fourth year BVSc 411 course

1. Inhalant Anaesthesia
Dr.Kamalesh Kumar K S
PhD Scholar (In-service)
ICAR-IVRI

2. Inhalation anaesthetic drugs are vapours or
gases that are directly breathed into respiratory
system
Absorbed from alveoli to blood stream
Eliminated by lungs
Depth of anaesthesia can be controlled easily.
General considerations

3.  Inhalation anesthetics, also known as inhalation anesthetics or volatile
anesthetics, are a class of drugs used to induce and maintain general
anesthesia by administering them through inhalation. These substances
are typically gases or volatile liquids that, when inhaled, produce a
reversible loss of consciousness, allowing medical procedures or
surgeries to be performed without causing pain or discomfort to the
patient.
 Inhalation anesthetics work by affecting the central nervous system,
specifically the brain, to depress the activity of the neurons
responsible for perception and response to pain. They are often
administered through a specialized anesthesia machine that controls the
precise concentration of the anesthetic agent in the inhaled air.

4.  Non irritating
 Adequate muscular relaxation
 Adequate analgesia
 Easily controlled
 No bleeding
 Minimal side effects
 Non toxic
 Non inflammable
 Compatible with other drugs
 Easy to deliver
 Economical
Desirable properties

5.  Safety
Excreted via the same route (lung)
Easier to adjust the depth of anesthesia
Rapid recovery ( fast excrete)
Easy to assist ventilation
Accurately controlling anesthetic depth
Safety to discontinue immediately if problem arise
Eliminated quickly through ventilation
Use high flow of oxygen during induction (rare to hypoxia)
ADVANTAGES OF INHALATION ANESTHETIC

6.  The anesthetic machine is expensive.
 Require more equipments.
 It is not suitable for healthy, unpremedicated dogs and Struggling
from slow induction and is also frequently accompanied with
vocalization, excitement, defecation, urination, and vigorous struggling.
 The pungent smell of the isoflurane or halothane may prompt the
animal to hold their breath during induction and therefore prevents
the uptake of the inhalant anesthetic and slows the speed of induction.
 Pollution of the work environment during induction. Waste inhalant
anesthetic gas may cause headaches and other health problems.
Disadvantages

7. Classification of Inhalational Anaesthetics:
Gaseous agents:
Nitrous oxide and Cyclopropane.
Volatile liquids:
Methoxyflurane, Halothane, Ether, Chloroform,
Enflurane, Isoflurane, Desflurane, Sevoflurane.

8. VOLATILE
LIQUIDS
OLD GENERATION
•Chloroform
•Diethyl Ether(1846)
•Ethyl chloride
•Trichloroethylene
NEW GENERATION
Halothane(1956)
Methoxyflurane(1959)
Enflurane (1972)
Isoflurane (1981)
Desflurane(1993)
Sevoflurane(1995)
Xenon
GASES
Nitrous oxide(1845)
cyclopropane
INHALANT
ANESTHESIA

9. Important properties to consider:-
 Minimum alveolar concentration (MAC)
 Vapor pressure
 Blood Gas Partition Coefficient
PhysicalandChemical Properties
ofInhalantAnesthetics

10. INHALATIONAL ANAESTHETICS
• MAC: The MAC is defined as the minimum alveolar
concentration of an anesthetic (in volume %) at 1
atmosphere produces immobility /shows no response
in 50% of patients exposed to a painful stimulus
• It is the measure of potency of inhalation
anaesthetics.
• The anaesthetic potency of an inhaled anaesthetic is
inversely related to MAC.
• Potency = 1/MAC; i.e. an agent having low anaesthetic
potency will have a high MAC value and vice-versa.

11. Comparison of anesthetic potency of inhalant anesthetics using MAC (volume %).
Dogs Cats Horses Human
Methoxyflurane 0.23 % 0.23 % ----- 0.16 %
Halothane 0.87 % 0.82 % 0.88 % 0.74 %
Isoflurane 1.30 % 1.63 % 1.31 % 1.15 %
Sevoflurane 2.1-2.36% 2.58% 2.31% 1.7%
Desflurane 7.2% 9.8% 7.6%
Nitrous oxide 222 % 255 % 205% 105 %

12. Is the amount of pressure exerted by the gaseous
form of a substance when in equilibrium.
i.e. – it’s ability to evaporate
Determines how readily an inhalation anesthetic
will evaporate in the anesthetic machine
vaporizer.
VaporPressure

13. Blood: Gas Partition Coefficient:
The measure of the solubility of an inhalation
anesthetic in blood as compared to alveolar gas (air)
The blood/ gas solubility is a measure of the speed
of anaesthetic induction, recovery and change of
anaesthetic levels.
 Lower the blood/ gas partition coefficient, the more
rapid the anaesthetic induction or rate of change of
anaesthetic level in response to a stepwise change in
anaesthetic delivery.

