teaching purpose; students quick and easy reference

1. - Inhalant anesthetics are generally absorbed, transported and excreted from
the body without change; except HALOTHANE which is metabolized and
excreted by the kidneys.
INHALANT
ANESTHESIA
Dr Warson Monsang

2.  The main reason of relative safety of INHALANT ANESTHESIA is that the anesthetic
effect can be reversed by elimination through LUNGS and no detoxification is necessary,
whereas, none of the INTRAVENOUS AGENTS can be voluntarily eliminated and they
require metabolic alteration or elimination via KIDNEYS or LIVER.
 The use of inhalant anesthesia is comparatively expensive and requires inhalation
equipment, however, it is the most controllable method of anesthesia and the depth of
anesthesia can be easily changed.
 Inhalation agents with high partition co-efficient have relatively slow induction and
slow recovery. On the other hand, agents with relatively low co-efficient has faster
induction and faster recovery.

3. PHASES OF INHALANT ANESTHESIA
1. PULMONARY PHASE 2. CIRCULATORY PHASE 2. TISSUE PHASE
- The gas or vapour is
introduced into the lungs
and is transferred through
the pulmonary epithelium
and to the capillary
endothelium to the blood.
- The gas or vapour are
in circulation and
distributed to the tissues
especially the CNS.
- Wherein the anesthetics
enters the tissues and
produces the effect.

4. NOTES.
 Vessel rich group (brain , heart, intestines, kidney , liver, spleen) and muscle group
(skeletal muscles) are the tissues which are primarily involved in uptake of inhalation
anesthetics.
ALVEOLAR CONCENTRATION is a readily measured index of brain anesthetic
tissues.
 A useful standard by which inhalation anesthetics may be defined is MINIMUM
ALVEOLAR CONCENTRATION (MAC).
MAC is the anesthetic concentration required to prevent gross muscular
movement in response to painful stimulus. In order to achieve clinical anesthesia, the
anesthetic concentration should be equal or above MAC values.

5. NOTES.
 the clinical signs of GENERAL ANESTHESIA result from the action of anesthetics on the
central nervous system (CNS).
The depth of anesthesia is related to the amount of anesthesia made available
to the nervous system.

6. ADVANTAGES OF INHALATION ANESTHESIA
1. Agents are not metabolized, therefore the recovery is not dependent upon body
detoxification mechanism.
2. Closed system of inhalation anaesthesia is available for thoracic surgery.
3. Good control over the level or depth of anaesthesia.
4. Early recovery of patient.
5. Deeper and safe anaesthesia available for long surgical procedures.

7. DISADVANTAGES OF INHALATION ANESTHESIA
1. Some of the inhalation agents flammable and when mixed with oxygen are
explosive.
2. Few are irritant to the respiratory system.
3. Require constant SURVEILLANCE by the anaesthetists

8. SIGNS / STAGES OF ANESTHESIA
- General anesthesia has been divided into 4-stages, depending upon the neuromuscular
signs.
(variation in response amongst patients due to their physical status, various
anesthetic agents, preanesthetic medications, etc. modify these signs).
STAGE-I (STAGE OF ANALGESIA OR VOLUNTARY MOVEMENT)
STAGE-II (STAGE OF DELERIUM OR INVOLUNTARY MOVEMENT)
STAGE-III (STAGE OF SURGICAL ANESTHESIA)
STAGE-IV (STAGE OF MEDULLARY PARALYSIS)

9. STAGE-I
(STAGE OF ANALGESIA OR VOLUNTARY
MOVEMENT)
STAGE-II
(STAGE OF DELERIUM OR INVOLUNTARY
MOVEMENT)
- Lasts from initial administration to the
loss of consciousness).
- Lasts from loss of consciousness to onset
of regular pattern of breathing.
- Animal may cry, neigh or whine depending
upon the species involved and reacts to
external stimuli by flex struggling, breath
holding and hyperventilation.
- Pupil may dilate.

10. STAGE-III
(STAGE OF SURGICAL ANESTHESIA)
STAGE-IV
(STAGE OF MEDULLARY PARALYSIS)
- Characterized by unconsciousness,
muscular relaxation, slow and regular
breathing, loss of swallowing reflexes.
- Characterized by extreme depression of
nervous system.
- Respiration ceases, however, the heart
continues to beat for sometime.
- Mucous membranes are blenched and
pupil may dilate widely.
- Relaxation of anal sphincter.
- DEATH may occur, if immediate resuscitate
steps are not taken.

