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?
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.
- 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
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.