14. Anesthetic Agent
Formula
Blood/gas
solubility
Vapor
Pressure
at 20oC (mmHg)
Preservatives
Methoxyflurane
12 23 Required
Halothane
2.4 243 Required
Isoflurane
1.4 240 None
Sevoflurane
0.69 160 None
Desflurane
0.42 664 None
Nitrous oxide
N O
0.47 --- None

15. Oil: Gas Partition Coefficient:
 A measure of fat solubility, determines the
potency of an anaesthetic and also influences
the kinetics of its distribution in the body, the
main effect being that high lipid solubility delays
recovery from anaesthesia.

16. BASIC COMPONENTS OF THE ANESTHETIC
MACHINE
 Gas source- compressed oxygen
 Pressure regulator (pressure reducing valves)
 Flowmeter
 Oxygen flush valve
 Vaporizer
 Patient breathing circuit (Endotracheal tube or
face mask)
 A means of eliminating carbon dioxide (CO₂ )
 Waste Gas Scavenger systems
 A compliant gas reservoir.

17. Anesthetic Machine
The anesthetic machine is a device
which prepares a precise, but
variable, gas mixture for delivery to
a breathing system.
Breathing system : provides oxygen
and anesthetic agent
 Removes carbon dioxide

18. PURPOSES OF THE ANESTHETIC MACHINE
 Provide oxygen (machine system)
 Deliver precise amounts of anesthetic agent (machine system)
 Remove CO2 (breathing system)
 Provide assisted or controlled ventilation (breathing system) to
the patient

19. Anesthetic machine
Functionally anesthetic machine is subdivided into four components
o High pressure system-
where pipe line and cylinder gas supplies are attached
o Low pressure system-
where oxygen and volatile anesthetics are mixed

20. o Breathing system-
where anesthetic gas mixture is delivered to the patient
o Scavenging system-
where excess gas from breathing system is collected and delivered
into waste gas evacuation system

21. Gas source- compressed oxygen
 Provides for the oxygen requirements of the patient and acts as a carrier
gas for the inhalation anesthetic agent.
 Oxygen is stored as a compressed
gas (100%) held under pressure
in metal cylinders.

22. Pin index safety system(PISS): avoids improper
cylinder connection

23. US and ISO Cylinder gas colour codes

24. Delivery of oxygen
 O2 delivered to patient either
by Flowmeter or o2 flush valve
 Regulator is attached to the tank
For most of the applications
Pressure is 40 to 50 psi

25. Flowmeter tubes Vaporizers
Flow rates are expressed in mililiters or litres of gas per minute (mL/min or L/min).

26. Oxygen enters the flowmeter and is delivered to the vaporizer at a
constant rate as indicated on the flowmeter dial.
Recommended oxygen flow rates for patients on a non-
rebreathing system are at least 200-300 ml/kg/min, with the
minimum flow rate being 1 L/min.
 Patients on a semi-closed (circle) system are run at a flow rate of
20-50 ml/kg/min with a maximum of 2 L/min.

27. Oxygen flush valve
It allows the gas bypass the vaporizer and delivery directly to the
anaesthetic circuit via the common gas outlet .
It allows O2 flow between 35 to75L/min
It is best to fill the breathing circuit using flow meter

28. Vaporizer
Oxygen exits the top of the flowmeter and continues via a low-
pressure hose to the vaporizer.
 The vaporizer is designed to convert liquid anesthetic to vapor and
to add a controlled amount of vapor to the carrier gas flowing
through the machine.
 If the oxygen flow is turned off, no anesthetic is delivered to the
patient.
 vaporizers are located near the flowmeters and can be either
out(VOC) or in (VIC) the anesthetic breathing circuit .

30. Non-precision vaporizers
(simple plenum)
 Simple glass containers which allow certain amount of fresh gas through the
space above the liquid anaesthetic agent.
 These do not compensate for drop in temperature or the surface area of
anaesthetic agent presented to the fresh gas.
 Hence, affecting the final output concentration of anaesthetic.
 Have low resistance to gas flow.
 Found within the breathing circuit.

31. NON-PRECISION VAPORIZER contd..
 There is no control deliver an exact concentration of anesthetic
gases.
They are acceptable for use with anesthetic that have a low
vapor pressure(e.g., methoxyflurane)
Gradations over it do not indicate the actual vapour concentration

32. Precision vaporizers
Agent specific.
Concentration-calibrated.
Placed out of circle/circuit (VOC).
Positioned between the flowmeters and breathing
circuit where the flow from flowmeters can cope with
this degree of resistance.
High resistance to gas flow.
Compensated for temperature, flow & back pressure.
Calibrated for one agent only.

