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.

Parameter	Patient Value	Adult Normal Range
pH	7.06*	7.32 – 7.43
PCO2	42 mmHg (5.5 kPa)	27 – 50 (3.5 – 6.6)
PO2	44 mmHg (5.8 kPa)	36 – 44 (4.7 – 5.8)
Bicarbonate	11 mmol/L*	22 – 38
Base Excess	-18 mmol/L*	-3 – +3
O2 Saturation	80%	70 – 80
Sodium	140 mmol/L	135 – 145
Potassium	3.8 mmol/L	3.5 – 5.2
Chloride	119 mmol/L*	95 – 110
Calcium Ionised	1.30 mmol/L	1.12 – 1.32
Glucose	10.6 mmol/L*	3.0 – 5.4
Lactate	1.0 mmol/L	< 1.5
Haemoglobin	116 g/L	115 – 160
Urea	9.3 mmol/L*	3.0 – 8.0
Creatinine	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)

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College answer

Not available.

Discussion

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.

The A-a gradient cannot be estimated, as the gas is venous.
There is acidaemia.
The PaCO₂ is not compensating for the acidaemia.
The SBE is -18, suggesting a severe metabolic acidosis
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)
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.

References

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.

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