Inhalational agents

1. INHALATIONAL ANAESTHETIC
AGENTS
Dr. S.O.A. Olateju
(MB,ChB, DA (WACS), MPH, FMCA, FICS)
Dept of Anaesthesia
OAU/OAUTHC, Ile-Ife
22nd February, 2016
UPDATE COURSE IN BASIC SCIENCES

2. OUTLINE
Introduction
Classification
Pharmacokinetics
-Uptake and distribution
Pharmacodynamics
Properties of an ideal inhalational anaesthetic
Clinical pharmacology of each agent

3. INTRODUCTION
 Inhalational Anaesthetics are volatile or gaseous
chemical compound possessing general
anaesthetic properties that can be delivered to the
patient via the respiratory tract
 In 1846, the 1st ether anaesthetic was given(G Morton)
 Single agent anaesthesia was used(Apparatus-schimmelbusch)
 A year later, chloroform was introduced(John Snow)
 Both agents were acceptable to the society but
none was good
 Inhalational anaesthetics remain popular
maintenance and under some circumstances for
induction.

4. Classification of Inhal anae (1)
 Old
Diethyl Ether, chloroform and nitrous oxide;
Ethylene,cyclopropane and divinyl ether,ethylchloride
trichloroethylene and halothane
Ethers, ethylene and cyclopropane vanished from
practice because of flammability.
Chloroform and ethylchloride (compds halogenated
with chlorine) disappeared because of their toxicity

5. Classification (1)cont……
 New or modern inhalational anaesthetics
-(Comps halogenated with fluorine)
The substitution of fluorine for chlorine or bromine
provided greater molecular stability (less toxicity) and
lower solubility
Examples are isoflurane, desflurane, enflurane,
sevoflurane and methoxyflurane
-Others e.g xenon

6. Classification cont…(2)
 Volatile anaesthetics (vapours). Require special
vapourizers.
Examples: all except nitrous oxide & xenon
 Gases. Examples are nitrous oxide, nitric oxide and
xenon
Alveolar levels and blood levels are easily controllable
by adjusting inspired concentration

7. Classification cont….(3)
 Halogenated Hydrocarbon
 Chloroform
 Cyclopropane
 Halothane
 Halogenated Ethers
 Isoflurane
 Enflurane
 Desflurane
 Sevoflurane
 Methoxyflurane
 Non-halogenated
 Nitrous oxide
 Nitric oxide
 Xenon

9. Fig.1

10. Mechanism of action
 Volatile anaesthetics exert their effects at multiple sites
throughout the central nervous system.
 It appears that volatile agents preferentially potentiate
GABAAreceptors and two-pore domain K+ channels,
whereas the anaesthetic gases nitrous oxide and xenon
inhibit N-methyl-D-aspartate channels.
 Uptake and removal of inhalation agents from the
body depends on the alveolar concentration of the
anaesthetic agent (FA) and its uptake from the alveoli
by the pulmonary circulation.

11. PHARMACOKINETICS
 Uptake and removal of inhalation agents from the body depends on
the alveolar concentration of the anaesthetic agent (FA) and its uptake
from the alveoli by the pulmonary circulation.
 Partition coefficient:This is the ratio of the amount of substance in
one phase to the amount in another phase at a stated temperature, with
the two phases being of equal volume and at equilibrium with each
other.
 The blood/gas coefficient is the ratio of the amount of anaesthetic in
blood and gas when the two phases are of equal volume and pressure
and in equilibrium at 37 oC
 The partial pressure of the agent in the blood and hence the brain that
gives rise to anaesthesia.
 Therefore, agents with a low blood:gas coefficient exert a high partial
pressure and therefore a more rapid onset/offset of action.

12. Pharmacokinetics cont….
 Oil/gas partition coefficient
The oil:gas coefficient is an index of potency and is
inversely related to MAC. The action of anaesthetic
agents is suggested to be related to the lipid solubility.

