- A metabolic acidosis is a process which, if uncorrected, would lead to an acidaemia.
- It is usually associated with a low bicarbonate concentration (or total CO2), but an acidosis may be masked by a co-existing metabolic alkalosis.
- accumulation of acids (measured, i.e. chloride hyperchloraemic metabolic acidosis] or unmeasured [increased anion gap metabolic acidosis])
- renal or gastrointestinal loss of bicarbonate (with absorption of chloride, resulting in hyperchloraemic metabolic acidosis).
ANION GAP AND METABOLIC ACIDOSIS
Anion gap = (Na + K) – (Cl + HCO3) simplified as (Na) – (Cl + HCO3)
- is usually determined primarily by negatively charged plasma proteins
- range = 10 to 16 mmol/L (8 to 12 mmol/L if K not included)
- AG decreases by about 2.5 mmol/L for every decrease in albumin by 10 g/L
- increased anion gap -> fall in unmeasured cations (Ca, Mg) or increase in unmeasured anions (lactate, ketoacids, formate (methanol), glycolate and oxlate (ethylene glycol))
High anion gap (HAGMA)
- Toxins – methanol, metformin, phenformin, paraldehyde, propylene glycol, pryroglutamic acidosis, iron, isoniazid, ethanol, ethylene glycol, salcylates, solvents
Normal anion gap (NAGMA)
- Acetazolamide and Addisons
- GI causes – diarrhoea, vomiting, fistulas (pancreatic, ureterostomies, small bowel, ileostomies)
- Extras – RTA
- Check delta ratio in HAGMA to determine if there is a coexistant NAGMA.
- osmolar gap can help as a screening test for methanol or ethylene glycol intoxication once alcohol has been excluded (calculated osmolality = 2*Na + Glucose + Urea + ethanol/4.6).
- urinary pH (inappropriately alkaline for an acidaemia) and electrolytes may facilitate eliciting the specific cause of the renal bicarbonate loss (e.g. renal tubular acidosis).
References and Links
- Acid-Base: ABG analysis – Anion Gap – SID – NAGMA
- Metabolic acidosis: Overview – evaluation – DDx
- Metabolic alkalosis: Overview – evaluation – DDx
- Respiratory acidosis: Overview – DDx
- Respiratory alkalosis: Overview – DDx