11. CLASSIFICATION OF INHALATION ANESTHETICS
OUTDATED GASES
VOLATILE
AGENTS
ETHER
TRILENE
METHOXYFLURANE
CYCLOPROPANE
CHLOROFORM
NITROUS OXIDE
XENON
HALOTHANE
ENFLURANE
ISOFLURANE
SEVOFLURANE
DESFLURANE

12. MAC (Minimum Alveolar Concentration) OF INHALANT ANESTHETICS
- It is used to compare the strengths and potency of anesthetic
vapours.
1 HALOTHANE 0.75
2. ISOFLURANE 1.17
3. ENFLURANE 1.68
4. DESFLURANE 6.60
5. SEVOFLURANE 1.80
6. NITROUS OXIDE 105.0
7. XENON 71.0

13. ANESTHETIC MACHINES
i. The primary function of anaesthetic machines is to deliver precise amount of oxygen
and volatile anaesthetics under controlled conditions to patients undergoing general
anaesthesia .
ii. The basic principle of operation of anaesthetic machine can be described as follows.
(a) A liquid anesthetic (eg. Isoflurane, Sevoflurane) is vaporized in a CARRIER GAS
(oxygen with or without nitrous oxide), which deliver the anaesthetics to the
patients via a BREATHING CIRCUIT. To achieve this result, the ANESTHETIC MACHINE
and BREATHING CIRCUIT must perform several important functions -

18. 1. The CARRIER GAS must be delivered at a controlled flow rate. OXYGEN is the primary
carrier gas used in all anesthetic machines. NITROUS OXIDE is another carrier gas that
was often used with oxygen to deliver older inhalant anesthetics, although it is
seldom used with anesthetics in currents use.
2. A precise concentration of liquid inhalant anesthetic (most commonly ISOFLURANE
and SEVOFLURANE) must be vaporized, mixed with the carrier gases, and delivered to
the patient.
3. Exhaled gases containing CARBON-DIOXIDE, must be moved away from the patient
and either removed through a scavenging system or recirculated to the patient. If the
gases are recirculated, the machine must remove the carbon-dioxide before retuning
them to the patient.,

19. COMPONENTS OF ANESTHETIC MACHINES
- The anesthetic machines consists of 4 (four) distinct systems -
1. The Compressed
gas supply
2. The Anesthetic
Vaporizer
3. The Breathing
circuit
4. The Scavenging
system
- Supplies CARRIER
GASES (oxygen and
sometimes nitrous
oxide).
-Vaporizes liquid
inhalant anesthetic
and mixes with
CARRIER gases.
- 2 types of
vaporizers viz:
i. VOC (vaporizer-
out-of-circuit)
ii. VIC (vaporizer-
out-in-circuit)
- Conveys the
CARRIER GASES and
INHALANT
ANESTHETICS to the
patient and removes
exhaled CO2.
- Breathing circuits
a. Rebreathing
circuits
b. Non-rebreathing
circuits
- disposes of excess
and waste
anesthetic gases.

20. ANESTHETIC MACHINES are used not only for inhalation anesthesia but also as
a means of delivering oxygen to critically ill patients.
In these situations, the VAPORIZER (i.e the anesthetic source) is turned off,
and the breathing circuit is used to deliver oxygen directly to the patient via
ENDOTRACHEAL TUBE or a MASK held over the patient’s muzzle.

21. A. COMPRESSED GAS SUPPLY – B. ANESTHETIC VAPORIZERS –
1. Compressed gas cylinders 1. VAPORIZER INLET PART
2. Tank pressure gauze 2. VAPORIZER OUTLET PART
3. Pressure-reducing valve 3. COMMON GAS OUTLET
4. Line-pressure gauze
5. Flow meter (s)
6. Oxygen flush valve
PARTS OF THE ANESTHETIC MACHINE SYSTEMS

22. C. BREATHING CIRCUITS
A) REBREATHING B) NON-REBREATHING
1. FRESH GAS INLET - (BOTH REBREATHING & NON-REBREATHING)
2. POP-OFF VALVE - (BOTH REBREATHING & NON-REBREATHING)
3. RESERVOIR BAG - (BOTH REBREATHING & NON-REBREATHING)
4. UNIDIRECTIONAL VALVES 7. AIR INTAKE VALVE
5. CO2 ABSORBER CANISTER 8. BREATHING TUBES
6. PRESSURE MANOMETER 9. Y-PIECE