13. Factors affecting uptake & distribution of inhalational
anaesthetics
 FGF (fresh gas flow) is determined by the vaporizer and
flowmeter setting.
1. DELIVERY TO THE LUNGS is dependent on:
Inhaled concn of anaesthetic. The higher the
inhaled concn, the more rapid is induction
Alveolar ventilation. Uptake of inhal anaesthetic is
directly proportional to alveolar ventilation.
Hyperventilation makes induction more rapid, but
hypoventilation will slow down induction

14. Factors cont…
2.UPTAKE OF ANAESTHETIC AGENT FROM THE LUNGS
 Blood/gas partition coefficient (solubility of the agent in
blood). It is defined as the ratio of its concentration in
blood to alveolar gas when their partial pressures are in
equilibrium
 Inhal anaesthetic with low partition coefficient i.e less
soluble in blood, the more rapid is induction because
alveolar tension of the anaesthetic will build up faster.
Highly soluble agents e.g ether are slow induction.
 Cardiac output : A high cardiac output slows down inhal
induction by reducing the rate of alveolar tension of
anaesthetic

15. Uptake & distribution cont..
3. UPTAKE BY THE TISSUES: Influenced by:-
Pulmonary blood flow. Pulmonary blood
flow equals cardiac output. Higher cardiac
output results in a greater uptake of
anaesthetic from the lungs and more
rapid delivery to the tissues including the
CNS
Alveolar tension of inhal anaesthetic also
builds up faster when pulmo bld flow is
reduced as in shock

16. METABOLISM OF VOLATILE AGENTS
LUNGS
LIVER-Halothane is 20-25% metabolised
-Sevoflurane 3 - 4%
-Enflurane 3%
-Isoflurane <0.2%
-Desflurane 0.1%.
Some metabolites are harmful e.g. metabolism of
Methoxyflurane liberates fluoride ions which may cause
renal failure as well as Enflurane does

17. PHARMACODYNAMICS OF INHALATIONAL ANAESTHETICS
Defn of Minimum Alveolar
Concentration (MAC)
Factors that reduce MAC
Factors that increase MAC
Factors that has no effect on MAC

18. MAC:Minimum Alveolar Concentration
The minimum alveolar concentration of inhalational
anaesthetic agent is the concentration that prevents
movement in response to skin incision in 50% of
(unpremedicated animals) subjects studied at sea level
(1 atmosphere), in 100% oxygen. Hence, it is inversely
related to potency.
The rationale for this measure of anaesthetic potency is:
 Alveolar concentration can be easily measured
Near equilibrium, alveolar and brain tensions are virtually
equal
The high cerebral blood flow produces rapid equilibration

19. Factors which reduce MAC
 Drugs-sedatives such as premedication agents,
analgesics, nitrous oxide, methyl dopa, clonidine,
lidocaine, pancuronium, Mg, CNS depressant drugs,
acute alcoholism
 Increasing age
 Higher atmospheric pressure, as anaesthetic potency is
related to partial pressure - for example, MAC for
enflurane is 1.68%
 Hypotension, Hypothermia Hypoxaemia
 Anaemia Myxoedema, Pregnancy
 Hypocapnia Hyponatraemia

20. Factors which increase MAC
 Drugs e.g presence of ephedrine or
amphetamine
 Chronic alcoholism
 Decreasing age
 Pyrexia
 Hypercapnia-induced sympathoadrenal
stimulation
 Thyrotoxicosis

21. Factors that do not affect MAC
Sex
Height
Weight
Duration of anaesthesia
Hypo/hyperkalaemia
Anaesthetic duration do not alter MAC

22. Pharmacodynamics cont…
 Effects on the respiratory system
 All halogenated agents depress ventilation by reducing
tidal volume.
 Effects on cardiovascular system
 All halogenated agents reduce mean arterial pressure
and cardiac output in a dose-dependent manner
 The arrhythmogenic potential of sevoflurane and
desflurane is lower than that of isoflurane.

23. Pharmacodynamics cont….
 Effects on the CNS: All inhalation agents decrease
cerebral metabolic rate and oxygen consumption.
 The vasodilation of cerebral vessels caused by
inhalation anaesthetics has the potential to increase
intracranial pressure.
 It has been shown that neither desflurane nor
isoflurane at one MAC concentration is associated
with a change in intracranial pressure in
normocapnoeic patients

24. air.
Pharmacodynamics cont…..
 Effects on the liver
 All inhaled anaesthetics reduce hepatic blood flow to some
degree. Only 0.2–5% of current volatile anaesthetics
isoflurane, sevoflurane, and desflurane are metabolized:
they are mainly excreted unchanged in exhaled air. 25% of
halothane is metabolised by oxidative phosphorylation
 Effects on the kidneys
 Older inhaled anaesthetics have differential effects on renal
blood flow and glomerular filtration rate that are
attenuated by renal autoregulation. Newer agents have
minimal effects on physiology.
 Methoxyflurane causes high output renal failure

25. STAGES OF ANAESTHESIA
 Stages have been modified from Guedel's classical
description for Ether
STAGE 1: From the commencement of induction to
loss of consciousness.
STAGE 2: Stage of excitement. From loss of
conciousness to onset of automatic breathing.
May be associated with breath-holding, coughing,
vomiting, struggling etc. It can be minimized by
adequate premedication, psychological
preparation and quiet surroundings. Emergence
delirium may occur during recovery from
anaesthesia.

