The document provides a comprehensive overview of anaesthesia machines, detailing their history, functions, types, components, and essential safety features. It explains the evolution of these machines from simple devices to modern anaesthesia workstations with sophisticated safety systems and monitoring capabilities. Additionally, it outlines the guidelines for machine obsolescence and the standards that govern their design and safety requirements.

1. Anaesthesia Machine,
Types and Components
Presenter: Dr. Suresh Pradhan
Moderator: Dr. Yogesh Dhakal

2. Outline
• Introduction
• Historical Perspective
• Functions of an Anaesthesia Workstation
• Types of anaesthesia Machine
• Standards for Anaesthesia Machines & Workstations
• Essential Safety Features On A Modern Anesthesia
Workstation

3. • Guidelines For Determining Anesthesia Machine
Obsolescence
• Functional Schematic Of An Anaesthesia Machine
• Components of Anaesthesia Machine

4. • a device which delivers a precisely known but
variable gas mixture, including anesthetizing and
life-sustaining gases

5. • function of the anaesthesia machine is to:
−receive gases from the central supply & cylinders
−meter them and add anesthetic vapors, and
−deliver them to the patient breathing circuit

6. • conceptually, a pump for delivering medical gases
and inhalation agents to the patient’s lungs
• the “pump” in the modern anaesthesia machine is
either:
−a mechanical ventilator, or
−lungs of the spontaneously breathing patient, or
−perhaps, a combination of the two

7. • no piece of equipment is more intimately
associated with the practice of anesthesiology
than the anaesthesia machine

8. • on the most basic level, the anesthesiologist uses
the anaesthesia machine to control the patient’s
ventilation and oxygen delivery and to administer
inhalation anesthetics
• proper functioning of the machine is crucial for
patient safety

9. • has evolved over the past 160 years from a rather
simple ether inhaler to a complex device of valves,
pistons, vaporizers, monitors, and electronic
circuitry

10. • modern anaesthesia machines have become very
sophisticated, incorporating
−many built-in safety features and devices
−monitors
−multiple microprocessors that can integrate and
monitor all components

11. • additional monitors can be added externally and
often still be fully integrated
• moreover, modular machine designs allow a wide
variety of configurations and features within the
same product line

12. • the term anaesthesia workstation is therefore
often used for modern anaesthesia machines
• consists of:
−the anaesthesia machine
−ventilator
−breathing system
−scavenging system
−monitors

13. −added to this may be:
− drug delivery systems,
− suction equipment, and
− a data management system

14. • there are two major manufacturers of anaesthesia
machines in the United States:
1)Datex-Ohmeda (GE Healthcare)
2)Dräger Medical

15. Historical Perspective
• Early anaesthesia: no definitive airway control
∙ mask anaesthesia, inhalers, drop mask techniques
were all equally capable of producing an unconscious
patient
∙ but offered no airway protection or control against
apnea or emesis
• WTG Morton, in 1846, first publicly demonstrated
‘general anaesthesia’, at Massachusetts General
Hospital, preserved for posterity as the ‘ether
dome’

16. • several improvements in drugs, apparatus and
techniques have made anaesthesia safe over the
years
• 1877: Joseph Clover describes jaw-thrust technique
for opening airway
• performed surgical airway with metal canula (first
cricothyrotomy by anaesthesia provider)

17. • Frederick Hewitt developed a device for preventing
the tongue from obstructing the airway in the
unconscious patient
• he called this device the “air-way restorer”
• device was a direct precursor to modern oral
airways.

18. • the original concept of Boyle's
machine was invented by the
British anaesthetist H.E.G. Boyle in
1917
• was eventually patented by British
Oxygen Company as ‘Boyle’s
Machine’
• the British Army used a portable
version during the First World War

19. • it was a modification of the American Gwathmey
apparatus of 1912 and became the best-known
early continuous flow anaesthetic machine
• there were several features in the simple machine,
which made anaesthesia easier to administer and
safer, compared to earlier methods

20. • several advances were incorporated in Boyle’s
machines also over the years from the earlier Boyle
Basic to
∙Boyle E
∙Boyle F
∙Boyle G
∙Boyle H
∙Boyle M
∙Boyle major and
∙Boyle International models

22. • 1920 – a vaporizing (Ether) bottle was incorporated
to the machine
• 1926 – a second vaporizing bottle (Chloroform) and
by-pass controls were incorporated
• 1930 – a Plunger device was added to the
vaporizing bottles
• 1933 – a dry-bobbin type of flowmeter was
introduced and replaced the water sight feed
bottles

