Question 6.1

The following blood gases, electrolytes and full blood count relate to a 32-year-old woman post-extubation, following an emergency lower segment Caesarian section at 38 weeks gestation for foetal distress during labour:

Parameter	Result	Normal Values
Barometric pressure	760 mmHg
FiO2	0.5
pH	7.31*	7.35 – 7.45
PO2	150 mmHg (19.7 kPa)
PCO2	42 mmHg (5.5 kPa)	35 – 45 (4.6 – 6.0)
HCO3	20.3 mmol/L*	22 – 27
Standard BXS	-5.0 mmol/L*	-2 – +2
Sodium	137 mmol/L	135 – 145
Potassium	4.3 mmol/L	3.2 – 4.5
Chloride	106 mmol/L	100 – 110
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	1.8 x 109/L	1.5 – 4.0

Comment on and interpret the arterial blood gases and the acid-base status.
What is the significance of the haemoglobin concentration and white cell count?

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College Answer

Raised A-a gradient of 154 mmHg suggestion shunt and/or V/Q mismatch. Potential explanations are loss of FRC post abdominal surgery, segmental collapse/consolidation or aspiration.

Acute respiratory acidosis – normal PCO2 for 38 weeks gestation is 30 mmHg with compensatory reduction in HCO3. CO2 retention is possibly due to pain, narcotics and/or sedation from anaesthetic agents

Normal anion gap

Anaemia and leukocytosis – mild anaemia is physiological in pregnancy. Neutrophil leukocytosis is a normal feature during labour and early post-partum.

Discussion

This question is very similar to Question 6.2 from the first paper of 2013.

Let us dissect these results systematically.

The A-a gradient is high;
PAO2 = (0.5 × 713) - (42 × 1.25) = 304
Thus, A-a = (304-150) = 154mmHg.
There is acidaemia
The PaCO2 is contributing to the acidosis
The SBE is -5, suggesting a metabolic acidosis
The respiratory compensation is inadequate - the expected PaCO₂(20.3 × 1.5) + 8 = 38.45mmHg, and thus there is also a mild respiratory acidosis (especially considering that in pregnancy the normal CO₂ value is around 30mmHg)
The anion gap is normal:
(137) - (106 + 20) = 11, or 15.3 when calculated with potassium

Thus, the main disorder here is respiratory acidosis (as the metabolic contribution is minimal - the bicarbonate is close to its normal value in pregancy).

The leukocytosis and anaemia are quite normal peripurpureal findings.

References

Oh's Intensive Care manual:

Chapter 64 (pp. 684) General obstetric emergencies by Winnie TP Wan and Tony Gin

Chapter 65 (pp. 692) Severe pre-existing disease in pregnancy by Jeremy P Campbell and Steve M Yentis

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

Milne, J. A. "The respiratory response to pregnancy." Postgraduate medical journal 55.643 (1979): 318-324.

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