40 year old female with a background of alcohol abuse presents to ED with a headache, abdominal pain and vomiting. The patient is not on oxygen. Below is the patients ABG:

pH 7.59                                            Na 135 mmol/l

pCO2 18 mmHg                               K 3.6 mmol/l

pO2 107 mmHg                               Cl 101 mmol/l

HCO3 18mmol/l                               BSL 6.2 mmol/l

B/E -1.7                                             Lactate 4.5 mmol/l

Cr 60 umol/l

Describe and interpret the ABG

Answer:

pH 7.59 Moderate alkalaemia

pCO2 18 mmHg – respiratory alkalosis

Expected HCO3 = 0.7xHCO3 + 20 = 32 mmHg, actual HCO3 = 18mmol/l

AG = Na – (Cl+HCO3) = 16

Delta Ratio = Change in AG/Change in HCO3 = 0.6

Expected PAO2 = (713 x FiO2) – (pCO2x1.25) =126

A-a gradient = Expected PAO2 – PaO2 = 19 (Normal for age = Age/4 +4 =14)

The above ABG shows a mixed respiratory alkalosis and HAGMA. The delta ratio of 0.6 also indicates an underlying NAGMA. There is also a low normal potassium and sodium, with a moderately raised lactate and a normal A-a gradient.

A combined respiratory alkalosis and HAGMA in an alcoholic could be caused by hepatic encephalopathy, drugs like stimulants or salicylates. The patient has also complained of a headache, and might be hyperventilating secondary to an underlying brain injury or cerebral oedema.  The cause of the HAGMA is a lactic acidosis which could be due to Type A mechanism like seizures or sepsis, but other Type B causes need to considered like liver failure, thiamine deficiency. The underlying NAGMA in this clinical context might be due to an underlying pancreatic fistula, chloride excess or adrenal insufficiency (sodium is low normal, potassium could be low instead of raised due to vomiting.)