23. • 1937 – rotameters displaced dry-bobbin type of
flowmeters
• 1952 – pin-index system
• 1979 – Standards for anaesthesia machines

24. • the early Boyle’s machine had five elements which
are still present in all modern machines:
• a high-pressure supply of gases: It housed two
oxygen and two nitrous oxide cylinders in a
wooden box
• pressure gauges on oxygen cylinders and fine-
adjustment reducing valves
• these produced a manageable breathing system
pressure
• it had a spirit flame to warm these and prevent
obstruction of gas flow from ice

25. • flowmeters to control gas flow rate and adjust
proportions of gas delivered
• a metal and glass vaporizer bottle for ether
• a breathing system comprising a Cattlin bag,
three-way stopcock and facemask

26. • safety features were incorporated sequentially
over a huge time span of nearly 100 years of
evolution of the anaesthesia machine (since 1917),
with more and more safety systems being added as
realization of the problems and mishaps surfaced
• the traditional pneumatic anaesthesia machine has
evolved into a complex electrical, mechanical and
pneumatic multi component workstation

27. Functions of a WORKSTATION
• Safe provision, selection and delivery of anesthetics
• Provision of back up supplies of gases
• Respiratory support
• Monitoring of machine function
• Monitoring of patient
• Record Keeping

28. • Anesthetic Gas Scavenging System (AGSS)
• Suction regulator
• Supplemental oxygen
• Work surface and storage facility for “everyday
items”
• Electricity sockets

29. Types of anaesthesia Machine
• can be divided into two classes based on the flow
of gas through the machine
Intermittent gas flow type
Continuous gas flow type

30. Intermittent Gas Flow Type
• gas flows only during inspiration
• they operate on demand
• gas flow is drawn by inspiration/inhalation
• are useful for short surgical procedures

31. • Examples:
i. Mackessons apparatus
ii. Walton V Machine
iii.Modified Walton V Machine (Lucy Baldwin’s
apparatus)
iv.Entonox apparatus

32. • Entonox apparatus:
• intermittent gas flow machine by Rovenstein
• it has premixed cylinder of oxygen (50%) with
nitrous oxide (50%)
• cylinder has body of French blue with top white
with pin index of 7
• pressure regulator

33. • pressure gauge
(2000 psi)
• demand valve
• key to open the
cylinder
• circuit provided with
facemask, expiratory
valve, corrugated
tube & metal hand
piece to hold mask

34. • oxygen & nitrous oxide mixture available in gas
form due to Poynting effect (at 2000 psi pressure &
at room temperature, oxygen has solvent action,
keeps nitrous oxide in gaseous form)
• the mixture separates into component parts, at -7
degree celsius known as pseudocritical
temperature, carries risk of hypoxia due to nitrous
oxide

35. • this is prevented by:
 proper storing
 avoiding excessive cooling
 rewarming of cylinder, shaking , inverting several
times before use

36. • Used very effectively as analgesic for :
⌐ Dressing of surgical wounds
⌐ Dressing of burns
⌐ Labour analgesia
⌐ Dentistry
⌐ Pain relief for trauma patients
⌐ Post-operative pain relief
⌐ In ophthalmological examination
⌐ During cardiac catheterization

37. • Other intermittent gas flow machines :
1. EMOTRIL
• E.M.O. apparatus with trilene
• grey in colour
• draw-over giving air + trilene (0.3 to 0.5 %)
2. TECOTA
• temperature compensated trilene apparatus giving air +
trilene

38. 3. CYPRANE INHALER
• Giving air+ methoxyflurane (0.35 %) for labour analgesia
4. CARDIFF INHALER
• giving air+ methoxyflurane (0.35 %) for labour analgesia
• inhaled during painful uterine contractions in first stage
of labour

39. Continuous Gas Flow Type
• gas flows both during inspiration and expiration
• Examples:
⌐Boyle Machine
⌐Forregar
⌐Dragger

40. Datex-Ohmeda Aestiva

41. Datex-Ohmeda Aestiva

42. Dräger Medical Fabius GS
Anaesthesia Workstation

43. Dräger Apollo Anesthesia
Workstation

44. GE Healthcare Aisys Anaesthesia
Workstation

45. Standards for Anaesthesia Machines
and Workstations
• standards for anaesthesia machines and workstations
provide guidelines to manufacturers regarding their:
a. minimum performance
b. design characteristics
c. safety requirements

