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Metabolic acidosis in poisoning

Learn about the mechanisms, calculations, and causes of metabolic acidosis in poisoning cases, including classical and toxicological origins. Discover the role of Anion Gap and Osmol Gap in diagnosis and treatment options for high AG metabolic acidosis.

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Metabolic acidosis in poisoning

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  1. Metabolic acidosis in poisoning • Taken from Metabolic acidosis: differentiating the causes in the poisoned patient B Judge Med Clinic N Am 2005, 89:1107

  2. Metabolic acidosis • Definition: process that lowers serum HCO3- • Occurs when H+ ion production exceeds body’s ability to compensate adequately via buffering or ventilation Mechanisms of metabolic acidosis in poisoning • Increased acid production • Impaired acid elimination

  3. Calculations • Note the low pH (or high H+) • Then calculate Anion Gap (AG) AG = [Na+] – ([Cl-] + [HCO3-]) Usual range = 12 +/- 4 m/Eq/L (more recently 7 +/- 4) • If toxic alcohols suspected, calculate osmolality: 2 x [Na+] + [glucose] + [urea] and request a measured osmolality on a blood sample Osmol Gap = measured osmolality – calculated osmolality

  4. AG & metabolic acidosis • High AG Occurs when an acid is paired with an unmeasured anion (eg lactate, formate) • Normal AG Occurs with gain of both H+ and Cl- ions, or a loss of HCO3- and retention of Cl-, preserving electroneutrality • However, AG can be affected by errors of calculation or assay, and by numerous disease states. So the lack of a high AG does not exclude any particular cause

  5. Classical causes of high AG metabolic acidosis • Methanol • Uremia • Diabetic ketoacidosis, alcoholic ketoacidosis, starvation ketoacidosis • Paraldehyde • Iron, isoniazid • Lactic acidosis • Ethylene glycol • Salicylates

  6. Common toxicological causes of high AG metabolic acidosis • Paracetamol • Amphetamines • Carbon monoxide • Cocaine • Toluene, benzene • Valproate • Salicylates • [NSAIDs, metformin, glycols] • [ARVs: Zidovudine, didanosine, stavudine]

  7. Use of the osmol gap in patients with a high AG metabolic acidosis • Osmol gap may provide extra information if a toxic alcohol is suspected. • However, be aware that other medical conditions such as ketoacidosis and renal failure also cause a raised OG • Normal osmol gap = less than 10 +/- 6 mOsm/L • However, normal range has problems due to wide variability between people and assays

  8. Toxins associated with a high osmol gap • Mannitol • Alcohols: ethanol, etylene glycol, isopropanol, methanol, propylene glycol • Diatrizoate (amidothizoate) • Glycerol • Acetone • Sorbitol

  9. Ethylene glycol Glyceraldehyde Glycolate Glyoxylaye Oxalate Methanol Formaldehyde Formate Metabolism of toxic alcohols

  10. The mountain Mycyk & Aks, 2003

  11. Acetazolamide Acids (NH4 Cl, HCl) Cholestyramine Mg Cl Mafenide acetate Topiramate Ureteroenterostomy Diarrhoea Hyper-alimentation Pancreatic fistula Post-hypocapnia Rapid IV fluid administration Renal tubular acidosis Common causes of normal AG metabolic acidosis

  12. Mechanisms of increased acid production • Toxins are acids (eg HCl vs. sulphuric acid) • Toxins have acid metabolites (eg metabolism of alcohols to acids) • Toxins affect ATP consumption/production in mitochondria (eg pcm, valproate, ARVs, metformin, CO, cyanide, formate, +++ adrenergic stimulation) [uncoupling oxidative phosphorylation or inhibiting cytochromes of the electron transport chain] • Toxins create ketoacids (eg ethanol, isoniazid)

  13. Mechanisms of impaired acid elimination • Toxin metabolites damage kidneys (ethylene glycol) • Toxin causes distal RTA (eg toluene)

  14. Treatment • Give supportive care and stop offending drug • HCO3- administration generally not recommended Appropriate for increasing poison elimination (eg salicylates) and countering Na channel block (eg TCA) • Consider antidotes where available (eg ethanol or fomepizole for toxic alcohol ingestion) • Thiamine, pyridoxine and folate for toxic alcohols

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