General Anaesthetics

Question 1

What is general anaesthesia ?

Answer 1

Reversible, drug induced loss of consciousness, usually to allow a surgical procedure to be performed. May also involve elements of:

• Analgesia
• Muscle relaxation

Question 2

Identify non-surgical procedure GA may be used in.

Answer 2

At extremes of age, and learning disability patients, for MRI/CT scans

Children, in treatments that require them to be still

Patients for electroconvulsive therapy

Question 3

Identify the main administration routes for general anaesthetic agents.

Answer 3

Intravenous induction

Inhlational induction

Question 4

Identify commonly used IV induction agents. Which of these is most frequently used ?

Answer 4

• Propofol (most frequently use)
• Thiopentone
• Etomidate

Question 5

What is the function of IV induction agents ?

Answer 5

Induce loss of consciousness in one arm brain circulation time

Question 6

Identify commonly used inhlational anesthetic agents. Which of these is most frequently used ?

Answer 6

• Nitrous oxide (gas)
• Isoflurane (vapour)
• Sevoflurane (vapour)
• Desflurane (vapour)

Question 7

What is the function of inhalational anesthetic agents ?

Answer 7

• May be used to induce anaesthesia (children)

* More commonly used to maintain anaesthesia

Question 8

How are inhalational anesthetic agents delivered to the patient ?

Answer 8

Via a breathing circuit

Question 9

Which of the inhalational anesthetic agents can be used to induce anaesthesia ? Which cannot ?

Answer 9

Sevoflurane can be used to induce anesthesia. Isoflurane and Desflurane cause cough and laryngospasm and should not be used to induce general anaesthesia.

Question 10

Identify the two most commonly used drugs to induce anesthesia.

Answer 10

1) Propofol

2) Sevoflurane

Question 11

Which situations may inhalational anesthetics be used to induce anesthesia ?

Answer 11

If worried about compromised airway that may be obstructed, and worried that may not be able to maintain airway when patient is unconscious

This may constitute Total intravenous anesthesia (TIVA), “a technique of general anesthesia which uses a combination of agents given exclusively by the intravenous route without the use of inhalation agents”

Question 12

Is it possible to maintain anesthesia with IV agents ?

Answer 12

Yes, propofol only.

Question 13

Identify the three most commonly used drugs to maintain anesthesia.

Answer 13

Sevoflurane
Other volatile agent
Nitrous Oxide

Question 14

What is Entonox ?

Answer 14

• 50:50 nitrous oxide : oxygen
• Good analgesic agent, used in labour (to manage childbirth) and in trauma
• Self-administered, patient holds mouth piece (safety mechanism, if too drowsy let it go and patient wakes up)

Question 15

Explain the pharmacodynamics of General Anesthetics.

Answer 15

• The more lipid soluble: the more potent: the lower the MAC (minimum alveolar concentration, i.e. concentration required for 50% to fail to respond to surgical stimulus for any one drug)
• The more blood soluble: the slower the onset

IDEAL DRUG insoluble in blood (speedy onset) and lipid soluble (potent)
BUT some drugs don’t fit lipid solubility theory (e.g. Isoflurane)

• Anesthetic agents also act on different receptors to inhibit or excite them (potentiate inhibitory receptors and dampen down excitatory receptors)

Question 16

Identify the main receptors acted upon by anesthetic agents.

Answer 16

GABA A
NMDA Receptors
Sodium Channels
Background Potassium Channels

Question 17

What current research is being done to develop future anesthetic agents ?

Answer 17

• Research using knock-in mice
• However, unlikely to be any new anaesthetics…economics! (small proportion of population will have an anesthetic, and only have one, V of drug used low, no incentive for more drugs to be developed)

Question 18

What are the specific functions of some of the subunits of the GABA receptor wrt anesthetics ?

Answer 18

• Intravenous anaesthetics mediated by β3 subunit

* β2 subunit mediating IV hangover

Question 19

Identify the receptor(s) acted upon by the following drugs:

• Etomidate
• Propofol
• Barbiturates
• Volatile anesthetics
• Nitrous Oxide
• Xenon
• Ketamine

As well as the general effect of each on consciousness and movement.

Answer 19

Refer to slide 21.

Question 20

What are the main uses of Ketamine ?

Answer 20

Useful in field anesthesia:

Very CV stable (no negative inotropic effects, does not drop BP) and does not depress respiration (keeps them breathing). Also very good analgesic. Often in the field, bad accidents and patient lost blood and heading towards shock, and not much equipment to maintain airway or breathing for them.

Question 21

Identify any side effects of Ketamine.

Answer 21

• Emergence phenomena, patients hallucinate afterwards, have bad trips
• In misuse, bladder problems (stone bladder, cannot expand or contract, patients often have to have cystectomies)
• Dissociative “k-hole” (in misuse)

Question 22

Identify anesthetics which are also drugs of misuse.

