Question 3.3

Highest mark
9.7
Pass rate
94.7%
Other SAQs in this topic:
Other SAQs in this paper:

A 65-year-old male is found wandering and incoherent in the street before presenting to hospital. His arterial blood gas is shown below:

Parameter

Patient Value

Adult Normal Range

FiO2

0.3

pH

7.50*

7.35 – 7.45

pO2

105 mmHg (14 kPa)

pCO2

20.0 mmHg (1.9 kPa)*

35.0 – 45.0 (4.6 – 6.0)

SpO2

98%

Bicarbonate

15.0 mmol/L*

22.0 – 26.0

Base Excess

-7.3 mmol/L*

-2.0 to +2.0

Lactate

2.3 mmol/L*

0.5 – 1.6

Sodium

120 mmol/L*

135 – 145

Potassium

3.9 mmol/L

3.5 – 5.0

Chloride

71 mmol/L*

95 – 105

Glucose

11.0 mmol/L*

3.5 – 6.0

a)  Comment on the acid base status.    (30% marks)

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College answer

Respiratory alkalosis, concomitant high anion gap metabolic acidosis (AG 34, delta-delta ratio 2.44) and pre-existing metabolic alkalosis.    (3 marks)

Discussion

Systematically:

  1. The A-a gradient is raised: (0.3 x 713) - (20 x 1.25) - 105 = 83.9 mmHg
  2. There is alkalaemia
  3. The PaCO2 is contributing to the alkalaemia
  4. There is a mild metabolic acidosis: the SBE is -7.0
  5. There is a respiratory alkalosis: the actual PaCO2 is way lower than the expected PaCO2, which is supposed to be (15 ×  1.5) + 8 = 30.5, or 33mmHg by the Copenhagen rules.
  6. The anion gap is significantly elevated:  (120) - (71 + 15) = 34, or 37.9 when calculated with potassium.
  7. The delta ratio, without using potassium and assuming a normal anion gap is 12 and a normal bicarbonate is 24, would therefore be (34 - 12) / (24 - 15) = 2.44. This suggests that there is a pre-existing metabolic alkalosis here.

But why? Why is this 65 yo male wandering and incoherent? How did they come to have this fascinating mixed disorder? And what are all these extra anions? Is it a toxic alcohol? Did this person imbibe a vast quantity of essential oils? Is a salicylate overdose responsible for the respiratory alkalosis?  Exploring these issues would have made a much more interesting question, one which might even test some sort of higher-order abilities. For the record, the ABG interpretation in the "systematically" section above was generated automatically using a script written in Google Sheets.

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