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Neonatal acidosis and alkalosis.


Physiology, pathology, diagnosis, and management of acid-base pathology.


Acid-base homeostasis is the regulation of the pH in the extracellular space of the body and is crucial for physiologic cellular function. The pH at which proteins optimally function is very narrow (7.35–7.45); therefore, very complex mechanisms are in place to control it. In the immediate postnatal period as the baby is transitioning to extrauterine life, the body is susceptible to numerous conditions that may disturb the balance in the pH. In addition, in the newborn period, the adaptive responses to reach equilibrium of acid base balance are still maturing, making the newborn more susceptible to these changes in the pH homeostasis. At the time of delivery, the baby experiences an abrupt increase in the sensitivity of the central respiratory control to changes in pH compared to that of fetal life. There is also a steady but gradual increase in the intracellular compartment after birth which enhances the body’s buffering system. In general, the body’s pH is tightly regulated by three basic systems: the respiratory system, the renal system, and the chemical buffers. Imbalance in this balance or homeostasis is known as acidosis when the acidity is high (low pH), or alkalosis when the acidity is low (high pH). Soon after delivery, there are a series of changes in the cardiac and respiratory systems that allow the lungs to serve for gas exchange, establishing their role as the end organ of respiratory compensation for acid-base homeostasis. The respiratory cycle controls the carbonic acid concentration in the extracellular fluid by blowing off or retaining carbon dioxide (CO2) and ultimately, carbonic acid from the plasma. Respiratory acidosis implies an elevated CO2 level in the plasma. This may be due to respiratory distress, as seen in pneumonia, or surfactant deficiency or poor respiratory drive due to medications or perinatal depression. The respiratory acidosis triggers an immediate increase in ventilation, as CO2 will diffuse freely across the blood-brain barrier and stimulates the respiratory drive center in the central nervous system. Metabolic acidosis implies a net low level of base in the plasma. This can be due to loss of bicarbonate in the kidneys or gastrointestinal tract, or increased levels of fixed acids such as lactate, etc. in the plasma, as seen in states of shock. When acidosis is metabolic in origin, the compensation is delayed for hours due to the time needed to equilibrate plasma and cerebrospinal fluid bicarbonate. Neonates with an immature pulmonary system, especially premature infants, have a decreased capacity for respiratory compensation of the acid-base balance. The renal system’s role in maintaining the acid-base balance is through adding or removing bicarbonate ions from the extracellular fluid. The renal tubular cells enzymatically convert CO2 into carbonic acid, which dissociates into ...

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