Metabolic acidosis

Metabolic acidosis is an acid-base disorder characterised by a metabolic pathological process which decreases the pH of the blood. If the acidosis is severe, it can overcome the body's defense against acidosis, causes the pH in the blood to fall below 7.35. This is called acidaemia.

Respiratory acidosis is similar but is rather due to a respiratory problem. Metabolic alkalosis is sort of the opposite of metabolic acidosis. Metabolic acidosis is the most common acid-base disorder.

Types

There are two types of metabolic acidosis: Normal anion gap metabolic acidosis (NAGMA) and high anion gap metabolic acidosis (HAGMA). In both types there is decreased concentration of bicarbonate, as bicarbonate buffers the increased acid as part of the bicarbonate buffer system. The two types are not mutually exclusive; both can occur simultaneously.

HAGMA = High anion gap metabolic acidosis. NAGMA = Normal anion gap metabolic acidosis. The figure to the left shows the normal situation

Anion gap

The anion gap is a fancy term we use to include all anions in the serum that are not bicarbonate and chloride. The reason they’re called that is because the levels of these anions aren’t measured directly, but rather calculated by the following calculation:

Anion gap = [Na] – ([Cl–] + [HCO3–])

The reason this works is because we know that the plasma is electroneutral, meaning that there must be an equal amount of positive and negative charges. Almost all the positive charge in plasma comes from sodium, while the negative charges come from chloride, bicarbonate and other anions that aren’t measured in the lab, like lactate, phosphate, sulphate, proteins.

Acids in the blood usually exist in their anionic form. When there is an increased level of anions other than chloride and bicarbonate, we can assume that there is an increased level of acids as wells. The anion gap can be increased due to elevated levels of acids that are normally found in the body, like lactic acid or ketone bodies, or due to ingestion of compounds not usually found in the body, like ethylene glycol, etc. The extra acids increase the “gap”.

The anion gap is normal in case of increased loss of bicarbonate or decreased loss of H+. No extra acids means that there is no increased “gap”.

High anion gap metabolic acidosis

High anion gap metabolic acidosis (HAGMA) or elevated anion gap metabolic acidosis indicates a metabolic acidosis where the concentration of the anions that comprise the anion gap is increased. It occurs when there is overproduction or ingestion of organic acids or decreased acid excretion in the kidney. The concentration of HCO3– is reduced because of buffering. HAGMA is more common than NAGMA.

Normal anion gap metabolic acidosis

Normal anion gap metabolic acidosis (NAGMA) occurs when HCO3–, a base, is lost in significant amounts, and no excess acid accumulates in the blood. When bicarbonate is lost, chloride will be retained to uphold the electroneutrality. Because of the increased chloride it is sometimes called hyperchloraemic metabolic acidosis. One can distinguish between a hypokalaemic and a hyperkalaemic type of NAGMA, but this distinction is rarely used.

Etiology

Different etiologies produce different types of metabolic acidosis.

Etiology of HAGMA

• Lactic acidosis
• Ketoacidosis
• Ingestion of acids (methanol, ethylene glycol)
• End-stage kidney disease

Methanol and ethylene glycol are organic acids which can be ingested, usually accidentally. Methanol is a common byproduct of homemade alcohol production.

In end-stage chronic kidney disease, the kidney cannot excrete acidic compounds like phosphate, sulphate, urate, and hippurate, causing HAGMA. However, chronic kidney disease may also cause NAGMA.

Etiology of NAGMA

• Diarrhoea
• Renal tubular acidosis
• Chronic kidney disease
• Excessive saline infusion

Diarrhoea causes excessive loss of bicarbonate.

Renal tubular acidosis (RTA) is a collection of three rare disorders characterised by disorder of the renal tubules which cause metabolic acidosis.

Chronic kidney disease may cause NAGMA due to decreased capacity for retention (and therefore increased excretion) of bicarbonate.

