A 69-year-old female with a past history of multiple bowel surgeries and severe rheumatoid arthritis presents to the ICU with hypotension. The following results are obtained:


Patient Value

Adult Normal Range





7.35 – 7.45


79.7 mmHg (10.6 kPa)


22.0 mmHg (2.9 kPa)*

35.0 – 45.0 (4.6 – 6.0)




12.0 mmol/L*

22.0 – 26.0

Base Excess

-12.0 mmol/L*

-2.0 – +2.0


3.9 mmol/L*

0.5 – 1.6


133 mmol/L*

135 – 145


5.3 mmol/L*

3.5 – 5.0


109 mmol/L*

95 – 105


4.1 mmol/L

3.5 – 6.0

  1. Describe the acid base abnormality.   (10% marks)
  2. Give three potential causes for this patient’s hypotension consistent with these results. Provide a rationale for each cause.    (30% marks)

[Click here to toggle visibility of the answers]

College answer

Describe the acid base abnormality: (10% marks)

Normal anion gap metabolic acidosis and respiratory alkalosis

Give three potential causes for this patient’s hypotension consistent with this ABG. Provide a rationale for each cause. (30% marks)

  1. Addisonian crisis. Hypotension, normal anion gap acidosis, high potassium and low sodium all fit. Patient likely to be on long term steroid treatment for rheumatoid making her vulnerable to this condition. (Note: if this was secondary adrenal insufficiency the potassium would not normally be raised; )
  2. Hypovolemia from fistula – Na consistent, and history of bowel surgery suggestive. Again, would expect a non-anion gap acidosis.
  3. Hypovolemia from diarrhoea: patients history makes her vulnerable to infective causes especially. NAGMA fits.

Coexistent respiratory alkalosis likely to be secondary to hyperventilation from pain/distress (any other plausible explanation acceptable – note candidates not required to comment on this)

Examiners Comments:

Some candidates paid insufficient attention to the clinical information in the stem, leading to generic responses and inappropriate prioritisation of information. Some failed to list potential causes of hypotension consistent with the ABG and lost potentially easy marks as a result of not slowing down and reading the question.


Let us dissect these data;

  • The A-a gradient is raised:  (0.3 x 713) - (22.0 x 1.25) - 79.7 = 106.7 mmHg
  • There is acidaemia
  • There is a metabolic acidosis (SBE = -12)
  • The CO2 is lower than it should be: the expected value is 28 mmHg (using the SBE method) or 26 mmH (using Winter's formula).
  • The anion gap is (133  + 5.3 ) - (109  + 12.0) = 17.3, or 12 when calculated sans potassium. Considering the college answer is NAGMA, we can assume they also omitted potassium in their calculation and used the older value range for the anion gap. 

Thus: this is a metabolic acidosis, which is either completely or almost completely a normal anion gap phenomenon, with some respiratory alkalosis. The college suggests that this alkalosis may be due to pain or distress, which is plausible. There is an A-a gradient but the patient is not particularly hypoxaemic, i.e. hypoxia is not driving this tachypnoea.

Potential causes for this patient’s hypotension:

The college suggestions make sense, and the college answer is well-reasoned: 

  • Addisonian crisis
  • Hypovolemia from fistula 
  • Hypovolemia from diarrhoea

An alternative suggestion could be basic bog-standard sepsis of abdominal origin. The raised lactate, suggestive history, impaired immunity - these raise sepsis as a possibility. Being resuscitated with saline in the ward would account for the hyperchloraemia.