Acid base disturbances are traditionally
classified as,
1. Metabolic
acidosis/alkalosis
2.
Respiratory acidosis/alkalosis.
Last 2
digit of pH = pCO2 (e.g., if pCO2 = 28, pH = 7.28)
cHCO3-
+ 15 = last 2 digit of pH (cHCO3- = 15, pH = 7.30)
Acidosis occurs as a result of one (or a combination) of
three mechanisms:
1. Increased
addition of acid,
2. Decreased
elimination of acid, and
3. Increased
loss of base.
Alkalosis occurs only by
1. Increased
addition of bases
2. Decreased
elimination of bases, and
3. Increased
loss of acid.
Metabolic
acidosis (primary bicarbonate deficit):
Indicated by decreased plasma
bicarbonate. Bicarbonate is lost in the buffering of excess acid. Causes
includes:
·
Excess production of organic acids (e.g. ketoacidosis,
lactic acidosis)
·
Reduced excretion of acids (H+) as occurs
in renal failure, some RTAs
·
Excessive loss of bicarbonate due to decreased tubular
reclamation like in RTA-II, loss due to vomiting, diarrhea. Fall in bicarbonate
is associated with rise in inorganic anion mostly Cl- or a
concomitant fall in sodium concentration.
Metabolic acidosis are classified
as those associated with either an increased anion gap or normal anion gap.
Anion gap = Na+ + K+
- (HCO3- + Cl-)
The difference between unmeasured cation
and unmeasured anion is called anion gap and this is about 8-16 mmol/L. This
apparent gap is due to unmeasured anions (e.g., proteins, SO42-,
H2PO42-) that are present in plasma and
cations. The presence of elevated anion gap is the first indication of
metabolic acidosis.
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