46. • during the past decades, the progression of
anaesthesia machine standards has been as
follows:
• 1979: American National Standards Institute
• 1988: American Society for Testing and
Materials
• 1994: ASTM F1161-94 (reapproved in 1994 and
discontinued in 2000)
• 2005: International Electrical Commission (IEC)

47. • 2005: ASTM (reapproved) F1850
• current standards: Standard Specification for
Particular Requirements for Anesthesia
Workstations and Their Components F1850-00,
found within ASTM volume 13.02, September
2012, Medical and Surgical Materials and
Devices
• European standard is EN740

48. Essential Safety Features On A Modern
Anesthesia Workstation

49. Essential Safety Features On A
Modern Anesthesia Workstation

50. • changes in equipment design have been directed at
minimizing the probability of breathing circuit
misconnects and disconnects and automating
machine checks
• because of the durability and functional longevity
of anesthesia machines, the ASA has developed
guidelines for determining anesthesia machine
obsolescence

51. Guidelines For Determining
Anesthesia Machine Obsolescence
Criterias:
• Absolute Criteria
• Relative Criteria

52. Absolute Criteria
• an anaesthesia machine shall be considered to be
obsolete if any of the following criteria apply
I. Lack of Essential Safety Features
i. minimum oxygen ratio device (O2 and N2O
proportionating system) on a machine that can
deliver N2O
ii. Oxygen failure safety/fail safe device
iii. Oxygen supply pressure failure alarm
iv. Vaporizer interlock device
v. Pin index safety system
vi. Non-interchangeable, gas-specific (DISS) connectors
on the gas pipeline inlets

53. II. Presence of Unacceptable Features
III. Adequate maintenance no longer possible

54. Relative Criteria
• consideration should be given to replacing an
anaesthesia machine if any of the following apply
I. Lack of certain safety features

55. II. Problems with maintenance
III. Potential for human error
IV. Inability to meet practice needs

56. Functional Schematic Of An
Anaesthesia Machine/Workstation

57. Components of Anaesthesia Machine
• Master Switch
• Power Failure Indicator
• Reserve Power
• Electrical Outlet
• Circuit Breakers
• Data Communication Ports
Electrical
Components
• High Pressure System
• Intermediate Pressure System
• Low Pressure System
Pneumatic
Components

58. Index

60. Electrical Components

61. Master Switch
• activates both the pneumatic and electrical
functions
• on most machines, when the master switch is in
the OFF position, the only electrical components
that are active are the backup battery charger and
the electrical outlets
• on some machines, electrical components can be
activated without pneumatic power

62. • a standby position may be
present to allow the machine
to be quickly powered up
• electronic machines utilize a
complicated power-up
procedure that includes a
system checkout
• in addition to an electronic
checkout, the computer
gathers data that are
necessary for proper function

63. Power Failure Indicator
• most machines are equipped with a visual and/or
audible indicator to alert the anesthesia provider to
the loss of mains electrical power

64. Reserve Power
• since electricity is crucial for most anesthesia
machines, a backup source is provided
• will provide power for about 30 minutes, depending
on usage
• the anesthesia provider should check the battery
status during the pre-use checkout procedure
• a non interruptible power source may be added to
the anaesthesia machine to extend the backup period

66. Electrical Outlet
• most modern anesthesia machines have electrical
outlets at the back of the machine
• these are intended to power monitors and other
devices
• as a general rule, these outlets should only be used
for anaesthesia monitors
• usually cannot supply electricity if there is a power
failure
• other appliances should be connected directly to
mains power

68. Circuit Breakers
• there are circuit breakers for both the anesthesia
machine and the outlets
• when a circuit breaker is activated, the electrical
load should be reduced and the circuit breaker
reset

69. Data Communication Ports
• most modern anesthesia
machines have data
communications ports
• these are used to communicate
between the anesthesia
machine, monitors, and the data
management system

70. Pneumatic Components

71. • gases are supplied under tremendous pressure for
the convenience of storage and transport
• the anaesthesia machine receives medical gases
from a gas supply; controls the flow of desired
gases reducing their pressure to a safe level
• so, the pressure inside a source (cylinder or
pipeline) must be brought to a certain level before
it can be used for the purpose of ventilation

