Metabolic acidosis

Definition of metabolic acidosis: low pH and low P_HCO3

Metabolic acidosis may be due to:

• Accumulation of acids (= high anion gap) or
• Loss of bicarbonate (= hyperchloraemic metabolic acidosis, normal anion gap) 

Anion gap =  (Na + K) - (Cl + HCO₃)

The normal anion gap is 14-16 mmol/l

• Causes of a high anion gap metabolic acidosis: MUDPILES
• Methanol (formic acid)
• Uraemia
• Diabetic ketoacidosis
• Paraldehyde (acetic and chloroacetic acid)
• Isoniazid
• Iron
• Inborn errors of metabolism
• Glucose 6 phosphate deficiency
• Lactic acid
• Cyanide poisoning
• Carbon monoxide poisoning
• Short bowel syndrome
• Excess inhaled beta agonists
• Metformin
• Seizures
• Shock
• Ethylene glycol (oxalic acid)
• Salicylates
• Also:
• Toluene OD (an aromatic hydrocarbon used as a solvent in paints, glues etc)
• Paracetamol OD – either due to lactic acidosis or pyroglutamic acidosis

Pyroglutamic acid (=intermediate in the gamma glutamyl cycle). Excess associated with ingestion of several medications, including flucloxacilin, vigabatrin and paracetamol.

• Causes of a hyperchloraemic (=normal anion gap) metabolic acidosis
• Gastrointestinal bicarbonate loss:
• Diarrhoea
• Intestinal fistula
• Urinary tract diversion to intestine
• Ileal conduit
• Ureterosigmoidoscopy
• Medications
• Magnesium sulphate
• Laxative abuse
• Cholestyramine
• Renal causes
• Renal tubular acidosis
• Renal failure
• Drug-induced hyperkalaemia with renal insufficiency
• spironolactone
• ACE-i
• Administration of fluid containing chloride

Small print gem: a major decrease in extra cellular fluid volume will raise the HCO₃, so the diagnosis of metabolic acidosis may be missed in the diabetic patient who has vomited lots and is severely dehydrated with concurrent DKA, or the cholera patient who has had massive diarrhoea 

References

Dickson, R. and Luks, A. Toluene toxicity as a cause of elevated anion gap metabolic acidosis. Respir Care. 2009; 54(8): 1115-1117. 

Green, T., Bijisma, J. and Sweet, D. Profound metabolic acidosis from pyroglutamic academia: an underappreciated cause of high anion gap metabolic acidosis. CJEM. 2010; 12(5): 449-452. 

Halperin, M. and Kamel, K. Some observations on the clinical approach to metabolic acidosis. J Am Soc Nephrol. 2010; 21(6): 894-897. 

Lim, S. Metabolic acidosis. Acta Med Indones. 2007; 39(3): 145-150.

Maccari, C. et al. The patient with a severe degree of metabolic acidosis: deductive analysis. QJM. 2006; 99(7): 475-485.