33. Common gas outlet
It exit from the anaesthetic machine for blended gas mixtures of
carrier gas and volatile anaesthetics
Most machine outlets have a 15mm inner diameter slip joint
connection with a 22mm connection for outer diameter
It is frequent source of gas leaks ,so some machines come with a
retaining device at the connection to make it harder to disengage

34. Rebreathing bag
An open-ended or side-hole bag is
attached to the corrugated tubing.
 Does not influence the mechanics of
the circuit - allows artificial ventilation
to be performed.

35. The bag should have minimum volume of 60 ml/kg.
 Bags are available in various sizes, For small animal
1,2,3,and 5L bags
Large animals 15, 20, 30 L bags

36. Scavenge system
It directs waste gases from anaesthetic
breathing circuit out of workspace into
atmosphere.
It includes:
APL valve
Waste gas elimination system

37. 2 types
a) Passive scavenging system
b) active scavenging system
In passive system the expired gas is allowed to displace out by the
expiratory effort and the elastic recoiling of the lung of the animal.
In this system the pipeline should be shorter.
 In active system the expired air is sucked by negative pressure
induced by pumps, or injector.

38. Adjustable pressure-limiting valve
(pop-off valve)
 It is one way valve.
 It is a safety valve allowing excess gas to escape from the patient’s circuit.
 It maintains system pressure between 1-3 mm H2O.
 Normally it is left open unless positive pressure ventilation is being used (closed).
 APL is partially closed to prevent collapse of reservoir bag due to the negative pressure
from scavenging system.

39. Absorber canister
 Sodalime Canister - for the chemical absorbent of CO2
94% calcium hydroxide
5% sodium hydroxide
1% potassium hydroxide
Baralyme contain 80% calcium hydroxide, 20% barium hydroxide

40. Adsorbent
Fresh granules of calcium hydroxide are soft and easily
crushed
Expended granules are hard ( changed to calcium
carbonate)
Ethyl violet is used as indicator
Soda lime changes color
Amount of CO2 absorption is about 26L/100g of adsorbent

41. Breathing Systems

42. Breathing Systems:
 It is a system through which the patient breathes & carries the volatile anaesthetic
agent in vapour form from anaesthetic machine to the patient.
 There are mainly two types of breathing systems:
Rebreathing system
Non-rebreathing system

43. Breathing systems
NON-REBREATHING SYSTEM
• These are for patients weighing <7 kg
• Oxygen flow rates are 200–300 mL/kg/min
• Advantages
•Minimal resistance
•Less dead space
•Do not require CO2 absorbents
•High oxygen flow rates allow rapid change in
inspired anesthetic concentration
REBREATHING SYSTEM
(CIRCLE SYSTEM)
•These are for patients weighing >7 kg
• Oxygen flow rates are 10-30 ml/kg/min
• Advantages
•Low oxygen flow rates
•Less expensive
•Less body heat loss
•Rebreathing of expired carbon dioxide doesn’t occurs if
all components are operational

44. NON-REBREATHING SYSTEM
• Disadvantages
• High heat & water loss
• If O2 flow is too low, patient will rebreathe
expired gasses
•More expensive
REBREATHING SYSTEM
(CIRCLE SYSTEM)
• Disadvantages
• Increased resistance in circuit (unidirectional valves
and carbon dioxide absorber)
• More components of anesthetic machine can lead to
risk of malfunction
• Large volume of breathing circuit requiring longer time
between changes in inspired anesthetic concentration

45. Non-rebreathing Rebreathing
1. Oxygen “E” Cylinder; 2. Cylinder Pressure Gauge; 3. Pressure Regulator; 4.
Oxygen Flowmeter; 5. Vaporizer; 6. Common Gas Outlet; 7. APL or “Pop-off” Valve;
8.Oxygen Flush Valve; 9. Unidirectional Valves; 10. Reservoir Bag Connection; 11.
Co2-absorbent Canister; 12. Pressure Manometer.
Non-rebreathing system (modified Bain circuit) with adaptor

46. Non-rebreathing system/circuits
 These are characterized by the absence of unidirectional valves and a carbon dioxide
absorber.
 These depends on high fresh gas flow rates to flush CO2 from the circuit.
 It consists of fresh gas‐conducting hose, patient connection, exhalation conducting
tubing (normally corrugated), excess gas venting system, and a reservoir bag.

47. Types of non-breathing circuits
 Ayre’s T-piece
 Bain circuit
 Modified Jackson Rees circuit

48. Rebreathing
Circuit
circle and to and fro breathing system use a chemical absorbent for
exhaled carbon dioxide. they are called rebreathing system because the part
or all of exhaled gases after extraction of carbon dioxide, flows back to
patient.

49. Y piece

50. Breathing tubes