Understanding High Anion Gap Metabolic Acidosis (HAGMA)
The anion gap is calculated as:
\(Anion\ Gap = Na^+ - (Cl^- + HCO_3^-)\)
A high anion gap indicates accumulation of unmeasured acid anions, classically remembered by the mnemonic MUDPILES (Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol/Paraldehyde, Iron/Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates). In a child, acute iron poisoning from accidental ingestion of iron tablets is a well-recognized and classic pediatric cause of HAGMA.
Why Iron is the Correct Answer
- Iron is directly corrosive to the GI mucosa and, once absorbed in toxic amounts, uncouples oxidative phosphorylation in mitochondria. This forces cells into anaerobic metabolism, generating excess lactic acid, which consumes bicarbonate and widens the anion gap.
- Iron toxicity also causes hypotension and poor tissue perfusion (shock), further worsening lactic acidosis.
- Free iron in the blood can also directly generate hydrogen ions when it hydrolyzes, adding to the acid load.
- This makes iron ingestion a classic exam-favorite cause of HAGMA in children, alongside toxins like salicylates and ethylene glycol.
Why the Other Options Are Incorrect
- Rifampicin: An antitubercular drug known for hepatotoxicity, orange discoloration of body secretions, and drug interactions (CYP450 induction), but it is not a recognized cause of high anion gap metabolic acidosis.
- Digoxin: Digoxin toxicity classically presents with GI symptoms (nausea, vomiting), visual disturbances (xanthopsia/halos), and cardiac arrhythmias (especially with hyperkalemia in acute toxicity). It does not typically produce a high anion gap acidosis.
- Beta blockers: Overdose classically causes bradycardia, hypotension, and hypoglycemia. Beta blockers are not associated with a high anion gap metabolic acidosis; if acidosis occurs, it is more related to poor perfusion/shock in severe overdose rather than being a classic direct cause.
Hence, in a child presenting with high anion gap metabolic acidosis, poisoning with iron should be strongly suspected.
Important Questions from Acid-Base Disorders
- The most common acid-base disturbance in critically ill patients is :
- Salicylate intoxication in adults usually causes:
1. Severe respiratory alkalosis.
2. Mixed metabolic acidosis respiratory alkalosis.
3. Hyperchloremic metabolic acidosis.
Select the correct answer using the code given below - Which one of the following pairs is not correctly matched?
Consider the following:
1. Normal anion gap
2. Hyperchloraemic acidosis
3. Inappropriately high urinary pH (>5.4)
Which of the above are the characteristic features of renal tubular acidosis (RTA)?- Which one of the following is not an example of metabolic acidosis with high anion gap?
