Pre-Anaesthetic Medication

Question 1

Aims of pre-anaesthetic medication?

Answer 1

-Facilitate anaesthesia induction, less dose required

-Reduce anxiety and pain(anxiolytic and analgesic)

-Promote amnesia(memory loss)

-Minimise secretions

-Reduce gastric acidity and acid volume

-Reduce nausea and vomitting(antiemitic)

-Suppress vagal and autonomic reflex

Question 2

Relieve fear/anxiety(sedative hypnotic and anxiolytic agents): Benzodiazepines?

Answer 2

Diazepam(long DOA), LORAZEPAM(modetrate DOA), Midazolam(short DOA)

MOA: Binds to allosteric site of GABAa Receptors, GABAa channel easier to open by GABA

GABAa open more frequently

more Cl- influx
hyperpolarisation
less nerve signals

Adverse effects: weakness,GI symptoms, incoordination, ataxia, confusion, depression, hypotension, decreased heart rate and respiratory rate.

Question 3

Relieve fear/anxiety(sedative hypnotic and anxiolytic agents): Antihistamines?

Answer 3

HYDROXYZINE, Diphenhydramine, Promethazine

MOA: crosses BBB, antagonizes brain H1 receptors, sedation

Adverse effects: Dry mouth, throat, reflex tachycardia, pupil dilation, urinary retention, constipation

-Hallucinations, delirium(high doses)

*Hydroxyzine has lower affinity for muscarinic Ach receptors, lower anticholinergic side effects and better anxiolytic effects

Question 4

Sedation and pre-/post-operative analgesia: Opioids?

Answer 4

Morphine, FENTANYL, Pethidine

MOA:
Binds to presynaptic mu receptors
-close Ca2+ channel, decrease neurotransmitter release, decrease nerve signal

Binds to postsynaptic mu receptors
-open K+ channel, nerve hyperpolarisation, decrease nerve signal

Adverse effects: Euphoria, nausea and vomiting, respiratory depression, constipation, histamine release, vasodilatation

*Fentanyl preferred because of rapid onset and very potent

Question 5

Reduce bronchial and salivary secretions: Anticholinergic agents?

Answer 5

Atropine, Hyoscine, GLYCOPYRROLATE

MOA: Block M3 receptors of exocrine glands(bronchial, salivary) , decrease bronchial and salivary secretion, decrease sweating, fever/flushing.

Adverse effects:
-sedation(M1 blocked)
-tachycardia(M2 blocked)
-nausea and vomiting, constipation, blurry vision, urinary retention(M3 blocked)

*Glycopyrrolate preferred because
-does not cross BBB, no confusion and sedation
-less tachycardia effects compared to atropine

Question 6

Reduce vomiting(antiemetic agents): Dopamine receptor antagonists?

Answer 6

Metoclopramide
-block dopamine D2 and 5-HT3 receptors in the chemical trigger zone of the brain, suppress signalling to vomiting center
-block GI tract’s D2 receptor, enhance Ach release, trigger receptors of GI smooth muscle, improve GI motility and reduce emesis

Adverse effects: Sedation, dizziness, diarrhoea, extrapyramidal symptoms(worsen symptoms in Parkinson’s disease pts)

DROPERIDOL
-selectively inhibit peripheral dopamine D2 receptor

Adverse effects: Dry mouth, abdominal cramps, hyper-prolactinemia

*Droperidol preferred because doesn’t aggravate extrapyramidal symptoms

Question 7

Other agents that reduce vomiting?

Answer 7

H1 receptor antagonist- hydroxyzine

Muscarinic antagonist- Hyoscine

Question 8

Reduce gastric acidity and acid volume?

Answer 8

SODIUM CITRATE(buffering agent), Ranitidine, CIMETIDINE(block histamine H2 receptor, decrease gastric secretion and acidity)

Adverse effects:
-mild and transient
diarrhea, fatigue, constipation, muscle pain

Question 9

Why drugs to reduce gastric acidity and acid volume?

Answer 9

Patients undergoing prolonged operation, caesarean section, obese patients are at increased risk of gastric regurgitation and aspiration pneumonia

Question 10

Drugs prescribed during anaesthesia induction?

Answer 10

Muscle relaxants
-non-depolarising neuromuscular blocking agents (d-tubocurarine, atracurium, rocuronium, vecuronium)
-facilitate endotracheal intubation in wisdom teeth extraction or extensive dental surgeries
*depolarising neuromuscular blocking agent not encouraged due to the risk in causing muscle pain

Question 11

How to avoid using neuromuscular blocking agents?

Answer 11

-Intubating patient whilst breathing spontaneously in a deep plane of anaesthesia

-Administering a high dose of alfentanil(painkiller)

*maintenance of a deep plane of anaesthesia throughout the procedure will be necessary to facilitate tolerance of the endotracheal tube (risk: respiratory depression and hypercabia)

Question 12

Drugs prescribed during recovery period(after anaesthesia)?

Answer 12

-Sugammadex(cyclodextrin), Neostigmine, both reverse neuromuscular blocking

-Paracetamol, analgesia in simple dental xtraction

-Dexamethasone+NSAIDS( eg. diclofenac), analgesia in more xtensive dental surgery