Pediatric Physiology

1. Pediatric Physiology Presented by- Dr Kamal Prakash Sharma Moderator-Prof. Dr Surinder Singh

2. Aspects of the pediatric Physiology • The neonatal oxygen consumption is approximately 6-7 ml/kg/min versus 3-4 ml/kg/min in the adult • Even under normal circumstances the immature cardiac and respiratory systems must function near maximum to support this metabolic demand.

3. Respiratory Physiology • The respiratory system is not fully developed at birth and continues through early childhood. • Airways fully developed at 16 wks of gestation • Alveolarize at 24-28 wks with complete maturation at 8 - 10 yrs.

4. Respiratory Physiology (cont’d) • Control of respiration matures rapidly in neonatal period • Ventilatory response to raised CO2 increases with postnatal age. • Response to hypoxemia is somewhat more complex during first 3 weeks as compared to CO2 & depends upon temperature.

5. Respiratory Physiology (cont’d) • In normothermia, hypoxemia causes transient hyperventilation(via peripheral chemoreceptors) followed by decrease in ventilation. • In hypothermia, hypoxemia decreases ventilation. • By the end of 1st month hypoxemia causes hyperventilation irrespective of temperature.

6. Respiratory Physiology (cont’d) Differences in airway anatomy • Head is relatively large with prominent occiput, retrognathic chin in neonate. • Nasal passages are relatively narrow, predisposing them to obstruction • Infants are obligate nasal breathers. Almost all infants can easily convert to oral breathing by 5 months of age. • Most infants can convert to oral breathing if the obstruction lasts longer than 15 seconds.

7. Respiratory Physiology (cont’d) Differences in airway anatomy • Relatively large size of the infant's tongue in relation to the oropharynx increases the likelihood of airway obstruction and technical difficulties during laryngoscopy • Short neck & Larynx is located higher (more cephalic) in the neck

8. Respiratory Physiology (cont’d) • Larynx is located higher (more cephalic) in the neck at C3-4 level as compared to C5-6 level in adults. • Epiglottis is shaped differently, long, floppy & omega shaped & angled over laryngeal inlet; control with laryngoscope blade is more difficult

9. Respiratory Physiology (cont’d) • Vocal cords are Bow shaped being cephalad ant. & rostralposteriorly, so a “blindly” passed endotracheal tube may easily lodge in the anterior commissure rather than slide into the trachea • Infant larynx is funnel shaped, the narrowest portion (3-5 mm) occurring at the cricoid cartilage & is covered with loose pseudo-stratified columnar epithelium.

10. Respiratory Physiology (cont’d) • Small diameter of the airways increases resistance to airflow; resistance is inversely proportional to the radius raised to the fourth power for laminar flow and to the fifth power for turbulent flow. • The airway of infants is highly compliant and poorly supported by surrounding structures. • Interstitium in neonates contains less elastin than in adult, leads to greater lung compliance, greater closing capacity.

11. Respiratory Physiology (cont’d) • The diaphragm(primary respiratory muscle) has fewer (type-1) high-oxidative muscle fibers which are more resistant to fatigue, until child is of 2 years age. • Mechanical efficiency of diaphragm is decreased in neonates. • Intercostal muscles are poorly developed, has fewer high-oxidative muscle (type-1) fibers.

12. Respiratory Physiology (cont’d) • Tidal volume & Dead space ventilation per kg body weight is proportionally similar to that in adults. • FRC per kg body weight is similar to those in adults. • Total alveolar surface area is 50 fold less in neonate as compared to adults • Spontaneous resp. rate decreases from 35-40/min to adult values (12-16 b/min) with increasing age.

13. Respiratory Physiology (cont’d) • Alveolar ventilation decreases as age increases from 100-150 ml/kg/min to 60 ml/kg/min • Periodic breathing with central apnea (of 5-15 seconds)may be present in preterm neonates • Apnea of duration >15 seconds are associated with desaturation episodes and bradycardia • Resolves at 50-55 weeks of postconceptual age.

14. Developmental Changes of the Rib Cage

15. Respiratory Physiology (cont’d) • Chest wall development • Ribs oriented parallel and unable to effectively increase the thoracic volume during inspiration • At 2 years of age ribs are oriented oblique • Highly compliant chest wall, ribs provide little support for the lungs; that is, negative intrathoracic pressure is poorly maintained. • Thus, each breath is accompanied by functional airway closure.

