Question 6.2

A 29-year-old female is admitted to ICU extubated following an emergency Caesarian section at 38 weeks gestation for pre-eclampsia and failure to progress. The following data were taken on admission to ICU: 



Normal Adult Range

Barometric Pressure

760 mmHg (100 kPa)





7.35 – 7.45


42 mmHg (5.6 kPa)

35– 45 (4.6 – 5.9)


110 mmHg (14.5 kPa)


20.5 mmol/L

24– 32

Standard Base Excess*

-4.9 mmol/L

-2.0 – +2.0

Comment on this arterial blood gas report and explain the abnormalities.

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College Answer

  1. a)

Increased A-a DO2 secondary to decreased FRC post surgery or possible collapse/consolidation or aspiration or pulmonary oedema.

Acute respiratory acidosis on background compensated respiratory alkalosis of pregnancy. CO2 retention secondary to e.g. hypoventilation post anaesthetic.


Let us dissect these results systematically.

  1. The A-a gradient is high:
    PAO2 = (0.4 × 713) - (42 × 1.25) = 232.7
    Thus, A-a = ( 232.7 - 110) = 122.7mmHg.
  2. There is acidaemia
  3. The PaCO2 is contributing to the acidosis(if one recalls that in pregnancy, the PaCO2 is usually somewhat lower than normal)
  4. The SBE is -4.9, suggesting a metabolic acidosis
  5. The respiratory compensation is inadequate - the expected PaCO2(20.5 × 1.5) + 8 = 38.75mmHg, and thus there is also a mild respiratory acidosis. 
    One can also view this in terms of metabolic compensation. The chronic respiratory alkalosis of pregnancy drives the PaCO2 down to about 32-30mmHg, and thus the bicarbonate decreases by 5mmol/L to 19-20mmol/L. Knowing this normal range, one can surmise that in this patient the bicarbonate is actually within the normal range.
  6. The anion gap and delta gap cannot be calculated

Thus: this post-LSCS patient is hypoxic and acidotic, with a predominantly respiratory acidosis. This is not a completely unexpected picture. If she had a spinal, one would be tempted to check her sensory level, to ensure that the respiratory muscles are unaffected. However, the college tells us there was a general anaesthetic. Atelectasis, loss of FRC and opiates are to blame.


Loverro, G., et al. "Indications and outcome for intensive care unit admission during puerperium." Archives of gynecology and obstetrics 265.4 (2001): 195-198.