# Question 9.2

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.

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.