Metabolic Acidosis - Definition, Causes, Symptoms & Treatment

Metabolic acidosis is an acid-base disorder that occurs when the body accumulates excess acid, loses too much bicarbonate, or cannot eliminate acids effectively. These changes lower the bicarbonate concentration in the blood and may cause blood pH to fall below its normal range.

Metabolic acidosis refers to a primary reduction in blood bicarbonate caused by a metabolic disturbance. Normally, the body maintains blood pH within a narrow range of approximately 7.35 to 7.45. The lungs regulate carbon dioxide levels, while the kidneys help control bicarbonate and eliminate excess acids.

Understanding the meaning of metabolic acidosis requires distinguishing between acidosis and acidemia. Acidosis describes a process that tends to increase acidity, whereas acidemia refers to a measured blood pH below 7.35 due to a metabolic disturbance. A person may have an acidosis process without acidemia if another process raises the pH and offsets the change.

Metabolic Acidosis Causes

The causes of metabolic acidosis generally involve increased acid production, reduced acid excretion, bicarbonate loss, or the ingestion of certain toxic substances. Identifying the underlying cause is essential for selecting appropriate treatment.

Common reasons for metabolic acidosis include the following:

• Diabetic ketoacidosis: When insulin is insufficient, the body breaks down fat and produces ketones. Excess ketone acids can lower blood pH, particularly in people with diabetes.
• Lactic acidosis: Lactate may accumulate during severe illness, shock, reduced tissue perfusion, or certain other medical conditions. Some medications and disorders can also contribute.
• Kidney disease: The kidneys normally eliminate acids and maintain bicarbonate balance. Impaired kidney function can interfere with these processes.
• Severe diarrhoea: Excessive bicarbonate loss through the gastrointestinal tract can cause metabolic acidosis.
• Renal tubular acidosis: Certain kidney tubular disorders prevent adequate acid excretion or bicarbonate conservation.
• Toxin exposure: Substances such as methanol and ethylene glycol can cause serious acid-base disturbances. Salicylate poisoning may also produce metabolic acidosis, often alongside respiratory alkalosis.
• Medications: Certain medicines can contribute to metabolic acidosis, depending on the drug, dosage, and the patient's medical condition.

Metabolic Acidosis Symptoms

Metabolic acidosis symptoms vary according to the severity of the acid-base disturbance and the underlying condition. Mild cases may produce few noticeable symptoms, while severe cases can affect breathing, circulation, and brain function.

Possible symptoms include:

• Rapid or deep breathing
• Fatigue and general weakness
• Nausea and vomiting
• Reduced appetite
• Headache or general discomfort
• Confusion or reduced alertness
• Rapid heartbeat in some cases

Deep breathing may occur as the body attempts to remove carbon dioxide and compensate for the increased acidity. Symptoms such as vomiting and weakness may also arise from the underlying illness rather than from acidosis alone.

Severe breathing changes, confusion, fainting, or rapid deterioration require urgent medical assessment. Suspected poisoning or diabetic ketoacidosis also requires immediate medical attention.

High Anion Gap Metabolic Acidosis

High anion gap metabolic acidosis occurs when acids accumulate in the blood and release negatively charged ions that are not routinely included in standard electrolyte measurements. These unmeasured ions increase the calculated anion gap.

The anion gap helps clinicians narrow down possible causes of metabolic acidosis. It is commonly calculated using the following formula:

Anion gap = Sodium − (Chloride + Bicarbonate)

The reference range varies between laboratories and depends on the measurement method. Potassium is usually excluded from the standard calculation.

Common causes of a high anion gap include diabetic ketoacidosis, lactic acidosis, advanced kidney failure, and poisoning from certain substances. The presence of a high anion gap does not establish a diagnosis by itself. Additional blood tests, medical history, and clinical findings are needed to identify the cause.

Low blood albumin can reduce the measured anion gap and may conceal an underlying increase. Healthcare professionals may therefore consider albumin levels when interpreting results.

Normal Anion Gap Metabolic Acidosis

Normal anion gap metabolic acidosis occurs when bicarbonate decreases while chloride increases, keeping the calculated anion gap within the laboratory's reference range. It is also called hyperchloremic metabolic acidosis.

