Metabolic alkalosis

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Background

Metabolic alkalosis generally occurs as a primary increase in serum bicarbonate (HCO3-) concentration, which can occur due to loss of H+ from the body or a gain in HCO3-.

Main Causes

Clinical Features

Symptoms are nonspecific and usually result from the concomitant hypokalemia or hypocalcemia. Common signs and symptoms include

Differential Diagnosis

Chloride-Responsive

Urine Cl < 20 mEq/L

  1. Loss of gastric secretions;
  2. Loss of colonic secretions
    • Diarrhea
    • Congenital chloridorrhea
    • Villous adenoma
  3. Thiazides/loop diuretics
  4. Cystic fibrosis

Chloride-resistant

Urine Cl > 20 mEq/L

Evaluation

  • pH > 7.42 = alkalemia
  • HCO3 > 28 = metabolic alkalosis
  • Always determine if there is also a concurrent primary respiratory process
    • expected pCO2 = 40 + 0.6(measured HCO3 - 24)
    • if pCO2>pCO2 expected, then there is also a primary respiratory acidosis
    • if pCO2<pCO2 expected, then there is also primary respiratory alkalosis
  • Always calculate AG to determine if concurrent primary metabolic acidosis

Management

  1. Correct volume depletion
    • Normal saline
      • Repletion of extracellular volume decreased need for Na reaborption
      • Delivery of Cl to distal tubule increases Cl/bicarb exchange
  2. Correct potassium depletion
    • Giving K+ leads to movement of H+ out of cells → acidosis
    • Giving K+ stops hypokalemia-induced distal H+/K+ pump
  3. Correct chloride depletion
    • Must give a reabsorbable anion to replace HCO3
  4. Correct mineralocorticoid excess
  • Note: if patient is edematous (CHF, cirrhosis), do NOT give normal saline
    • If patient is hypokalemic, KCl will correct both hypoK AND alkalosis

See Also

References

Authors:

Ross Donaldson