THE BASIC ANAESTHESIA
Prof. & Head
Deptt. of Anaesthesiology and Critical Care, GMCH
• Anaesthesia machine is a device which delivers
precisely known but variable gas mixture, including
anaesthetizing and life sustaining gases.
• Standard guidelines have been given to
manufacturers for minimum performance, design,
characteristics and safety requirements of machine.
• The current standard for anesthesia workstation as
promulgated by American society for testing and
materials (ASTM) is F1850. European standard is
Types of anesthesia machine
• Intermittent-Gas flows only during
• E.g: Entonox apparatus
• Continuous-Gas flows both during inspiration
and expiration. E.g : Boyle Machine, Forregar
• The original concept of Boyle's
machine was invented by the
British anaesthetist H.E.G.
Boyle in 1917
• 1920 – A vapourizing bottle
isincorporated to the machine.
• 1930 – Circle absorption system
was introduced by Brian Sword
• 1933 – A dry-bobbin type of
flowmeter is introduced.
• 1937 – Rotameters replaced dry-
bobbin type of flowmeters
• 1952-Pin index safety system
(PISS) by Woodbridge
2.Power Failure Indicator
6.Data Communication Port
1.High Pressure System
3.Low Pressure System
• Master Switch
Master (main power) switch activates both the
pneumatic and electrical functions .
•Standby position - allows
the system to be powered up
•Computer-driven machines should
be turned OFF and restarted with a
full checkout at least every 24
•STANDBY mode is not used for an
Power failure indicator:
Visual or Audible indicator to alert provider of
• Backup source of power for the occasional outage
• The anesthesia provider should check the battery
status during the preuse checkout procedure.
• While some older anesthesia machines used
replaceable batteries, most new machines use
• It usually takes a number of hours to fully
recharge a battery after it has completely
Most modern anesthesia machines have electrical
These are intended to power monitors and other
As a general rule, these outlets should only be
used for anesthesia monitors.
Other appliances should be connected directly to
Fig: Next to each outlet is a circuit breaker.
• 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
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
Pressure units to remember
100 kPa = 1000 mbar = 760 mm Hg =
1030 cm H2O = 14.7 psi = 1 atm
1 psi = 6.8 kpa
Psig = pounds per square inch gauge
BRIEF NOTE ON CYLINDERS
Valve – Port, stem
Pin index safety
• Most medical gas cylinders are made of steel
with various alloys added
Chromium (If + 0.8-1.1%)
used to increase strength and to minimise
weight and wall thickness
▫ MRI compatible cylinders are made of aluminium
• Walls of the cylinder vary from 5/64 to ¼ inch
thickness on an average
• Cylinders that have a marking 3AA are made
from steel. The marking 3AL or 3ALM indicates
that the cylinder is made from aluminium
• Cylinder have a flat or a concave base. The other
end may taper into a neck that is fitted with
tapered screw threads that attach to the cylinder
Cylinder valves are of 2 types –
packed type and diaphragm type
•Capable of withstanding
•A.k.a direct acting valve
•Stem is sealed by resilient
packing such as TEFLON
which prevents leaks
around the threads
•It is opened by 2-3 turns
•Used in most of the
• This valve uses a two piece stem separated by
non-perforated diaphragms. These diaphragms
prevent leakage along the valve stem.
• The lower part of the stem is encased in a
spring, which forces the stem away from the
seat when the valve is opened. The upper stem
is threaded into the diaphragm retainer nut.
• When the handwheel is rotated to the closed
position, the upper stem pushes on the
diaphragms, which deflect downward, forcing
the lower stem against the valve seat.
• Can be opened fully by ½ to ¾ turns
• Generally preferred when pressures are
relatively low and no leaks can be allowed
•It is the point of exit for the gas
It fits into the nipple on the hanger yoke of the
•When installing a cylinder on anesthesia
machine, it is important for the user not to
mistake the port for the conical depression
Closes the valve by sealing against the seat.