16. Cardiovascular Physiology • Fetal circulation – high pulmonary vascular resistance, low systemic resistance (placenta) and right to left shunt via PFO and DA. • Soon after birth aeration of the lungs – decrease pulmonary vascular resistance, mediated by NO, increase systemic resistance by placenta removal • PVR in neonate is less than that in fetus & is 82-240 dynes.sec/cm5 and in case of adults is 30-120 dynes.sec/cm5 • SVR in neonate is around 800 dynes.sec/cm5

17. Developmental Changes of the Rib Cage

18. Cardiovascular Physiology (cont’d) • Factors required for transition from fetal to neonatal circultaion are Oxygen, Nor-epinephrine, epinephrine, Acetylcholine, Braykinin. • Functional closure of FO occurs immediately after birth & of DA within few hours • Anatomical closure of FO is complete at 1 year & of DA at age of 3 months • Hypoxemia, hypercarbia, acidosis can cause pulmonary vasoconstriction and opening of the FO/DA.

19. Cardiovascular Physiology (cont’d) • During this critical period, the infant readily reverts from the adult to a fetal type of circulation; this state is called transitional circulation. • When such a “flip-flop” occurs, PAP increases to systemic levels, blood is shunted past the lungs via the PFO, and the DA may reopen and allow blood to shunt. • A rapid downhill course may occur & lead to severe hypoxemia, which explains why hypoxemic events in infants are often prolonged despite adequate ventilation with 100% O2

20. Cardiovascular Physiology (cont’d) • Risk factors increasing the likelihood of prolonged transitional circulation include prematurity, infection, acidosis, pulmonary disease resulting in hypercapnia or hypoxemia, acidosis, hypothermia & congenital heart disease. • Care must be directed to keeping the infant warm, maintaining normal PaO2,PaCo2 and minimizing anesthetic-induced myocardial depression.

21. Cardiovascular Physiology (cont’d) • Neonatal myocardium contains immature contractile elements & more connective tissue & is less compliant than the adult myocardium • This developmental immaturity of myocardial structures accounts for the tendency toward biventricular failure, sensitivity to volume loading, Poor tolerance of increased afterload, and Heart rate–dependent cardiac output

22. Cardiovascular Physiology (cont’d) • At birth autonomic innervation of heart & peripheral vasculature is primarily parasympathetic • Balance of autonomic innervation matures as child matures, with increasing innervation by sympathetic nervous system.

23. Cardiovascular Physiology (cont’d) • The infant cardiovascular system maintains lower catecholamine stores and displays a blunted response to exogenous catecholamines. • The vascular tree is less able to respond to hypovolemia with vasoconstriction d/t immaturity of baroreceptor reflex. • The hallmark of intravascular fluid depletion in neonates and infants is therefore hypotension without tachycardia.

24. Cardiovascular Physiology (cont’d) • Cardiac calcium stores are reduced because of immaturity of the sarcoplasmic reticulum • consequently, neonates have a greater dependence on exogenous (ionized) calcium and probably increased susceptibility to myocardial depression by potent inhaled agents that have calcium channel blocking activity

25. Cardiovascular Physiology (cont’d) • To meet increased metabolic demand, the Cardiac output relative to body weight is twice that of the adult. • CO is 200ml/kg/min in newborns, 150 ml/kg/min at 2 months of age • Resting Stroke volume remains fairly constant at about 1ml/kg, increased CO is achieved mainly by increase in Heart rate( average HR at birth140 b/min)

26. Central Nervous System • Soft & pliable cranium, non-fused sutures, two open fontanels. • Brain is structuraly complete but incompletely myelinated until 2 years of age, cerebral cortex that is poorly developed. • In neonate, predominant constituent of brain is water. • During infancy and childhood fraction of water decrease steadily and fraction of myelin, protein increases

27. Central Nervous System • Decreasing fraction of water is reflected in inverse changes in partition coefficients that is increase in coeff. with increasing age. • Blood-brain barrier is immature after birth, facilitate passage of lipid soluble drugs (anaesthetics) into brain.

28. Central Nervous System • One third of total CO perfuses brain as compared to one seventh in adults. • Cerebrovascular responses to changes in Co2 & O2 are attenuated in neonate. • CBF autoregulation is present in healthy neonate & less developed in preterm neonate. • Neuroendocrine axis is well developed even in preterm neonates • Spinal cord ends at L3/L4 • Dural sac ends at the level of S3 .

