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.
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