Metabolic alkalosis

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

• Hydrogen ion loss (via vomiting)
• Shift of hydrogen ions intracellularly (from hypokalemia)
• Bicarbonate administration
• Contraction alkalosis (Diuretic administration, diarrhea, or any excessive loss of volume)

Differential Diagnosis

Chloride-Responsive

Urine Cl < 20 mEq/L

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

Chloride-resistant

Urine Cl > 20 mEq/L

• Exogenous alkali (Nabicarb + renal failure, metabolism of lactic acid, or ketoacids)
• Milk alkali syndrome
• Hypercalcemia (inability to concentrate urine leads to hypovolemia)
• Intravenous penicillin
• Refeeding alkalosis
• Massive blood transfusion
• Primary hyperaldosteronism
• CAH (11-Hydroxylase or 17-hydroxylase deficiency)
• Cushing syndrome
• Exogenous steroids
• Liddle syndrome
• Renovascular HTN
• Bartter syndrome (pediatrics)
• Gitelman syndrome (pediatrics)
• Severe K+ depletion
• Hypomagnesemia

Diagnosis

• 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

Treatment

1. Correct volume depletion
   • Normal Saline
     • Repletion of extracellular volume decr 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
   • Aldostorone antagonists if indicated (i.e. spironolactone)

• Note: if pt is edematous (CHF, cirrhosis), do NOT give normal saline
  • If pt is hypokalemic KCl will correct both hypoK AND alkalosis

See Also

• Acid-base disorders