General Anesthetics: Intravenous Agents
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intravenous agents
Propofol
- Propofol potentiates the effects of the GABA(A) receptor. Remember that GABA is an inhibitory neurotransmitter.
- It is an antiemetic, which helps prevent postoperative nausea.
- It can produce marked hypotension through reduction in peripheral vascular resistance, and thus it can cause cerebral hypoperfusion.
- It produces a rapid onset and recovery of consciousness due to its lipophilicity and its ability to rapidly redistribute into fat stores.
Propofol infusion syndrome (PRIS)
- The complexities of this syndrome are beyond our scope here, but as a simple heuristic, consider that propofol can cause various toxicities through varying mechanisms – short-term high doses can cause metabolic acidosis from mitochondrial uncoupling; prolonged infusions can interfere with fatty acid oxidation; high doses can cause rhabdomyolysis; and the redistribution into fat cells can produce hypertriglyceridemia.
Thiopental & Methohexital
- Thiopental and methohexital are barbiturates.
- Like propofol, they act on GABA(A) receptors and are highly lipid soluble, so they have a rapid onset.
- They are respiratory and circulatory depressants and promote decreased cerebral blood flow, thus they decrease intracranial pressure.
Etomidate
- Etomidate also acts on GABA(A) receptors and produces rapid onset and recovery.
- Notably, indicate that it is NON-analgesic (it does not have pain suppressing properties).
Midazolam
- Midazolam is a benzodiazepine. Accordingly, it has slower onset and longer duration of action with greater risk of postoperative respiratory depression.
- Flumazenil is a benzodiazepine receptor antagonist that can rapidly help reverse the effects of midazolam.
- Note that flumazenil carries the risk of seizures, so it is used with caution.
Ketamine
- Is an NMDA receptor antagonist.
- It can produce a dissociative anesthesia with preserved consciousness but the patient experiences catatonia, analgesia, and amnesia. Unfortunately, note that possible emergence reactions are common with psychotic, sometimes injurious behaviors. Also, note that it is a cardiovascular stimulant, so it can produce increased intracranial pressure.
Dexmedetomidine
- Dexmedetomidine is a centrally-acting alpha-2 adrenergic agonist that is used as an adjuvant agent to help provide analgesia and hypnotic effects in order to reduce the required doses of other medications.
Opioids
- Note that opioids are often used to help with pain suppression in anesthesia, especially when a general anesthetic cannot be safely administered; we discuss them in detail, elsewhere.