26. STAGES OF ANAESTHESIA Cont---
STAGE 3: Stage of surgical anaesthesia. From onset of
automatic respiration to respiratory paralysis. This was
originally divided into 4 planes (Guedel), but is now more
conveniently divided into:-
i) Light anaesthesia - until the eyeballs become
fixed.
ii) Medium anaesthesia - with increasing
intercostal paralysis.
iii) Deep anaesthesia - with diaphragmatic
breathing only.
STAGE 4: Stage of medullary paralysis. From the onset of
diaphragmatic paralysis to apnoea and death. All reflex
activity lost and pupils widely dilated.

27. Properties of an Ideal Inhal Agent
 Physical properties
 Biological properties

28. Ideal: Physical properties
1. Non-flammable, non-explosive at room
temperature
2. Stable in light.
3. Liquid and vaporisable at room temperature i.e.
low latent heat of vaporisation.
4. Stable at room temperature, with a long shelf life
5. Stable with soda lime, as well as plastics and
metals
6. Environmentally friendly - no ozone depletion
7. Cheap and easy to manufacture & administer

29. Ideal:Biological properties
1. Pleasant to inhale, non-irritant, induces
bronchodilatation
2. Low blood:gas solubility - i.e. fast onset
3. High oil:gas solubility - i.e. high potency
4. Minimal effects on other systems - e.g. cardiovascular,
respiratory, hepatic, renal or endocrine & should not
interact with other drugs used commonly during
anaesthesia e.g pressor agents or catecholamines.
5. No biotransformation - should be excreted ideally via the
lungs, unchanged
6. Non-toxic to operating theater personnel

30. Properties of Inhal Anae Agents
N2O Halotha
ne
Isoflura
ne
Enfluran
e
Desflura
ne
Sevoflur
ane
Mol wt 44 197 184 184 168 200
Boiling
pt. oc
-88 50.2 48.5 56.5 23.5 58
SVP at
20oc
(mmHg)
43643 243 238 175 664 157
MAC in
100% O2
105 0.75 1.15 1.7 6 2.05
Blood/gas 0.45 2.5 1.4 1.91 0.45 0.6
Oil/gas 1.4 224 98 98.5 28 47

31. HALOTHANE
 Synthesized in 1951 and introduced into clinical practice in
the UK in 1956.
 Colourless liquid with a relatively pleasant smell.
 Decomposed by light.
 Addition of 0.01% thymol and storage in amber-coloured
bottles render it stable.
 Although, decomposed by soda lime, it may be used safely
with this mixture.
 Should be store in a closed container away from light and
heat.

32. HALOTHANE:
UPTAKE AND DISTIBUTION
 Blood/gas solubility coefficient of 2.5
 Not irritant to the airways.
 It may take at test 30min for the alveolar concentration
to reach 50% of inspired concentration.
 20% of halothane is metabolised in the liver
 Recovery is slower than with other agents and
prolonged with increasing duration of anaesthesia

33. HALOTHANE:
METABOLISM
 Approximately 20% in the liver usually by oxidative
pathways.
 End products are excreted in the urine.
 Major metabolites are bromine, chlorine, trifluoroactic
acid and trifluoroacetylethanol amide.
RESPIRATORY SYSTEM
 Non-irritant and pleasant to the breath.
 Rapid loss of pharyngeal and laryngeal reflexes and
inhibition of salivary and bronchial secretions.
 Causes a dose-dependent decrease in mucocilliary
function. May contribute to post operative sputum
retention.

34. HALOTHANE cont.
 Antagonizes bronchospasm and reduces airway resistance
in pxs with bronchoconstriction (B-mimetic effect on
bronchial smooth muscle).
CVS
 Potent depressant of myocardial contractility and
myocardial metabolic activity (inhibition of glucose uptake
by myocardial cells)
 Depression of CO with little effect in PR. Thus reduction in
arterial pressure.
 Hypotensive effect is augmented by a reduction in heart
rate.
 Antagonism of bradycardia by administration of atropine
frequently leads to increase in arterial pressure.