Answer 22

Ketamine

Nitrous Oxide

Question 23

What is the route of administration when Ketamine is misused ?

Answer 23

Sniffed/snorted

Question 24

Which class of controlled drugs does Ketamine belong to ?

Answer 24

Class B

Question 25

Which parts of the CNS do anesthetic agents act on ?

Answer 25

Anesthetic agents acts on Thalamus, but also cortex, brainstem, and on spinal cord for muscle relaxation

Question 26

Explain the pharmacokinetics behind waking up.

Answer 26

Patients wake up because of redistribution of the drug. -Giving drug into bloodstream (either, by IV injection or inhaled into lungs) -Brain has big blood supply so large amount of drug gets into brain and concentration in brain rises -When brain concentration is high, patient becomes unconscious -When cease administration, concentration in brain falls, drug comes out of brain, re-enters bloodstream, and goes to the intermediate compartment (muscles, skin, other organs), or into vessel poor compartment which is essentially fat (fat acts a large reservoir in body, drugs are lipid soluble). When this occurs, and brain concentration drops, patient wakes up STILL DRUG IN THE BODY, JUST REDISTRIBUTED

Question 27

Briefly state how most anesthetic agents are metabolised, and excreted.

Answer 27

Mosy of drugs metabolised by liver, excreted by kidneys.

Question 28

Identify side effects of most anesthetics.

Answer 28

1) Hangover effect (because anticipate low levels of drug hanging around so patient reaction time slowed) 2) CV effects (virtually all anaesthetic agents will to a greater or lesser effect have a negative inotropic effect on the heart and will reduce systemic vascular resistance, i.e. drop BP) 3) Respiratory Depression (may lead to obstruction of the airway, partly due to lost muscle tone)

Question 29

Identify any anesthetic agents without a negative inotropic effect on the heart.

Answer 29

Ketamine (positive effect on sympathetic system, maintains BP)

Question 30

What is the mechanism of respiratory depression of anesthetic agents ?

Answer 30

IV agents: fall in respiratory rate Inhalational agents: Fall in tidal V Hence minute V falls (for both), pCO2 rises

Question 31

What is the clinical implication of the respiratory depression effect of anesthetic agents ?

Answer 31

Must be prepared to maintain patient airway

Question 32

Identify the properties of an ideal anesthetic agent.

Answer 32

``` • Act rapidly • Pleasant (no pain if IV, good smell if inhalational) • Cheap to manufacture • Stable (soda lime) • Analgesic effect • Amnesic effect • Minimal “hangover” (quick discharge) • NOT Irritant on veins or airways • NOT Emetic • Minimal effects on other systems - Breathing - Cardiovascular • NOT produce toxic metabolites • NOT cause histamine release / anaphylaxis ```

Question 33

What is the function of soda lime with anesthetic agents ?

Answer 33

Use soda lime to absorb CO2 in anesthetic breezing circuits (so want anesthetic agents to interact with that)

Question 34

Why do we want to avoid emesis in the context of anesthetic agents ?

Answer 34

Partially unconscious patient and emesis, may have inhalation aspiration (which may result in pneumonia)

Question 35

Identify the main pros and cons of Thiopentone as an anesthetic agent, and state the clinical situation where it may be used.

Answer 35

THIOPENTONE - Pros: Quicker for getting patients off to sleep - Use if patient in on full stomach and is an aspiration risk

Question 36

Identify the main pros and cons of Etomidate as an anesthetic agent, and state the clinical situation where it may be used.

Answer 36

ETOMIDATE - Pros: CV Stable - Cons: Painful injection + patients gets twitchy and wriggly as unconsciousness occurs - Patient with septic shock or CV unstable in general

Question 37

Identify the main pros and cons of Propofol as an anesthetic agent, and state the clinical situation where it may be used.

Answer 37

PROPOFOL - Pros: Dampens down laryngeal reflex (patients don't go into laryngospasm when intubated or given masks) - Cons: Drops BP and causes respiratory depression - Painful injection

Question 38

To what extent is Ketamine ever given for analgesia ?

Answer 38

Ketamine is a very good analgesic agent, sometimes given in subanaesthetic doses for analgesia.

Question 39

What is the inhalational induction drug of choice ? for obese patients ?

Answer 39

Sevoflurane (least irritant on the airway) Desflurane used in obese patients

Question 40

Identify other non-anesthetic agents which may be used in conjunction with anesthetics. What is their function ?

Answer 40

Midazolam (to give less inductive anesthetic e.g. popofol, also because latter may cause drop in BP) Anti-emetic Morphine (analgesic)

Question 41

Identify the main factors which affect selection of anesthetic drugs and techniques.

Answer 41

1. health and requests of the patient (e.g. if on full stomach use Thiopentone) 2. the properties of the drugs (children, gas or needle ?) 3. the requirements of surgery (does surgery need more muscle relaxant? does it require patient to be fully still?)