Study Questions 1.6 Maintenance Of Anaesthesia

Front 1

What are teh 3 benefits of using inhalational agents for maintenace of anaesthesia?

Back 1

1. Most of the commonly used agents have a short onset of action. This allows rapid change in depth or anaesthesiaby adjusting the amount of gas being delivered.

2. Most of the commonly used agents are relatively short acting compared to the IV agents regardless of duration of anaesthesia. Thus onset and recovery is generally rapis and predictable.

3. These agents are eliminated by exhalation and thus require minimal metabolism. Asa result effects of these agents are rapidly eliminated once the delivery of the agent has been stopped.

Front 2

How do inhalational agents produce anaesthesia?

Back 2

They interfere with the nerve excitation and transmission of impulses at many sites within the CNS. These sites include cerebral cortex, RAS, basal ganglia, cerebellum, medulla, and motor pathways in the spinal cord.

Front 3

What influences the amount of anaesthetic vapour available for delivery to patient?

Back 3

For an agent to be inhaled they have to be in gas or vapour form . The difference between them is at room temperature and pressure, a gas exist as gas while a vapour is defined as a gaseous phase of a substance that is liquid at room temperature.

SVP and boiling points of an agent determines the amount being delivered to the patient.

Front 4

What is vapour pressure?

Back 4

It is pressure exerted by the molecules of the vapour on the wall it is contained in.

Front 5

What is saturated vapour pressure?

Back 5

The amount of an inhalational agent present as vapour is measured as the vapour pressure. The maximum amount of an agent that can be present as a vapour at a particular temperature and pressure . Measured as percentage of atmospheric pressure at 20C . Most agentshave high SVP and hence have to be diluted with oxygen.

Front 6

What determines the amount of vapour present? Use desflourane as an example.

Back 6

Boiling point of desflourane is 23.5 C which is close to room temperature. As a result the amount of vapour present may vary dramatically with small changes in room temperature. To ensure accurate delivery of desflourane , heated vaporizers is used to ensure that it is in vapour form all the time to enable accurate calculation of the amount of vapour being delivered to the patient.

Front 7

Describe the delivery of gases at the start of anaesthesia. 3 steps!

Back 7

Initially there is a pressure gradient from the alveoli to the blood and tissues of the animal which causes the anaesthetic gasses to diffuse acorss the alveoli membrane into the pulmonary circulaton > tissues.

Once in the tissues, the agents diffuses across the pressure gradient from blood into tissues.

Once the required pressure is acheived in the brain, anaesthesia occurs.

Front 8

What 3 factors influences the uptake of the anaesthetic agents?

Back 8

1. Concentration of anaesthetic agents being delivered to the animal.

2. Uptake of anaesthetic gases into the lungs.

3. Transfer of anaesthetic gases from alveoli to circulation.

Front 9

What 4 factors influences the inspired concentration to the patient?

Back 9

1)oxygen flow,
2) vaporiser settings,
3) position of vapouriser in the circuit and t
4) The type of breathing system used.

Front 10

What factors influences the uptake of anaesthetic gases by the lungs?

Back 10

Tidal volume must be sufficient size to ventilate the alveoli. Tidal volume could be affected by panting. Reduce tidal volume= reduced anaesthetic agent available.

Front 11

What are the 3 factors that afect transfer of anaesthetic gases from alveoli to circulation?

Back 11

1. Solubility and partition co-efficient
2. Pulmonary perfusion
3. V/q mismatch.

Front 12

What are the 2 main partition co-efficients used to describe the relative solubility of the anaesthetic gases? Which has the greatest effect on the onset and offset of each agent?

Back 12

blood/gas (B/G) << greatest effect.
oil/gas (O/G)

Front 13

what does the blood gas partition co-efficient describes? 3

Back 13

It describes the relative solubility of an inhalational agent in blood compared with gas.

In the body this generally refers to the relative solubility in the alveolar gas compared to pulmonary capillary blood.

At equilibrium the B:G coefficient describes the way in which the molecule partitions itself between the blood compartment (pulmonary vasculature) and the gas compartment (alveoli).

