2. History of Low flow Anaesthesia
As early as 1850 John Snow (1813-53) recognized that only a
small fraction that inhalation anaesthetics gets metabolized
and most of it exhaled unchanged in the expired air of the
anaesthetized patients.
He also concluded that the narcotic effect of the volatile
anaesthetics could be markedly prolonged by re-inhaling
these unused vapours.
About 75 years later, in 1924, the rebreathing technique with
carbon dioxide absorption was introduced into routine clinical
practice by Ralph Waters (1883-1979), the to-and-fro
absorption system.
Brian C. Sword introduced the circle (CO2 absorption) system in
the 1930s.
3. LOW FLOW ANESTHESIA
• Any technique that utilises a fresh gas flow (FGF) that is less than the
alveolar ventilation can be classified as „Low flow anaesthesia‟.
• Baum et al : had defined it as a technique wherein at least 50% of the
expired gases had been returned to the lungs after carbon dioxide
absorption in the next inspiration.
• Baker had classified the FGF used in anaesthetic practice into
the following categories:
• Metabolic flow : about 250 ml /min
• Minimal flow : 250-500 ml/min.
• Low flow : 500- 1000 ml/min.
• Medium flow : 1 - 2 l/min.
• For most practical considerations, utilization of a fresh gas flow less than 2
litres/min may be considered as low flow anaesthesia.
4. CONT.
• there exists a need for a system that provided the advantages of
the completely closed circuit and at the same time, reduced the
dangers associated with it.
• Low flow anaesthesia fulfilled these requirements.
• Low flow anaesthesia involves utilizing a fresh gas flow which is
higher than the metabolic flows but which is considerably lesser
than the conventional flows.
• The larger than metabolic flows provides for considerably greater
margin of safety and satisfactory maintenance of gas composition
in the inspired mixture.
• THE NEED FOR LOW FLOW ANAESTHESIA
• Completely closed circuit anaesthesia is based upon the reasoning that
anaesthesia can be safely maintained if the gases which are taken up
by the body alone are replaced into the circuit taking care to remove
the expired carbon dioxide with sodalime. No gas escapes out of the
circuit and would provide for maximal efficiency for the utilisation of
the fresh gas flows.
5. Requirements for LFA
1. A circle system with CO2 absorption.
2. A flow meter for the adjustments of FGF below 1 l/min.
3. Gas-tight breathing system, recommended test leakage
should be below 150 ml/min.
4. The breathing system must be having minimal internal
volume and a minimum number of components and
connections.
5. Continuous gas monitoring must be employed, essential in
controlling the patient's alveolar gas concentrations.
6. The vaporizers delivering the high concentrations must be
calibrated and must be accurate at low fresh gas flow.
6. Circle System
• The minimum requirement for conduct of low
flow anaesthesia is the effective absorption of
CO2 from the expired gas, so that the CO2 free
expired gas can be reutilized for alveolar
ventilation.
• In the past, two systems were in use; “To and
Fro” system introduced by Waters and the “circle
system” introduced by Brian Sword.
• But because of its bulkiness near the patient and other
disadvantages the “To and Fro” has gone out of favour
and the “circle system” using large sodalime canisters is
in use.
7. Describing a Circle System.
The circle system should have the
basic configuration with two
unidirectional valves on either side
of the sodalime canister, fresh gas
entry, reservoir bag, pop off valve,
and corrugated tubes and „Y‟ piece
to connect to the patient.
The relative position of fresh gas
entry, pop off valve, and reservoir
bag are immaterial as long as they
are positioned between the
expiratory and the inspiratory
unidirectional valves that functions
properly and CO2 absorption is
efficient at all times.
8. THE PRACTICE OF LOW FLOW
ANAESTHESIA:
The practice of low flow anaesthesia can be
dealt with under the following three
headings:
o 1. Initiation of Low flow anaesthesia
o 2. Maintenance of Low flow anaesthesia
o 3. Termination of Low flow anaesthesia.
9. INITIATION OF LOW FLOW ANAESTHESIA
Primary aim at the start of low flow anaesthesia is to
achieve an alveolar concentration of the anaesthetic
agent that is adequate for producing surgical
anaesthesia (approximately 1.3 MAC).
Factors influencing to buildup of alveolar concentration should be
considered . These factors can broadly be classified into three
groups :
1) Factors governing the inhaled tension of the anaesthetic agent,
2) Factors responsible for rise in alveolar tension,
3) Factors responsible for reducing the alveolar tension, uptake
from the lungs.
10. Factors governing the inhaled tension of
the anaesthetic.
Three most important factors responsible
for inhaled tension of anesthetics are :
1.BREATHING CIRCUIT VOLUME
2. RUBBER GAS SOLUBILITY
3. SET INSPIRED CONCENTRATION
11. Breathing Circuit Volume
Circle system is often bulky and volume roughly equal to 6-7 liters, FRC of the
patient, which is roughly 3 litres, together constitutes a reserve volume of 10
liters; to which the anaesthetic gases and vapours have to be added.
• With the addition of FGF, the rate of change of composition of the reserve
volume is exponential, time required for the changes to occur is governed by
the time constant, which is equal to this reserve volume divided by the fresh
gas flow.
• Three time constants are needed for a 95% change in the gas
concentration to occur.
• Hence, if a FGF of 1L/min is used, then 30 minutes will be required for the
circuit concentration to reflect the gas concentration of the FGF.
• If the FGF is still lower, then correspondingly longer time will be
required.
12. Rubber Gas Solubility of the agent
• The anaesthetic agent could be lost from
the breathing system due to solubility of the
agent in rubber, and permeability through
the corrugated tubes.
• The concentration will take a longer time if
solubility is more.
• Though the amount of loss will be minimal,
it should be considered at the start if the
aimed anaesthetic concentration is low.