23. C. SCAVENGING SYSTEM
1. DISCHARGE HOSE (passive or active system)
2. PRESSURE REGULATOR (active system)
3. OUTFLOW PIPE (passive or active system)
4. PUMP (active system)

24. Compressed Gas Cylinders
or Gas Tanks
UK & India – Black with White shoulders
Canada – Green & White

25. - After exiting the FLOWMETER, Oxygen enters the
vaporizer through the inlet-port.
- The function of the VAPORIZER is to convert a LIQUID
ANESTHETIC (such as ISOFLURANE and SEVOFLURANE)
to a gaseous state and to add controlled amounts of
this vaporized anesthetic to the CARRIER GAS (oxygen
and nitrous-oxide).
ANESTHETIC VAPORIZERS

26. - After exiting the VAPORIZER through the OUTLET PORT, the oxygen and anesthetic
mixture known as ‘FRESH GAS’ enters into the BREATHING CIRCUIT through a
connection referred to as ‘fresh gas-inlet’.
NOTE: Most vaporizers are designed for use with only one specific anesthetic agent,
which is purchased in a liquid form and poured into the vaporizer).

27. TYPES OF ANESTHETIC VAPORIZERS
– VOC and VIC are used to describe 2- different ways anesthetic
machines are configured based on the location of the
vaporizers in relation to the breathing circuits
VOC – stands for vaporizer-out-of-circuit
VIC – stands for vaporizer-in-circuit
VOC – indicates that the vaporizer is not located within the
breathing circuit. In this case, OXYGEN from the flow-
meters flows into the vaporizer before entering into the
breathing circuit.
VIC - indicates that the vaporizer is located within the breathing
circuit. In this case, OXYGEN enters into the breathing
circuit directly from the flow-meters without first
entering into the vaporizers.
- Exhaled gas enter into the vaporizer each time the
patient breathes.

28. – BREATHING CIRCUIT consists of a group of components that carry ANESTHETIC and
OXYGEN from the ‘fresh-gas inlet’ to the patient and conveyed expired gases away
from the patient.
- The breathing circuit may be incorporated into the anesthetic circuit (as in case with
REBREATHING SYSTEM), or it may be a separate unit (as in case with NON-
REBREATHING SYSTEM).
BREATHING CIRCUIT

29. – A rebreathing system is an anesthetic machine fitted with a rebreathing circuit.
- They are called as “CIRCLE SYSTEM’ because exhaled gases minus carbon-dioxide (CO2) are
recirculated and rebreathed by the patient, along with variable amounts of fresh oxygen and
anesthetics.
- The exhaled gases includes – oxygen (O2), anesthetic vapor, CO2, nitrogen, water vapour and
nitrous-oxide (if used).
- When a rebreathing system is used, the gases exhaled by the patient travel through the EXPIRATORY
BREATHING TUBE and the EXPIRATORY UNIDIRECTINAL VALVE then to the CO2 ABSORBER CANISTER.
Then they are directed to RESERVOIR BAG, POP-OFF VALVE and PRESSURE MANOMETER and back
towards the patient through the INSPIRATORY UNIDIRECTIONAL VALVE and INSPIRATORY
BREATHING TUBE.
- Fresh oxygen and anesthetic enter the circuit from the ‘fresh gas-inlet’ and mix with the patient’s
exhaled gases.
REBREATHING SYSTEMS
OR CIRCLE SYSTEM

30. REBREATHING SYSTEMS - CLASSIFICATION
1. CLOSED RE-BREATHING SYSTEM
(TOTAL RE-BREATHING SYSTEM)
2. SEMI - CLOSED RE-BREATHING SYSTEM
(PARTIAL RE-BREATHING SYSTEM)
= the main difference lies in the amount of ‘carrier gas’ that is delivered to the
breathing circuit (FLOW METERS) and the position of the POP-OFF VALVE =
- Here, the pop-off valve us kept nearby or
completely closed and the flow of oxygen
is relatively low, providing only the volume
necessary to meet the patient’s metabolic
needs
-Here, the pop-off valve is positioned
partially open, and more oxygen is added
than the patient requires.
- this type of machine is relatively easy to
use and meets the needs of the majority of
the patients.
- for this reason, it is the most common
machine configuration used in clinical
practice.