35. HALOTHANE cont.
 Arrythmias are very common during halothane anaesthesia
 Arrhythmias are produced by:
i) Increased myocardial excitability augmented by the
presence of hypercapnia, hypoxaemia or increase
circulating cathecolamines.
ii) Bradycardia caused by central vagal stimulation during
local infiltration with LA solutions containing
epinephrine, multifocal ventricular extrasystoles and
sinus tachycardia have observed and cardiac arrest has
been reported.

36. Recommendations
i) Avoid hypoxaemia and hypercapnia
ii) Avoid concentration of epinephrine > 1 in 100,000
iii) Avoid overdosage in adults.
CNS
 Anaesthesia without analgesia
 Increase CBF or increase ICP
 No seizure activity in EEG.
HALOTHANE cont.

37. HALOTHANE CONTD.
GIT:
 GI motility is inhibited
 PONV are seldom severe.
UTERUS:
 Relaxes uterine muscle and may cause PPH.
 Concentration <0.5% not associated with increased blood
loss in c/s
SKELETAL MUSCLE:
 Causes skeletal muscle relaxation and potentiates non-
depolarizing relaxants
 Post-op shivering is common.

38. HALOTHANE cont…
 May trigger Malignant Hyperthermia in susceptible
patients.
HALOTHANE IS ASSOCIATED HEPATIC
DYSFUNCTION.

39. HALOTHANE conts…
RECOMMENDATIONS OF COMMITTEE ON SAFETY
OF MEDICINE.
 Careful anaesthetic history
 Repeated exposure to halothane within a period of 3/12
should be avoided unless there are overriding clinical
circumstances.
 A patient of unexplained jaundice or pyrexia after
previous exposure is an absolutely C/I.
 Incidence is very low in children.

40. Summary: Halothane
Advantages
 Smooth induction.
 Minimal stimulation of salivary and bronchial
secretions.
 Bronchodilation.
Disadvantages
 Arrythmias
 Possibility of liver toxicity especially with repeated
administrations.
 Slow recovery.

41. NEWER INHALATIONAL
ANAESTHETICS
 In western countries, it is customary to use one of the 5
modern volatile Anaesthetics – desflurane, enflurane,
isoflurane and sevoflurane- vaporized in a mixture of
nitrous oxide in oxygen.
 The use of halothane has declined because of medicolegal
pressure relating to the rare occurrence and hepatotoxicity.
 Sevoflurane is increasing rapidly, particularly in paediatric
anaesthesia because of its superior quality as an
inhalational induction agent.
 Desflurane produces rapid recovery but is very irritant to
the airway.

42. ISOFLURANE
 An isomer of enflurane.
PHYSICAL PROPERTIES.
 Colourless, volatile liquid with a slightly pungent odour.
 Stable and does not react with metal or other substances.
 No preservative is required
 Non-inflammable in clinical concentrations
 MAC is 1.15% in oxygen & 0.56% in 70% N2O.

43. ISOFLURANE cont…
 Rate of induction is limited by this pungency of the
vapour and clinically may be faster than that which
may be achieved with halothane.
 Coughing or breath holding on induction is
significantly > with isoflurane and halothane.
 Not an ideal agent for inhalational induction.

44. ISOFLURANE cont...
METABOLISM
 ≈0.17%. The rest is excreted via the lungs
RS
 Dose-dependent depression of ventilation in common with other modern
volatile agents.
 Decrease in tidal volume but and increse in ventilatory rate in the absence of
opioid drugs.
CVS
 Systemic hypotension occurs predominantly as a result of reduction in SVR.
 Less depression of CO than with halothane and enflurane
 Arrythmias are common.
 Little sensitization of the myocardium to catecholamines
 Dilates systemic arterioles
 Coronary vasodilation.

45. ISOFLURANE cont….
Uterus.
 Similar to halothane and enflurane.
CNS:
 Speed of uptake is limited by its pungency
 Low concentration do not cause any change in CBF at
normocapnia.
 Superior to enflurane and halothane, both cause cerebral
vosodilatation.
 But higher inspired concentrations cause vasodilation and
increase CBF.
 No seizure activity in EEG.

46. Muscle Relaxation
 Depression of NMT with protection of Depolarising NB
drugs.
Summary:
Advantages
 Rapid recovery
 Minimal biotransformation with risk of hepatic or renal
toxicity.
 Muscle relaxation
Disadvantages
 Pungent odour. Makes inhalational induction relatively un-
pleasant, particularly in children.