Excessive rapid intravenous infusion of normal saline (0.9% NaCl) causes hyperchloraemic NAGMA, however usually a mild one. Because sodium chloride solutions do not contain bicarbonate but do contain chloride at a higher concentration than in the serum, a dilution of bicarbonate occurs, causing a decrease in bicarbonate concentration.

Lactic acidosis

Lactic acidosis is the most common cause of high anion gap metabolic acidosis, and is due to excess production of lactic acid, usually due to reduced oxygen delivery to peripheral tissues, which causes the tissues to switch from aerobic to anaerobic metabolism, which produces lactic acid. We distinguish two types of lactic acidosis, type A and type B.

Type A occurs when there is a global reduction in oxygen delivery due to circulatory shock or cardiopulmonary arrest. Type B occurs when there are other causes of increased lactic acid production, usually due to regional reduction in oxygen delivery or due to toxins impairing normal cellular metabolism.

The most common cause of type B lactic acidosis is improper use of metformin, an antidiabetic drug. Metformin changes cellular and intermediary metabolism in a way that makes tissues more likely to undergo anaerobic metabolism. Metformin-associated metabolic acidosis (MALA) may occur in case of metformin overdose, or if a person treated with metformin undergoes cellular stress. This can occur in case of kidney disease, liver disease, alcohol abuse, circulatory shock, surgery, or other severe acute illness. To prevent MALA, it's important to withold metformin treatment in case of such concomitant disease.

Ketoacidosis

Main article: Ketoacidosis

Ketoacidosis refers to metabolic acidosis due to accumulation of ketone bodies, which are acidic. Ketone bodies are the body's "reserve fuel" when glucose availability is limited or available glucose cannot be metabolised. Accumulation of ketone bodies may occur in case of severe fasting, for example due to alcohol abuse, or due to uncontrolled or untreated type 1 diabetes mellitus. In type 1 DM, there is hyperglycaemia, so lack of glucose is not a problem. Instead, there is insulin deficiency, and insulin is required for cells to take in and metabolise glucose.

Pathophysiology

Compensatory mechanism

The body compensates for a metabolic acidosis in two ways: It stimulates increased rate and depth of breathing, called Kussmaul respiration. This increases the exhalation of CO2, which increases pH back towards the normal range. The respiratory compensation is rapid, occuring within 30 minutes of the acidosis, reaching its maximum after a day. For every 1 unit HCO3- respiratory compensation decreases pCO2 by approximately 1.2 mmHg.

The other compensatory mechanism is due to the kidneys. Kidneys respond to metabolic acidosis by increasing urinary excretion of acids and decreasing urinary excretion of bicarbonate. This mechanism is slow, taking a few days to kick in.

Clinical features

Acidosis causes nonspecific clinical features. Hyperventilation may be present. In severe cases, mental status may be altered.

Diagnosis and evaluation

Arterial blood gas is essential in the evaluation of acid-base disorders. It will give the pH, bicarbonate level, pCO2, pO2, and lactate levels. In metabolic acidosis, the bicarbonate level is abnormally low (< 22 mmol/L). If there is acidaemia, the pH is < 7.35.

The anion gap should be calculated to help narrow down the possible causes. The following formula is used:

Anion gap = [Na] – ([Cl–] + [HCO3–])

In case of acidosis, the body will compensate by hyperventilating. This decreases pCO2. It's important to evaluate pCO2 to determine whether respiratory compensation is effective or not. The pCO2 should be reduced to approximately [HCO3-] + 15. One may also subtract 7 from the pH and multiply by 100 to get the optimal pCO2 level. For example, if the pH is 7.23, the pCO2 should be 23 mmHg. If the pCO2 is not at the expected level, there is an impairment of the body's compensatory response to acidosis, or there may be a mixed acid-base disorder.

Management

As metabolic acidosis is not a disease of itself but rather a consequence, the underlying disease must be identified and treated. This will reverse the metabolic acidosis.

Administration of intravenous bicarbonate is usually reserved for the most severe cases where pH < 7.1. However, there is controversy as to when exactly bicarbonate administration is indicated.