A 30 year old male BIB police found at the shops acting erratically. The RMO accidentally canulates the brachial artery and a ABG is obtained:

pH 7.56                                                             Na 142 mmol/l

pCO2 17mmHg                                                K 4.2 mmol/l

pO2 126mmHg (FiO2 0.21)                             Cl 105 mmol/l

HCO3 15 mmol/l                                              Glucose 5.5 mmol/l

Lactate 3.8mmol/l

Describe and Interpret the ABG

Answer

Alkalaemia pH 7.56

Respiratory alkalosis pCO2 17mmHg

Expected HCO3 = 24 – 2 x (40-pCO2)/10 = 19.4. Actual HCO3 15 mmol/l

AG = Na – (HCO3+CL) = 22

Delta Gap = (AG-12)/ (24-HCO3) = 1.1

Expected PAO2 = (FiO2x713) –(pCO2x1.25) =128.75

A-a gradient = PAO2 – PaO2 = 2.75

The ABG shows a primary respiratory alkalosis with an existing HAGMA.  There is a delta ratio of 1.1 consistent with a HAGMA. There is a raised lactate and a normal A-a gradient.

The gas in the clinical context is consistent with a primary hyperventilation causing a respiratory alkalosis.  The HAGMA is likely caused by a lactic acidosis that may be due to a toxilogical cause for example stimulants and salicylates, or sepsis with delirium. An intracranial pathology could also be the cause of the respiratory alkalosis resulting in increased ICP and respiratory drive.