47. SEVOFLURANE
Physical properties
 Non flammable and has a pleasant smell.
 Blood/gas partition coeffient is 0.65, about half that of
isoflurane (1.43), those of desflurane (0.45) and N2O
(0.45).
 MAC in adult is between 1.4 and 2% in oxygen and 0.66%
in 60% nitrous oxide.
 MAC in children is 2.6% in oxygen and 2.0% in nitrous
oxide and neonates (3.3%). Elderly(1.48%).
 Stable-stored in amber-coloured bottle.
 In the presence of water, it undergoes some hydrolysis and
also with soda lime.

48. Uptake and Distribution
 Non-irritant to the URT, therefore rate induction
should be faster than with any of the other agents.
 Rate of recovery is slower than that of desflurane
because of its higher partition coefficients in vessel
rich tissues, muscle and fat.

49. Metabolism
 Approximately 5% in the liver to 2 main metebolites.
 This molecule is potentially hepatotoxic, but
conjugation occurs so rapidly that clinically significant
liver damage seems theoretically impossible.

50. SEVOFLURANE cont…
RS
 Non irrittant to the URT.
 It produces dose dependent ventilation depression,
reduces respiratory drive in response to hypoxia and
increase CO2 partial pressure comparable with levels
achieved with other volatile agents.
 Relaxes bronchial smooth muscle but not as effectively
as halothane.

51. SEVOFLURANE contd...
CVS
 Similar to those of isoflurane with slightly smaller effects
on HR and less coronary vasodilatation.
 Decrease AP mainly by reducing PVR, mild myocardial
depression resulting from its effect on calcium channel.
 Does not defer from isoflurane in its sensitization of the
myocardium to exogenous catecholamines.
 Less potent coronary arteriolar dilator and does not cause
coronary steal.
 Lower HR: helps to reduce myocardial o2 consumption.

52. CNS
 Similar to those of isoflurane and desflurane.
 ICP increases at high F1 of sevoflurane but this effect is
minimal over the 0.5-1.0 MAC.
 Decreases CVR and CMR
 No excitatory effect on the EEG.
RENAL SYSTEM
 Serum fluoride concentrations > 50mol have been
reported.
 However renal toxicity does not appear to be related to
inorganic fluoride concentrations following anaesthesia as
opposed to that associated with methoxyflurane.

53. SEVOFLURANE cont…
 Apparent lack of renal toxicity with sevoflurane may be
related to its rapid elimination from the body.
 Renal blood flow is well preserved.
MSS
 In common with isoflurane, it potentiates NDNRs, and to a
similar extent.
 May trigger MH in susceptible patients and there have
been cases reported in the literature.
Obstetric Use
 Limited data
 In summary, is a newer IAA.

54. SEVOFLURANE cont…
Advantage
 Smooth, fast induction
 Rapid recovery
 Ease of use, requiring conventional vaporizers(particularly
when compared with desflurane).
Disadvantages .
 Production of potentially toxic metabolites in the body
(more a theoretical problem).
 Instability with CO2 absorbants.
 Relatively expensive.

55. DESFLURANE
 Colorless agent, store in amber-coloured bottles
without preservative.
 Not broken down by sodalime, light or metals.
 Nonflammable.
 Boiling point of 23.50c, vapour pressure of 88.5k pa
(664mmHg) at 20 degree centigrade and therefore, it
cannot be used in a standard vaporizer.
 A special vaporizer (the TEC’6) has been developed
which requires a source of electric power to heat and
vaporize it.
 Less pungent than Isoflurane.

56. ENFLURANE
 Clear, Colourless, Volatile anaesthetic agent with a
pleasant smell.
 Non-inflamable in clinical concentrations.
 No preservative is required.

57. ENFLURANE
HEPATOTOXICITY
 Jaundice
 Derangement of liver enzymes to a lesser extent than
after halothane.
 In summary, enflurane is a useful alternative agent to
halothane.

58. ENFLURANE CONT----
Advantages
 Low risk of hepatic disfunction.
 Low incidence of arrhythmias.
Disadvantages
 Seizure activity on EEG.
 Its use in patients with pre-existing renal disease or in

59. Table 1

60. OLDER AGENTS
Chloroform and ether were the first universally
accepted general anaesthetic agents.
Ethyl chloride, ethylene and cyclopropane were also
used
Cyclopropane was particularly popular because of its
fast induction and properties
Toxicity and flammability of the affected drugs lead to
their withdrawal from market
Methoxyflurane was also discontinued because of its
toxicity(cause high-output RF and was associated with
chloride toxicity)

61. OLDER AGENTS CONT---
DI-ETHYL ETHER
 Has been abandoned in western countries because of its
flammability but remains an agent of wide spread use in
underdeveloped countries.
 Colourless, highly volatile liquid with a characteristic smell.
 Flammable in air and explosive in oxygen.
 Decomposed by air, light and heat in acetaldehyde and
ether peroxide (must important product)
 Should be stored in cool environment in opaque
containers.