31. COMPONENTS OF RE-BREATHING CIRCUIT
1. FRESH GAS INLET
2. UNIDIRECTIONAL (or ONE-WAY) VALVES
3. POP-OFF (or PRESSURE RELIEF) VALVE
4. RESERVIOR BAG
5. CARBON-DIOXIDE ABSORBER CANISTER
6. PRESSURE MANOMETER
7. AIR INTAKE VALVE
8. BREATHING TUBES
9. Y-PIECE

33. - Is a rubber bag, often BLACK or GREEN which serves a number of functions - .
1. It serves as a flexible storage reservoir –
- Because the breathing circuit is essentially a system of tubes and parts with a fixed
volume, a space is necessary to accept expired air and provide air needed to fill the
lungs during inspiration.
2. It allows anaesthetist to observe the animal’s respirations –
- The bag expands as the patient exhales and contracts as the patient inhales.
- Both the respiratory rate and respiratory depth can be determined by observing the
movement of bag.
- A lack of movement indicates “APNEA’, a disconnected Y-piece or blockage of the
airway.
RESERVOIR BAG
(REBREATHING BAG)

35. - Inadequate movement indicates a leak in the system, a partial airway blockage, or a
decreased Vt.
3. It may be used to confirm proper Endotracheal Tube (ET) placement –
- Movement of the bag in concert with patient respiration indicates that the ET is
correctly placed within the trachea.
4. It allows delivery of anesthetic gases to the patient –
- By application of light pressure to the bag (also known as manual ventilation or
bagging), oxygen with or without anesthetic gas can be forced into the patient’s
lungs.

37. NOTE:
- The reservoir bags are available in various sizes.
500 ml – for very small patients , to
30 L – for adult HORSES and CATTLE
Ideally, the bag should hold a volume of at least 60ml/kg of patient weight.
- If the bag is UNDERSIZED or OVERSIZED, a number of complications may arise -
a. If the rebreathing bag is TOO SMALL, the patient may be unable to fill its lungs
completely during inspiration.
- an UNDERSIZED BAG may also become overinflated during exhalation, thereby
increasing air pressure in the patient’s airway
- on the other hand, movement of an OVERSIZED BAG is hard to see, impairing the
ability of the anesthetist to monitor respirations.

38. GUIDELINES FOR SELECTING A BAG
500 ml for up to 3 Kg
1 L for 4 -7 Kg
2 L for 8 – 15 Kg
3 L for 16 – 50 Kg
5 L for 51 – 150 Kg
30 L for LARGE ANIMALS OVER 150 Kg.

39. - The exhaled gases are directed to EXPIRATORY UNIDIRECTIONAL VALVE to the CO2 -
absorber canister before being returned to the patient.
- Gas may enter the canister through the bottom or top depending on the design.
- The canister contains ABSORBENT GRANULES called as CO2-GRANULES. The main
ingredient in these products is CALCIUM HYDROXIDE (Ca[OH]2), along with 14% to
19% water and small amounts of sodium-hydroxide (NaOH), potassium hydroxide
(KOH), calcium chloride (CaCl2), and or calcium sulfate (CaSO4), which activate the
chemical reaction. During this reaction, heat and water are produced and the pH
decreases.
CO2 + Ca (OH)2 CaCO3 + H2O + Heat
Carbon-DIOXIDE ABSORBER CANISTER

40. - When significant amount of CO2 are absorbed, the HEAT released by this reaction
may cause the CO2 – canister to become warm during use. The WATER produced by
this reaction may serve to humidify the fresh gas entering the breathing circuit from
the fresh gas inlet.
- CO2-absorbents are supplied as loose granules or in a repackaged cartridge. The
granules are of a size (4-8 mesh) large enough to allow gases to pass through
without excessive resistance, but small enough to provide adequate surface area
for absorption of CO2.

41. NOTE:
- CO2 absorbents do not last indefinitely.
After absorption of about 26 L of CO2/100 g of absorbent, the granules become
exhausted. The use of depleted granules will result in rebreathing of CO2, leading to
‘HYPERCAPNEA’.
- There are several ways in which the anesthetist may become aware of granules that
are exhausted and must be replaced, including the following –
1. FRESH GRANULES (mainly calcium hydroxide), can be chipped or crumbled with finger
pressure, whereas, saturated with CO2 (containing calcium carbonate) become hard
and brittle.

