# 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:

 Test Value Normal Adult Range Barometric Pressure 760 mmHg (100 kPa) FiO2 0.4 pH* 7.31 7.35 – 7.45 pCO2 42 mmHg (5.6 kPa) 35– 45 (4.6 – 5.9) pO2 110 mmHg (14.5 kPa) Bicarbonate* 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.

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.

## Discussion

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.

## References

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