Metabolic Acidosis Evaluation
OVERVIEW
- 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.
CAUSES
- 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)
- Lactate
- Toxins – methanol, metformin, phenformin, paraldehyde, propylene glycol, pryroglutamic acidosis, iron, isoniazid, ethanol, ethylene glycol, salcylates, solvents
- Ketones
- Renal
Normal anion gap (NAGMA)
- Chloride
- Acetazolamide and Addisons
- GI causes – diarrhoea, vomiting, fistulas (pancreatic, ureterostomies, small bowel, ileostomies)
- Extras – RTA
Other
- 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
Critical Care
Compendium
Chris is an Intensivist and ECMO specialist at The Alfred ICU, where he is Deputy Director (Education). He is a Clinical Adjunct Associate Professor at Monash University, the Lead for the Clinician Educator Incubator programme, and a CICM First Part Examiner.
He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives. He was one of the founders of the FOAM movement (Free Open-Access Medical education) has been recognised for his contributions to education with awards from ANZICS, ANZAHPE, and ACEM.
His one great achievement is being the father of three amazing children.
On Bluesky, he is @precordialthump.bsky.social and on the site that Elon has screwed up, he is @precordialthump.
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