Tuesday, September 4, 2007

Osmolar Gap

The set up

Patient without a significant medical history is admitted to the hospital comatose. The immediate differential includes alcohol ingestion

Na 140
K 4.0
Cl 99
HCO3 25
BUN 38
Cr 0.7
Glucose 90
7.34 / 47 / 167

Ethanol 574 mg/dL
Serum osmolality 442

Step one

What is the primary acid-base disorder:
The pCO2 is up and so is the bicarb, so this is respiratory acidosis.

Step two

Is the compensation appropriate:
for every 10 the pCO2 is increased the bicarb should rise 1 if the disease is acute and 3 if it is chronic. In this case we presume the respiratory disorder is due to the intoxication so it is acute, so the bicarb should rise 0.7 or close to one because the pCO2 is 7 above 40 (normal). The actual bicarbonate is 25 so this is an appropriately compensated acute respiratory acidosis.

If this patient had chronic respiratory acidosis, then the bicarbonate should rise to 26 (0.7 x 3 =2.1).

Step three

Is there an anion gap?
140 - (99+25) = 16. Yes. But it is very small.

Step four

Is there an osmolar gap?

Calculated osmolality = 2 x Na + Glucose / 18 + BUN / 2.8 + Ethanol / 3.7
2x140 + 90/18 + 15/2.8 + 574/3.7 = 280+5+5.4+155 = 445.5

Osmolar Gap = Measured Osm - Calculated osmolality
Osmolar Gap = 442 - 445 = -3

Step five

No significant anion gap and no osmolar gap means that this is just ethanol toxicity.
Many medical calculators use 4.6 as the divisor for the osmolar gap. However empiric data shows that ethanol does not act as ideal solute and the divisor should be 3.7. If you use 4.6 the osmolar gap comes out to be: 442 - 415 = 26.

Step six

This is simple alcohol intoxication. No indication for fomepizole or dialysis. Ethanol is highly dialyzable. The indications for dialysis is hemodynamic instability despite pressers and volume resuscitation. This patient has depressed mental status and depressed respiration. The treatment for this is supportive care, not dialysis.
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