40. 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

41. 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

42. 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