13. SET INSPIRED CONCENTRATION
The rate of rise of alveolar partial pressure of the
anaesthetic agent must bear a direct relationship
to the inspired concentration.
Higher the inspired concentration, the more
rapid is the rise in alveolar concentration.
The Inspired concentration of the agent
depends on several factors.
– Denitrogenation of the circuit volume.
– Alveolar ventilation.
– Concentration effect.
14. Denitrogenation of FRC & Breathing
Circuit Volume
The functional residual capacity of the lung
and the body as a whole contain nitrogen
which will try to equilibrate with the circuit
volume and alter the gas concentration if
satisfactory denitrogenation is not achieved at
the start of anaesthesia.
Hence, as a prelude to the initiation of closed or low flow
anaesthesia, thorough denitrogenation must be achieved with
either a non-rebreathing circuit or the closed circuit with a
large flow of oxygen and a tight fitting facemask.
15. Alveolar ventilation:
The second factor governing the delivery of
anaesthetic agent to the lung is the level of
alveolar ventilation.
The greater the alveolar ventilation, the
more rapid is the rise of alveolar
concentration towards the inspired
concentration.
• This effect is limited only by the lung volume, the larger
the functional residual capacity, the slower the wash. in
of the new anaesthetic gas.
16. Factors responsible for uptake from the lungs
thus reducing the alveolar tension
• 1. CARDIAC OUTPUT .
• 2. BLOOD GAS SOLUBILITY .
• 3. ALV – VENOUS GRADIENT.
17. Cardiac output
• Because blood carries anaesthetic away from the
lungs, the greater the cardiac output, the
greater the uptake, and consequently the slower
the rate of rise of alveolar tension.
• At low inspired concentration, the alveolar
concentration results from a balance between
the ventilatory input and circulatory uptake.
• If the later removes half the anaesthetic
introduced by ventilation, then the alveolar
concentration is half that inspired.
18. Blood gas solubility
• If other things are equal, the greater the blood gas
solubility coefficient, the greater the uptake of
anaesthetic, and slower the rate of rise of alveolar
concentration.
19. Alveolar to venous partial pressure gradient:
• During induction the tissues remove all the
anaesthetic brought to them by the blood.
Tissue uptake lowers the venous anaesthetic
partial pressure far below that of the arterial
blood.
• If tissue uptake is more then it results in a large alveolar
to venous anaesthetic partial pressure difference,
which promotes maximum anaesthetic uptake and
hence lowers the alveolar partial pressure.
20. Concentration effects
The concentration effect modifies this influence of uptake.
o When appreciable volumes are taken up rapidly, the lungs do not
collapse; instead the sub atmospheric pressure created in the lung by the
anaesthetic uptake causes passive inspiration of an additional volume of
gas to replace that lost by uptake, thus increasing the alveolar
concentration and offsetting the mathematical calculations.
o Similarly, if an insoluble gas (e.g., nitrogen) is present in the inspired
mixture, as the blood takes up the anaesthetic gas, the concentration of
the insoluble gas will go up in the alveoli, reducing the concentration of
the anaesthetic agent.
o Concentration effect: The concentration effect helps in raising the
alveolar tension towards the inspired tension, but hinders with it if an
insoluble gas is present in the mixture.
21. Methods to achieve desired gas and
agent concentration.
• High flow for short time
• Use of Prefilled Circuits with gases
• Use of large doses of anaesthetic agents.
• Injector technique.
22. Use of high flows for a short time
This is by and far the commonest and the most effective technique of initiating
closed circuit.
• By using high flows for a short time, the time constant is reduced thereby
bringing the circuit
concentration to the desired concentration rapidly.
• Often, a fresh gas flow of 10L of the desired gas concentration and 2 MAC agent
concentration is used so that by the end of three minutes (three time
constants) the circuit would be brought to the desired concentration.
Advantages:
--- Rapidity with which the desired concentration is achieved, the ability to
prevent unexpected raise in the agent concentration and the ability to
plenum vaporizers to achieve the desired concentration.
– This also has the added advantage of achieving better denitrogenation, so
vital to the conduct of the low
flow anaesthesia.
The chief disadvantage:
- Not economical & need scavenging systems to prevent theatre pollution
Mapleson3 using a spreadsheet model of a circle breathing system has
calculated that, by using a FGF equal to minute ventilation and setting the
anaesthetic agent partial pressure to 3 MAC, the end expired partial pressure
of halothane will reach 1 MAC in 4 minutes and that of isoflurane in
1.5 minutes.
23. Use of Prefilled circuit
• The second method is utilizing a
different circuit like Magill’s for pre-
oxygenation.
• Simultaneously, the circle is fitted with a test
lung and the entire circuit is filled with the gas
mixture of the desired concentration.
• Following intubation, the patient is connected to the circuit
thereby ensuring rapid achievement of the desired
concentration in the circuit.
24. Use of large doses of anaesthetic
agents.
The third method consists of adding large amounts of anaesthetic agent
into the circuit so that the circuit volume + FRC rapidly achieves the
desired concentration as well as compensates for the initial large
anaesthetic gas uptake.
To execute this, Pre-oxygenation after intubation, fresh gas flow is started
with metabolic flows of oxygen and a large amount of nitrous oxide often
in the range of 3-5 litres per minute.
Oxygen concentration in the circuit gradually falls, is continuously
monitor Oxygen and the nitrous oxide concentration, N2O flow is reduced
once the desired oxygen concentration (33 - 40%) is achieved .
Disadvantage of this method : is hypoxia if the oxygen monitor were to
malfunction.
o Hence this method is seldom used for N2O.
o In the contemporary anaesthesia machines it is not possible to administer
lesser than 25% of oxygen and the described technique cannot be
executed.
25. Injection techniques.
An alternative method for administering the large amounts of the
agents is by directly injecting the agent into the circuit. The high
volatility coupled with the high temperature in the circle results in
instantaneous vaporization of the agent.
