Metabolic alkalosis is an acid-base disorder 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 (HCO3-), 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)

Chloride-indeterminate alkalosis

Chloride-resistant (Urine chloride > 20 mEq/L)

Compensation

Compensation for metabolic alkalosis occurs mainly in the lungs, which retain carbon dioxide (CO2) through slower breathing, or hypoventilation (respiratory compensation). CO2 is then consumed toward the formation of the carbonic acid intermediate, thus decreasing pH. Respiratory compensation, though, is incomplete. The decrease in [H+] suppresses the peripheral chemoreceptors, which are sensitive to pH. But, because respiration slows, there is an increase in pCO2 which would cause an offset of the depression because of the action of the central chemoreceptors which are sensitive to the partial pressure of CO2 in the cerebral spinal fluid. So, because of the central chemoreceptors, respiration rate would be increased. Renal compensation for metabolic alkalosis, less effective than respiratory compensation, consists of increased excretion of HCO3− (bicarbonate), as the filtered load of HCO3− exceeds the ability of the renal tubule to reabsorb it. To calculate the expected p in the setting of metabolic alkalosis, the following equations are used:

Treatment

To effectively treat metabolic alkalosis, the underlying cause(s) must be corrected. A trial of intravenous chloride-rich fluid is warranted if there is a high index of suspicion for chloride-responsive metabolic alkalosis caused by loss of gastrointestinal fluid (e.g., due to vomiting).

Terminology