Question 18

A 35 year old woman with pre-eclampsia at 38 weeks gestation  is transferred to ICU post lower segment Caesarean section under general anaesthesia (performed because of failure to progress in labour).  Blood gases, electrolytes  and  full blood count  post extubation are as follows:

Normal values

Barometric pressure

760mm Hg







150mm Hg


42 mm Hg





Standard base excess




135 -145



3.2 - 4.5



100 -110



125 - 165


19.8x 109/L

4.0 - 11.0


17.3x 109/L

1.8 - 7.5


1.8x 109/L

1.5 - 4.0

Describe and  explain  the acid-base  status.   

Calculate  and  interpret the A-a gradient.

What is the likely significance of the anaemia  and leucocytosis?

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College Answer

The intention of this question is to test the candidates’ understanding of some important normal alterations in physiology due to pregnancy, and how this affects our interpretation of common ICU

Acid-base status: Acute respiratory acidosis. Anion gap normal. At 38 weeks pregnancy the normal PaCO2 is <30 mm Hg with a compensatory reduction in bicarbonate. The blood gases therefore indicate acute CO2 retention, probably due to pain and narcotics. In the non-pregnant state these values would indicate an uncompensated normal anion gap metabolic acidosis. However these data must be interpreted in the light of the normal changes of pregnancy.

A-a gradient: There is a raised A-a gradient of 154 mm Hg, suggesting shunt and/or V/Q mismatch. Potential explanations are the loss of FRC after abdominal surgery, segmental collapse or consolidation, or aspiration.

Anaemia and leucocytosis: The mild anaemia is physiological in pregnancy, and the neutrophil leucocytosis is a normal feature during labour and early post-partum. This lady has been in labour prior to Caesarean section.

Sixteen out of twenty-six candidates passed this question.


This question is virtually identical to Question 9.2 from the second paper of 2011.

Whatever the reason, one identical question has been placed in the acid base disorders section, and another in the O&G section.

In this incarnation of the discussion section, I will focus on the normal changes which take place during pregnancy.

In summary:

  • pH increases to 7.40-7.47
  • PaCO2 decreases to 30 mmHg
  • PaO2 increases to 105 mmHg
  • HCO3- decreases to 20 mmol/L
  • Maternal 2,3-DPG increases 
  • p50 remains the same because of alkalosis 

There is a mild respiratory acidosis. The normal CO2 of late pregnancy is around 30mmHg, which is generally sustained with a bicarbonate of 20. In this scenario the bicarbonate has not changed, and the CO2 is elevated by 12mmHg. The use of the standard equation yields an expected pH of 7.304 for this change in CO2- very close to the measured pH (7.31), so there really is no metabolic acid-base disturbance at all.

The anion gap is normal if you calculate it without the potassium. It is 15.3 with potassium included, trivially elevated (by 3.3).


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


Carlin, Andrew, and Zarko Alfirevic. "Physiological changes of pregnancy and monitoring." Best Practice & Research Clinical Obstetrics & Gynaecology 22.5 (2008): 801-823.


Chesnutt, Asha N. "Physiology of normal pregnancy." Critical care clinics 20.4 (2004): 609-615.


Silversides, Candice K., and Jack M. Colman. "Physiological changes in pregnancy." Heart Disease in Pregnancy 2 (2007): 7-16.