Question 18.2

You are performing clinical brain death testing on a 63-year-old male. Two arterial blood gas (ABG) results are presented below. ABG 1 was performed immediately prior to testing, and ABG 2 was performed at the end of the apnoea test.

a) Comment on the implication these results have for diagnosing brain death in this patient.  (20% marks)


Patient Value

Adult Normal Range









7.35 – 7.45


110 mmHg (14.7 kPa)

148 mmHg (19.7 kPa)


49.0 mmHg (6.5 kPa)*

62.0 mmHg (8.3 kPa)*

35.0 – 45.0 (4.6 – 6.0)





30.0 mmol/L*

31.0 mmol/L*

22.0 – 26.0 

Base Excess 

5.3 mmol/L*

4.9 mmol/L*

-2.0 – +2.0 


1.8 mmol/L*

1.8 mmol/L*

0.5 – 1.6


151 mmol/L*

152 mmol/L*

135 – 145 


4.2 mmol/L

4.1 mmol/L

3.5 – 5.0


103 mmol/L

102 mmol/L

95 – 105


7.5 mmol/L*

8.1 mmol/L*

3.5 – 6.0

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College answer

Although the CO2 has risen to above 60 mmHg, the pH remains above 7.3, and so brain death cannot be diagnosed. The Na of 152 does not preclude the diagnosis of brain death. 


This question is straight from the CICM ANZICS statement (version 3.2) which reads:

"At the end of the period without mechanical ventilation, apnoea must persist in the presence of an adequate stimulus to spontaneous ventilation, i.e. an arterial PaCO2 > 60 mmHg (8 kPa) and an arterial pH < 7.30"

The key point there is an arterial PaCO2 > 60 mmHg (8 kPa) and an arterial pH < 7.30. Both must be demonstrated in order for the clinical diagnosis of brain death to be valid.  As to why and how this was decided, the ANZICS statement is silent. The American guidelines do not contain this rule.

Interestingly, as a reader had pointed out, there is more material in this blood gas result to discredit the apnoea test result. Consider the pre-test bicarbonate value, which is 31 mmol/L. The PaCO2 is only 49 at this stage, but surely the high baseline bicarbonate means there must be some sort of chronic CO2 retention here. And if so, you'd want the PaCO2 to rise by at least 20 mmHg (i.e. up to 69 mmHg) in order to be able to declare the apnoea test as failed, considering that the ANZICS statement clearly says:

"In patients with pre-existing hypercapnia, it is recommended to wait for a PaCO₂ rise of >20 mmHg (2.7 Kpa) above the chronic level, with a pH <7.30."

This did not happen in the example, and the high initial bicarbonate may also to some extent account for the pH being higher than 7.30 at the end of the test.

 As for the sodium level, the old  ANZICS statement was not prescriptive; "marked derangements" were disqualifying, but there no was mention as to how marked these must be. The new Version 4 of the statement gives the following ranges:

  • Glucose <3 or > 25 mmol/L
  • Sodium <125 or >160 mmol/L
  • phosphate <0.5 mmol/L
  • magnesium <0.5 mmol/L
  • urea > 40 mmol/L
  • "untreated severe hypothyroidism or severe hypoadrenalism"

Other Intensive Care Societies differ slightly;  for example the Irish ICSI guidelines recommend 125-155mmol/L as the acceptable range for clinical testing.  


ANZICS Death and Organ Donation Committee, THE ANZICS STATEMENT ON DEATH AND ORGAN DONATION Edition 3.2 2013

Wijdicks, Eelco FM, et al. "Evidence-based guideline update: determining brain death in adults: report of the Quality Standards Subcommittee of the American Academy of Neurology." Neurology 74.23 (2010): 1911-1918.

Thanks to Simon Baylis for picking up the chronic hypercapnia which was hidden in plain sight here.