The injection is made through a self sealing rubber diaphragm
covering one limb of a metal t piece or a sampling port, inserted into
either the inspiratory or the expiratory limb.
o This is an old, time-tested method and is extremely reliable.
o Each ml of the liquid halothane, on vaporization yields 226 ml of
vapour and each ml of liquid isoflurane yields 196 ml of vapour at
20oC. Hence, the requirement of about 2ml of the agent is
injected in small increments into the circuit.
26. THE MAINTENANCE OF LOW FLOW
ANAESTHESIA
This is the most important phase as this is
stretched over a long period of time and financial
savings result directly from this; this phase is
characterized by :
1. Need a steady alveolar concentration of respiratory gases.
2. Minimal uptake of the anaesthetic agents by the body.
3. Need to prevent delivery of hypoxic gas mixtures.
Since the uptake of the anaesthetic agent is small in this phase,
adding small amounts of the anaesthetic gases to match the uptake
and providing oxygen for the basal metabolism should suffice.
27. Management of the oxygen and nitrous oxide flow
during the maintenance phase
The need to discuss the flow rates of N2O and O2 arises specifically because
of the possible danger of administration of a hypoxic mixture.
Let us analyze the following example. 33% oxygen is set using a flow of 500
ml of O2 and 1000 ml of N2O.
o Oxygen is taken up from the lungs at a constant rate of about 4 ml/kg/min.
o N2O is a relatively insoluble gas and after the initial equilibration with the FRC
and vessel rich group of tissues, the up take is considerably reduced.
o In this situation, there is a constant removal of O2 at a rate of 200 - 250
ml/min, where as the insoluble gas N2O uptake is minimal. Hence the gas that
is partly vented and partly returning to the circuit will have more N2O and less
of O2.
o Over a period of time, due to the mixing of fresh gas that has 66% N2O and the
expired CO2 free gas that has N2O much higher than that, the percentage of
N2O will go up and that of O2 will fall, sometimes dangerously to produce
hypoxic mixtures.
A high flow of 10 lit/min at the start, for a period of 3 minutes, is followed by a
flow of 400 ml of O2 and 600 ml of N2O for the initial 20 minutes and a flow of
500 ml of O2 and 500 ml of N2O thereafter.
This has been shown to maintain the oxygen concentration between 33 and 40 %
at all times.
28. Management of the potent anaesthetic agents during
maintenance phase
This is easily accomplished by dialling in the calculated
concentration on the plenum vaporizer for the flow being used.
For example, suppose the anaesthetic uptake for a desired
concentration of 0.5% halothane is 7.5ml/min.
o If a FGF of 500ml/min is being used, then the dial setting should be
1.5% for at this setting and for the used flow, the total vapour
output would be 7.5ml/min.
o If a flow of 1000ml/min is being used, then the dial setting should
be 0.8%.
In practice the actual dial setting often over estimates the actual
output since the plenum vaporizer under delivers the agent at low
flows.
Hence, the dial setting is fine-tuned depending on the endpoints
being achieved.
29. Weir and Kennedy recommend
infusion of halothane
Weir and Kennedy recommend infusion of
halothane (in liquid ml/hr) at the following rates
for a 50 kg adult at different time intervals.
• 0-5 min 27 ml/hr
• 5-30 min 5.71 ml/hr
• 30-60 min 3.33ml/hr
• 60-120 min 2.36 ml/hr
These infusion rates had been derived from the
Lowe's theory of the uptake of anaesthetic agent.
30. Salient points to be considered during the
maintenance phase
The other salient points to be considered
during the maintenance phase are the
following:
o a) Leaks must be meticulously sought for and prevented, flows must
be adjusted to compensate for the gas lost in the leaks.
o b) Most of the gas monitors sample gases at the rate of 200 ml/min,
which may be sometimes as high as half the FGF. Hence, care must be
taken to return the sample back to the circuit to maximize the economy of
FGF utilisation.
31. TERMINATION OF LOW FLOW
ANAESTHESIA.
Unlike the initiation or the maintenance of the closed circuit,
termination is less controversial. There are only two recognized
methods of termination of the closed circuit.
A. Use of high flow of gases: the circuit is opened and a high flow of gas is
used to flush out the anaesthetic agents .
B . Use of activated charcoal.
o Activated charcoal when heated to 220oC adsorbs the potent vapours
almost completely. Hence, a charcoal-containing canister with a bypass is
placed in the circuit, towards the end of the anaesthesia, the gas is
directed through the activated charcoal canister.
o This results in the activated charcoal adsorbing the anaesthetic agent
resulting in rapid recovery and at the same time, reducing theatre
pollution. Nitrous oxide, due to its low solubility is washed off towards the
end by using 100% oxygen.
32. Advantages of LFA system.
o 1.Enormous financial savings due to use of low fresh gas
flows as well as the agent.
o 2. High humidity in the system leads to fewer post
anaesthetic complications.
o 3.Maintenance of body temperature during prolonged
procedures due to conservation of heat.
o 4. Reduction in the theatre pollution.
o 5. Environmental Pollution ( Ozone depletion/green house
effect)
33. Disadvantages of LFA
• Inability to quickly alter the inspired concentrations.
• More attention is required
• Danger of hypercarbia.
• Uncertainty about inspired concentrations
• Faster absorbent exhaustion
• Accumulation of undesirable trace gases in the system like CO,
acetone, methane, hydrogen, ethanol, argon, nitrogen,
compound A, etc.
34. Concerns about Safety in LFA
Points of concern in providing safe
anaesthesia are:
o Hypoxia.
o Misdosage of the volatiles.
o Exhaustion of the absorbent.
o Gas volume deficiency.
o Reduced controllability.
35. Absorber assembly :
An absorber assembly consists of
► A canister containing an absorbent.
