Question 9.2

The following venous blood results are from a 52-year-old female who has had a prolonged ICU course following extensive surgery for resection of a pelvic sarcoma, complicated by sepsis and multi-organ dysfunction.


Patient Value

Adult Normal Range



7.32 – 7.43


42 mmHg (5.5 kPa)     

27 – 50 (3.5 – 6.6)


44 mmHg (5.8 kPa)

36 – 44 (4.7 – 5.8)


11 mmol/L*

22 – 38

Base Excess

-18 mmol/L*

-3 – +3

O2 Saturation


70 – 80


140 mmol/L

135 – 145


3.8 mmol/L

3.5 – 5.2


119 mmol/L*

95 – 110

Calcium Ionised      

1.30 mmol/L

1.12 – 1.32


10.6 mmol/L*

3.0 – 5.4


1.0 mmol/L

< 1.5


116 g/L

115 – 160


9.3 mmol/L*

3.0 – 8.0


244 mmol/L*

45 – 90

a) State the acid-base disturbance in the above results.

b) List three likely explanations for the acid-base status.

(30% marks)

[Click here to toggle visibility of the answers]

College answer

Not available.


This question, with the exception of trivial changes in the question wording, is identical to Question 4.2 from the first paper of 2016.

a) State the acid-base disturbance in the above results.

Let us dissect these results systematically.

  1. The A-a gradient cannot be estimated, as the gas is venous.
  2. There is acidaemia.
  3. The PaCO2 is not compensating for the acidaemia.
  4. The SBE is -18, suggesting a severe metabolic acidosis
  5. The respiratory compensation is inadequate- the expected PaCO2 (11 × 1.5) + 8 = 24.5mmHg, according to the Boston rules. Thus, there is also a respiratory acidosis.
    (Copenhagen rules can also be applied, and yield an expected PaCO2 of 22 mmHg)
  6. The anion gap is  140 - (119+11) = 10, though in the last version of this question the college gave us a pre-calculated anion gap, and it was 14 (because they included potassium in their calculation). In either case, the anion gap is essentially normal, and the delta ratio is either #DIV/0! or 0.15. 

Thus, this is a pure NAGMA and a respiratory acidosis. The interesting question of whether you can call respiratory acidosis on the basis of a venous CO2  is debated in the discussion section from Question 4.2 (tl;dr: yes, it's usually reasonable, but not when the patient is as shocked as this, where the A-v CO2 difference could be quite large).

b) List three likely explanations for the acid-base status.

The college only asked for possible explanations of the acid-base disturbance. The generic causes of NAGMA are given below. Any of them could potentially be applicable.


Ilkiw, Jan E., R. J. Rose, and I. C. A. Martin. "A Comparison of Simultaneously Collected Arterial, Mixed Venous, Jugular Venous and Cephalic Venous Blood Samples in the Assessment of Blood‐Gas and Acid‐Base Status in the Dog." Journal of veterinary internal medicine 5.5 (1991): 294-298.

SIGGAARD‐ANDERSEN, Ole, and Ivar H. Gøthgen. "Oxygen and acid‐base parameters of arterial and mixed venous blood, relevant versus redundant." Acta Anaesthesiologica Scandinavica 39.s107 (1995): 21-27.

Griffith, K. K., et al. "Mixed venous blood-gas composition in experimentally induced acid-base disturbances." Heart & lung: the journal of critical care 12.6 (1983): 581.

Berner, Barbara J. "The Use of mixed venous blood to assess acid-base status in states of decreased cardiac output when respiration is controlled." (1983).

Murphy, Janet A. "The use of mixed venous blood for assessment of acid-base status in states of decreased cardiac output." (1982).

Mallat, Jihad, et al. "Use of venous-to-arterial carbon dioxide tension difference to guide resuscitation therapy in septic shock." World journal of critical care medicine 5.1 (2016): 47.