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Metabolic Alkalosis

By

James L. Lewis III

, MD, Brookwood Baptist Health and Saint Vincent’s Ascension Health, Birmingham

Last full review/revision Jul 2021| Content last modified Jul 2021
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Metabolic alkalosis is primary increase in bicarbonate (HCO3) with or without compensatory increase in carbon dioxide partial pressure (Pco2); pH may be high or nearly normal. Common causes include prolonged vomiting, hypovolemia, diuretic use, and hypokalemia. Renal impairment of HCO3 excretion must be present to sustain alkalosis. Symptoms and signs in severe cases include headache, lethargy, and tetany. Diagnosis is clinical and with arterial blood gas and serum electrolyte measurement. The underlying condition is treated; oral or IV acetazolamide or hydrochloric acid is sometimes indicated.

Etiology

Metabolic alkalosis is bicarbonate (HCO3) accumulation due to

Regardless of initial cause, persistence of metabolic alkalosis indicates that the kidneys have increased their HCO3 reabsorption, because HCO3 is normally freely filtered by the kidneys and hence excreted. Volume depletion and hypokalemia are the most common stimuli for increased HCO3 reabsorption, but any condition that elevates aldosterone or mineralocorticoids (which enhance sodium [Na] reabsorption and potassium [K] and hydrogen ion [H+] excretion) can elevate HCO3. Thus, hypokalemia is both a cause and a frequent consequence of metabolic alkalosis.

The most common causes of metabolic alkalosis are

  • Diuretic use

  • Volume depletion (particularly when involving loss of gastric acid and chloride [Cl] due to recurrent vomiting or nasogastric suction)

  • Bicarbonate excess

  • Renal acid loss

Table
icon

Metabolic alkalosis can be

  • Chloride (Cl)-responsive: Involves loss or excess secretion of Cl; it typically corrects with IV administration of NaCl-containing fluid.

  • Chloride-unresponsive: Does not correct with NaCl-containing fluids, and typically involves severe magnesium (Mg) and/or potassium (K) deficiency or mineralocorticoid excess.

The 2 forms can coexist, eg, in patients with volume overload made hypokalemic by high-dose diuretics.

Symptoms and Signs

Symptoms and signs of mild alkalemia are usually related to the underlying disorder. More severe alkalemia increases protein binding of ionized calcium (Ca++), leading to hypocalcemia Hypocalcemia Hypocalcemia is a total serum calcium concentration 8.8 mg/dL ( 2.20 mmol/L) in the presence of normal plasma protein concentrations or a serum ionized calcium concentration 4.7 mg/dL ( 1.17... read more and subsequent headache, lethargy, and neuromuscular excitability, sometimes with delirium, tetany, and seizures. Alkalemia also lowers threshold for anginal symptoms and arrhythmias. Concomitant hypokalemia Hypokalemia Hypokalemia is serum potassium concentration 3.5 mEq/L ( 3.5 mmol/L) caused by a deficit in total body potassium stores or abnormal movement of potassium into cells. The most common cause is... read more may cause weakness.

Diagnosis

  • Arterial blood gas (ABG) and serum electrolyte measurements

  • Diagnosis of cause (usually clinical)

  • Sometimes measurement of urinary Cl and K+

Recognition of metabolic alkalosis and appropriate respiratory compensation is discussed in Diagnosis of Acid-Base Disorders Diagnosis Acid-base disorders are pathologic changes in carbon dioxide partial pressure (Pco2) or serum bicarbonate (HCO3−) that typically produce abnormal arterial pH values. Acidemia is serum pH 7... read more and requires measurement of ABG and serum electrolytes (including Ca and Mg).

Common causes can often be determined by history and physical examination. If history is unrevealing and renal function is normal, urinary Cl and K+ concentrations are measured (values are not diagnostic in renal insufficiency).

Urinary K and the presence or absence of hypertension help differentiate the chloride-unresponsive alkaloses.

Treatment

  • Cause treated

  • IV 0.9% saline solution for chloride-responsive metabolic alkalosis

Underlying conditions are treated, with particular attention paid to correction of hypovolemia and hypokalemia.

Patients with chloride-responsive metabolic alkalosis are given 0.9% saline solution IV; infusion rate is typically 50 to 100 mL/hour greater than urinary and other sensible and insensible fluid losses until urinary Cl rises to > 25 mEq/L (> 25 mmol/L) and urinary pH normalizes after an initial rise from bicarbonaturia.

Patients with chloride-unresponsive metabolic alkalosis rarely benefit from rehydration alone.

Patients with severe metabolic alkalosis (eg, pH > 7.6) sometimes require more urgent correction of blood pH. Hemofiltration or hemodialysis Hemodialysis In hemodialysis, a patient’s blood is pumped into a dialyzer containing 2 fluid compartments configured as bundles of hollow fiber capillary tubes or as parallel, sandwiched sheets of semipermeable... read more is an option, particularly if volume overload and renal dysfunction are present. Acetazolamide 250 to 375 mg orally or IV once or twice a day increases HCO3 excretion but may also accelerate urinary losses of K+ and phosphate (PO4); volume-overloaded patients with diuretic-induced metabolic alkalosis and those with posthypercapnic metabolic alkalosis may especially benefit.

In patients with severe metabolic alkalosis (pH > 7.6) and kidney failure who otherwise cannot or should not undergo dialysis, hydrochloric acid in a 0.1 to 0.2 normal solution IV is safe and effective but must be given through a central catheter because it is hyperosmotic and scleroses peripheral veins. Dosage is 0.1 to 0.2 mmol/kg/hour. Frequent monitoring of ABGs and electrolytes is needed.

Key Points

  • Metabolic alkalosis is bicarbonate (HCO3) accumulation due to acid loss, alkali administration, intracellular shift of hydrogen ion, or renal HCO3 retention.

  • The most common causes are volume depletion (particularly when involving loss of gastric acid and chloride (Cl) due to recurrent vomiting or nasogastric suction) and diuretic use.

  • Metabolic alkalosis involving loss or excess secretion of Cl is termed chloride-responsive.

  • Treat the cause and give patients with chloride-responsive metabolic alkalosis 0.9% saline IV.

  • Chloride-resistant metabolic alkalosis is due to increased aldosterone effect.

  • Treatment of chloride-resistant metabolic alkalosis involves correction of hyperaldosteronism.

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