► two ports for connection to breathing
tubes
► a fresh gas inlet.
► Other components that may be
mounted are inspiratory and expiratory
unidirectional valves , an adjustable
pressure limiting (APL) valve, and a bag
mount.
36. Canisters :
The absorbent is held in canisters.
► Single or two canisters in series can be used.
► Side walls are transparent so that color changes can be easily recognised.
► Comes in large and small sizes.
37. Absorbent
Types
► High alkali absorbents
High amount of NaOH or KOH
When desiccated react with anesthetic agent to form CO or compound-A
Do not change color when dry.
► Low alkali absorbents
Less amount of KOH or NaOH
Less amount of CO or compound-A when desiccated.
► Alkali free absorbent
Consists mainly Ca(OH)2 and other added agents No evidence of CO
formation
Little or no compound-A formation
38. Different formulations
► 1. Sodalime
2. Baralyme
3. Sofnolime
4. Amsorb plus
5. Ca(OH)2 lime etc,.
6. LiOH lime
39. ► Sodalime is made of
► Ca(OH)2 - 80%
► NaOH – 4%
► KOH – 1%
► Water – 14% to 19 %
Silica or Kieselguhr – helps in hardening and reduced dust formation.
Indicator
40. ► CO2 absorption is exothermic reaction
► Principle – acid base neutralization
►End product – carbonic acid and water.
CO2 + H2O → H2CO3
H2CO3 + 2NaOH → Na2CO3 (sodium carbonate)+ 2H2O
Na2CO3 + Ca(OH)2 → 2NaOH or 2KOH + CaCO3 .
41. Pattern of CO2
absorption
A.Unused absorbent in the canister.
B.After limited use- absorption of CO2 has occurred primarily at the inlet and to lesser
extent along the sides.
C.After extensive use – granules at the inlet and along the sides are exhausted.
D.Exhausted sodalime – few granules at the distal 1/3rd are still capable of absorbing
CO2.
E.Channeling effect
42. Indicators
color When fresh when exhausted
Phenolphthalein white Pink
Ethyl violet White Purple
Clayton yellow Red Yellow
Ethyl orange Orange Yellow
Mimosa Z Red White
43. ► Shape and size of absorbent
pellets or granules form
Advantage - small granules increases the surface area, decreases the
gas channelling along low resistance pathway.
Disadvantage –
increased
resistance,caking.
4 to 8 mesh more commonly used.
44. REACTIONS BETWEEN ABSORBENTS AND
ANESTHETIC AGENTS
► Haloalkene Formation
Halothane degradation most often occurs during closed-circuit anesthesia and
produces the haloalkene 2-bromo-2- chloro-1, 1-difluoroethene
(BCDFE).Nephrotoxic in rats.
►Compound A Formation
Sevoflurane decomposes in the presence of carbon dioxide absorbents to
compound A. Compound A is vinyl ether and has dose dependent
nephrotoxic effects in rats.
45. Carbon monoxide formation
► when desflurane, enflurane, or isoflurane is passed through
desiccated absorbent containing a strong alkali (potassium or
sodium hydroxide)
► When sevoflurane is degraded by desiccated absorbent, carbon
monoxide is formed if the temperature exceeds 80°C.
46. Factors associated with carbon
monoxide formation:
► Absorbent Composition – NaOH , KOH
► Absorbent Desiccation ( dehydrated absorbent).
► Anesthetic Agent – Desflurane > Enflurane>Isoflurane .Halothane produces
very less amount of CO.
► Increased temperature Inside the Absorber.
► Fresh Gas Flow - FGF leads to more dessication of absorbent and more CO
production.
► Carbon Dioxide Absorption – Increases absorption leads to decreased CO
production.
47. The APSF recommendations to prevent absorbent
desiccation if the department continues to use
strong alkali absorbents with volatile anesthetic
agents:
► All gas flows should be turned OFF after each case.
► Vaporizers should be turned OFF when not in use.
► Canisters on an anesthesia machine that is commonly not used for a long
period of time should not be filled with absorbent that contains strong alkali
or should be filled with fresh absorbent before each use.
► The absorbent should be changed routinely, at least once a week, preferably
on a Monday morning, and whenever fresh gas has been flowing for an
extensive or indeterminate period of time.
48. ► The canister should be labelled with the filling date.
► Checking this date should be part of the daily machine checklist.
► If a double-chamber absorber is used, the absorbent in both canisters
should be changed at the same time.
► The practice of supplying oxygen for administration to a patient who is not
receiving general anesthesia through the circle system should be strongly
discouraged.
► Using fresh gas to dry breathing system components should be
discouraged.
49. WHEN AND HOW TO CHANGE THE ABSORBENT
► Inspired Carbon Dioxide - Most reliable method ( indicated by capnograph)
► Indicator Color Change
► Heat in the Canister
50. HUMIDITY
general term used to describe the amount of water vapor in a gas
may be expressed several ways:-
Absolute Humidity
- the mass of water vapor present in a volume of gas
- commonly expressed in milligrams of water per liter of gas.
Humidity at Saturation
-maximum amount of water vapor that a volume of gas can hold
- varies with the temperature.The warmer the temperature, the more water
vapor can be held in a gas
HUMIDIFICATION
51. RELATIVE HUMIDITY
the amount of water vapor at a particular temperature expressed as a
percentage of the amount that would be held if the gas were saturated.
DEW POINT
The temperature at which a gas is 100% saturated
WATER VAPOR PRESSURE
Humidity may also be expressed as the pressure exerted by water vapor
in a gas mixture.
52. normal breathing :upper respiratory tract warms humidifies, and filters
inspired gases, primarily in the nasopharynx
Even at extremes of inspired temperature and humidity, gas that
reaches the alveolar level is 100% saturated at body temperature.
