Inhalational anaesthetic agents

Question 1

What is the difference between a vapour and a gas

Answer 1

A vapour is a substance that can exist as a liquid at room temperature (i.e. room temperature is below its critical temperature). A substance whose critical temperature is below room temperature cannot exist as a liquid at room temperature regardless of the amount of pressure exerted and is considered a gas.

Question 2

What does the dial indicator of ‘1%’ mean

Answer 2

The vapour occupies 1% of the outflow from the vaporizer

Question 3

Define MAC

Answer 3

This is the mean alveolar concentration of the volatile agent required to prevent 50% of patients moving when subjected to a standard midline incision

Question 4

What determines potency: partial pressure of VA administered or concentration of VA delivered?

Answer 4

Partial pressure of volatile agent delivered

Question 5

Why is partial pressure of VA delivered independent of atmospheric pressure

Answer 5

If 1% is dialled into the vaporizer then 1% of isoflurane is delivered at 20ºC and 100kPa (SVP of isoflurane is 32kPa)

Inside the vaporizer:
32kPa/100kPa x 100/1 = 32%

1/100 x 100kPa/1 = 1kPa

If 1% is dialled into the vaporizer then 0.5% of isoflurane is delivered at 20ºC and 200kPa (SVP of isoflurane is 32kPa)

Inside the vaporizer:
32kPa/200kPa x 100/1 = 16%

0.5/100 x 200kPa/1 = 1kPa

Question 6

As vaporizers are calibrated in % and because one atmosphere is approximately 100kPa, there is a simple relationship between % and partial pressure of the agent: 1% = 1kPa. How is it possible for N2O to have a MAC of 103 (above 101.325 =atm pressure)

Answer 6

This means that the MAC of N2O cannot be achieved at atmospheric pressure.

The MAC values are quoted in terms of % concentration but their potency is actually determined by PARTIAL PRESSURE rather than % concentration.

Partial pressure is independent of atmospheric pressure. In a hyperbaric chamber a MAC of 103kPa can be obtained because the atmospheric pressure is increased e.g. 150 kPa

103kPa/150kPa = 69%

Is a concentration of 69% of N2O is delivered to a patient at Pb of 150 –> MAC of 103 kPa is obtained.

Question 7

How do vaporizer adjust for changes in altitude?

Answer 7

They have mechanisms which ensure the delivery of a set partial pressure of anaesthetic agent (not concentration as displayed on the dials. The percentage of agent delivered will vary with altitude but the Partial pressure and the MAC remain unchanged

Question 8

How are MAC values calculated

Answer 8

In volunteers just breathing anaesthetic agent n oxygen. (No other gases present)

Question 9

How does the MAC change when N2O is added?

Answer 9

The MACs are additive e.g. half MAC N2O + half MAC Isoflurane = 1 MAC. (same calculation is releavant for sevo and iso together but this is NOT done in practice

Question 10

What factors reduce MAC

Answer 10

IV anaesthetics used by infusion
N2O
Adjuvant medications (benzodiazepines, opioids, a2-adrenergic agonists)
Acute alcohol intoxication
Chronic amphetamine use (depleted CATS)

Reduced GCS
Hypothermia
Hypothyroidsim
Increasing age

Question 11

What factors increase MAC

Answer 11

Chronic alcohol dependence
Exogenous catecholamine use and stimulus
Anxiety and stress

Hyperthyroidism
Hyperthermia
Young age

Question 12

What effects the speed of onset of anaesthesia when using a volatile agent for induction?

Answer 12

How quickly FA/Fi approaches 1

How rapidly the alveolar concentration equals the inspired concentration of volatile anaesthetic

Question 13

What is overpressure with regard to inhalation induction

Answer 13

Overpressure involves setting the initial concentration on the vaporizer above that needed for maintenance during induction and then, over 5 minutes or so, reducing it towards a maintenance value of approximately 1 MAC.

Incremental increases over a minute or so will minimize airway irritation (also airway irritation much less with sevoflurane)

Question 14

Why do we use an intravenous induction in adults?

Answer 14

To avoid the unpleasantness of breathing a volatile –> do not delay initiating the volatile

Question 15

What are the main reasons why N2O is used?

Answer 15

It is used as a carrier gas as it reduces the MAC required for a volatile agent and has useful analgaesic action

Question 16

Explain the second gas effect

Answer 16

N2O has a very low B:G partition co-efficient (0.47)
Nitrogen has a relatively high B:G part. coeff. (0.015)

The B:G partition co-efficient for N2O is 30 x GREATER than that of Nitrogen.

Rate of diffusion = ∆P x SA x Solubility / √MW x L

∆P = partial pressure gradient
SA = Surface Area for gas exchange
Solubility = B:G partition co-efficient
MW = Molecular weight
L = Thickness of the diffusion barrier

In comparing N2 to N2O in lungs –> since N2O will replace N2 the ∆P is the same (constant), SA, L - constant (same lungs).

The variables are therefore MW and Solubility of each gas

For N2:
MW = 28 g/mol
B:G = 0.015

Relative Rate of diffusion = 0.015/√28 = 0.0028

For N2O
MW = 44 g/mol
B:G = 0.47

Relative Rate of diffusion = 0.0708 –>THIS IS 25 TIMES FASTER THAN N2.

Question 17

Define the concentration effect and draw the FA/Fi graph for all the volatile agents and N2O to illustrate this effect

Answer 17

The concentration effect is an observed phenomenon that describes the disproportionate
rate of rise of the alveolar fraction compared with the inspired fraction when high
concentrations of N2O are inspired.