When the valve is opened, the stem moves
upward,allowing the gas to flow to the port
▫ Conical depression is situated on the
opposite side of the port on the cylinder
valve and is situated above the safety relief
▫ It is present on those cylinders which are
designed to fit on anesthesia machine
▫ Conical depression is designed to receive
the retaining screw on the yoke of the
Screwing the retaining screw into the
port may damage the port
Pressure Relief Devices:
▫ Every cylinder is fitted with pressure relief devices
whose purpose is to vent the cylinder’s contents to
if the pressure of enclosed gas increases
to dangerous levels
Combination of Both
Pressure Relief valve (spring loaded)
When pre-determined pressure is reached the disc
ruptures and allows the gas contents to be
It is a non- reclosing device held against an orifice
It protects against excess pressure as a result of high
It is thermally operated
It is a non-reclosing pressure relief device where the plug
is held against the discharge channel
It provides protection against excess pressure due to high
temperature but not overfilling
Woods metal alloy is most commonly used
YIELD TEMPERATURE: Temperature at which fusible
material becomes sufficiently soft to extrude from its
holder- so that cylinder contents are discharged
Spring loaded pressure relief device:
It is a reclosing device
When set pressure is
exceeded, the pressure
in the cylinder forced
the spring to open the
channel for letting out
▫ It is used to open or close a cylinder valve
▫ It is turned counter-clockwise to open and clock-
wise to close
this causes the stem to turn
▫ A good practice is to attach a handle to each
anesthesia machine or other apparatus for which
it may be needed
GAS SHOULDER BODY
OXYGEN WHITE BLACK
NITROUS OXIDE BLUE BLUE
CYCLOPROPANE ORANGE ORANGE
CARBON DI OXIDE GREY GREY
AIR WHITE GREY
NITROGEN BLACK BLACK
ENTONOX WHITE BLUE
Pin Index Safety System
• It consists of holes on the cylinder
valve positioned in an arc below the
A series of pins on the hanger yoke
is made to fit into the
• The seven hole positions are on the
circumference of a circle of 9/16
inch (14.3mm) radius centered on
GAS PIN INDEX
NITROUS OXIDE 3,5
O2-CO2 (CO2 <7.5%) 2,6
O2-CO2 (CO2 >7.5%) 1,6
Contents and Pressure
• In a cylinder containing a non-liquefied gas
Pressure declines as the contents are withdrawn
Hence pressure can be used to measure cylinder
In a cylinder containing a liquified gas, the pressure
depends on VAPOUR PRESSURE of the liquid and is not
an indication of the amount of gas remaining in the
cylinder as long as the contents are partly in the liquid
Pressure remains nearly constant till all liquid has
After which pressure declines till cylinder is
• Consists of:
– Hanger Yolk
– Check valve
• Usually not
supply is off
Hanger Yoke Assembly
1) Orients and supports the cylinder
2) Provides a gas-tight seal
3) Ensures uni-directional gas flow
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.
Threaded into frame of
Hinged Swinging gate
• RETAINING SCREW:
Threaded into the distal end
Tightening the screw – gas
Conical point fits into conical
depression on cylinder
Projects from yoke and fits
into cylinder port
Entrance of gas into machine
• INDEX PINS
Component of pin
index safety system
4mm in diameter and
6mm long (except pin
7 which is slightly
Fit into the
on the cylinder
• Bodok seal
• -cylinders are fitted with yoke with a sealing
washer called BODOK SEAL
• -it is made up of non combustible material and
has a metal periphery which make it long lasting.
• -it should be less than 2.4mm thick prior to
• -only one seal should be use between the valve &
It is used to prevent particulate matter from
entering the machine.
It is to be placed between the cylinder and
the pressure reducing device.
Bourdon Pressure gauge
• Displays cylinder pressure for each gas
• The gauge is usually colour coded and name and
symbol of gas are written over the dial. Blue colour for
nitrous oxide and white for oxygen.
• The scale must be at least 33% greater than the
maximum filling pressure of the cylinders or the full
• Gauge is calibrated in (kilopascal)kPa or (pound per
square inch)psi or Kg/cm2 .
Check valve assembly
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.
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.
It is not designed to act as a permanent seal for
empty yoke and may allow small amount of gas to
As soon as a cylinder is exhausted it should be
replaced by a full one or a dummy plug.
Pressure reducing device
The pressure in a cylinder varies. The anesthesia machine is fitted
with devices (reducing valves, regulators, reducing regulators,
reduction valves, regulator valves) to maintain constant flow with
changing supply pressure.
These reduce the high and variable pressure found in a cylinder to a
lower (40 to 48 psig, 272 to 336 kPa) and more constant pressure
suitable for use in an anesthesia machine.
The machine standard requires reducing devices for each gas
supplied to the machine from cylinders.
• BASIC PRINCIPLE
A larger pressure acting over a small area is
balanced by a smaller pressure acting over a
A1 x P1=A2 x P2
Safety features on pressure
• Pressure regulators have safety relief valves
• If due to any reason there is build up of pressure
in pressure regulator then the safety valve blow
off at a set pressure of 525 k pa(70psi)
Begins at the regulated
cylinder supply source
at 45 psig includes the
pipeline sources at 50
to 55 psig and extends
to the flow control
INTERMEDIATE PRESSURE SYSTEM
Pipeline inlet connections
Pipeline pressure indicators
Gas power outlet
Oxygen pressure failure devices
Additional pressure regulators
Flow control valves
PIPELINE INLET CONNECTIONS
It is the entry point for gases from the pipelines.
