Neonatal Physiologic Characteristics
1. Water metabolism
Water represents 70 to 80% of the body weight of the normal neonate and premature baby respectively. Total body water (TBW) varies inversely with fat content, and prematures have less fat deposits. TBW is distributed into extracellular fluid (ECF) and intracellular fluid (ICF) compartment. The ECF compartment is one-third the TBW with sodium as principal cation, and chloride and bicarbonate as anions. The ICF compartment is two-third the TBW with potassium the principal cation. The Newborn's metabolic rate is high and extra energy is needed for maintenance of body temperature and growth. A change in body water occurs upon entrance of the fetus to his new extrauterine existence. There is a gradual decrease in body water and the extracellular fluid compartment with a concomitant increase in the intracellular fluid compartment. This shift is interrupted with a premature birth. The newborn's body surface area is relatively much greater than the adults and heat loss is a major factor. Insensible water loss are from the lung (1/3) and skin (2/3). Transepithelial (skin) water is the major component and decreases with increase in post-natal age. Insensible water loss is affected by gestational age, body temperature (radiant warmers), and phototherapy.
Neonatal renal function is generally adequate to meet the needs of the normal full-term infant but may be limited during periods of stress. Renal characteristics of newborns are a low glomerular filtration rate and concentration ability (limited urea in medullary interticium) which makes them less tolerant to dehydration. The neonate is metabolically active and production of solute to excrete in the urine is high. The kidney in the newborn can only concentrate to about 400 mOsm/L initially (500-600 mOsm/L the full-term compared to 1200 mOsm/L for an adult), and therefore requires 2-4 cc/kg/hr urine production to clear the renal solute load. The older child needs about 1-2 cc/kg/hr and the adult 0.5-1 cc/kg/hr.
2. Fluid and Electrolytes Concepts
Cellular energy mediated active transport of electrolytes along membranes is the most important mechanism of achieving and maintaining normal volume and composition of fluid compartments. Infants can retain sodium but cannot excrete excessive sodium. Electrolytes requirements of the full-term neonate are: Sodium 2-3 meq/kg/day, potassium 1-2 meq/kg/day, chloride 3-5 meq/kg/day at a rate of fluid of 100 cc/kg/24 hrs for the first 10 kg of weight. As a rule of thumb, the daily fluid requirements can be approximated too:
Prematures 120-150 cc/kg/24 hrs
Neonates (term) 100 cc/kg/24 hrs
Infants >10 kg 1000 cc+ 50 cc/kg/24 hrs.
Special need of preterm babiesí fluid therapy is: conservative approach, consider body weight changes, sodium balance and ECF tonicity. They are susceptible to both sodium loss and sodium and volume overloading. High intravenous therapy can lead to patent PDA, bronchopulmonary dysplasia, enterocolitis and intraventricular hemorrhage. Impaired ability to excrete a sodium load that can be amplifies with surgical stress (progressive renal retention of sodium). Estimations of daily fluid requirements should take into consideration: (1) urinary water losses, (2) gastrointestinal losses, (3) insensible water losses, and (4) surgical losses (drains).
Blood Volumes estimates of help during surgical blood loss are: premature 85-100 cc/kg, term 85 cc/kg, and infant 70-80 cc/kg. The degree of dehydration can be measured by clinical parameters such as: body weight, tissue turgor, state of peripheral circulation, depression of fontanelle, dryness of the mouth and urine output. Intravenous nutrition is one of the major advances in neonatal surgery and will be required when it is obvious that the period of starvation will go beyond five days.
Oral feeding is the best method and breast is best source. Newborn infants require 100-200 calories/kg/day for normal growth. This is increased during stress, cold, infection, surgery and trauma. Minimum daily requirement are 2-3 gm/kg of protein, 10-15 gm/kg of carbohydrate and small amount of essential fatty acids.
B. Variations in Individual Newborns
1. Types of Newborns Infants
a) The full-term, full-size infant with a gestational age of 38 weeks and a body weight greater than 2500 grams (TAGA)- they received adequate intrauterine nutrition, passed all fetal tasks and their physiologic functions are predictable. b) The preterm infant with a gestational age below 38 weeks and a birth weight appropriate for that age (PreTAGA); c) The small-for-gestational-age infant (SGA) with a gestational age over 38 weeks and a body weight below 2500 grams- has suffered growth retardation in utero. d) A combination of (b) and (c), i.e., the preterm infant who is also small for gestational age.