Question 18

What implications does the difference in B:G coefficients for N2O and N2 have for anaesthetists

Answer 18

1. Second gas effect
2. Diffusion hypoxia

N2O diffuses 25 x faster than N2. When an anaesthetic agent is co-administered with N2O the effect of rapid diffusion of N2O out of the alveoli with relative slow diffusion of Nitrogen into the alveoli leads to a pressure gradient between the alveoli and the adjacent respiratory bronciles and other airways bringing more alveolar gas into the alveoli –> the effect is concentration of both the N2O and the volatile agent and increased FA of both and the increase concentration of the latter is the outcome of the second gas effect.

Diffusion hypoxia occurs because, during emergence in a patient in whom N2O was administered N2O leaving the blood and moving into the alveoli occurs at a faster rate than Nitrogen which is leaving the alveoli and entering the blood –> this has the effect of diluting the gases in the alveoli.

Question 19

What effects do all volatile agents have on RR and Vt

Answer 19

Increase RR and decrease VT
As tidal ventilation decreases dead space remains unchanged and hence alveolar ventilation decreases and may become inadequate with high concentrations of volatile.

Question 20

What effect does N2O have on minute ventilation

Answer 20

Minimal

Question 21

Briefly describe the technique for inhaled induction with sevoflurane

Answer 21

Minute 1: incremental dial up of sevo to 8 % starting with 0% but arriving at 8% by one minute

When patient is almost asleep (at about 2 mins) dial down to 3% to MAINTAIN TIDAL VENTILATION. If 8 % is continued –> Vt could drop to 40 ml and RR up to 22 but this means VA = 0

Question 22

Which volatile agents cause decreased SVR and reduced myocardial contractility

Answer 22

All volatile agents

Isoflurane

• drops SVR slightly more than sevoflurane + accompanied by a reflex tachycardia
• Also has been associated with coronary steal

Sevoflurane
- less pronounced drop in SVR, BP and HR better preserved

Question 23

What effect does N2O have on the CVS

Answer 23

Minimal reduction in cardiac contractility - usually offset by increased SNS activity

Question 24

What are the implications related to the vasodilation of coronary vessels caused by volatile anaesthetic agents

Answer 24

Volatile anaesthetic associated coronary vasodilation may reduce perfusion in coronary vessels distant to stenosed regions of these vessels precipitating ischaemia.

Question 25

In practice is coronary steal a factor in perioperative myocardial ischaemia?

Answer 25

As long as coronary perfusion pressure is maintained, no.

Question 26

What is the extent of metabolism of N2O, isoflurane and sevoflurane and what is the clinical relevance?

Answer 26

N2O - 0%

Isoflurane - 0.2%

Sevoflurane - 3 - 5% (High number of Fl- ions produced which are known to cause renal impairment –> despite this sevoflurane does not appear to impair renal impairment even after prolonged exposure

Question 27

What compounds are synthesized when sevoflurane interacts with CO2 absorbers (NaOh, KOH containing) and which of these are potentially toxic - describe the toxicity

Answer 27

Compounds A to E

Compound A - potential to produce renal toxicity but there have been no reports of actual renal toxicity despite potentially toxic levels being reached

Question 28

Is N2O a trigger for malignant hyperthermia

Answer 28

NO. All volatiles are

Question 29

What other important side effect is possible with N2O

Answer 29

Megaloblastic anaemia

Question 30

Which volatile agent causes some SNS stimulation at a MAC >1.5 or when the concentration is increased too rapidly

Answer 30

Desflurane

Question 31

Which volatile agent can sensitize the heart to catecholamines which may lead to arrhythmias

Answer 31

Halothane

Question 32

Which volatile agent causes coronary artery vasodilation and may lead to the ‘coronary steal syndrome’ and is therefore avoided in patients with marked coronary artery disease

Answer 32

Isoflurane

Question 33

Which volatile agent has the reduces myocardial contractility the most

Answer 33

Halothane

Question 34

What effect do the following agents have on heart rate
Isoflurane
Desflurane
N2O
Sevoflurane
Halothane

Answer 34

Isoflurane - Marked increase HR
Desflurane - Moderate increase HR
N2O - Mild increase HR
Sevoflurane - no effect
Halothane - Decrease HR

Question 35

Summarize the effects of the volatile effects on the CVS

Answer 35

HR
Isoflurane increases HR significantly
Halothane decreases HR
Desflurane increase HR moderately
Sevoflurane - no effect
N2O - mild increase HR

BP
All agents reduce BP

Contractility
All agents reduce contractility and Halothane has the greatest reduction. N2O the mildest.

SVR
All agents reduce SVR except N2O with a mild increase in SVR

Question 36

What are the CVS effects of muscle relaxants

Answer 36

SUX may cause bradycardia after 2nd dose from stimulation of cardiac M receptors. Prevented by giving atropine prior to second dose of SUX

Atracurium and mivacurim
- my cause histamine release causing BS and hypotension after RAPID IV injection –> reduced by slow injection

Question 37

How can the CVS response during extubation be minimized

Answer 37

1. Ensure adequate reversal of muscle relaxation
2. Optimize analgaesia
3. Deep extubation as an option
4. LMA less SNS than ETT
5. Short acting BB

Question 38

Why does hypotension occur during central neuraxial blockade

Answer 38

Denervation of the sympathetic outflow, which causes a decrease in vascular resistance. The higher the block the more pronounced the hypotension, because of the greater SNS blockade leading to greater vasodilatation.

Question 39

When does bradycardia occur in neuraxial blocks

Answer 39

Bradycardia occurs when the cardiac sympathetic fibres are blocked (T1-T4).