Common causes include significant diarrhoea, renal tubular acidosis, and certain medications. Some kidney disorders reduce the ability to conserve bicarbonate or excrete acid, while gastrointestinal bicarbonate loss can produce a similar pattern.

The distinction between normal and high anion gap metabolic acidosis helps guide further investigations. However, a normal anion gap does not rule out a serious medical condition. The patient's symptoms, kidney function, electrolyte levels, and medical history must also be considered.

Metabolic Acidosis ABG: Diagnosis and Testing

A metabolic acidosis ABG assessment involves analysing an arterial blood gas test. This test measures blood pH, the partial pressure of carbon dioxide (PaCO₂) and, depending on the analyser, bicarbonate and other parameters.

Metabolic acidosis typically involves a reduced bicarbonate concentration. Blood pH is often below 7.35, although mixed acid-base disorders can produce more complicated results.

Doctors may also request electrolyte measurements, kidney function tests, blood glucose, ketone testing, lactate measurements, and investigations for possible toxin exposure. The tests selected depend on the suspected cause.

Anion gap analysis helps distinguish between different metabolic acidosis patterns. Additional investigations may be necessary when the clinical findings and laboratory results do not match.

Metabolic Acidosis Compensation

Metabolic acidosis compensation refers to the body's response to the primary reduction in bicarbonate. The lungs increase ventilation to remove more carbon dioxide, which helps limit the fall in blood pH. The kidneys play an important role in long-term acid-base balance but may be impaired in some causes of metabolic acidosis.

Clinicians can estimate the expected respiratory response using Winter's formula:

Expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2 mmHg

Here, HCO₃⁻ represents the bicarbonate concentration measured in millimoles per litre. If the measured PaCO₂ is higher than expected, an additional respiratory acidosis may be present. If it is lower than expected, a concurrent respiratory alkalosis may be contributing.

Compensation does not necessarily return blood pH to the normal range. Interpreting the pH, bicarbonate, PaCO₂, and clinical findings together helps identify mixed acid-base disorders.

Metabolic Acidosis Treatment

Metabolic acidosis treatment depends on the underlying cause, the severity of the condition, and the patient's overall clinical status. Correcting the cause is generally the priority rather than treating the blood pH in isolation.

Treatment may involve:

• Diabetic ketoacidosis: Management commonly includes intravenous fluids when indicated, insulin, and careful monitoring of electrolytes, particularly potassium.
• Lactic acidosis: Treatment focuses on addressing the underlying cause, such as shock, severe infection, or impaired tissue perfusion.
• Kidney-related acidosis: Management may include treating the underlying kidney disorder, reviewing medications, and using prescribed alkali therapy in selected cases.
• Bicarbonate loss: Fluid and electrolyte replacement may be needed when significant diarrhoea or another cause of bicarbonate loss is present.
• Toxic exposure: Specific treatment depends on the substance involved and may require antidotes, specialist care or dialysis.

Bicarbonate therapy is not appropriate for every case of metabolic acidosis. Its use depends on factors such as the severity of acidemia, the underlying cause, and the patient's clinical condition. Self-treatment with bicarbonate or other remedies can be harmful and should be avoided.

When Should You Seek Medical Attention?

Metabolic acidosis can develop because of conditions that require prompt treatment. Seek urgent medical attention if you experience deep or rapid breathing, confusion, severe weakness, persistent vomiting, or a sudden decline in your condition. People with diabetes should seek immediate assessment if they suspect diabetic ketoacidosis, particularly when accompanied by vomiting, abdominal pain, dehydration, or altered consciousness.

Early diagnosis allows healthcare professionals to identify the underlying cause, assess the severity of the acid-base disturbance, and provide appropriate treatment.

Reader Information: This article is for informational purposes only and does not replace professional medical advice, diagnosis, or treatment. If you experience symptoms associated with metabolic acidosis, such as rapid breathing, persistent weakness, nausea, or confusion, consult a qualified healthcare professional for appropriate evaluation and care. Seek urgent medical attention if symptoms are severe or worsen suddenly.