72. • also, it needs to be supplied in a constant pressure,
otherwise the flow meter would need continuous
adjustment
• this is achieved by bringing down the pressure of a
gas supply in a graded manner with the help of
three pressure reducing zones

73. • thus, the pneumatic part of the machine can be
conveniently divided into three parts-
1. High Pressure System: from cylinder to pressure
reducing valves
2. Intermediate Pressure System: from pressure
reducing valves to the flowmeters
3. Low Pressure System: from the flowmeters to
the common gas outlet

74. Components of
High Pressure System
1. Hanger Yoke
2. Check Valve
3. Cylinder Pressure Gauge (Indicator)
4. Pressure Reducing Device/ Regulator

75. 1. Hanger Yoke
the hanger yoke assembly-
 orients and supports the cylinder
 provides a gastight seal
 ensures a unidirectional gas flow

76. • it is composed of several parts:
a) the body, which is the principal framework and
supporting structure
b) the retaining screw, which tightens the cylinder
in the yoke
c) the nipple, through which gas enters the
machine

77. d) the Pin Index Safety System pins, which prevent
an incorrect cylinder from being attached
e) the washer (bodock seal), which helps to form a
seal between the cylinder and the yoke
f) a filter to remove particulate matter that could
come from the cylinder
g) a check valve assembly, prevent retrograde flow
of gases

78. • the workstation standard recommends that there
be at least one yoke each for oxygen and nitrous
oxide
• if the machine is likely to be used in locations that
do not have piped gases, it is advisable to have a
double yoke, especially for oxygen

79. Body
• is threaded into the frame of the machine
• provides support for the cylinder
• commonly the swinging gate type is used
• When a cylinder is mounted onto or removed from
a yoke, the hinged part can be swung to side

80. Retaining Screw
• it is threaded into the distal end of the yoke
• tightening the screw presses the outlet of the
cylinder valve against the washer and the nipple so
that a gas tight seal is obtained
• the cylinder is then supported by the retaining
screw, the nipple, and the index pins
• the conical point of the retaining screw is shaped to
fit the conical depression on the cylinder valve

82. Nipple
• it is a part of the yoke through
which the gas enters the
machine
• it fits into the port of the
cylinder valve
• if it is damaged, it may be
impossible to obtain a
tight seal with the cylinder
valve

83. Index Pins
• are situated below the nipple
• these help to prevent mounting of incorrect
cylinder to yoke
• the holes into which the pins are fitted must be of a
specific depth
• if they extend too far into the body of the yoke, it
may be possible to mount a incorrect cylinder

84. Pin Index Safety System
• it consists of-
• two holes on the cylinder valve positioned in an arc
below the outlet port
• pins on the yoke or the pressure regulator to fit into
these holes
• pins are: 4mm in diameter & 6mm in length
• the seven holes are in the circumference of a circle
of 9/16 inch radius with the outlet port as center
• the position of the pins and corresponding holes
are different for different gases
• prevents the placement of wrong gas cylinder in
the yoke

87. Washer (Bodock Seal)
• cylinders are fitted with yoke with a sealing
washer called Bodock Seal
• it is made up of non combustible material
• has a metal periphery which make it long lasting
• it should be less than 2.4mm thick prior to
compression
• only one seal should be use between the valve &
yoke

88. Filter
• is used to prevent particulate matter from entering
the machine
• is placed between the cylinder and the pressure
reducing device

89. Placing a Cylinder in a Yoke
• cylinder valves and yokes should not be
contaminated with oil or grease
• persons placing a cylinder in a yoke should always
wash their hands first
• Pin Index Safety System pins should be present
• the retaining screw should be retracted
• the washer is placed over the nipple

90. • the cylinder is supported by the foot
and guided into place manually
• the port on the cylinder valve is guided
over the nipple and the index pins
engaged in the appropriate holes
• the retaining screw is tightened
• should make certain the cylinder is full
and that there is no leak

91. 2. Check Valve
• it allows gas from a cylinder to enter the machine
but prevents gas from exiting the machine when
there is no cylinder in the yoke
• it allows an empty cylinder to be replaced with a
full one without having to turn off the cylinder in
use
• prevents transfer of gas from one cylinder to the
other with a lower pressure in a double yoke
• it consists of a plunger that slides away from the
side of the greater pressure

92. • it is not designed to act as a permanent seal for
empty yoke and may allow small amount of gas to
escape
• as soon as a cylinder is exhausted it should be
replaced by a full one or a dummy plug

