Expert: Dr Ian Jackson - please note UK based - 12/6/2009

Question
QUESTION: I'm a night nurse on a med-surg floor (I'm a new grad).  Recently, I had an epileptic patient with a serum phenytoin level of only 5.7 who was scheduled to have a hip replacement surgery the next morning at 0730.  MD was aware of the patient's phenytoin level, but had ordered no phenytoin beyond the dose the patient had already been taking (the dose that had resulted in this low serum level).
As I passed the patient and his information on to the day shift nurse, we asked ourselves the question, "What if the patient were to have a seizure while under general anesthesia?"  If a neuromuscular blocking agent were used, and the patient ventilated, what, if any, would be the effects of a seizure while under general anesthesia?  Also, do inhalation anesthetics and their adjuncts raise the seizure threshold?  Enough so that low serum levels of anticonvulsants are not a deterrent to surgery?

Thank you so much for being available to answer these questions.
Sincerely,
Rebecca

ANSWER: Hi Rebecca
Good questions!
Firstly it is interesting that the levels were checked - we wouldn't in the UK if the patient was stable on therapy and hadn't missed any doses. This is because phenytoin has a plasma half life of 22hrs - so it hangs around a long time and has fairly stable levels. So if your patient hadn't missed their medication then the level is what they were used to. Some seizure types respond well to these lower levels of phenytoin.
So move on to the next bit.
In the UK we seldom paralyse and ventilate patients who are having hip replacements - most are done with sedation under a spinal. However that is side stepping your question - let's imagine the patient is paralysed and ventilated. What is the affect of the anaesthetic agents on seizure threshold?
Induction agents thiopentone, propofol tend to raise the threshold i.e. they can control seizures. Volatile agents are a mixed bag and some of the old ones were felt to lower the threshold - however todays agents don't really effect it much, probably raise it a bit.
It is important to state that paralysis and ventilation does not control seizures e.g. status epilepticus - it merely controls the signs but the seizure continues! Hence on ICU if we are ventilating someone who was having a seizure we like to monitor their brainwaves as continued seizure activity can damage the brain.
I hope this overview helps a bit.
Kind regards
Dr Ian Jackson



---------- FOLLOW-UP ----------

QUESTION: How would a seizure while under general anesthesia affect a patient's cardiac function?  (i.e., does seizure activity reach the brainstem?)  Also, how does continued seizure activity damage the brain, if not via hypoxia?  Is it via hypoglycemia/low ATP levels?

Thank you again for giving your time to answer these questions.
Kind regards,
Rebecca  

Answer
Hi Rebecca
Seizure under general anaesthesia would have no effect on cardiac function. The seizure activity occur at cortical level (hence we see the motor effects) and does not effect brain stem level.
Continued seizure activity damages the brain by increasing metabolic requirements - oxygen and glucose and production of metabolites which causes acidosis. The continued release of neurotransmitters leads to increased calcium entry into the neuronal cells and this is bad news!
Much of the brain damage in head injuries etc revolves around calcium metabolism and its uncontrolled entry into neuronal cell.
Hope this helps
Kind regards
Ian