The lower the B:G coefficient the faster equilibration is acheived (and therefore the faster the uptake of the anaesthetic agent).

Front 14

How does the adequacy of perfusion ( compare high and low perfusion) of the pulmonary circulation influence the uptake of anaesthetic agent. ??

Back 14

If cardiac output is high, agent delivered to alveoli will be removed rapidly preventing the pressure in the alveoli from increasing. This will delay the development of the presseure gradient required to drive the transfer of anaesthetic agent from the alveoli to blood and tissues.
Reduced perfusion = slower removal of agentand pressure in alveoli increases. This can lead to a dangerously high level of anaesthesia being achieved to rapidly.

Front 15

What is V/Q mismatch?

Back 15

When alveoli collapse, it creates a shunt which means that blood perfusing the collapsed alveoli contains no agent and thus decreased uptake of agent into the lungs, blood containing no agent dilutes blood containing agent resulting in lower concentration of agent in the systemic blood. As a result the concentration gradient is lower and onset in anaesthesia is slower. Alveolar collapse is associated with small lung volumes that occurs when GIT is distended.

Front 16

What 2 factors does the amount of anaesthetic agent delivered to the body tissues depend on?

Back 16

Total cardiac output and relative perfusion of each organ.

Front 17

When an animal is in hypovolaemic shock would we expect faster or slower onset of anaesthesia? Why?

Back 17

Fast because when CO is low, more of the circulation is contained within the central vessels which is delivered to vital organs like the brain.

Front 18

When an animal is in pain would we expect faster or slower onset of anaesthesia? Why?

Back 18

Slow. Because cardiac output is high and circulation will be distributed throughout the body, relatively less anaesthetic agent available.

Front 19

What is MAC and what MAC a measure of?

Back 19

MAC= minimum alveolar concentration. MAC is deifined as th eminimum concentration of an inhalational agent that prevents response to specified stimulus in 50% of the patients.

MAC is a measure of potency which is defined as the dose of an agent required to achieve a specific effect. For anaesthetic agents, the more potent an agent is the lower the does required to produce anaesthesia.
O:G coefficient and MAC are inversely related.

Front 20

What are the pharmalogical factors that decrease MAC ? (3)

Back 20

1) Any agent that decreases the activity of sympathetic nervous system

2) Administration of sedtive and analgesia agents (alpha 2 agonists, phenothiazines, benzodiazepines, opiods, N2O.

3) Increased activity of the sympathetic nervous system acts to increase MAC.

Front 21

What are the 6 physiological factors that decrease MAC?

Back 21

1) Decrease body temperature
2) Marked decreased in blood pressure
3) decreased thyroid function
4)old age
5) pregnancy
6) illness

Front 22

What 3 factors increase MAC?

Back 22

1) young animals
2) hyperthermia
3) increased thyroid function

Front 23

What is MAC not affected by? 3

Back 23

1) Elevated PaCO2,
2) decreased PaO2
3)Mild hypotension or anaemia.

Front 24

what are the 8 characteristics of an ideal inhalational agent?

Back 24

1) Does not degrade in presence of heat or light
2) Does not react or dissolve in rubber, plastic and other materials
3) non- flammable and non- explosive
4) allow rapid, smooth and predictable onset and recovery of anaesthesia.
5) cause minimal toxicity particularly
6) minimally depressant on function of heart and lungs
7) eliminated completely unchanged by exhalation
8)compatible with all other anaesthetic agents used during anaesthesia

Front 25

What is the rate of onset of NO2?

Back 25

It has low blood:gas partition co-efficient (0.7) This results in rapid increase in alveolar concentration. Extremely rapid onset and offset.

Front 26

Why is NO2 a poor anaesthetic agent?