94. • in order to minimize losses:
• yokes should not be left vacant for extended
periods
• an empty cylinder should be replaced as soon as
possible
• an yoke plug can be used to prevent gas leak
• an empty cylinder can be left behind after
closing the valve

96. 3. Cylinder Pressure Gauge
(Indicator)
• displays cylinder pressure for each gas
• indicators may be near the cylinder or on a panel in
front of machine
• calibration in kilopascals or pounds per square inch
• indicators are of bourdon tube type

98. • most new anaesthesia machines indicate cylinder
pressure digitally
• light-emitting diodes may also be used to indicate
adequate pressure in the cylinder

101. 4. Pressure Reducing
Device/Regulator
• reduces the high and variable pressure delivered
from a cylinder to a lower, more constant pressure
suitable for use in an anesthesia machine
• without a regulator, it would be necessary for the
anesthesia provider to constantly alter the flow
control valve to maintain a constant flow through
the flowmeter as the pressure in the cylinder
decreases
• the machine standard requires reducing devices for
each gas supplied to the machine from cylinders

102. • pressure regulators used in anesthesia machines
are preset at the factory
• the pressure at the regulator outlet is set lower
than the pipeline pressure
• this ensures that pipeline gas is used preferentially
to the cylinder supply if the cylinder valve is open
while oxygen from the piping system is being used

103. • this differential pressure may not always prevent the
cylinder from becoming exhausted since pressure
fluctuations in the pipeline may cause the pressure
in the machine to drop below the pressure from the
pressure regulator
• physical principle- a large pressure acting over a
small area is balanced by a small pressure over a
large area

104. Components of
Intermediate Pressure System
1. Master Switch (Pneumatic
Component)
2. Pipeline Inlet Connections
3. Pipeline Pressure Gauges
4. Oxygen Pressure Failure Devices
a)Oxygen Pressure Failure Safety Device
b)Oxygen Supply Failure Alarm

105. Components of
Intermediate Pressure System
5. Gas Selector Switch
6. Second-Stage Pressure
Regulator
7. Oxygen Flush
8. Flow Adjustment Control
a)Mechanical Flow Control Valves
b)Electronic Flow Control Valves
9. Alternate Oxygen Control

106. 1. Master Switch (Pneumatic
Component)
• the pneumatic portion of the master switch is
located in the intermediate-pressure system
downstream of the inlets for the cylinder and
pipeline supplies
• the oxygen flush is usually independent of this
switch
• the master switch may be a totally electronic switch
that when activated controls the various pneumatic
components in the anesthesia machine

107. • when the master switch is turned off, the pressure
in the intermediate-pressure system will drop to
zero

108. 2. Pipeline Inlet Connections
• pipeline inlet connections are the entry points for
gases from the pipelines
• the anesthesia workstation standard requires
pipeline inlet connections for oxygen and nitrous
oxide
there is usually connection for air in most machines
• the pipeline inlets are fitted with threaded non
interchangeable Diameter Index Safety System
(DISS) connectors

109. • each inlet contains
• a unidirectional/check valve to prevent reverse
gas flow from the machine into the piping
system (or to atmosphere if no hose is
connected)
• a filter to prevent debris from the pipeline
entering the anesthesia machine

111. 3. Pipeline Pressure Gauges
• gauges are present to monitor the pipeline pressure
of each gas
• they are usually on a panel on the front of the
machine (or the information screen, if present) and
color coded for the gases they monitor
• the workstation standard requires that the indicator
be on the pipeline side of the check valve in the
pipeline inlet
• some machines have digital pressure gauges that
display pressure either continuously or on demand

112. • some use light-emitting diodes (LEDs) to indicate
adequate pipeline pressure
• the sensing point for the pipeline pressure gauge is
located on the pipeline side of the check valve
• if the hose is disconnected or improperly
connected, it will read “0” even if a cylinder valve is
open

113. • pipeline pressure indicators should always be
checked before the machine is used
• the pressure should be between 50 and 55 psig
(345 and 380 kPa)
• indicators should be scanned repeatedly during use

116. 4. Oxygen Pressure Failure
Devices (OPFD)
• one of the most serious mishaps that occurred with
earlier anaesthesia machines was depletion of the
oxygen supply (usually from a cylinder) without the
user’s awareness
• the result was delivery of gas containing no oxygen
to the patient
• to remedy this problem, a device that turns off the
supply of gases other than oxygen (oxygen failure
safety device) is used

