Inhalation anesthesia

Question 1

The ideal inhalation anesthetic should (9)

Answer 1

¡ provide good quality anaesthesia
¡ be practicable to give

¡ non-flammable
¡ stable chemically under all circumstances (in and out of the animal)

¡ allow a rapid induction and recovery
¡ minimal metabolism

¡ have minimal effects on circulation and respiration

¡ be non-toxic for animal and human
¡ no environmental hazard

Question 2

Advantages of Inhalation anaesthesia: (4)

Answer 2

¡ Depth of anaesthesia can be readily altered

¡ Elimination of inhalation agents occurs mainly through the lungs without metabolism

¡ Allows the constant delivery of highly concentrated oxygen

¡ Considered to have a greater margin of safety

Question 3

¡ Disadvantages of Inhalation anaesthesia: (3)

Answer 3

¡ Requires an anaesthetic machine

¡ Induction with inhalation agents alone is slow

¡ Potential for the escape of waste anaesthetic gas into room air

Question 4

The potency of an inhalant anesthetic agent is expressed as

Answer 4

the minimum alveolar concentration
(MAC)

Question 5

MAC means what exactly

Answer 5

alveolar concentration of an anaesthetic
that prevents muscular movement in response to a noxious stimuli in 50% of animals.

The concept of MAC allows a standard comparison of the potency of different inhalant anaesthetics.

Question 6

An inhalant agent concentration of 1 MAC will still allow

Answer 6

50% of animals to move in response to a
noxious stimulation (e.g. surgery).

1.3 times a given MAC value will prevent movement in approximately 95% of anaesthetized animals.

Question 7

The MAC of isoflurane is different to sevo and des how?

Answer 7

isoflurane MAC 1.7 / 1.3

sevo 3.0 / 2.3
des 9.8 / 7.6

This means that isoflurane is more potent than sevo and des.

An anesthetic with a lower MAC value is more potent, meaning it requires a lower concentration to achieve the desired effect compared to an anesthetic with a higher MAC value.

For example, if one anesthetic has a MAC of 1% and another has a MAC of 2%, it means that the second anesthetic is less potent, as it requires a higher concentration to achieve the same level of anesthesia.

Question 8

Factors that increase MAC (require a higher concentration of anesthetic in order to induce anesthesia) (4)

Answer 8

¡ Hyperthermia
¡ Catocholamines/sympathomimetics
(ephedrine)

¡ Hyperthyroidism
¡ Hypernatraemia

Question 9

Factors that decrease MAC (9)

Answer 9

¡ Hypothermia
¡ Hypoxemia
¡ Hypercapnia

¡ Drugs causing CNS depression
¡ Sedatives
- Injectable anaesthetic agents
- Analgesic agents

¡ Pregnancy
¡ Old age

¡ Hypotension
¡ Hypothyroidism

Question 10

The rate of diffusion of an anesthetic into a patient is controlled by

Answer 10

the concentration gradient between the alveolus and the bloodstream, as
well as the lipid solubility of the drug.

¡ During the induction period, the concentration of the agent in the alveoli is high and the concentration in the blood is low.

¡ Diffusion of anaesthetic from the alveoli into the blood is rapid during this period.

Question 11

Tissues with greater blood flow (brain, kidneys) are more quickly saturated with anaesthetic than

Answer 11

tissue with lesser blood flow (muscle, fat).

Because of their high lipid solubility, inhalation agents readily leave the circulation and enter the brain, inducing anaesthesia by increased GABA activity.

Question 12

Because of their high lipid solubility, inhalation agents readily leave the circulation and enter the brain, inducing anaesthesia by

Answer 12

increased GABA activity.

Question 13

Review this image.

Answer 13

Question 14

When the concentration of the inhalation agent administered is reduced or discontinued by adjusting the anesthetic machine vaporizer, the amount of anaesthetic in the alveolus is reduced.

Because the blood level is still high, the
concentration gradient now…

Answer 14

favors the diffusion of anaesthetic from the blood into the alveoli.

Administration of 100% oxygen increases elimination of the anaesthetic from the blood.

As the concentration of the anaesthetic in the blood falls, the agent leaves the brain and the patient wakes up.

Question 15

The blood:gas solubility coefficient (or partition coefficient) is a measure of

Answer 15

the distribution of the inhalation agent between the blood and gas in the
body.

Question 16

Blood:gas solubility coefficient indicates

Answer 16

indicates the speed of induction and recovery.

Question 17

Low blood:gas solubility coefficient = ?

High blood:gas solubility coefficient = ?

Answer 17

low = less soluble, more effect on the brain - fast induction and recovery.

high = more soluble, less effect on the brain - slower induction and recovery.

Question 18

Isoflurane blood:gas solubility coefficient is

Answer 18

low.

This, combined with relatively low tissue solubility, results in rapid induction and recovery.

Recovery is rapid – return of consciousness may occur within 1 - 2 min after isoflurane administration is discontinued.

Question 19

Isoflurane drawbacks (7)

Answer 19

Irritating to respiratory system

¡ Causes marked dose-depended fall in arterial blood pressure and systemic vascular resistance.

¡ Not arrhythmogenic
¡ Depresses respiration

¡ Little hepatic metabolism and very little renal excretion.

¡ Has no analgesic effect
¡ Trigger to malignant hyperthermia

Question 20

Isoflurane anesthesia is induced and maintained at what concentrations?

Answer 20

Anesthesia is induced at a concentration 3 - 5% and maintained in most patients at of 1.5% to 2.5% isoflurane to oxygen.

Question 21

Isoflurane pros (2)

Answer 21

¡ Blood flow to most vital organs is well maintained. (not a drawback, just a detail)

¡ Induces adequate good muscle relaxation

Question 22

Sevoflurane Solubility coefficient is lower than

Answer 22

isoflurane, allowing even more rapid induction.

¡ The differential in recovery between isoflurane and sevoflurane is not marked.

Question 23

describe Sevoflurane

Answer 23

¡ Not so irritant as isoflurane
¡ Best choice for mask induction

¡ Less potent than isoflurane and higher concentrations are required to induce and maintain anesthesia.

¡ Cardiac output is usually well maintained at MAC.

¡ Does not increase cerebral blood flow - can be used in patient with head trauma or brain tumors.

¡ 6 - 8% for mask induction
¡ 2.5 - 4% for maintenance

Question 24

Sevoflurane drawbacks (5)

Answer 24

Dose-dependent depression of both
cardiovascular and respiratory systems.

¡ Hypotension is the most common side effect.

¡ Can be trigger for malignant hyperthermia.

¡ Not arrhythmogenic
¡ Has no analgesic effect

Question 25

Desflurane Has the lowest

Answer 25

solubility coefficient of the volatile
anesthetics.

¡ “Single breath” induction

¡ Irritating to breathe
¡ Less potent than isoflurane

¡ Increases heart rate and blood pressure in humans (called “sympathetic storm”) – but this effect has not been reported in dogs.

¡ Respiratory depression
¡ Trigger to malignant hyperthermia

Question 26

Review this image.

Answer 26

Question 27

Waste anesthetic gas is defined as

Answer 27

gas that is leaked from the anesthetic delivery system into the surrounding environment (includes gas exhaled by patient during recovery).

Question 28

Short-term exposure to high concentrations of volatile agents is associated with

Answer 28

headache, irritability, fatigue, nausea, drowsiness and impaired judgment and coordination.