Back 26

low lipid solubility

Front 27

list 5 characteristics of NO2

Back 27

Gas at 20 C (high BP)

Rapid onset/offset (B:G = 1.4)

Poor anaesthetic (high MAC 104%)
Elimination: 100% exhalation

Good analgesia

Teratogenic

Front 28

What are the adverse effects of NO2?2

Back 28

Hypoxaemia
Inspired concentration > 70% decreases inspired oxygen
Diffusion hypoxia .
2. Distension of gas filled space

Front 29

What are the 4 contraindications for N2O?

Back 29

CNS disease
GDV
Respiratory disease
Cows and horses

Front 30

What are the 4 important characteristics of Methoxyflurane?
- solubility
- vaporiser
- potency
-onset

Back 30

Highly soluble in rubber and plastic

Uncalibrated vaporiser ( SVP 23 mmHg)

Extremely potent (MAC 0.23%)

Very slow onset ( BG = 13)

Front 31

What are the 5 physiological effects of Methoxyflurane?

Back 31

1. Toxic to kidneys
2. High liver metabolism
3.Decrease minute ventilation
4. Increased ICP
5. Increased Arrthmias , Decreased CO

Front 32

What are the 4 physiological effects of methoxyflurane?

Back 32

1. Decrease minute ventilation
2. increased ICP
3. Arrythmias
4. Decreased CO

Front 33

What are the 3 clinical use of meth?

Back 33

Not commonly used

Mostly used in research laboratories

Provides good analgesia

Front 34

What are the 3 contraindications for meth?

Back 34

1. Heart disease
2. Liver disease
3. CNS disease

Front 35

What are the 8 important characteristics of halothane?
- form at room temperature
-what type of vaporiser
- onset
- potency
- metabolism
- toxicity
- other?

Back 35

Liquid at room temperature
Degraded by light
Precision vaporiser (SVP 243 mmHg)
Very potent (MAC = 0.8-0.9)
Relatively slow onset (B:G = 2.5)
Binds plastic and rubber
Metabolism 20% hepatic
Toxicity: liver ???

Front 36

What are the 8 physiological effects of halothane?

Back 36

1. Increased ICP
2. Interferes with blood flow regulation.
3. Arrthmias
4. Decreased CO
5. Decreased minute ventilation
6. Increased CO2 Threshold
7. Non irritant
8. Relaxes airways.

Front 37

What are the 2 contraindications of halothane?

Back 37

CNS disease e.g head trauma, brain tumours


Liver disease

Front 38

List 4 characteristics of Isoflurane.
- form at room temperature?
- potency
- onset
- elimination?

Back 38

Liquid at room temperature
Precision vaporiser: (SVP 240 mmHg)
Less potent than halothane (MAC 1.2-1.6)
Quicker onset/offset halothane (B:G 1.46)
Elimination: exhaled gases

Front 39

What are the physiological effects of iso? 2

Back 39

As with halothane but irritant and pungent

Front 40

What are the 6 characteristics of sevoflurane?
- form at room temperature
- onset
-potency
- elimination
- toxicity

Back 40

Liquid at room temperature

Precision vaporiser (SVP: 160)

Onset/offset > isoflurane (B:G 0.68)

Potency < isoflurane (MAC = 2.3-2.6)

Elimination: exhalation

Not toxic at clinical doses

Front 41

What are the physiological effects of sevoflurane? 3

Back 41

LIke halothane but

1. Decreased BP
2. Non irritant
3. Non pungent

Front 42

What are the 3 clinical uses of sevoflurane?

Back 42

Can be used in healthy and critically ill animals

Additional cost limits use

Most common use: chamber/mask induce cats

Front 43

What are the 6 characteristics of desflurane?
- form
- vaporiser
- onset
- potency
- elimination
- toxicity

Back 43

Liquid at room temperature

Heated precision vaporiser (SVP= 700)

Onset/offset > sevoflurane (B:G 0.42)

< potent than sevoflurane (MAC 7-8%)

Elimination: expired gases

Not toxic at clinical doses

Front 44

What are the clinical use and caution of using desflurane?

Back 44

Clinical use
Healthy and critically ill animals

Cost of drug and vaporiser limits use

Caution
Careful use in animals with brain disease/injury