The characteristic that most significantly affects the survival of the preterm infant is the immature state of the respiratory system. Between 27 and 28 weeks of gestation (900-1000 grams), anatomic lung development has progressed to the extent that extrauterine survival is possible. It is only after 30 to 32 weeks of gestation that true alveoli are present. Once there is adequate lung tissue, the critical factor that decides extrauterine adaptation and survival of the preterm infant is his capabilities to produce the phospholipid-rich material, surfactant that lines the respiratory epithelium.
2. Metabolic and Host Defenses
Handling of the breakdown products of hemoglobin is also a difficult task for the premature infant. The ability of the immature liver to conjugate bilirubin is reduced, the life span of the red blood cell is short, and the bilirubin load presented to the circulation via the enterohepatic route is increased. 'Physiologic' jaundice is, therefore, higher in the preterm infant and persists for a longer period. Unfortunately, the immature brain has an increased susceptibility to the neurotoxic effects of high levels of unconjugated bilirubin, and kernicterus can develop in the preterm baby at a relatively low level of bilirubin.
Other problems affecting the baby include the rapid development of hypoglycemia (35 mg %), hypocalcemia and hypothermia. Newborn have a poorly developed gluconeogenesis system, and depends on glycolysis from liver glycogen stores (depleted 2-3 hrs after birth) and enteral nutrition. Immature infants can develop hyperglycemia from reduced insulin response to glucose causing intraventricular hemorrhage and glycosuria. The preterm and surgical neonate is more prone to hypocalcemia due to reduced stores, renal immaturity, and relative hypoparathyroidism (high fetal calcium levels). Symptoms are jitteriness and seizures with increase muscle tone. Calcium maintenance is 50 mg/kg/day.
Human beings are homeothermic organisms because of thermoregulation. This equilibrium is maintained by a delicate balance between heat produced and heal lost. Heat production mechanisms are: voluntary muscle activity increasing metabolic demands, involuntary muscle activity (shivering) and non-shivering (metabolizing brown fat). Heat loss occurs from heat flow from center of the body to the surface and from the surface to the environment by evaporation, conduction, convection and radiation. There is an association between hypothermia and mortality in the NICU's. The surgical neonate is prone to hypothermia. Infant produce heat by increasing metabolic activity and using brown fat. Below the 35 C the newborn experiences lassitude, depressed respiration, bradycardia, metabolic acidosis, hypoglycemia, hyperkalemia, elevated BUN and oliguria (neonatal cold injury syndrome). Factors that precipitate further these problems are: prematurity, prolonged surgery, and eviscerated bowel (gastroschisis). Practical considerations to maintain temperature control are the use of humidified and heated inhalant gases during anesthesia, and during all NICU procedures use radiant heater with skin thermistor-activated servo-control mechanism.
The newborn's host defenses against infection are generally sufficient to meet the challenge of most moderate bacterial insults, but may not be able to meet a major insult. Total complement activity is 50% of adultís levels. C3, C4, C5 complex, factor B, and properdin concentration is also low in comparison to the adult. IgM, since it does not pass the placenta, is absent.
3- Surgical Response of Newborns
The endocrine and metabolic response to surgical stress in newborns (NB) is characterized by catabolic metabolism. An initial elevation in catecholamines, cortisol and endorphins upon stimulation by noxious stimuli occurs; a defense mechanism of the organism to mobilize stored energy reserves, form new ones and start cellular catabolism. Cortisol circadian responsiveness during the first week of life is diminished, due to inmaturation of the adrenal gland. Cortisol is responsible for protein breakdown, release of gluconeogenic amino acids from muscle, and fat lipolysis with release of fatty acids. Glucagon secretion is increased. Plasma insulin increase is a reflex to the hyperglycemic effect, although a resistance to its anabolic function is present. During surgical stress NB release glucose, fatty acids, ketone bodies, and amino acids; necessary to meet body energy needs in time of increase metabolic demands. Early postoperative parenteral nutrition can result in significant rate of weight gain due to solid tissue and water accumulation. Factors correlating with a prolonged catabolic response during surgery are: the degree of neuroendocrinological maturation, duration of operation, amount of blood loss, type of surgical procedure, extent of surgical trauma, and associated conditions (hypothermia, prematurity, etc.). They could be detrimental due to the NB limited reserves of nutrients, the high metabolic demands impose by growth, organ maturation and adaptation after birth. Anesthetics such as halothane and fentanyl can suppress such response in NB.
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