A 20 year old male is brought into your ED by police highly agitated

pH 7.60 with PCO2 15mmHg – respiratory alkalosis likely acute

Compensation – expected HCO3 = for very 10 decrease in PCO2 there should be an decrease in HCO3 of 2. Expected HCO3 of 19 mmol/l

AG = Na – (Cl +HCO3) =24

Delta Gap = AG – 12 /24-HCO3 = 1.2

A-a gradient = PAO2 – PaO2. Expected PAO2 = 150 – (PCO2x1.25) =131.25

= 131.25 – 125 = 6.25

The above ABG shows a alkalaemia secondary to a primary respiratory  alkalosis with a coexisting HAGMA.  The delta gap of 1.2 is consistent with a HAGMA. The A-a gradient is within normal limits, there for oxygenation is appropriate.  Sodium, lactate and creatinine are all mildly elevated.

In this clinical context there are a number of causes of the combined respiratory alkalosis and HAGMA. Salicylate poisoning gives this typical VBG, but other causes need to be considered for example stimulants, sepsis with delirium, increased ICP driven by meningitis/encephalitis. The raised lactate, sodium and Cr is likely due to dehydration.