29. Hematologyc Physiology • Blood vol in preterm neonate is greatest 90-100 ml/kg, in term neonate 80 ml/kg, in adult 70 ml/kg. • Hb conc. at birth is between 14-20 gm% in term neonate, conc. decreases during infancy reaches nidus of 10 gm% at 10-15 wk & adult conc. at end of infancy. • At birth 80% is HbF type and only 5% remains at age of 6 months & rest is HbA

30. Renal Physiology • Renal function is immature in neonates because of low perfusion pressure and low glomerular filtration rate and poor tubular function • Poor concentrating ability • Complete maturation of renal function takes place by about 2 years of age. • Ability to handle free water and solute loads may be impaired in neonates • Half-life of medications excreted by means of glomerular filtration will be prolonged.

31. Electrolyte • Sodium conc is similar to adults in fullterm neonate. • Potassium conc may be as high as 7.5 meq/l, decreases after birth, reaching adult level during infancy. • Hypocalcemia may be present, particularly in preterm.

32. Thermoregulation • The thermoregulatory range is the ambient temperature range within, an unclothed subject can maintain normal body temperature • The lower limit of the thermoregulatory range is 10C for an adult, 230C and 280C for the full term and premature infant, respectively.

33. Thermoregulation • Enhanced heat loss due to: • relatively larger surface area to body weight ratio 2:1 (Body wt is 1/10 of adult & surface area is 1/5 of adult) • thinner layer of insulation(skin) • limited capability of heat production • Thermogenesis in brown fat is mediated by the sympathetic system and stimulated by norepinephrine, resulting in triglyceride hydrolysis.

34. Thermoregulation • It is very important to address all aspects of possible heat loss during anesthesia, as well as during transport to and from the operating room. • Placing the baby on a warming mattress and warming the operating room (=/>27°C) reduce heat lost by conduction. • Keeping the infant in an incubator, covered with blankets, minimizes heat lost through convection. The head should also be covered.

35. Thermoregulation • Heat lost from radiation is decreased with the use of a double-shelled Isolette during transport. • Heat lost through evaporation is lessened by humidification of inspired gases, the use of plastic wrap to decrease water loss through the skin and warming of skin disinfectant solutions.

36. Thermoregulation • Hot air blankets are the most effective means of warming children. • Anesthetic agents can alter many thermoregulatory mechanisms, particularly nonshivering thermogenesis in neonates.

37. Gastrointestinal System • At birth, gastric pH is alkalotic; by the second day of life, pH is in the normal physiologic range for older children. • Ability to coordinate swallowing with respiration not fully mature till infants is 4 to 5 months of age • high incidence of gastro-esophageal reflux in newborns (common in preterm infants).

38. Changes in Body Composition Reproduced from - R. S. Litman: Pediatric Anesthesia – The Requisites in Anesthesiology, Elsevier Mosby 2004

39. Hepatic Physiology • Hepatic functions are immature at birth • Hepatic synthesis of vitamin K dependent clotting factors in term neonate is between 20-60% of adult values, less in preterm. • Activity of phase-1 cytochrome P450-dependent mixed function oxidaes is immature in neonate but matures to adult values by 6 months of age.

40. Hepatic Physiology • Activity of phase-2 reactions, primarily conjugative immature at birth & matures gradually. • Based on above considerations, elimination half life of drugs dependent on hepatic biotransformation may be prolonged in neonate. • Albumin and alpha-1acid glycoprotein are low in neonate, so free fraction of protein bound drugs(opioids) also increase

41. Glucose Homeostasis • Neonates have low glycogen stores that predispose them to hypoglycemia. Impaired glucose excretion by the kidneys may partially offset this tendency. • Neonates at greatest risk for hypoglycemia are premature or small for gestational age, have been receiving hyperalimentation, and were born to diabetic mothers.

42. Glucose Homeostasis • Hypoglycemia is defined as <30 mg/dl in term neonate & <20 mg/dl in preterm neonate during first 3 days and <40 mg/dl after 3 days. • To maintain euglycemia 3-5 mg/kg/min of glucose in term neonate & 5-6 mg/kg/min in preterm neonate is required. • Basal energy requirement of neonate is very high about 120 kcal/kg/d as compared to 35-50 kcal/kg/d in adults.

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