Question 15.2

Angoff score

5.79

Highest mark

7.5

A 35-year-old female with pre-eclampsia is admitted to the ICU following an emergency Caesarian section under general anaesthesia for failure to progress during labour at 38 weeks gestation. Arterial blood gas, full blood count and electrolytes post extubation are as follows:

Parameter	Patient	Reference
FiO2	0.5
pH	7.31*	7.35 – 7.45
PaO2	150 mmHg (19.7 kPa)
PaCO2	42 mmHg (5.5 kPa)	35 – 45 (4.6 – 6.0)
HCO3	20.1 mmol/L*	22 – 27
Base excess	-5 mmol/L	-2.0 – +2.0
Sodium	137 mmol/L	135 – 145
Potassium	4.3 mmol/L	3.5 – 5.0
Chloride	106 mmol/L*	95 – 105
Haemoglobin	110 g/L*	125 – 165
White cell count	19.8 x 109/L*	4.0 – 11.0
Neutrophils	17.3 x 109/L*	1.8 – 7.5
Lymphocytes	2.5 x 109/L	1.5 – 4.0

a) Explain the acid-base status (2 marks)

b) Calculate and interpret the A-a gradient (2 marks)

c) What is the likely significance of the anaemia and the leukocytosis (2 marks)

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College comments

Syllabus topic/section: 2.1.5 Respiratory Intensive Care: Interpretation of arterial blood gases and 2.1.7 Renal Intensive Care: Blood gas analysis

Discussion:

This question assessed core principles of acid-base physiology but proved challenging for many candidates, as it required applying these principles to term pregnancy. In pregnancy, a mild compensatory metabolic acidosis is physiologically normal due to chronic respiratory alkalosis (driven by progesterone- induced hyperventilation).

In part a) many candidates misinterpreted the presence of metabolic acidosis as pathological. Furthermore, the relatively elevated PaCO₂ (e.g., 42 mmHg vs. the expected 30 mmHg in pregnancy) was overlooked as a critical abnormality. Instead of recognizing this as hypercapnic respiratory acidosis (e.g., from respiratory depression or wound pain), candidates often attributed the elevated PaCO₂ to "inadequate compensation" for a perceived primary metabolic acidosis.

Part b) some candidates either did not interpret the A-a gradient or provided wrong calculations. Candidates are advised to re-visit normal physiology related to different stages of pregnancy.

Interpretation

There is acidaemia
The CO₂ is unexpectedly high, as in late pregnancy it should be trending towards 30.
The base deficit and bicarbonate are fairly normal, as might be expected for this stage of pregnancy
There are no rules of compensation to cover this population, but an unfair misapplication of the common rules yields an expected CO₂ of around 35, which would make this a respiratory acidosis.

b) The A-a gradient is

= (FiO₂ × (760 - 47)) - (PaCO₂ × 1.25) - PaO₂

= (0.5 × (760 - 47)) - (42 × 1.25) - 150

= 304 - 150

= 154 mmHg

It can only be interpreted in one way, which is to say that the hypoxia is not due to hypoventilation. That is literally the only information one can derive from the A-a gradient equation. What the actual cause of the hypoxia is, we were not asked to comment on, but the differentials wuold have to be broad.

c) The anaemia and the leukocytosis are potentially completely normal for the late stage of pregnancy, and not exactly "normal" but totally expected in the contest of surgery.

References

Hegewald, Matthew J., and Robert O. Crapo. "Respiratory physiology in pregnancy." Clinics in chest medicine 32.1 (2011): 1-13.

Fadel, Hossam E., et al. "Normal pregnancy: a model of sustained respiratory alkalosis." Journal of Perinatal Medicine-Official Journal of the WAPM 7.4 (1979): 195-201.

Jensen, Dennis, et al. "Physiological mechanisms of hyperventilation during human pregnancy." Respiratory physiology & neurobiology 161.1 (2008): 76-86.

AWE, ROBERT J., et al. "Arterial oxygenation and alveolar-arterial gradients in term pregnancy." Obstetrics & Gynecology 53.2 (1979): 182-186.

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