ACID BASE DISORDERS:

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, SO₄^2-, H₂PO₄^2-) that are present in plasma and cations. The presence of elevated anion gap is the first indication of metabolic acidosis.