ISOTHERMIC SATURATION BOUNDARY (ISB) : The point at which gases
reach alveolar conditions (37°C and 100% relative humidity)
Over the course of a normal day, the respiratory tract loses
approximately 1470 J of heat and 250 mL of water.
IMPORTANCE OF HUMIDIFICATION
53. Water is intentionally removed from medical gases so that gases delivered
from anaesthesia machine are dry and at room temperature
Gases must therefore be warmed to body temperature and saturated
with water by upper respiratory tract
normal nasal breathing : temperature in upper trachea is between 30-
33o C with RH-98% & AH-33 mg/L
: at alveolar level 37 degreeC , RH- 100% ,AH – 44 mg/L
Tracheal intubation and high fresh gas flows bypass this normal
humidification by upper airways and expose lower airways to dry (< 10 mg
H2O/L), room temperature gases
54. EFFECTS OF INHALING DRY GASES
DAMAGETOTHE RESPIRATORYTRACT
Mucosa dries
Temperature drops
Secretions thicken if not cleared atelectasis / airway obstruction
Thick plugs loci for infection
Ciliary function reduced
Surfactant activity impaired
Mucosa more susceptible to injury
Dry gases bronchoconstriction
Body heat loss
Absorbent desiccation
Trachealtube obstruction
55.
56. SOURCES OF HUMIDITY
CO2 absorbent – reaction ofCO2 with absorbent releases water
-- absorbent granules also contain water
Exhaled gases – some rebreathing is present in tracheal tube, supraglottic
airway device, and connections to breathing system
-almost half of humidity in expired gases is preserved in this manner
Moistening/Rinsing breathing tubes and reservoir bag before use
Low fresh gas flows – conserve moisture
Coaxial breathing circuits – increase humidity more quickly than a system with
2 separate limbs, when combined with low flows
-not very efficient : Bain system (coaxial Version of Mapleson D) does not meet
optimal humidification requirements because of high fresh gas flow required
58. HME
•SYNONYMS :- condenser
humidifier, swedish nose,
artificial nose, nose humidifier,
passive humidifier, regenerative
humidifier, moisture exchanger,
and vapor condenser
•When combined with filter for
bacteria & viruses :- HMEF
59.
60. disposable devices
exchanging medium enclosed in a plastic housing
vary in size and shape
Each has a 15-mm female connection port at the patient
end and a 15-mm male port at the other end; patient port
may also have a concentric 22-mm male fitting
May have a port to attach the gas sampling line for a
respiratory gas monitor or an oxygen line
One type utilizes a ceramic heating element with a water
input port, a membrane, and an aluminum grid that
vaporizes the water
Preferred for short term use.
61. Hydrophobic
hydrophobic membrane with small
pores
membrane is pleated to increase the
surface area
provides moderately good inspired
humidity
may be impaired by high ambient
temperatures
efficient bacterial and viral filters
prevent the HCV from passing
allow the passage of water vapor but
not liquid water at usual ventilatory
pressure
associated with small increases in
resistance even when wet
Hygroscopic
contain a wool/foam/paperlike material
coated with moisture-retaining chemicals
medium may be impregnated with a
bactericide
Composite hygroscopic HMEs -- a
hygroscopic layer plus a layer of thin,
nonwoven fiber membrane that has been
subjected to an electrical field to increase its
polarity -- improves filtration efficiency and
hydrophobicity.
composite hygroscopic HMEs are more
efficient than hydrophobic ones
lose their airborne filtration efficiency if they
become wet; microorganisms held by the
filter medium can be washed through the
device
Their resistance can increase greatly when
wet
62. Type
Heat and moisture
exchangingefficiency
Hygroscopic
Excellent
Hydrophobic
Good
Effect of increased tidal
volume on heat and
moisture exchange
Slightdecrease Significantdecrease
Filtrationefficiency
when dry
Good Excellent
Filtrationefficiency
when wet
Poor Excellent
Resistance when dry Low Low
Resistance when wet Significantlyincreased Slightlyincreased
Effect of nebulized
medications
Greatly increased
resistance
Little effect
63. CONTAINDICATIONS
thick, copious, or bloody secretions
expired tidal volume less than 70% of the delivered tidal volume (eg, those
with large bronchopleurocutaneous fistulas or incompetent or absent
endotracheal tube cuffs)
body temperatures less than 32°C
high spontaneous minute volumes (> 10L/min)
Spontaneously breathing, difficult to wean patients – relativeC/I
MOISTUREOUTPUT
•Amount of Heat & Moisture provided.
•Currently no standards for minimum MO for HME’s
•Depends upon –TV, insp time, RR, system leak, initial humidity.
64. ADVANTAGES DISADVANTAGES
Temp preservation
Dead space rebreathing
IncreasedWOB
Resistance
ETT obstruction
Not for all patients
Disconnection
CO2 absorbent dessication
Avoid using with nebulizers
Short term use
Inexpensive
Easy to use
Portable,
light weight
Less risk of contamination
Efficient filter
Do not require external heat,
water, temp monitoring
No condensate- thus no
ventilator malfunction
65. HAZARDS
ineffective low-pressure alarm during disconnection due to
resistance through HME.
Suspected high resistance Peak airway pressure should be
measured withAND without HME in place
possible increased resistive work of breathing due to mucus plugging
of HME
In Mapleson system – resistance causes diversion of FGF down the
expiratory limb.
hypothermia
airway obstruction– ETT blockage/ ETT kinking/ detachment
Should not be used with Heated Humidifiers
nebulized medications increase resistance of hygroscopic HME.
66. •Ineffective filtration:- liquid can break through a hygroscopic HME
•Foreign body aspiration – detached parts inhaled
•Rebreathing due to dead space (more in paed)
•Hypothermia
•Leaks & disconnections
•dryCO2 absorbent- absorb volatile agents, hence delay in induction.
