Metabolic alkalosis is a metabolic condition in which the pH of tissue is elevated beyond the normal range (7.35–7.45). This is the result of decreased hydrogen ion concentration, leading to increased bicarbonate, or alternatively a direct result of increased bicarbonate concentrations. The condition typically cannot last long if the kidneys are functioning properly.
Signs and symptoms
Mild cases of metabolic alkalosis often cause no symptoms. Typical manifestations of moderate to severe metabolic alkalosis include abnormal sensations, neuromuscular irritability, tetany, abnormal heart rhythms (usually due to accompanying electrolyte abnormalities such as low levels of potassium in the blood), coma, seizures, and temporary waxing and waning confusion.
Causes
The causes of metabolic alkalosis can be divided into two categories, depending upon urine chloride levels.
Chloride-responsive (Urine chloride < 25 mEq/L) Loss of hydrogen ions – Most often occurs via two mechanisms, either vomiting or via the kidney. Vomiting results in the loss of hydrochloric acid (hydrogen and chloride ions) with the stomach contents. In the hospital setting this can commonly occur from nasogastric suction tubes. Severe vomiting also causes loss of potassium (hypokalemia) and sodium (hyponatremia). The kidneys compensate for these losses by retaining sodium in the collecting ducts at the expense of hydrogen ions (sparing sodium/potassium pumps to prevent further loss of potassium), leading to metabolic alkalosis. Congenital chloride diarrhea – rare for being a diarrhea that causes alkalosis instead of acidosis. Contraction alkalosis – This results from a loss of water in the extracellular space, such as from dehydration. Decreased extracellular volume triggers the renin-angiotensin-aldosterone system, and aldosterone subsequently stimulates reabsorption of sodium (and thus water) within the nephron of the kidney. However, a second action of aldosterone is to stimulate renal excretion of hydrogen ions (while retaining bicarbonate), and it is this loss of hydrogen ions that raises the pH of the blood. Diuretic therapy – loop diuretics and thiazides can both initially cause increase in chloride, but once stores are depleted, urine excretion will be below < 25 mEq/L. The loss of fluid from sodium excretion causes a contraction alkalosis. Diuretic abuse among athletes and people with eating disorders may present with metabolic alkalosis. Posthypercapnia – Hypoventilation (decreased respiratory rate) causes hypercapnia (increased levels of ), which results in respiratory acidosis. Renal compensation with excess bicarbonate retention occurs to lessen the effect of the acidosis. Once carbon dioxide levels return to base line, the higher bicarbonate levels reveal themselves putting the patient into metabolic alkalosis. Cystic fibrosis – excessive loss of sodium chloride in the sweat leads to contraction of the extracellular volume in the same way as contraction alkalosis, as well chloride depletion. Alkalotic agents – Alkalotic agents, such as bicarbonate (administered in cases of peptic ulcer or hyperacidity) or antacids, administered in excess can lead to an alkalosis.
Chloride-indeterminate alkalosis
Milk alkali syndrome
Blood product administration since this contains sodium citrate which is then metabolized into sodium bicarbonate. Typically, this is seen with large volume transfusions such as more than 8 units. Decreases in albumin and phosphate will cause metabolic alkalosis.