The anesthesia workstation standard requires pipeline inlet connections for
oxygen and nitrous oxide.Most machines also have an inlet connector for air.
These inlets are fitted with threaded non interchangeable Diameter Index Safety
System (DISS) fittings
A unidirectional (check) valve prevents reversed gas flow from the machine into
the piping system
Each pipeline inlet is required to have a filter with a pore size of 100μm or less.
The filter may become clogged, resulting in a reduction in gas flow.
PIPELINE PRESSURE INDICATORS
Indicators to monitor the pipeline pressure of each gas are required by
the anesthesia workstation standard.
They are usually found on a panel on the front of the machine and may
be color coded
The workstation standard requires that the indicator be on the pipeline
side of the check valve in the pipeline inlet.
If the indicator is on the pipeline side of the check valve, it will monitor
pipeline pressure only. If the hose is disconnected or improperly
connected, it will read “0” even if a cylinder valve is open
If a cylinder valve is open and the pipeline supply fails, there will be no
change in the pressure on the indicator until the cylinder is nearly
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). The indicators should be scanned repeatedly during use.
• Piping is used to connect components inside the
• It must be able to withstand four times the
intended service pressure
• Leaks between the pipeline inlet or cylinder
pressure reducing system and the flow control
valve not exceed 25 mL/minute
GAS POWER OUTLET
• One or more gas power (auxiliary gas) outlets may
be present on an anesthesia machine. It may serve
as the source of driving gas for the anesthesia
ventilator or to supply gas for a jet ventilator.
Either oxygen or air may be used.
• The ventilator is an integral part of the modern
machine and the breathing system and is connected
to the ventilator with internal piping. Therefore,
the power outlet is not found in many anesthesia
Oxygen Pressure Failure devices-
• 1.Oxygen Failure safety devices- (Oxygen Failure
safety valve,low pressure guardian system, Oxygen
Failure protection devices, pressure sensor shutoff
system,fail safe,nitrous oxide shutoff valve)
• This valve shuts off or proportionally decreases and
ultimately interrupts the supply of nitrous oxide if
the oxygen supply pressure decreases.
• The anaesthesia workstation standard requires that
whenever the oxygen supply pressure reduced below
the manufacturer-specified minimum,the delivered
oxygen concentration shall not decrease below 19%
at the common gas outlet.
The oxygen flush (oxygen bypass, emergency oxygen
bypass) receives oxygen from the pipeline inlet or
cylinder pressure regulator and directs a high
unmetered flow directly to the common gas outlet.
It is commonly labeled “02+.”
On most anesthesia machines, the oxygen flush can
be activated regardless of whether the master
switch is turned ON or OFF.
A flow between 35 and 75 L/minute must be
The button is commonly recessed or placed in a
collar to prevent accidental activation.
It consists of a button and stem
connected to a spring loaded ball .The
ball is in contact with the seat .When
the button is depressed, the ball is
forced away from the seat , allowing
the oxygen to flow to the outlet. A
spring opposing the ball will close the
valve when the button is not
•May cause barotrauma
•Dilution of inhaled anaesthetic
•Flush may stick and obstruct flow of
gases from flowmeter
Second-stage Pressure Regulator
• Some machines have pressure regulators in the
intermediate pressure system just upstream of the flow
• Reduce the pressure further to around 26 psi (177 kPa)
for nitrous oxide and 14 psi (95 kPa) for oxygen
• The purpose of this pressure regulator is to eliminate
fluctuations in pressure supplied to the flow indicators
• By reducing the pressures below the normal fluctuation
range, the flow will remain more constant.
• Not all anesthesia machines are equipped with this
• It is advantageous to have stops for the OFF and
MAXIMUM flow positions. A stop for the OFF
position avoids damage to the valve seat. A stop for
the MAXIMUM flow position prevents the stem from
becoming disengaged from the body.
Control Knob :
• The control knob is joined to the stem. If it is a
rotary style knob, the oxygen flow control knob must
have a fluted profile and be as large as or larger than
that for any other gas. All other flow control knobs
must be round.
• The knob is turned counterclockwise to increase
flow. If other types of flow control valves are present,
the oxygen control must look and feel different from
the other controls.
When a machine is not being used, the gas
source (cylinder or pipeline) should be closed or
The flow control valves should be opened until
the gas pressure is reduced to zero and then
If the gas source is not disconnected, the flow
control valve should be turned OFF to avoid the
fresh gas desiccating the carbon dioxide
absorbent and to conserve gas.
Before machine use is resumed, the control
valves should be checked to make certain that
they are closed.