Lecture 10: Inhalation and IV anaesthetics

Question 1

The triad of anaesthesia consists of?

Answer 1

IV or Volatile agents - Amnesia

Opoids - Autonomic areflexia

Muscle relaxant - immobility

Question 2

How do volatile agents work?

Answer 2

We don’t know

but…

likely due to GABA modulation in the brain and glycine modulation in the spinal cord

Question 3

What is MAC?

Answer 3

Minimum alveolar concentraion (%) producing immobility on standard surgical stimulus (forearm incision) in 50% of patients.

More potent agents have a lower MAC (doesn’t inherently mean they are better)

Is not a taget but rather just a dose index to a clinical effect

Question 4

Dose response curve?

How can you change the dose response curve?

Answer 4

Is very steep and a small increase will heavily increase the effect

This graph will be heavily altered by the use of other drugs

Question 5

How is dosing measured and monitored?

Answer 5

F_{inspired —-minute volume—>} F_{A- —-flow—>} Blood

We monitor the expired breath and then titrate the amount to reach the aquired amount. Remember we can alter the minute volume as the patient is ventilated.

Question 6

Pharmacodynamics of volatile agents? (CNS)

Answer 6

1. hypnosis, immoblity, amnesia
2. decrease CMRO₂ (cerebral metabolic rate) = neuroprotective
3. Dose dependent increase cerebral BF anf ICP meaning care with neurosurgery

Question 7

Pharmacodynamics of volatile agents (CVS)

Answer 7

1. peripheral vasodilation, lower BP
2. HR unchanged

Question 8

Pharmacodynamics of volatile agents (RS)?

Answer 8

1. respiratory depressants - impair ventilatory response to hyoxia and to CO₂
2. Bronchodilation - (though desflurane is an airway irritant)

Question 9

Nitrous oxide use, benefit and adverse effects?

Answer 9

Odourless

low potency (MAC 101%)

Rapid onset

analgesic

AE: nausea + vomiting

Question 10

Isoflurane use, benefits?

Answer 10

Pungent

Pontent (MAC 1.1%)

Itermediate solubility - medium onset speed

very cardiovascular stable

Question 11

Sevoflurane use, benefits and AE?

Answer 11

Non-pungent and least resp depressant

cheap and good for gas induction in children

intermediate potency (MAC 1.7%)

low sol. - rapid onset

AE: reacts with CO₂ so theoretical renal toxicity

Question 12

Desflurane use, benfits

Answer 12

Intermediate potency (MAC 6%)

rapid onset and offset (ideal for long cases)

Pungent and is known to be an airway irritant

Question 13

5 types of IV anaesthetic agents

Answer 13

1. Barbituates (Thiopentone)
2. Phenols (Propfol)
3. Imidazoles (etomidate)
4. Phencyclidine derivitives (ketamine)
5. Benzodiazapines (midazolam)

Question 14

Mechanism of action of IV agents?

Answer 14

Virtually all except ketamine enchance GABA in the brain and hyperpolarise the neurons shutting the brain down

Ketamine binds to PCP receptor to antaganise glutamate, supressing exitation. Also has an analgesic effect as well

Question 15

Pharmacokinetics of IV agents?

Answer 15

Highly lipid solubiluty and cross BBB

Drug from IV blous taken up by VRG organs, then leaves these organs as lean tissues take up drug and conc in blood falls

Offest after single IV dose is therefore due primarily to redistribtion (patient may be wide away even though total amount in body is basically unchanged)

Question 16

Thiopentone?

Answer 16

very rapid (10sec) - eergency c section

rapid offset by redistrubtion

slow clearance - will accumulate

some decr. in PVR and BP (careful in shocked patients)

Resp depression and loss of airway reflexes

CV instability

Question 17

Propofol

Answer 17

moderately rapid onset (30-40sec)

rapid offset by redispribution

Fast clearance - clean offset, no accumulation, can be infused for maintenance

significant decrease in PVR and BP

Resp depression and loss of airway reflexes

Has replaced Thiopentone but still experience CV instability

Question 18

Etomidate

Answer 18

Remarkable CV stability

Less resp depression

BUT: adrenocortical inhinition decreases stress hormone production thought to be useful for recovery from surgery so not used unless needed.

Question 19

Ketamine?

Answer 19

Analgesic

CVS simulant - good for shoced patients

prserves airway reflexes and resp drives

Increases CMRO₂ and CBF and ICP so not good for neurosurgery

puts patients in a dissociated state, emerge slower and complicated by dysphoria (can have nightmares)

Question 20

Total intrvenous anaesthesia?

Answer 20

Avoids inhalation route

Avoids complication of vapours - malignant hyperthermia + POVN (post op vomiting and nausea) + intracranial hypertension

BUT.. = expensive

no agent monitoring (normally monitor expelled breath) but do have EEG to mointor how awake a patient is