62. DI-ETHYL ETHER cont…
Uptake and Distribution
 High blood/gas solubility coefficient of 12.
 Induction and recovery are slow.
CNS
 Because induction is slow, the classical stages of
anaesthesia are seen.
 Stimulation of the sympathoadrenal system and
increased level of catecholamines which offset the
direct myocardial depressant effect.

63. DI-ETHYL ETHER conts…
RS
 Irritant to the RT and provokes cough, breath-holding
and profuse secretions from all mucus-secretion
glands.
 Laryngeal spasm is not uncommon during induction.
 During established anaesthesia, there is dilatation of
the bronchi and bronchioles.
 One time recommended for the reaction of
bronchospasm.

64. DI-ETHYL ETHER conts…
CVS
 Myocardial depression.
 light name of anaesthesia. This is sympathetic stimulation
and this often results in little change in CO, AP or PR.
 In deep planes, CO decreases as a result of myocardial
depression.
 Arrythmias rarely occur.
 No sensitization of the myocardian to circulating
catecholamines.
Alimentary canal
 Salivary and gastric secretions are increased during light
anaesthesia but decrease during deep.

65. DI-ETHYL ETHER conts…
Skeletal Muscle
 Potentiates the effects of NDNBs
Uterus and Placenta
Pregnant uterus not affected during light anaesthesia but
relaxation occurs with deep.
Metabolism.
 15% of CO2 and water.
 ≈4% is metabolised in the liver acetaldehyde and ethanol.
 Stimulates gluconeogenesis and therefore causes
hyperglycaemia.

66. Clinical uses of ether.
 Much higher therapeutic ratio than halothane,
enflurane or isoflurane.
 Induction is very slow.
 Vapour strength of up to 20% are required for
induction, light anaesthesia may be maintained with
3-5% and deep anaesthesia with 5-6%.

67. ANAESTHETIC GASES
NITROUS OXIDE (N2O)
Manufacture
 Prepared commercially by heating ammonium nitrate to a
temperature of 245-270c.
 Various impurities are produced after cooling, ammonia
and nitric acid are reconstituted to ammonium nitrate
which is returned to the beginning of the process.
 The remaining gases then pass through a series of
scrubbers.
 Nitrogen escapes as a gas.

68.  N2O is then evaporated, compressed and passed through
another aluminium dryer before being stored in cylinders.
 Higher oxides of nitrogen dissolve in water to form nitrous
and nitric acids.
 These substances are toxic and produce
methaemoglobinaemia and pulmonary oedema if inhaled.
Storage.
 Stored in compressed form as a liquid in cylinders at a
pressure of 44bar(4400kpa, 638ibm)
 Cylinders are painted blue in the UK.

69. N2O conts…
 Because the cylinder contains liquid and vapour, the
total quantity of nitrous oxide contained in a cylinder
may be ascertained only by weighing.
 Thus the cylinder weights are stamped on the
shoulder.

70. PHYSICAL PROPERTIES
 Sweet smelling, non irritant colourless gas.
 MW is 44. Bp is 88c
 Critical temperature of 36.5c
 Critical pressure of 72.6bar.
 Not flammable but it supports combination of fuels in
the absence of oxygen.
Phamacology.
 Good anagelsic but a weak anaesthetic.
 MAC is 105%
 Oil/water solubility coeffient is 3.2.

71. N2O conts…
 Used in combination with other agents.
 When using in relaxant technique, the inspired gas
mixture should be supplemented with a low
concentration of volatile agent.
Side effects
 Diffusion hypoxia
 Effect on closed gas spaces.
 Cardiovascular depression
 Toxicity.
 Teratogenic changes.

72. Xenon
 A gas
 No taste or odour
 Blood gas partition coeff is 0.2
 Rapid pulmonary uptake and elimination
 No hepatic or renal metabolism
 Minimal cardiovascular depression
 Minimal arrhythmogenicity
 Near ideal but it is expensive to produce

73. Nitric Oxide (NO)
 A anaesthetic gas
 It is also naturally occuring
 It works by relaxing smooth muscle to dilate blood
vessels especially in the lungs
 It is used with mechanical ventilator to treat
respiratory failure in premature infants.

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