POSITION INCIRCUIT : -
Patient – HME – filter – sampling line – machine/ ventilator
67. HUMIDIFIERS
- Add water to gas by passing the gas over a water chamber
(passover) / through a saturated wick (wick humidifier) /bubbling
it through water (bubble-through) / mixing it with vaporized
water (vapor-phase)
- do not filter respiratory gases
UNHEATED
Disposable, bubble-through
devices used in oxygen supplied to
patients via facemask or nasal
canula
Simple containers containing
distilled water through which
oxygen is passed and it gets
humidified
Maximum humidity that can be
achieved is 9mg H2O/L
HEATED
Incorporate a device to warm
water in the humidifier, some
also heat inspiratory tube
Humidification chamber
Heat source
Temperature monitor
Thermostat
Inspiratorytube
69. HEATED HUMIDIFIERS
Humidification chamber – contains liquid water, disposable/
reusable, clear (easy to check water level)
Heat source – heated rods immersed in water/ plate at bottom of
humidification chamber
Temperature monitor – to measure gas temperature at patient end
of breathing system
Thermostat –
1. Servo-controlled units – automatically regulates power to heating
element in response to temperature sensed by a probe near
patient connection/ humidifier outlet
2. Non servo-controlled units – provides power to heating element
according to setting of a control, irrespective of delivered
temperature
70. Inspiratory tube – conveys humidified gas from humidifier outlet
to patient
If unheated gas will cool and lose some of its moisture as it
travels to the patient, water trap necessary to collect condensed
water
If Heated or insulated more precise control of temperature
and humidity delivered to patient, avoids moisture rainout
Controls – most allow temperature selection at end of delivery
tube or at humidification chamber outlet
Alarms – to indicate temperature deviation by a fixed amount,
displacement of temperature probe, disconnection of heater
wire, low water level in humidification chamber, faulty airway
temperature probe , lack of gas flow in the circuit
Standard requirements -An international and aU.S. standard on
humidifiers have been published
71. In circle system, heated humidifier is placed in the inspiratory limb
downstream of unidirectional valve by using an accessory
breathing tube
Must not be placed in the expiratory limb
Filter, if used, must be placed upstream of humidifier to prevent it
from becoming clogged
In Mapleson systems, humidifier is usually placed in fresh gas
supply tube
Humidifier must be lower than patient to avoid risk of water
running down the tubing into the patient
72. Condensate must be drained periodically or a water trap
inserted in the most dependent part of the tubing to prevent
blockage or aspiration
Heater wire in delivery tube should not be bunched, but strung
evenly along length of tube
Delivery tube should not rest on other surfaces or be covered
with sheets, blankets, or other materials; a boom arm or tube
tree may be used for support
73. Advantages –
1. Capable of delivering saturated gas at body temperature or
above, even with high flow rates
2. More effective humidification than an HME
Disadvantages –
1. Bulky and somewhat complex
2. Involve high maintenance costs, electrical hazards, and
increased work (temperature control, refilling the reservoir,
draining condensate, cleaning, and sterilization)
3. Offers relatively little protection against heat loss during
anaesthesia as compared to circulating water and forced-air
warming
74. NEBULISERS
Aerosol generators/ atomizers/ nebulizing humidifiers
Emit water in the form of an aerosol mist (water vapor plus
particulate water)
Used for producing humdification and delivery of drug directly into
respiratory tract
Drugs delivered by nebulizers – Bronchodilators, decongestants,
mucolytic agents, steroids
Optimal particle size of droplet (aerosol) = 0.5 to 5 µm
Particles > 5 µm – unable to reach peripheral airways, deposited in
main airways
Particles < 0.5 µm - very light, come back with expired gases
without being deposited in airways
75. Most commonly used 2 types –
1. Pneumatically driven (gas-driven, jet, high pressure, compressed
gas)
2. Ultrasonic
76. Pneumatically driven nebulizer works by pushing a jet of high-
pressure gas into a liquid, inducing shearing forces and breaking
the water up into fine particles
Should be placed in the fresh gas line (high flow of gas must be
used with pneumatic nebulizer)
Produces particles of size 5 to 30 µm (only 30 to 40% of particles
produced are in optimal range) most of the particles get
deposited in wall of main airways
77. Ultrasonic nebulizer
produces a fine mist by subjecting the liquid to a high-frequency,
electrically driven resonator
Can be used in the fresh gas line or the inspiratory limb (No need
for a driving gas)
Frequency of oscillation determines the size of the droplets
Creates a denser mist than pneumatic ones
produces mist with aerosol size of 1 to 10 µm (95% of particles
produced are in optimal range) particles get deposited directly
in airways very useful for delivery of bronchodilators directly in
peripheral airways
Can nebulize 6 mL of water or drug in 1 minute
78.
79. HAZARDS
1. Nebulized drugs may obstruct an HME or filter in the
breathing system
2. Overhydration
3. Hypothermia
4. Transmission of infection
5. Case reports where a nebulizer was connected
directly to a tracheal tube without provision for
exhalation resulted in pneumothorax in one case
80. Advantage - can deliver gases saturated with water
without heat and, if desired, can produce gases carrying
more water
Disadvantages –
1. Somewhat costly
2. Pneumatic nebulizers require high gas flows
3. Ultrasonic nebulizers require a source of electricity and
may present electrical hazards
4. May be considerable water deposition in the tubings,
requiring frequent draining, water traps in both the
inspiratory and exhalation tubes, and posing the dangers
of water draining into the patient or blocking the tubing
82. ● Twodifferent classesof
chemicals:
nitrousoxide and
halogenated agents
● In1977NationalInstitute for
Occupational SafetyandHealth
(NIOSH) –
recommendedexposurelimits
(RELs) for both nitrousoxide and
halogenatedagent
83. Scavenging and Waste Anesthetic
Gases (WAGs)
SCAVENGING
● Definition
● Scavengingisthe collectionand
removal ofvented anesthetic
gases from the OR.
