Physiological Effects of Acidemia

Acidosis

PHYSIOLOGICAL EFFECTS OF ACIDEMIA

[H⁺] is strictly regulated in the nanomole/liter (36–43 nmol/L) range, as H⁺ ions have high charge densities and “large” electric fields that can affect the strength of hydrogen bonds that are present on most physiological molecules. Biochemical reac-tions are very sensitive to changes in [H +]. The over-all effects of acidemia seen in patients represent the balance between its direct biochemical effects and the effects of acidemia-induced sympathoadrenal activation. With severe acidosis (pH < 7.20), direct depressant effects predominate. Direct myocardial and smooth muscle depression reduces cardiac con-tractility and peripheral vascular resistance, result-ing in progressive hypotension. Severe acidosis can lead to tissue hypoxia, despite a rightward shift in hemoglobin affinity for oxygen. Both cardiac and vascular smooth muscle become less responsive to endogenous and exogenous catecholamines, and the threshold for ventricular fibrillation is decreased. Progressive hyperkalemia as a result of the movement of K+ out of cells in exchange for extracellular H+ is also potentially lethal. Plasma [K+] increases approximately 0.6 mEq/L for each 0.10 decrease in pH.

Central nervous system depression is more prominent with respiratory acidosis than with meta-bolic acidosis. This effect is often termed CO₂narco-sis. Unlike CO₂, H⁺ions do not readily penetrate theblood–brain barrier.