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

a)

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

b)

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.

  1. The A-a gradient is high; 
    PAO2 = (0.5 × 713) - (42 × 1.25) = 304
    Thus, A-a = (304-150) = 154mmHg.
  2. There is acidaemia
  3. The PaCO2 is contributing to the acidosis
  4. The SBE is -5, suggesting a metabolic acidosis
  5. 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)
  6. 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.