aka Metabolic Muddle 007

76 year old male living alone. Found obtunded at home by neighbors. Breathing rapidly and muttering incoherently. No other history available.

Admission arterial blood gas shown (note that despite the labeling this specimen is arterial):


Q1:  Describe the arterial blood gas results

Answer and interpretation
The first step: is the patient acidaemic or alkalaemic?

there is severe acidaemia

The second step: is there a metabolic acidosis or a respiratory acidosis or both?

there is a severe metabolic acidosis

The third step: is there appropriate compensation?
  • The estimated expected CO2 is 1.5xHCO3 + 8
  • i.e. approximately 13 so its pretty close
The fourth step: what is the nature of the metabolic acidosis?

The anion gap is markedly elevated at 35 [(Na+ K+) – (Cl-+HCO3-)]

The fifth step: is there a coexistent normal anion gap acidosis or pre-existing metabolic alkalosis?

If the high anion gap acidosis is the only metabolic disturbance, the bicarbonate drops by the same degree that the anion gap rises.

  • In this case, assuming a normal anion gap of 12, the anion gap has increased by 23 while the HCO3- has decreased by 23.
  • Assuming a normal HCO3- of 26 the bicarbonate has decreased by 23 to finish up at 3.
  • So the high anion gap metabolic acidosis is the only metabolic disturbance.

If the bicarbonate drops less than anticipated, it must have started off at a higher level than you normally expect (i.e. there must be a pre-existing metabolic alkalosis)

If the bicarbonate drops more than anticipated, there must be another source of acidosis (i.e. a co-existent normal anion gap acidosis)

The final step: summarise

There is a severe high anion gap metabolic acidosis (HAGMA) with appropriate respiratory compensation

Q2:  What are the causes of metabolic acidosis with raised anion gap?

Answer and interpretation

The simple way of remembering is to remember only four, using the mnemonic LTKR (“left total knee replacement”):

  • Lactate
  • Toxins
  • Ketones
  • Renal failure

The complicated way is to use the mnemonic CAT-MUDPILES:

  • Cyanide, carbon monoxide
  • Alcoholic ketoacidosis
  • Toluene
  • Methanol, metformin
  • Uraemia
  • Diabetic ketoacidosis
  • Phenformin, pyroglutamic acid, paraldehyde, propylene glycol, paracetamol
  • Iron, isoniazid
  • Lactate (numerous causes)
  • Ethanol, ethylene glycol
  • Salicylates

Q3. What happened in this case?

Case Outcome

In this case the cause was renal failure:

Metabolic Muddle 007 02

Metabolic Muddle 700


Metabolic Muddle

Intensivist in Wellington, New Zealand. Started out in ED, but now feels physically ill whenever he steps foot on the front line. Clinical researcher, kite-surfer  | @DogICUma |

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