● Since the amount ofanestheticgas
supplied usuallyfar exceedsthe
amount necessary for the patient,
OR pollution isdecreased by
scavenging.
● Scavengerandoperating room
ventilationefficiencyare the two
most important factors in reduction
ofwaste anestheticgases(WAGs).
84. ● Theuseofscavengingdeviceswith anesthesiadeliverysystemsisthe
most effectivewayto decrease waste anestheticgases.
● Anefficientscavengingsystemiscapableof reducing ambient
concentrations ofwastegasesbyup to 90%.
● Anesthesiamachinesandbreathingsystemsdelivering
halogenatedhydrocarbonanestheticsand/ornitrous
oxideshouldnotbeoperatedunlesstheyareequipped
withafunctional scavengingsystem
.
● Tobe effective,ascavengingsystemmustnot leakandmustcontrol
theconcentrationoftrace anestheticsin ambient air.
● Wastegasesshould not bedischarged into the outsideair in an
areawhere reentryinto the buildingislikely.
85. Types
● Scavengingmaybe active(suctionapplied) or passive(waste gasesproceed
passivelydown corrugated tubing through the room ventilation exhaust
grill ofthe OR).
● Activesystemsrequire ameansto protect the patient's airwayfrom
the applicationofsuction, or buildup ofpositive pressure.
● Passivesystems require that the patient be protected from
positivepressure buildup only.
86. ● The 2 majorcausesof wastegascontamination in the O.R :
1. The anesthetic technique used and
2. The equipment issues
Regardinganesthesiatechnique the followingwill increase anesthetic gas
pollution wheninhalantanestheticsare used:
● Administeringinhalantanestheticsbyopen drop (eg, periodically
drippingliquid volatileanesthetic onto agauzesponge) or
insufflation(eg, delivery ofarelativelyhighflowof anesthetic in
oxygeninto the tracheaor pharynx throughacatheter) techniques.
● Turning on flowmetersandvaporizersbefore attachingthe
breathingsystemto the patient.
● Allowingflowmetersandvaporizersto remain on after the patient
isdisconnected fromthe breathingsystem.
87. ● Using uncuffed endotracheal tubes that do not create a
completely sealed airway or using cuffed tubes without inflating
the cuff.
● Disconnecting apatient from abreathing system without eliminating as
much of the residual gases as reasonably possible through the
scavenging system.
● Spillingliquidanesthetic duringthe filling ofvaporizers,especially
duringan anestheticprocedure.
● Poorlyfittingmasks.
● Flushingthe circuit.
● Use ofbreathing circuits that are difficult to scavenge, suchasJackson-
Rees.
89. 1.Gas collecting assembly
● Capturesexcessanesthetic
gases and deliversit to the
transfer tubing.
● WAGare vented fromanesthesia
systemthrough either
adjustable pressure limiting
valveor ventilator reliefvalve.
● Gaspassingthroughthese valves
accumulatesin the gas
collecting assemblyand is
directed to the transfer
means.
90. 2.Transfer tubing
● The transfer tubescarriesexcessgasfromgascollecting assembly
to scavenging interface.
● The tubes mustbe either 19 or 30 mm, sometimesyellowcolor-coded .
● The tubesshould be sufficientlyrigidto prevent kinkingand asshort as
possibleto minimizethe chanceofocclusion.
● Separatetubes from the APLvalveandventilator relief valvemerge into a
singlehosebefore they enter scavenginginterface.
● Ifthe transfer meansisoccluded, baselinebreathing circuit pressure
willincrease and barotrauma can occur.
91. 3.Scavenging interface
● The scavengerinterfaceisthe most important component because
it protects the breathing circuit or ventilator from excesspositiveor
negativepressure.
● Positive-pressurereliefismandatory, irrespective ofthe typeof
disposable systemused, to vent excessgasin caseofocclusiondistal
to interface.
● scavenger interfacesmaybe open
closed
93. OPEN INTERFACE:
● It containsno valvesand isopen to
the atmosphere, allowingpositive
and negativepressure relief.
● Open interfacesshould be used
only withactive disposable systems
that haveacentralevacuation
system.
● open interfaces require areservoir
becausewastegasesare
intermittently discharged insurges
whereasflow fromthe evacuation
systemis continuous.
94. ● The efficiencyofit dependson several factors:
A. Thevacuum flowrate per min must equalor exceed
the minute volume ofexcessgasesto prevent spillage.
B. Spillagewill occur ifthe volume ofasingle exhaled
breathexceedsthe capacityofreservoir.
Openinterfaces
aresaferforthepatients.
95. ● CLOSEDINTERFACE:
● It communicateswith the atmosphere throughvalves.
● Twotypesofclosedinterfacesare commerciallyavailable:
positive pressure reliefonly
positive and negativepressure relief
POSITIVEPRESSURERELIEFONLY:
● It hasasingle positive pressure relief valve and is designated to be used
onlywithpassivedisposable systems.
● Transferofthe wastegasfromthe interface to the disposablesystem
reliesonweightor pressureofthe wastegasitselfbecauseanegative
pressure evacuation systemisnot used.
● In thissystemreservoir bagisnot required.
96.
97. POSITIVEANDNEGA
TIVEPRESSURERELIEF:
● It haspositive and negative pressure relief valvein addition to areservoir
bag.
● It isused with active disposable systems.
Theeffectivenessofaclosed systeminpreventing spillagedependson:
the rate ofwaste gasinflow
the evacuation flow
rate the size of the
reservoir
● Leakage occurs only when the reservoir bag becomes fully inflated and
pressure increases sufficiently to open the positive pressure relief valve
.
