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
Let us dissect these results systematically.
The college suggests the following explanations for this gas:
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.
Pavlakis, Steven G., et al. "Mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes: a distinctive clinical syndrome." Annals of neurology 16.4 (1984): 481-488.
Orringer, Carl E., et al. "Natural history of lactic acidosis after grand-mal seizures: a model for the study of an anion-gap acidosis not associated with hyperkalemia." New England Journal of Medicine 297.15 (1977): 796-799.
Bateman, Lisa M., Mark Spitz, and Masud Seyal. "Ictal hypoventilation contributes to cardiac arrhythmia and SUDEP: Report on two deaths in video‐EEG–monitored patients." Epilepsia 51.5 (2010): 916-920.