Metabolic alkalosis

Background

Pathophysiology

  • Chloride-Responsive (volume depletion)
    • Condition that produces chloride loss also tends to reduce extracellular volume
      • Reduction in extracellular volume increases mineralocorticoid activity
        • Enhances Na reabsorption and K+/H+ secretion in renal tubule
          • K/H+ secretion -> HCO3 generation
            • Resulting urine is alkaline with little chloride (<10)
  • Chloride-Resistant
    • Excess mineralocorticoid activity leads to same cascade as above
      • No hypovolemia so urine chloride is generally normal (>10)

Clinical Features

Differential Diagnosis

  • Chloride-Responsive (urine Cl < 20 mEq/L)
    • Loss of gastric secretions
    • Loss of colonic secretions
      • congenital chloridorrhea
      • villous adenoma
    • Thiazides/loop after D/C
    • Post hypercapnia
    • Cystic fibrosis
  • Chloride-resistant (urine Cl > 20 mEq/L)
    • With HTN
      • Primary hyperaldo
        • adrenal adenoma
        • bilateral adrenal
        • hyperplasia
        • adrenal carcinoma
      • 11B-HSD2
        • genetic, licorice
        • chewing tobacco
        • carbenoxolone
      • CAH (11-Hydroxylase or 17-hydroxylase deficiency)
      • Current diuretics + HTN
      • Cushing syndrome
      • Exogenous steroids
      • Liddle syndrome
      • Renovascular HTN
    • Without HTN
      • Bartter syndrome^
      • Gitelman syndrome^
      • Severe K+ depletion
      • Current thiazides/loop
      • Hypomagnesemia
  • Other causes

^in children

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

Source

Emedicine, Tintinalli, UpToDate, Kaji 2011