Physiologic differences between infants, children and adults



Physiologic differences between infants, children and adults

I. Central Nervous System

- Myelination of nerve fibers incomplete in newborn, muscle tone and reflexes are different (rooting/suckling reflex etc.), cerebral cortex less developed

- Pain threshold of young children is lower than that of older children or adults because of more nerve endings/cm2 and immature inhibitory pathways

- Infant skull much less rigid than adult; can estimate ICP by palpation of fontanelle

- Autoregulation of CBF impaired in the sick newborn

- Intraventricular hemorrhage (IVF) common in pre-term infants. Predisposing factors are hypoxia, hypercarbia, hypernatremia, fluctuation in arterial/venous pressure, low Hct, rapid administration of hypertonic fluids.

- Retinopathy of Prematurity (ROP, older name retrolental fibroplasia) – believed to be from increased oxygen tension in the immature retina of the preterm infant (but occasionally happens in full-term infant never receiving oxygen). Other risk factors: hypoxia, hypercarbia, hypocarbia, blood transfusion, exposure to light, recurrent apnea, sepsis, other systemic illness. For infants 15 sec or a shorter apneic pause which is accompanied by bradycardia (HR R, R>L or bidirectional – newborn may revert to this under stress such as sepsis, hypoxia, hypercarbia, acidosis, congenital heart disease. By having a pre-ductal and post-ductal pulse-oximeter, you can detect if PDA is open and shunting R>L (i.e. lower extremities have lower O2 sat than upper extremities). When PDA shunt is exclusively R>L, this is called “persistent fetal circulation”.

- Neonatal myocardium – fewer contractile elements, limited ability to change stroke volume so CO is rate-dependent, ventricle is less compliant, much better ability to tolerate hypoxia

- Blood– volume is 80ml/kg in the term infant (20% higher in preterm infant). 70-90% of Hb present at birth is of the fetal type which has a leftward shift on the oxyhemoglobin dissociation curve. (This enables it to “hold on” to oxygen better while circulating through the placenta.) During first few months of life, Hct drops (due to suppression of erythropoiesis and increased plasma volume) to nadir at 2-3 months of age (Hct ~30, even lower in premature infants).

IV. Hepatic function

- Term neonate has stores of glycogen in the liver and myocardium, premature infant has smaller stores and is unable to establish adequate gluconeogenesis. Hypoglycemia is common in the stressed neonate (level ................
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