Metabolic Alkalosis: Causes

Alkalemia is the condition of having a higher blood pH. Metabolic alkalosis is the condition of having elevated bicarbonate in the setting of alkalemia; or, an increased strong ion difference (the difference between positively-charged strong ions and negatively-charged strong ions). Several mechanisms can contribute to metabolic alkalosis, including intracellular hydrogen ion shift, GI hydrogen ion loss, retention or administration of bicarbonate ions, and volume contraction.

• Intracellular hydrogen ion shift – This occurs most frequently in the setting of hypokalemia. Hypokalemia will cause intracellular potassium ions to move to the extracellular fluid; this in turn will cause bicarbonate ions to relocate into the cell to maintain electroneutrality. This will cause renal tubular cells to secrete more hydrogen ions into the lumen, thereby promoting increased blood levels of bicarbonate.

• GI hydrogen ion loss – This usually occurs from patients who lose gastric secretions, either through vomiting or through vigorous NG suction. Loss of hydrogen ions means that the bicarbonate that is secreted by the pancreas, liver, and intestines to respond to the normally low gastric pH will not be neutralized. In rare circumstances, namely in villous adenoma and laxative abuse, diarrhea can also induce metabolic alkalosis; however, under more normal circumstances diarrhea leads to non-anion gap metabolic acidosis.

• Retention or administration of bicarbonate ions – Administration of bicarbonate, even large amounts, does not usually increase the serum bicarbonate level, as renal excretion of bicarbonate increases in response to the bolus. In the setting of hypovolemia and decreased GFR, administration of sodium bicarbonate can cause metabolic acidosis, as these conditions decrease bicarbonate excretion. Similarly, giving large quantities of blood products to a hypovolemic patient or a patient with renal dysfunction may cause metabolic acidosis, as the sodium citrate in these products is converted to bicarbonate.

• Volume contraction – Contraction alkalosis occurs from the loss of fluid with high sodium chloride content but low bicarbonate concentration, thus increasing the concentration of bicarbonate in the remaining fluid.