Electrolyte and Metabolic Derangements

Abstract

Acid – base disorders are very common in the NCCU. The normal pH range is 7.35–7.45; alkalosis is defined as pH >7.45, and acidosis is defined as pH <7.35. pH is a measure of the hydrogen ion concentration in the extracellular fluids and is determined by the pCO₂ and HCO₃ concentration. [H²] (meq/L) = 24 × (PCO₂/HCO₃). The initial change in PCO₂ or HCO₃ is called the primary disorder; the subsequent change is called the compensatory or secondary disorder. Compensatory changes frequently will not return the pH to the normal range but will serve to limit the effect of the primary derangement. Acid – base disorders are of particular concern in neurophysiology because of their effects on cerebral blood flow (CBF). Acidosis (decrease in pH) results in cerebral vasodilation, whereas alkalosis (increase in pH) results in cerebral vasoconstriction. As pH increases, cerebral vasoconstriction also increases, resulting in decreased CBF and therefore decreased cerebral blood volume and ICP. Changes in acid – base status within the blood are transmitted across the blood-brain barrier (BBB) via CO₂ rather than by H⁺ ions; the BBB is impermeable to H⁺, but CO₂ crosses freely. The subsequent change in the CSF pH is a result of the conversion of CO₂ + H₂O to H⁺ and HCO₃ by carbonic anhydrase. The pH of the CSF returns to normal after 6–8 h, as HCO₃ is either retained or extruded across the BBB despite ongoing hyper- or hypocapnia, respectively. As the CSF pH returns to normal, CBF also trends toward normal