117. • in addition, an alarm to warn when oxygen
pressure has fallen to a dangerous level is used
• can be described as
• Oxygen Pressure Failure Safety Device
• Oxygen Supply Failure Alarm

118. Oxygen Pressure Failure Safety Device:
• ASTM standard requires that whenever the oxygen
supply pressure fails, the delivered oxygen
concentration shall not decrease below 19% at the
common gas outlet
• the oxygen failure safety valve (fail safe) placed in
the piping system for nitrous oxide (and on some
machines air) shuts off or proportionally decreases
and ultimately interrupts the supply of the other
gas if the oxygen supply pressure decreases

119. • when the pneumatic system is activated, oxygen
pressure opens the oxygen pressure failure safety
device, allowing other gases to flow
• a decrease in oxygen pressure causes it to interrupt
the flow of other gases to their flow control valves

121. Pressure Sensor Shut-off Valve: Datex Ohmeda
• operates in a threshold manner: either open or
shut
• oxygen pressure moves the piston and pin upward
and the valve opens for N2O
• when pressure of oxygen falls below preset value,
force of the valve return spring completely closes
the valve

123. Oxygen Failure Protection Device: Drager
• based on a proportioning principle rather than a
threshold principle
• pressure of N2O falls in proportion of decrease of
Oxygen
• total cutoff seen at <12psig
• seat nozzle assembly connected to a spring loaded
conical tapered piston

126. • an important concept to be understood with these
particular fail-safe devices is that
• the older Datex- Ohmeda Pressure Sensor Shut off
Valve is threshold in nature (all-or-nothing),
whereas
• the GE balance regulator and Dräger Oxygen Failure
Protection Device are variable, flow type
proportioning systems

127. Oxygen Supply Failure Alarm
• ASTM standard specifies that whenever the oxygen
supply pressure falls below a certain threshold
(usually 30 psig), alarm must get activated within 5
seconds
• it should not be possible to disable this alarm
• aid in preventing hypoxia caused by problems
occurring upstream in the machine circuitry
• disconnected oxygen hose
• low oxygen pressure in the pipeline
• depletion of oxygen cylinders

128. 5. Gas Selector Switch
• many machines have a gas selector switch that
prevents air and nitrous oxide from being used
together

130. 6. Second-Stage Pressure
Regulator
• some machines have additional pressure regulators
in the intermediate pressure system just upstream
of the flow adjustment controls
• the second-stage regulator receives gas from either
the pipeline or the cylinder pressure regulator
• reduces it further to around 26 psi (177 kPa) for
nitrous oxide and 14 psi (95 kPa) for oxygen

131. • purpose of this pressure regulator is to eliminate
fluctuations in pressure supplied to the flow
indicators
• reducing the pressures below the normal
fluctuation range causes the flow from the
flowmeters to remain more constant
• not all anesthesia machines incorporate this device

132. 7. Oxygen Flush
• also known as oxygen bypass, emergency oxygen
bypass
• receives oxygen from the pipeline inlet or cylinder
pressure regulator and directs a high (35 to 75 L/
minute) unmetered flow directly to the common
gas outlet
• is commonly labeled “O2+”

134. • the button is commonly recessed or placed in a
collar to prevent accidental activation
• on most anesthesia machines, the oxygen flush can
be activated regardless of whether the master
switch is turned ON or OFF

135. • the anesthesia workstation standard requires that
the connection of the flush valve delivery line to
the common gas outlet be designed so that
activation does not increase or decrease the
pressure at the vaporizer outlet by more than 10
kPa or increase the vapor output by more than 20%

136. Reported hazards associated with the oxygen flush
• accidental activation and internal leakage, which
result in an oxygen-enriched mixture being
delivered with anaesthetic dilution
• flush valve may get stuck in the ON position and
obstruct flow from the flowmeters
• barotrauma and awareness during anesthesia

137. 8. Flow Adjustment Control
• controls flow of gas through it’s associated
indicator by manual adjustment of a variable orifice
• current standard requires that:
 there must be only one flow control for each gas
 must be adjacent to or identifiable with its
associated flowmeter
•there are two types of flow adjustment controls:
mechanical and electronic

142. 9. Alternate Oxygen Control
• when using a computer-controlled anesthesia
machine, there is always the possibility that the
electronics will fail
• different machines deal with this problem in
different ways
• usually accomplished by a means to administer
oxygen
• is separate from the auxiliary (courtesy) flowmeter

