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metabolic acidosis


  • a primary metabolic acidosis is a derangement of normal physiology characterised by a low arterial pH and a low arterial bicarbonate level.
  • if arterial bicarbonate level is low but pH is high, then this is a compensatory metabolic acidosis which is the body's attempt to correct a prolonged primary respiratory alkalosis event - see acid-base physiology
  • there are many possible causes of a primary metabolic acidosis, and we often resort to looking at the anion gap as a starting point to determining the main cause.

causes of a primary metabolic acidosis with a normal anion gap

  • this is associated with hyperchloraemia and is due to either loss of bicarbonate or excessive intake of proton donors
  • it may also occur excessive dilution with rapid extracellular fluid expansion

associated with hypokalaemia

  • diarrhoea;
  • small bowel, biliary, or pancreatic tube or fistula drain;
  • ureteral diversion with bowel (ileal conduit):
  • bowel augmentation cystoplasty;
  • exogenous chloride intake: CaCl2, MgCl2, NH4Cl, cholestyramine, HCl, arginine HCl;
  • impaired urinary acidifying capacity:

associated with hyperkalaemia

causes of a primary metabolic acidosis with a raised anion gap

  • ie. arterial chloride concentration will be normal
  • anion gap = ([Na] + [K]) − ([Cl] + [HCO3]) = 20 mEq/L


  • Methanol
  • Uraemia
  • Diabetic ketoacidosis
  • Paraldehyde and Phenformin
  • Isoniazid and Iron
  • Lactic acidosis
  • Ethanol and Ethylene Glycol
  • Salicylates

with ketones in urine

with raised serum creatinine > 300

  • renal failure most likely

with raised lactate levels

  • if +ve urine Phenystix or raised serum salicylate ⇒ salicylate toxicity
  • if raised RBC transketolase ⇒ thiamine deficiency

type A lactic acidosis (tissue hypoxia)

  • shock - hypovoalemic/cardiogenic/septic;
  • severe hypoxia
  • severe anaemia
  • severe asthma ⇒ work of breathing increased
    • ⇒ decreased preload due to increased i/thoracic pressure
  • seizures (takes >1hr to be metabolised back to normal)
  • severe exercise

type B1 lactic acidosis (disease conditions)

  • diabetes mellitus (as well as causing acidosis via ketoacidosis)
  • liver disease
  • malignancy
  • sepsis
  • phaeochromocytoma
  • thiamine deficiency

type B2 lactic acidosis (drugs/toxins)

  • biguanides such as Metformin
  • ethanol/methanol/ethylene glycol/fructose/sorbitol
  • salicylate/paracetamol/salbutamol/nitroprusside/isoniazid/adrenaline

type B3 lactic acidosis (inborn errors of carbohydrate/pyruvate metabolism)

  • Glucose-6-Pase deficiency
  • hypoglycaemia

with raised osmolal gap

  • osmolal gap = measured - calculated osmolality
  • calculated osmolarity = 2 x Na + urea + glucose if all in mmols/L


  • ethanol (each 0.05% blood ethanol contributes 12mOsmols to osmolal gap)
  • methanol
  • ethylene glycol
  • isopropyl alcohol
metabolic_acidosis.txt · Last modified: 2018/09/11 05:18 by

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