Neonatal Physiology and Anesthesia

1. Neonatal Physiology and Anesthesia Elena Brasoveanu, MD Boston University March 2, 2006

2. Pediatric Anesthesia • Origin in the 1930’s at Toronto Hospital for Sick Children when Robson described techniques only for children • Recognized subspecialty starting with 1946 when Robert Smith became director of anesthesia at Boston Children’s Hospital • Now, approximately 49 fellowship programs

3. Aspects of the Neonatal Physiology • The neonatal oxygen consumption is approximately 6 ml/kg/min versus 3 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.

4. 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 y.o.

5. Respiratory Physiology (cont’d) • Increasing respiratory rate rather than tidal volume is more efficient to increase alveolar ventilation • The diaphragm is the primary respiratory muscle – has fewer high-oxidative muscle fibers and is less resistant to fatigue than the adult diaphragm

6. Respiratory Physiology (cont’d) • Awake FRC is similar, when normalized to body weight, to that of an adult • FRC declines during anesthesia and combined with hypoventilation and increased consumption causes a rapid desaturation

7. Respiratory Physiology (cont’d) • Central apnea- self limited in newborns, approximately 5 seconds • Apnea of prematurity – 20 seconds or associated with desaturation episodes and bradycardia • Resolves at 50- 55 weeks gestational age.

8. Respiratory Physiology (cont’d) • Chest wall development • Ribs oriented parallel and unable to increase the thoracic volume during inspiration • At 2 y.o., associated with standing and walking, ribs are oriented oblique • Cartilaginous structure with inward movement during inspiration

9. Developmental Changes of the Rib Cage Reproduced from - R. S. Litman: Pediatric Anesthesia – The Requisites in Anesthesiology, Elsevier Mosby 2004

10. Respiratory Physiology (cont’d) • Production of surfactant begins at 23 – 24 wks of gestation and reaches maturation after 35 wks of gestation • Decreased surfactant production in preterm babies decreases the compliance – risk for respiratory distress syndrome, bronchopulmonary dysplasia and pulmonary hypertension

11. Respiratory Physiology (cont’d) • Respiratory Distress Syndrome • Perinatal asphyxia • Maternal diabetes • Multiple pregnancies • Cesarean section delivery

12. Respiratory Physiology (cont’d) • Respiratory Distress Syndrome – treatment • Steroids to the mother • Surfactant intratracheally • Protective strategies for lung ventilation

13. Cardiovascular Physiology • Fetal circulation – high pulmonary vascular resistance, low systemic resistance (placenta) and right to left shunt via PFO and DA. • Aeration of the lungs – decrease pulmonary vascular resistance, mediated by NO, increase systemic resistance by placenta removal

14. Fetal Circulation Reproduced from - R. S. Litman: Pediatric Anesthesia – The Requisites in Anesthesiology, Elsevier Mosby 2004

15. Cardiovascular Physiology (cont’d) • The neonatal pulmonary circulation is very reactive • Hypoxemia, hypercarbia, acidosis can cause pulmonary vasoconstriction and opening of the DA.

16. Cardiovascular Physiology (cont’d) • Neonatal myocardium contains immature contractile elements and is less compliant than the adult myocardium • Limited increase in CO by volume load in a normovolemic newborn • CO rate dependent

17. Cardiovascular Physiology (cont’d) • Adrenergic receptors mature at birth but sympathetic innervation is incomplete • To meet increased metabolic demand, the CO relative to body weight is twice that of the adult. • CO is 350ml/kg/min in newborns, 150 ml/kg/min at 2 months of age • The baroreflex activity is impaired and a 10% reduction in blood volume will cause a 15-30% decrease in MAP

18. Cardiovascular Physiology (cont’d) • The electrocardiogram shows an axis oriented to the right due to right dominance.

19. Newborn EKG Reproduced from - R. S. Litman: Pediatric Anesthesia – The Requisites in Anesthesiology, Elsevier Mosby 2004

20. Open Sutures of the Newborn Cranium Reproduced from - R. S. Litman: Pediatric Anesthesia – The Requisites in Anesthesiology, Elsevier Mosby 2004

21. Central Nervous System • IVH occurs in 40 to 50% of premature infants and is a major cause of neonatal morbidity and mortality • At the level of germinal matrix • Careful anesthetic management to avoid hypercarbia, hypoxia, fluctuation in BP

22. Central Nervous System • Retinopathy of prematurity – abnormal growth of retinal vessels that leads to scarring and blindness • Risk factors: • Prematurity • Hyperoxia • Hypocarbia • Vitamin E deficiency • Acidemia

23. Thermoregulation • Enhanced heat loss due to: • relatively larger surface area, • thinner layer of insulation and • limited capability of heat production • Thermogenesis in brown fat is mediated by the sympathetic system and stimulated by norepinephrine, resulting in triglyceride hydrolysis.

24. Thermoregulation • The thermoregulatory range is the ambient temperature range within un 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.

25. Fluid, Electrolyte, and Renal Physiology • Increased • total body water • extracellular fluid volume • water turnover rate • Reduced glomerular filtration rate • Obligate sodium losers

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

27. Fluid, Electrolyte, and Renal Physiology • Decreased glycogen stores – prone to hypoglycemia • The term neonate requires 3 to 5mg/kg/min of glucose to maintain euglycemia • Glucose should be maintained between 35 and 125 mg/dl. • Hyperglycemia predisposes to intracranial hemorrhage

28. Hematologyc Physiology • At birth Hb is 19g/dL represented by HbF • Physiologic anemia of infancy – at 6 to 10 weeks of life, the HbF is replaced by HbA • There is no indication for transfusion in the absence of clinical signs of anemia • Coagulation factors are decreased at birth but normalize during the first year of life

29. Bibliography: • S. C. Hillier, G. Krishna, E. Brasoveanu: Neonatal Anesthesia. Seminars in Pediatric Surgery: Pediatric Anesthesia, vol 13/3, August 2004. • R. S. Litman: Pediatric Anesthesia – The Requisites in Anesthesiology, Elsevier Mosby 2004, pg3 to 18, 68 to 72. • E.K. Motoyama, P.J. Davis: Smith’s Anesthesia for Infants and Children, Elsevier Mosby 2006, pg 12 to 148.