144. Components of
Low Pressure System
1. Flowmeters
2. Hypoxia prevention safety devices
3. Unidirectional (Check) Valve
4. Pressure Relief Device
5. Common (Fresh) Gas Outlet

145. 1. Flowmeters
• also known as flow indicators, flow tubes, rotameters
• indicate the rate of flow of a gas passing through
them
• they may be mechanical or electronic
• electronic flowmeters usually have a representation
of a mechanical flowmeter on a screen or a number
representing the flow

146. Physical Principles
• traditional mechanical flow indicators used in
anaesthesia machines are the variable orifice
(variable area, Thorpe tube) type
• a vertical glass tube has an internal taper with its
smallest diameter at the bottom
• it contains an indicator that is free to move up and
down inside the tube
• when the flow control valve is opened, gas enters
at the bottom and flows up the tube, elevating
the indicator

148. • the gas passes through the annular opening
between the indicator and the tube wall and on to
the outlet at the top of the tube
• the indicator floats freely in the tube at a position
where the downward force caused by gravity
equals the upward force caused by the gas pressure
on the bottom of the indicator
• as gas flow increases, the number of gas molecules
hitting the indicator bottom increases, and it rises

149. • because the tube is tapered, the size of the annular
opening around the indicator increases with height
and allows more gas flow
• when the flow is decreased,
gravity causes the indicator
to settle to a lower level
• a scale on or beside the tube
indicates the gas flow rate

150. • the flow rate through the tube will depend on three
factors:
• the pressure drop across the constriction
• the size of the annular opening, and
• the physical properties of the gas

151. • as gas flows around the indicator, it encounters
frictional resistance between the indicator and the
tube wall
• there is a resultant loss of energy, so the pressure
drops
• this pressure drop is constant for all positions in the
tube and is equal to the weight of the float divided
by its cross-sectional area

152. • for this reason, these flowmeters are often called
constant-pressure flowmeters
• increasing the flow does not increase the pressure
drop but causes the indicator to rise to a higher
position in the tube, thereby providing greater flow
area for the gas
• the physical characteristics of the gas affect the gas
flow through the flowmeter tube

153. • when a low gas flow passes through the tube, the
annular opening between the float and the tube
wall will be narrow
• as flow increases, the annular opening becomes
larger
• the physical property that relates gas flow to the
pressure difference across the constriction varies
with the form of the constriction

154. • with a longer and narrower constriction (low
flows), flow is laminar and is mainly a function of
the viscosity of the gas (Hagen–Poiseuille equation)
• when the constriction is shorter and wider (high
flows), flow is more turbulent and depends more
on the gas density (Graham’s law)

156. • flowmeters are calibrated at atmospheric pressure
(760 mm Hg) and room temperature (20°C)
• temperature and pressure changes will affect the
gas viscosity and density and so influence the
indicated flow rate accuracy
• variations in temperature as a rule are slight and do
not produce significant changes

157. Flowmeter Assembly
• consists of:
⌐a tube through which the gas flows
⌐an indicator inside the tube
⌐a stop at the top of the tube, and
⌐a scale that indicates the flow

158. • lights are available on most anesthesia machines to
allow the flowmeters to be observed in a dark room
• each assembly is marked with the appropriate color
and name or chemical symbol of the gas measured
• flowmeters are usually protected by a plastic shield
• flowmeter assembly empties into a common
manifold that delivers the measured amount of
gases into the low-pressure system

159. • flowmeter tube can have a single or double taper
• single-taper tubes have a
gradual increase in diameter
from the bottom to the top
• they are usually used where
there are different tubes for
low and high flows

160. • dual-taper flowmeter tubes have
two different tapers on the inside
of the same tube
• lower taper is more gradual and is
used when fine flows are in use
• the less gradual taper is used for
higher flows
• these tubes are used when only
one tube is used for a gas

161. • the indicator (float, ball, rotameter or bobbin) is a
free-moving device within the tube
• it is important to observe the indicator frequently
and especially when the flow is altered
• if the indicator moves erratically, the readings may
be inaccurate
• the widest diameter of the indicator is the point
where the flow should be read

164. Flowmeter Tube Arrangement
• flowmeter tubes for different gases are side by side
• the various gas flows meet at the common
manifold (mixing chamber) at the top
• sometimes, there are two flowmeters for the same
gas: one for low and one for high flows

