In practically every CICM exam paper, the candidates are presented with an ABG (sometimes several) which expect the diagnosis of a metabolic acidosis of some sort. It is therefore essential to develop some sort of mnemonic aid to recall the differential diagnosis of metabolic acidosis. In spite of its many obvious disadvantages, the anion gap classification of metabolic acid-base disorders is useful tool.
High anion gap |
Normal anion gap |
MUD PILES |
PANDA RUSH |
The MUDPILES mnemonic is showing its age. Nowhere is there space for citrate, for instance. This is a problem, as several SAQs (eg. Question 3.3 from the second paper of 2013) present the candidate with a citrate-based acidosis. The "P" in PILES used to be "Paraldehyde", but paraldehyde has fallen out of favour since the 1980s, and so "Pyroglutamic acidosis" is probably a better substitute. Iron is not a cause of a high anion gap on its own (in fact, ionised iron is cationic) but it can cause the anion gap to increase when it is removed from solution by its conversion to ferric hydroxide. Also, methanol and ethylene glycol are insufficiently distinct to merit their own individual places in the mnemonic (both are toxic alcohols). In spite of these concerns, the MUDPILES meme is still propagated throughout the medical school curricula, as the educators cynically expect their graduates to never have to seriously think about acidosis. It's simply irrelevant in the skin lesion clinic, cath lab or endoscopy suite. After all, you can always call the ICU if you can't make sense of the blood gas; they will sort it out.
Though a metabolic acidosis of one sort or another appears in virtually every written paper, the actual approach to metabolic acidosis has only been asked about once, in the dark past of the CICM fellowship papers - namely, Question 11(16) from the second paper of 2003.
1) Calculate the anion gap
2) Calculate the delta ratio
3) Measure urinary electrolytes and calculate the urinary anion gap (for normal anion gap metabolic acidosis)
4) Measure the serum osmolality and calculate the osmolar gap
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Fencl, Vladimir, et al. "Diagnosis of metabolic acid–base disturbances in critically ill patients." American journal of respiratory and critical care medicine162.6 (2000): 2246-2251.
Moviat, M. A. M., F. M. P. Van Haren, and J. G. Van Der Hoeven. "Conventional or physicochemical approach in intensive care unit patients with metabolic acidosis." Critical Care 7.3 (2003): R41.
Park, M., et al. "Clinical utility of standard base excess in the diagnosis and interpretation of metabolic acidosis in critically ill patients." Brazilian Journal of Medical and Biological Research 41.3 (2008): 241-249.