Clinical relevance

• The four clinical scenarios in which you will encounter metabolic alkalosis the most are:
  • Volume depletion, particularly, those due to vomiting or over-diuresis.
  • Volume overload, particularly, those who are being actively diuresed.
  • Chronic respiratory acidosis when CO2 is acutely reduced with BIPAP or mechanical ventilation (post hypercapnic alkalosis) if associated with with stimulant for HCO3 gain typically volume depletion.
  • Iatrogenic from bicarbonate therapy oral or IV.

Pathophysiology

• For metabolic alkalosis to develop, two conditions must present:
  • HCO3 gain (direct & indirect).
  • Sustained HCO3 renal absorption (Direct & indirect).
• Almost all of the filtered HCO3 is reabsorbed in the proximal tubule. This process is very effective to keep the HCO3 22-26, any HCO3 above this concentration will be expelled in the urine returning the HCO3 level to normal.
• The following factors increase proximal tubule HCO3 reabsorption:
  • Decreased intravascular volume whether from Volume depletion or volume overload.
  • Severe hypokalemia
  • Hypercapnia

Diagnosis

• Metabolic alkalosis is defined by a serum HCO3 > 26 in the abscess of a primary respiratory acidosis.

Treatment

• Stop the HCO3 gain (Treat the underlying cause).
• Excrete the excess HCO3.
• Discontinue any HCO3 or HCO3 precursors (Acetate or lactate) treatment

Hypokalemia

• Potassium must be repleted to correct metabolic alkalosis.
• Use KCL not K-acetate or lactate (Acetate and lactate are metabolized to HCO3 in the liver).
• Don’t forget to check Mg and replete if needed.

Volume depletion

• Volume depletion: Resuscitation with 0.9 NS (Saline responsive).
• Urine Cl < 10 (Unlike Na, Urine chloride isn’t affected by diuretics).

Volume overload

Diuretics

Acetazolamide: Strong HCO3 urinary loss, modest diuresis, exacerbate hypokalemia.

Spironolactone/Amiloride/Triamterene: Weak HCO3 urinary loss, weak diuresis, potassium-sparing.

Post hypercapnic

• Acute reduction of CO2 in patients with chronic CO2 retention.
• Most of these patients have volume depletion which sustains the elevated HCO3 and minimizes urinary excretion.
• Resuscitation with 0.9 NS.
• Replete K & Mg

Other causes of metabolic alkalosis

• Renal failure: rarely leads to metabolic alkalosis rather than acidosis → dialysis.
• Barter syndrome = loop diuretics → Potassium repletion and Amiloride.
• Gittleman syndrome = Thiazides diuretics → Potassium repletion and Amiloride.
• Cushing syndrome.
• Liddle syndrome → Potassium repletion and Amiloride/triamterene.
• Primary hyperaldosteronism.