165. • the tubes are arranged in series (tandem) with one
flow control valve for both the flowmeter tubes
• gas from the flow control valve first passes through
a tube calibrated up to 1 L/minute and then passes
through a second tube that is calibrated for higher
flows
• the total flow through two-series flowmeters is not
the sum of the two tubes but that shown on the
higher flow tube

167. Flow indicator sequence

168. Auxiliary Oxygen Flowmeter
• an auxiliary (courtesy) oxygen flowmeter is a self-
contained flowmeter with its own flow control
valve, flow indicator, and outlet
• it is not affected by computer control
• it usually has a short tube with a maximum flow of
10 L/minute and a barbed fitting on the outlet for
connection to a face mask or nasal cannula
• it is used to supply oxygen to the patient without
turning ON the anesthesia machine

170. Flowmeter Problems
• the flowmeter scale, tube, and indicator must be
regarded as an inseparable unit
• the tube assembly calibrated for one gas cannot be
used for a different gas
• if a tube, indicator, and scale calibrated for one gas
are used for another gas, they will deliver an
incorrect gas flow

171. • if any part of the flowmeter is damaged, the
flowmeter, scale, and indicator all need to be
replaced with a new set
• flowmeter components are calibrated as a set and
if only one component is changed there will be
inaccuracy
• damage to the flow indicator can result if it is
suddenly propelled to the top of the tube when a
cylinder is opened or a pipeline hose is connected
while the flow control valve is open

172. Electronic Flowmeters
• most of the electronic anesthesia machines
available at this time use a conventional flow
control valve and an electronic flow sensor
• different technologies are used to measure gas flow
• the flow measured by the sensor is then
represented digitally and/or by a simulated
flowmeter on the anesthesia machine screen
• an advantage of electronic flow measurement is
that this information is available in a form that can
be sent to a data management system

173. 2. Hypoxia Prevention Safety
Devices
• one of the hazards associated with flowmeters is
the possibility that the operator will set the flows
that could deliver a hypoxic mixture
• various devices have been developed to prevent
this problem

174. 1. mandatory minimum oxygen flow
2. minimum oxygen ratio
a) Datex-Ohmeda Link 25 proportioning system
b) Drager- Oxygen Ratio Monitor Controller/
Sensitive Oxygen Ratio Controller
3. unidirectional check valves
4. alarms

175. 3. Unidirectional (Check) Valve
• when ventilation is controlled or assisted, positive
pressure from the breathing system can be
transmitted back into the machine
• using the oxygen flush valve may also create a
positive back pressure
• this pressure can affect flowmeter readings and the
concentration of volatile anesthetic agents
delivered from the vaporizers on the machine
• some machines have a unidirectional (check) valve
to minimize these effects

176. • this valve is located between the vaporizers and the
common gas outlet, upstream of where the oxygen
flush flow joins the fresh gas flow
• this valve will lessen the pressure increase but not
prevent it, because gas will be continually flowing
from the flowmeters toward a blocked valve
• these valves are not present on all machines
• if present, they will interfere with checking for leaks
upstream of the machine outlet

177. 4. Pressure Relief Device
• some machines have a pressure relief device near
the common gas outlet to protect the machine
from high pressures
• this valve opens to atmosphere and vents gas if a
preset pressure is exceeded
• a pressure relief device may limit the ability of an
anesthesia machine to provide adequate pressure
for jet ventilation through a catheter inserted
through the cricothyroid membrane

178. 5. Common (Fresh) Gas Outlet
• the common (fresh) gas outlet receives all of the
gases and vapors from the machine and delivers
the mixture to the breathing system at the pressure
of 5-8 psig
• some outlets have a 15-mm female slip-joint fitting
(that will accept a tracheal tube connector)
• they may also have a manufacturer-specific fitting

179. • there is a mechanism to prevent a disconnection
between the machine and the hose to the
breathing system
• machine standard mandates that it be difficult to
accidently disengage the delivery hose from the
outlet
• many new anesthesia machines have internal
connections to the breathing system
• these machines may not have the conventional
common gas outlet described

180. • the common gas outlet should not be used to
administer supplemental oxygen to a patient
• this is will delay conversion to the breathing system
if an emergency arises
• another potential problem is that a vaporizer on
the back bar may be accidentally left ON, leading to
undesired administration of the inhalation agent
• the auxiliary oxygen flowmeter or a separate
flowmeter should be used for this purpose

181. THE END!!!