A 28-year-old female has presented with a severe asthma attack. She is 26 weeks pregnant. 

        a) Comment on her arterial blood gas result shown below.                                                           (40% marks)

Parameter

Patient Value

Adult Normal Range

FiO2

0.6

pH

7.31*

7.35 – 7.45

pO2 

120 mmHg (16 kPa)

pCO2

42.0 mmHg (5.6 kPa)

35.0 – 45.0 (4.6 – 6.0)

SpO2

98%

Bicarbonate 

20.0 mmol/L*

22.0 – 26.0 

Base Excess 

-4.9 mmol/L*

-2.0 – +2.0 

Lactate 

3.0 mmol/L*

0.5 – 1.6

Sodium 

136 mmol/L

135 – 145 

Potassium 

3.2 mmol/L*

3.5 – 5.0

Chloride 

105 mmol/L

95 – 105

Glucose 

8.1 mmol/L*

3.5 – 6.0

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College answer

a) 

Respiratory acidosis 
Metabolic acidosis 
Normal anion gap High A-a gradient 
Suggests imminent fatigue, as CO2 should be lower in pregnancy. The reduced bicarbonate may indicate chronic compensation for preexisting respiratory alkalosis of pregnancy. 
The elevated lactate and glucose are likely secondary to B2 agonist treatment and stress response. 
 

Discussion

"Comment on her arterial gas result shown below" is a curious choice of SAQ wording, and certainly the (usually essay-related) instruction word  "comment" is not a part of the standard repertoire of CICM SAQ stems (for example, it does not appear in the glossary of terms at the front of the paper). That violation of SAQ design is probably forgivable because the candidates (being intelligent people) would have done past papers before, and will have worked out quickly that the CICM examiners really just wanted them to interpret the results and list the abnormalities.

Thus, in a systematic fashion:

1) There is a widened A-a gradient. The gradient is  (0.6  × 713) - (42 / 0.8) - 120 = 255.3 mmHg

2) There is mild acidaemia

3) There is a mild metabolic acidosis (SBE -4.9)

4) The respiratory compensation in non-existent, as the CO2 in pregnancy is expected to be lower. For a non-pregnant patient, the CO2 should be 35.1 mmHg by the SBE method, or 38 mmHg by Winter's rule. In short, there is respiratory acidosis. 

5) The anion gap calculated with potassium is 14.2, or 11 without it. In other words, it is essentially normal, insofar as it would be embarrassing to try to calculate the delta ratio.

6) Other abnormalities are well explained by the (no doubt continuous) salbutamol.

The salient feature pointed out by the college here is the respiratory acidosis, which- no matter pregnant or not - is a bad sign in severe acute asthma.

References

References