● In contrast, the effectiveness of aopen system in preventing spillage
depends not only on the volume of the reservoir but also on the
98. 4.Gas disposal tubing
● The gas disposable tubing conducts waste gas from the Scavenging
interface to the gas disposable assembly.
● It should be collapse proof and should run overhead, if possible to
minimize the chance of accidental occlusion.
● It ultimately eliminates excess waste gas.
● It is of two types: Active and Passive
● Active assembly:
● most commonly used. It uses central evacuation system.
● A vacuum pump serves as mechanical flow inducing device that removes the
waste gases.
● An interface with a negative pressure relief valve is mandatory because the
pressure within the system is negative.
● A reservoir is very desirable and the larger the reservoir, the lower the
suction flow rate needed.
5.Gas disposal assembly
99. - PipedVacuum
- Active duct system - employsflow inducingdevices(fans,
pumps, blowersetc..) to move evenat low pressures
Advantages
● Convenientin largehospitals, where manymachinesare in
use in different locations
Disadvantages
● Vacuumsystemandpipe workisamajor expense
● Needlevalve mayneedcontinualadjustment
100. ● Passivedisposablesystem
:
● doesnot use amechanical flowinducingdevice.
● The weightor pressure from the heavier-than-air anesthetic
gasesproducesflowthroughthe system.
● Positive pressure reliefismandatory, but anegative pressure reliefand
reservoir are unnecessary.
● Passive– simpler and lessexpensivebut not effective
- Roomventilation system
- Nonrecirculatingor – Recirculating
- Pipingdirect to atmosphere
- Adsorption devices
- Catalyst decomposition
101. Charcoal canisters :
● activated charcoal
● connected to the outlet ofthe breathing system
● removeshalogenated anaestheticsby filtration
CatalyticDecomposition :
• Catalyticdecompositioncanbe
used
to convert nitrousoxide to nitrogen
andoxygen, reducingits
contribution to the greenhouse
effect
102. Advantages
● No set-up cost
● Mobile - moveswiththe
machine
Disadvantages
● Continuingcost ofreplacement.
● Continuosreplacement ismessy
● Doesnot remove nitrousoxide
● Heatingthe canister causesthe release of
the
inhalational
agents
103. Evaluation of Anesthetic Equipment
● Eachpieceofequipment involved in the deliveryofinhalant
anestheticsshouldbe evaluated regularly to assureits function and
integrity.
Proceduresforcheckoutofanesthesiaequipment,dependingontheequipmentto
beused,shouldincludethefollowing:
● Statusofthehigh-pressuresystem,includingtheoxygensupplyandnitrousoxide
supply- The nitrous oxide supply should not leak when the cylinder valve
ison and the nitrousoxideflowmeter isoff.
● Statusofthelow-pressuresystem(flowmeterfunction)- Anegative-pressure
leaktest should be performed at the commongasoutlet or the outlet
ofthe vaporizer immediatelyupstreamfromthe breathingsystem.
104. ● Statusofthebreathingsystem- Anappropriateleaktest for acircle system
andNoncirclesystemscanusuallybe done. The quantityofleakagecan
be measuredbydeterminingthe flowrate ofoxygen necessaryto
maintainaconstant pressureinthe system,andthe leakrate should be
lessthan300ml/ minat 30cmofh2O.
● Statusofthescavengingsystem- Thescavengingsystemshouldbeproperly
attachedat allconnectors, andthe appropriate vacuumshould be
assured for activesystems.Ifcharcoalcanistersare employedfor
scavenging,theyshould bechangedat appropriateintervals.
● Statusofmechanicalventilators- Ventilatorsshouldbeconnected properly
to the anesthesiamachine, andanabsence ofleaksshould be assured.
105. Monitoring of the Effectiveness of
Antipollution Techniques
● Monitoring trace-gas concentrations in the workplace provides a
quantitative assessment of the effectiveness of a waste-gas
control program.
● Measuringthe concentration ofanesthetic in the breathing zoneof
the most heavilyexposed workersisthe usual procedure.
● Anair-monitoring programismostappropriately startedafter
anesthesiadeliverysystemshavebeen equipped with scavenging
systemsandafter other techniquesfor minimizingwastegaspollution
are in place.
● Anidealapproachwouldincludefrequentairmonitoring,preferably at least
semiannual evaluations.
106. Effectiveness
● Unscavengedoperatingrooms show10-70 ppmhalothane, and 400-
3000ppmN2O.
● scavengingbrings these levels downto 1 and60 ppmrespectively.
● Addingcarefulattention to leaksandtechniquecanyieldlevelsaslowas
0.005 and 1 ppm.
107. HAZARDS
● Scavengingsystemfunctionallyextendsthe anesthesiacircuit all the way
fromthe anesthesiamachine to the disposable site.
● Obstruction of scavengingpathwaycancauseexcessivepositivepressure in
the breathing circuit andbarotraumacanoccur.
● excessivevacuum appliedcanresult in undesirablenegativepressures
withinbreathingsystem.
● LossofMonitoringInput – it maymaskthe strong odor ofavolatile
anesthetic agent, delaying recognition ofanoverdose
● Alarmfailure – neg. pressure from the scavengingsysteminterface
prevent the bellows from collapsingwhen breathing systemis
disconnected . Low
airwaypressure alarmnot activated.
108. Avoiding waste gas exposure
● Good maskfit.
● Avoidunscavengeable techniquesifpossible(insufflation).
● Prevent flowfrom breathing systeminto room air (onlyturn on agent
andnitrous oxide after maskison face, turn them offbefore suctioning).
● Washoutanesthetics(into the breathing circuit) at the end of
the anesthetia.
● Don’t spill liquidagent.
● Use low flows.
● Usecuffedtracheal tubes whenpossible.
● Checkthe machine regularlyfor leaks.
● Disconnectnitrous oxide pipeline connection at wallat the day’send.
● Total intravenousanesthesia.
