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.
The causes of metabolic alkalosis can be divided into two categories, depending upon urine
chloride levels.[1]
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.[2]
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[4] and people with
eating disorders[5] may present with metabolic alkalosis.
Posthypercapnia – Hypoventilation (decreased respiratory rate) causes hypercapnia (increased levels of CO2), 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.[6]
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.
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.[6]
Decreases in albumin and phosphate will cause metabolic alkalosis.[7]
Chloride-resistant (Urine chloride > 20 mEq/L)
Retention of
bicarbonate – Retention of bicarbonate would lead to alkalosis.
Shift of hydrogen ions into
intracellular space – Seen in
hypokalemia. Due to a low extracellular potassium concentration, potassium shifts out of the cells. In order to maintain electrical neutrality, hydrogen shifts into the cells, raising blood pH.
Hyperaldosteronism – Loss of hydrogen ions in the urine occurs when excess
aldosterone (
Conn's syndrome) increases the activity of a sodium-hydrogen exchange protein in the kidney. This increases the retention of sodium ions whilst pumping hydrogen ions into the
renal tubule. Excess sodium increases extracellular volume and the loss of hydrogen ions creates a metabolic alkalosis. Later, the kidney responds through the
aldosterone escape to excrete sodium and chloride in urine.[8]
Bartter syndrome and Gitelman syndrome – syndromes with presentations analogous to taking diuretics characterized with normotensive patients
Liddle syndrome – a gain of function mutation in the genes encoding the epithelial sodium channel (ENaC) which is characterized by hypertension and hypoaldosteronism.
Aminoglycoside toxicity can induce a hypokalemic metabolic alkalosis via activating the calcium sensing receptor in the thick ascending limb of the nephron, inactivating the NKCC2 cotransporter, creating a Bartter's syndrome like effect.
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[citation needed] 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 pCO2 in the setting of metabolic alkalosis, the following equations are used:
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
Alkalosis refers to a process by which the pH is increased.
Alkalemia refers to a pH which is higher than normal, specifically in the blood.
^Mascolo, Margherita; Chu, Eugene S.; Mehler, Philip S. (April 2011). "Abuse and clinical value of diuretics in eating disorders therapeutic applications". International Journal of Eating Disorders. 44 (3): 200–202.
doi:
10.1002/eat.20814.
PMID20186716.