Metabolic acidosis

In medicine, metabolic acidosis is a state in which the blood pH is low (under 7.35) due to increased production of H⁺ by the body or the inability of the body to form bicarbonate (HCO₃^-) in the kidney. Its causes are diverse, and its consequences can be serious, including coma and death. Together with respiratory acidosis, it is one of the two general types of acidosis. more...

Signs and symptoms

Symptoms are aspecific, and diagnosis can be difficult unless the patient presents with clear indications for arterial blood gas sampling. Symptoms may include chest pain, palpitations, headache, altered mental status, decreased visual acuity, nausea, vomiting, abdominal pain, altered appetite (either anorexia or excessive eating) and weight loss (longer term), muscle weakness and bone pains. A slightly specific finding is when the patient reports rapid breathing, not due to shortness of breath but an unmotivated drive to hyperventilate. Kussmaul respiration is rare, but may indicate ketoacidosis.

Exteme acidosis leads to neurological and cardiac complications:

• Neurological: lethargy, stupor, coma, seizures.
• Cardiac: arrhythmias (ventricular tachycardia), decreased response to epinephrine; both lead to hypotension (low blood pressure).

Physical examination occasionally reveals signs of disease, but is otherwise normal. Cranial nerve abnormalitites are reported in ethylene glycol poisoning, and retinal oedema can be a sign of methanol (methyl alcohol) intoxication. Longstanding chronic metabolic acidosis leads to osteoporosis and can cause fractures.

Diagnosis

Arterial blood gas sampling is essential for the diagnosis. The pH is low (under 7.35) and the bicarbonate levels are decreased (<12 mmol/l). In respiratory acidosis (low blood pH due to decreased clearance of carbon dioxide by the lungs), the bicarbonate is elevated, due to increased conversion from H₂CO₃. An ECG can be useful to anticipate cardiac complications.

Other tests that are relevant in this context are electrolytes (including chloride), glucose, renal function and a full blood count. Urinalysis can reveal acidity (salicylate poisoning) or alkalinity (renal tubular acidosis type I). In addition, it can show ketones in ketoacidosis.

To distinguish between the main types of metabolic acidosis, a clinical tool called the anion gap is considered very useful. It is calculated by subtracting the chloride and bicarbonate levels from the sodium plus potassium levels.

Anion gap = ( + ) - ( + )

As sodium and potassium are the main extracellular cations, and chloride and bicarbonate are the main anions, the result should reflect the remaining anions. Normally, this concentration is about 8-16 mmol/l. An elevated anion gap (i.e. > 16 mmol/l) can indicate particular types of metabolic acidosis, particularly certain poisons, lactate acidosis and ketoacidosis.

As the differential diagnosis is narrowed down, certain other tests may be necessary, including toxicological screening and imaging of the kidneys.

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