Metabolic Acidosis

What happens in metabolic acidosis

Your blood pH is kept in a narrow range, 7.35 to 7.45. The body does this with buffers (mainly bicarbonate), the lungs (which breathe out carbon dioxide, an acid) and the kidneys (which excrete acid and make new bicarbonate).

In metabolic acidosis, bicarbonate falls below about 22 mEq/L because:

• the body produces too much acid (ketones, lactate)
• an acid is swallowed or formed from a poison
• the kidneys cannot excrete acid
• bicarbonate is lost from the gut or kidneys

The lungs try to compensate by breathing faster and deeper to blow off carbon dioxide. That is why rapid deep breathing (Kussmaul breathing) is a hallmark. "Metabolic" distinguishes it from respiratory acidosis, where acid builds up because the lungs cannot remove enough carbon dioxide.

Causes: high vs normal anion gap

Doctors calculate the anion gap from routine blood tests: sodium minus (chloride plus bicarbonate). It estimates unmeasured acids in the blood. A normal value is roughly 8 to 12 mEq/L, depending on the lab.

High anion gap metabolic acidosis

An added acid is present. Common causes:

Normal anion gap (hyperchloremic) metabolic acidosis

Bicarbonate is lost, and chloride rises to replace it. Common causes:

• Diarrhea, especially severe or long-lasting, and fluid losses from fistulas or ileostomies
• Renal tubular acidosis, where the kidneys cannot excrete acid or hold on to bicarbonate
• Large volumes of saline IV fluid
• Medicines: acetazolamide and topiramate
• Early chronic kidney disease
• Urinary diversion surgery that connects the ureters to bowel

A useful memory aid doctors use for high-gap causes is GOLDMARK: glycols, oxoproline, L-lactate, D-lactate, methanol, aspirin, renal failure, ketoacidosis.

Symptoms

Symptoms often reflect the underlying cause. Acidosis itself can cause:

• deep, rapid breathing or feeling short of breath
• nausea, vomiting and belly pain
• headache, tiredness and weakness
• confusion, drowsiness and, in severe cases, coma
• a fast heart rate, low blood pressure and irregular heartbeat
• in chronic acidosis: bone thinning, muscle wasting, kidney stones and, in children, poor growth

How metabolic acidosis is diagnosed

1. Blood gas (arterial or venous): confirms low pH and low bicarbonate, and shows whether breathing is compensating as expected.
2. Basic metabolic panel: sodium, potassium, chloride and bicarbonate (often reported as "CO2" or "total CO2"), plus kidney function and glucose.
3. Anion gap, then specific tests: ketones, lactate, salicylate and acetaminophen levels, serum osmolality (the "osmolar gap" suggests toxic alcohols), and urine tests.
4. Urine anion gap and urine pH: help separate diarrhea from renal tubular acidosis in normal-gap acidosis.

A low CO2 on a routine blood test is not always metabolic acidosis. It can also be the body's response to over-breathing (respiratory alkalosis), which is why a blood gas is needed.

Treatment

Treatment depends on the cause and severity.

• DKA: IV fluids, insulin and potassium.
• Lactic acidosis and sepsis: restore circulation with fluids and blood pressure support, treat infection, stop metformin.
• Toxic alcohols: fomepizole blocks the formation of toxic acids; dialysis removes the poison.
• Aspirin overdose: IV bicarbonate to help the kidneys excrete salicylate, and dialysis in severe cases.
• Diarrhea: fluids and electrolytes, and treating the cause.
• Renal tubular acidosis: daily oral alkali (sodium bicarbonate or potassium citrate).
• Chronic kidney disease: oral sodium bicarbonate or a diet richer in fruits and vegetables is recommended when bicarbonate stays below about 18 to 22 mEq/L, to protect bones and muscle and possibly slow kidney decline. Dialysis corrects acidosis in kidney failure.

IV sodium bicarbonate is not routinely given for DKA or lactic acidosis, because treating the cause works better. It may be used when pH is extremely low or for specific causes such as some poisonings and severe bicarbonate loss.

Complications

• dangerous heart rhythms and reduced heart pumping
• low blood pressure and shock
• high potassium, as acid shifts potassium out of cells
• bone loss and kidney stones with chronic acidosis
• slow growth in children

Outlook

The outlook depends on the cause. Acidosis from diarrhea or DKA usually corrects within a day or two of treatment. Acidosis from septic shock or late-treated poisoning carries a higher risk of death. Chronic acidosis from kidney disease is managed long term with alkali treatment and monitoring.

Frequently asked questions

What is the most common cause of metabolic acidosis?

In hospitals, the most common causes are lactic acidosis from sepsis or shock, diabetic ketoacidosis, kidney failure and severe diarrhea. In outpatients, chronic kidney disease is the most common cause of a persistently low bicarbonate.

What are the symptoms of metabolic acidosis?

The classic sign is deep, rapid breathing as the lungs blow off carbon dioxide. Other symptoms include nausea, vomiting, tiredness, headache, confusion, a fast heartbeat and low blood pressure. Many symptoms come from the illness causing the acidosis. Mild chronic acidosis may cause no obvious symptoms.

What does a high anion gap mean?

A high anion gap means an unmeasured acid is present in the blood, such as ketones, lactate, a poison (methanol, ethylene glycol, aspirin) or acids retained in kidney failure. A normal anion gap with acidosis points to bicarbonate loss, as with diarrhea or renal tubular acidosis.

How is metabolic acidosis treated?

Treatment focuses on the cause: insulin and fluids for DKA, restoring circulation in sepsis, dialysis or antidotes for poisoning, and fluids for diarrhea. Bicarbonate tablets are used for chronic kidney-related acidosis and renal tubular acidosis. IV bicarbonate is reserved for specific or severe situations.

What is the difference between metabolic and respiratory acidosis?

Metabolic acidosis comes from too much acid or too little bicarbonate, caused by problems such as kidney failure, ketoacidosis or diarrhea. Respiratory acidosis comes from the lungs not removing enough carbon dioxide, for example in severe COPD, opioid overdose or weak breathing muscles.