42. 2. FRESH GRANULES are white, whereas, exhausted granules are slightly off-white
(visible if crystals are examined carefully)
3. Most granules contain pH indicator, that will cause the granules to change colour
when exhausted, form white to violet or in some cases, pink to white depending on
the brand.
- Granules that have changed colour (indicating saturation with CO2) may return to
the original colour after a few hours. Thus, it is important that the anesthetist
remove granules that have changed colour as soon as possible after using an
anesthetic machine.

44. 4. When a CAPNOGRAPH is used to monitor the patient, the concentration of CO2 during
peak inspiration should be at or near ZERO (0) mmHg, if the absorbent is working
correctly. A higher CO2 level at peak inspiration indicates possible exhaustion of the
absorbent.
- Granules should be discarded when no more than one-third or one-half have
changed colour.
- Granules should be always changed after 6-8 hrs of use.

45. NOTE:
SIGNS THAT CO2-GRANULES MUST BE CHANGED ARE AS FOLLOWS -
1. Hard, brittle granules.
2. Granules that are slightly off-white in colour.
3. Colour change of one-half to one-third of granules.
4. A CO2 level greater than 0 mmHg during peak inspiration as measured with a
capnograph.
5. Granules should always be changed after 6-8 hrs of use.

46. - The INSPIRATORY and EXPIRATORY BREATHING TUBES (corrugated breathing tubes)
complete the breathing circuit by carrying the anesthetic gases to and from the
patient.
- Each tube is connected to a UNIDIRECTIONAL VALVE at one end and to the Y-PIECE
at the other end.
- They are made of RUBBER or PLASTIC and available in three (3) sizes –
i. Small animal (22 mm diameter)
ii. Large animal (50 mm diamter)
iii. Small animal pediatric (15 mm diameter)
REBREATHING TUBES & Y- PIECE

47. - One end of each tube must be firmly attached to the INSPIRATORY and EXPIRATORY
UNIDIRECTIONAL VALVES , and the other end of each tube to the Y-PIECE which
connects the two-tubes together.
- The remaining portion of the Y-PIECE is then connected to a MASK or ET
(Endotracheal tube) tube.

48.  Although the REBREATHING SYSTEMS are suited to many patients; very small
patients (under 2.5 to 3 Kg BW) require the use of a machine configuration called as
NON-REBREATHING SYSTEM.
 In this system, little or no exhaled gases are returned to the patient but instead
evacuated by a SCAVENGER connected to POP-OFF valve, or to an EXIT-PORT.
NON- REBREATHING SYSTEMS
NON-REBREATHING SYSTEM
OPEN NON-REBREATHIN SYSTEM SEMI-OPEN NON-REBREATHING SYSTEM
-System in which no gases returned - system in which some gases returned
to the patient to the patient

49. - just as in a rebreathing system –
 OXYGEN (nitrous oxide, if used) flows from the TANK through a FLOW METER and
into the VAPORIZER (i.e. tank flow-meter vaporizer)
FRESH GAS PATIENT
 Exhaled gases enter the RESERVOIR BAG and ultimately released into the scavenging
system through POP-OFF/OVERFLOW VALVE or other EXIT-PORT.
NOTE: In this system, a CO2-absorbent is not needed, because exhaled gases are vented
from the system immediately after exhalation.

50. - Several components that are used in REBREATHING CIRCUITS are not present in a
NON-REBREATHING CIRCUIT. These includes –
i. Carbon-dioxide absorber canister
ii. Pressure manometer
iii. Unidirectional valves (inspiratory and expiratory)
NON- REBREATHING circuits

51. - NON-REBREATHING CIRCUIT are available in various configurations -
1. Brain co-axial circuit
2. Ayers T-Piece
3. Magill circuit
4. Lack circuit
5. Jackson-Rees circuit, and
6. Norman mask elbow
- These circuits are lightweight and easy to move and position, and are comparatively
inexpensive)
- Non-rebreathing circuits are grouped using the MAPLESON CLASSIFICATION SYSTEM
(only class A, modified A, modified D, E and F are in common use in veterinary
patients).