75 year old male with a back ground of metastatic prostate cancer, presents with marked shortness of breath.

Metabolic acidosis pH 7.2, HCO3 13mmol/l

Compensation – Expected pCO2 = 1.5 x HCO3 +8 = 27.5 mmHg

AG = Na – (Cl+ HCO3) = 19

Delta ratio = Change in AG/ Change in HCO3 =0.64

PAO2 = FIO2 x 713 – CO2/0.8 = 260

A-a gradient : PA02 – PaO2 = 200mmHg (expected 22mmHg for a 75 year old)

The above ABG shows a very unwell patient with a  HAGMA and an underlying respiratory alkalosis (expected PCO2 27mmHg, actual pCO2 20mmHg). The delta gap of 0.6 suggests there is also an underlying NAGMA.  There is also a markedly elevated A-a gradient.  The ABG also shows a markedly elevated lactate and markedly decreased Hb. The creatinine is the upper limit of normal.

In this clinical context the above HAGMA is most likely due to the lactic acidosis. The cause of the lactic acidosis is multifactorial – septic shock (low BP), hypovolaemic shock both due to decreased oral intake and bleeding (low Hb). If the Hb is chronic it will add to the lactic acidosis due to poor oxygen delivery to the tissues. A PE in this context could cause obstructive shock adding to the high lactate.

The associated respiratory alkalosis can be due to underlying PE, pneumonia or ARDS. Central causes are unlikely in this context. The possible cause for the NAGMA might be due to normal saline infusion, RTA secondary to obstructive nephropathy associated with hypercalcaemia due to bone mets, as well as Addisons due to metastatic spread to the adrenals.

The markedly elevated A-a gradient could be due to PE, ARDS or an underlying bronchopneumonia.

The low Hb can be due to metastatic infiltration of the bone marrow, but an upper GI bleed needs to be excluded. An upper GI bleed in this context can be from PUD secondary to hypercalcaemia.

Ceiling of care discussion needs to be had with the patient and family. The patient needs to demonstrate the following in order to be deemed competent to make decisions about their own treatment:

-maintain and communicate a choice

• understand relevant information that is conveyed to them

-appreciate the situation and the consequences of different treatment options

-weigh the information in a rational fashion.