You  are  asked  to  review  a  44  year  old  male  known  epileptic  following  a prolonged  generalised  tonic-clonic  convulsion.     He is  intubated  and ventilated. Arterial blood gas analysis is as follows:

Test

Value

Normal Range

FiO2

0.5

pH*

7.15

7.35 – 7.45

pCO2

35 mmHg (4.6 kPa)

35 – 45 (4.6 – 6)

pO2*

105 mmHg (14 kPa)

75 – 98 (10 – 13)

HCO3-*

10.3 mmol/l

22 – 26

a)  List the abnormalities  on the blood gas and give the most likely cause of each abnormality.

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College Answer

a)  List the abnormalities  on the blood gas and give the most likely cause of each abnormality.

•    Metabolic acidosis – lactic acidosis secondary to prolonged seizures
•    Respiratory  acidosis  (or  inadequate  compensation)  –  central  hypoventilation  or inadequate mechanical ventilation
•    Increased A-a gradient  - aspiration pneumonia

Discussion

Let us dissect these results systematically.

  1. The A-a gradient is high:
    PAO2 = (0.5 × 713) - (35 × 1.25) = 312.75
    Thus, A-a = ( 312.75 - 105) = 207.75mmHg.
    In fact, the PaO2/FiO2 ratio is (105/0.5) = 210, which puts this man's hypoxia in a "mild ARDS" category by the 2012 Berlin definition.
  2. There is acidaemia
  3. The PaCO2 is compensatory
  4. The SBE is not offered, but the bicarbonate is low, suggesting a severe metabolic acidosis
  5. The respiratory compensation is inadequate - the expected PaCO2 = (10.3 × 1.5) + 8 = 23.5 mmHg, and thus there is also a respiratory acidosis
  6. The anion gap cannot be calculated.

The college suggests the following explanations for this gas:

  • Hypoxia = aspiration
  • Hypercapnea = obtundation
  • Metabolic acidosis = seizure-associated rise in lactate.

This explanation is certainly within reason. One could even get into a discussion about MELAS. Often in the postictal state there is still appropriate respiratory compensation for the lactic acidosis, with most postictal patients averaging a PaCO2 around 17mmHg in a 1977 study of post-seizure lactic acidosis. However, postictal hypopnoea is a known cause of respiratory acidosis after a seizure, and may be a factor in SUDEP - Sudden Unexplained Death in Epilepsy.

To consider alternative explanations, this mixed acid-base disturbance can also be accounted for by the complications of toxic alcohol ingestion, a hypoglycaemic episode, opiate overdose, and numerous others.

References