A 35-year-old woman with pre-eclampsia is admitted to 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 value    Normal range
 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      -2.0 – +2.0         
Sodium                        137 mmol/l     135 – 145               
 Potassium                        4.3 mmol/l   3.5 – 5.0                
 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) Describe and explain the acid-base status

b) Calculate and interpret the A-a gradient

c) What is the likely significance of the anaemia and the leukocytosis?

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College Answer

a)
Acute respiratory acidosis

At 38 weeks pregnancy the normal PaCO2 is <30 mmHg with a compensatory reduction in bicarbonate. The blood gases therefore indicate acute CO2 retention probably due to pain and narcotics. 
In the non-pregnant patient these results would indicate an uncompensated normal anion gap metabolic acidosis.

b)
A-a gradient – this is raised at 154 mmHg, suggesting shunt and/or V/Q mismatch. Possible explanations are the loss of FRC after abdominal surgery, segmental collapse/consolidation or aspiration

c)
Hb and WCC – the mild anaemia is physiological in pregnancy and the neutrophil leukocytosis is a normal feature during labour and early post-partum.

Discussion

This ABG interpretation question is testing the candidate's knowledge of normal changes in the biochemistry of the pregnant woman.

This question closely resembles Question 18 from the first paper of 2006.

Instead of trying to justify why one identical question ended up in the O&G section and the other among the acid-base questions, I will instead focus on the acid-base and gas exchange abnormalities, for a second forgetting that the patient is pregnant. The obstetric flavour is dealt with in the other version of this discussion section.

For b)

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.

That makes one think: is the pH influenced by any other disorder? One of the standard equations comes to mind. Every 1mmHg of change in Pa CO2 leads to a 0.008 change in pH. The use of the standard equation yields an expected pH of 7.304 for this 12mmHg change in CO2- very close to the measured pH (7.31)

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

Now, as for the A-a gradient:

A-a gradient = (FiO2 x (760-47) - (PCO2 x 1.25) - PaO2

The humidity is always 100% so the left side of the equation is always FiO2 x 713.

So: (0.5 x 713) - (42 x 1.25)

Thus, the alveolar oxygen concentration is 303 mmHg, and the A-a gradient is 153 (303 - 150)

Given that there is a history of abdominal surgery and pregnancy, the causes of this could be
- Post-operative atelectasis 
- Aspiration 
- pulmonary thromboembolism or amniotic fluid embolism

for c), I note that a 15% drop in Hb is normal in pregnancy.

Normal acid-base changes in pregnancy are discussed elsewhere.

References

References

For a review of physiological changes of pregnancy, one is directed to any obstetrics textbook. I found Obstetric Evidence Based Guidelines By Vincenzo Berghella to have a nice chapter on it.

 

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