Merck Manual

Please confirm that you are a health care professional

Loading

Metabolic Alkalosis

By

James L. Lewis, III

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

Last full review/revision Jan 2020| Content last modified Jan 2020
Click here for Patient Education
Topic Resources

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

  • Acid loss

  • Alkali administration

  • Intracellular shift of hydrogen ion (H+—as occurs in hypokalemia)

  • Renal HCO3 retention

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

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

  • Diuretic use

Among other causes of metabolic alkalosis are disorders that cause

  • Renal acid loss

  • Bicarbonate excess

Table
icon

Causes of Metabolic Alkalosis

Cause

Comments

Gastrointestinal acid loss*

Congenital chloridorrhea

Fecal Cl loss and HCO3 retention

Gastric acid loss due to vomiting or nasogastric suction

Loss of HCl and acid coupled with contraction alkalosis due to release of aldosterone and subsequent resorption of HCO3

Villous adenoma

Probably secondary to K depletion

Renal acid loss

Includes congenital adrenal hyperplasia

Occurs with volume depletion, heart failure, cirrhosis with ascites, nephrotic syndrome, Cushing syndrome or disease, renal artery stenosis, or renin-secreting tumor

Use of glycyrrhizin-containing compounds† (eg, licorice, chewing tobacco, carbenoxolone, Lydia Pinkham’s vegetable compound)

Glycyrrhizin inhibition of enzymatic conversion of cortisol to less active metabolites

Rare congenital disease causing hyperaldosteronism and hypokalemic metabolic alkalosis that manifests in early childhood with renal salt wasting and volume depletion

Similar to Bartter syndrome

Characterized in addition by hypomagnesemia and hypocalciuria

Manifests in young adults

Diuretics (thiazide and loop)‡

Multiple mechanisms: Secondary hyperaldosteronism due to volume depletion, Cl depletion, or contraction alkalosis; may be Cl-unresponsive because of concomitant K depletion

Stimulate K and Mg reabsorption and H excretion; alkalosis unresponsive to NaCl and volume replacement until deficiencies corrected; low K causing H to shift into cells, raising extracellular pH

HCO3 excess

Posthypercapnic*

Persistent elevation of compensatory HCO3 levels, often with volume, K, and Cl depletion

Postorganic acidosis

Conversion of lactic acid or ketoacid to HCO3 worsened by HCO3 therapy for acidosis

NaHCO3 loading

Occurs with overzealous loading or with loading in patients who have hypokalemia; serum becomes more alkalotic as H shifts back into cells

Chronic ingestion of calcium carbonate antacids provides Ca and HCO3 load; hypercalcemia lowers PTH, increasing HCO3 reabsorption

Contraction alkalosis*

Diuretics (all types)

Sweat loss in cystic fibrosis

NaCl loss concentrates a fixed amount of HCO3 in a smaller total body volume

Other

Carbohydrate refeeding after starvation

Resolution of starvation ketosis or acidosis with improved cellular function

Laxative abuse*

Unclear mechanism

Some antibiotics (eg, carbenicillin, penicillin, ticarcillin)

Contain nonreabsorbable anion, which increases K and H excretion

* Chloride-responsive.

† Chloride-unresponsive.

‡ May be either chloride-responsive or chloride-unresponsive.

Ca = calcium; Cl = chloride; H = hydrogen; HCl = hydrochloric acid; HCO3 = bicarbonate; K = potassium; Mg = magnesium; NaCl = sodium chloride; NaHCO3 = sodium bicarbonate; PTH = parathyroid hormone.

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 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 and subsequent headache, lethargy, and neuromuscular excitability, sometimes with delirium, tetany, and seizures. Alkalemia also lowers threshold for anginal symptoms and arrhythmias. Concomitant hypokalemia 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 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 Cl < 20 mEq/L (< 20 mmol/L) indicates significant renal Cl reabsorption and hence a Cl-responsive cause (see table Causes of Metabolic Alkalosis)

  • Urinary Cl > 20 mEq/L (> 20 mmol/L) suggests a chloride-unresponsive form.

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

  • Urinary K < 30 mEq/day (< 30 mmol/day) signifies hypokalemia or laxative misuse.

  • Urinary K > 30 mEq/day (> 30 mmol/day) in a patient without hypertension suggests diuretic abuse or Bartter syndrome or Gitelman syndrome.

  • Urinary K > 30 mEq/day (> 30 mmol/day) in a patient with hypertension requires evaluation for hyperaldosteronism, mineralocorticoid excess, and renovascular disease.

Tests in patients with hypertension typically include plasma renin activity and aldosterone and cortisol levels (see Diagnosis of Cushing Syndrome and Diagnosis of Primary Aldosteronism).

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 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.

Drugs Mentioned In This Article

Drug Name Select Trade
No US brand name
DIAMOX
Click here for Patient Education
NOTE: This is the Professional Version. CONSUMERS: Click here for the Consumer Version
Professionals also read

Also of Interest

Videos

View All
Overview of Acid-Base Maps and Compensatory Mechanisms
Video
Overview of Acid-Base Maps and Compensatory Mechanisms
3D Models
View All
Female Endocrine System
3D Model
Female Endocrine System

SOCIAL MEDIA

iOS Android
iOS Android
iOS Android
TOP