Question 9.1

A previously fit and well 41-year-old male underwent an anterior resection under general anaesthesia with regional blockade. In recovery he required additional analgesia for escalating pain and treatment for nausea, following which he had an apparent seizure.

The following arterial blood gas sample was taken during resuscitation:

Parameter     

Patient Value         

Adult Normal Range   

FiO2

0.6

pH

6.91*

7.35 – 7.45

pCO2

64 mmHg (8.5 kPa)*     

35 – 45 (4.6 – 6.0)

pO2

158 mmHg (21 kPa)*

75 – 98 (10 – 13)

SaO2

96%

Bicarbonate

12 mmol/L*

22 – 26

Base Excess

-18 mmol/L*

-2 – +2

Sodium

145 mmol/L

135 – 145

Potassium

4.1 mmol/L

3.5 – 5.2

Chloride

110 mmol/L

95 – 110

Lactate

16 mmol/L*

< 2

  1. State the acid-base abnormality, and show your calculations for Anion Gap, A-a gradient and Delta gap.
  2. List three reasons for the lactataemia.

(30% marks)

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College answer

Not available.

Discussion

This question is identical to Question 4.1 from the first paper of 2016, except instead of "Give six possible causes for this clinical and biochemical scenario", the question itself was revised into something a lot more reasonable, with clearer expactations.

Let us dissect these results systematically.

  1. The Aa gradient is high; the PaO2 is 347.8 mmHg and so the A-a gradient is 189.8 mmHg
    (713 × 0.6) - (64 × 1.25)
    The P/F ratio is 263.
  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 essentially non-existent - the expected PaCO2 (12 × 1.5) + 8 = 26mmHg, 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 raised:  (145) - (110 + 12) = 23, or 27.1 when calculated with potassium (note that in the old Question 4.1 the college examiners included the potassium value in their anion gap calculation, even though the Chair of the Examination Committee has previously instructed against this in a letter to the candidates).
    The delta ratio, using potassiumless values and assuming a normal anion gap is 12 and a normal bicarbonate is 24, would  be (23 - 12) / (24 - 12) = 0.91

Thus, this is an almost completely pure HAGMA and a respiratory acidosis.

Why the lactate? Only three reasons? 

  • Seizures
  • Sepsis
  • Mesenteric ischaemia

In the last iteration of this SAQ, where six reasons for the clinical scenario were asked for, the college had also included local anaesthetic toxicity,  dystonic drug reaction, intra-abdominal catastrophe, myocardial infarction, anaphylaxis and subarachnoid haemorrhage.

References