Low pressure system in anaesthesia machine

Dr. Swadheen kumar Rout
1st year P.G
Dept. of Anaesthesiology
M.K.C.G College & hospital

Anaesthesia machine:- Function is to deliver a precisely-known but
variable gas mixture, including anesthetizing
and life-sustaining gases at a controlled &
known pressure.
Types
Intermittent( gas flows only during inspiration)
Ex. – Entonox apparatus
Continuous(Gas flows both during
inspiration and expiration)
Ex.- Boyle`s machine ,.

 Boyle`s anaesthetic machine is a continuous-flow type
machine used for administration of anaesthetic gases.
 was pioneered by Henry Gaskin Boyle (1875-1941).
 the original machine from 1917 was carried around in a
wooden box and used ether and nitrous oxide.
 It has undergone various modifications with time to increase its
safety & utility.

SYSTEM COMPONENTS
ELECTRICAL SYSTEM PNEUMATIC SYSTEM
- Master switch
- Battery back up
- Battery recharge
- Electric outlet for in built monitor
- Circuit breaker
- High pressure system
- Intermediate pressure system
- Low pressure system

 Extends from the flow control valves to the common gas
outlet.
 Consists of : - Flowmeters
- Hypoxia prevention safety devices
- Unidirectional check valve
- Pressure relief valves
- Common gas outlet
- Vaporiser & their mounting devices

TUBE: made of glass (THORPE tube).
 Tubes are contained in a chromium plated
metal casing protected by a plastic
window.Backplate is luminous & detachable.
 Anti-static coating.
INDICATOR: Float / Bobbin made usually of
aluminium.
 It is free moving device & must not stick to
tube wall.
 If it moves erratically, readings may be
inaccurate.
TYPES Non-rotating H type
Rotating type
Slanted grooves
Ball type
STOP: Prevents float from plugging the outlet &
prevents rising to a point it cannot be seen.

Types of flowmeters
1. Variable orifice flowmeters (fixed pressure difference)
2. Fixed orifice flowmeters (variable pressure difference)
• The area between the outside of the bobbin and the inside of the tapered glass
tube represents a orifice / annulus. It can be considered an equivalent to a
circular channel of the same cross-sectional area
Variable orifice flowmeters type used mostly today in modern machines.
(Synonym- Rotameters)
• The glass tube, slightly smaller on
cross-section at bottom than at top (tapered tube)
•Can be single or double taper.
Single taper have gradual increase in diameter from bottom
to top, used when there are different tubes for high & low flows.
Dual taper have two different taper inside the same tube .one for
fine flows (200mL/min to 1L/min) & one for coarse flows.
-used when only one tube is used for a gas.

variable orifice flowmeters (Thorpe type). A: single taper design. B: Dual taper
design

Low pressure system in anaesthesia machine

 Flowmeters adjust the proportions of medical gases controlled by the
anesthesia machine as well as the total gas flows delivered to the patient
circuit .
 Flowmeters measure the drop in pressure that occurs when a gas passes
through a resistance and correlates this pressure drop to flow .
 When the flow control valve is opened the gas enters at the bottom and
flows up the tube elevating the indicator.
 The indicator floats freely at a point where the downward
force on it (gravity) equals the upward force caused by gas
molecules hitting the bottom of the float.
 As the bobbin rises with increased flow, the size of the annulus between it
and the glass tube increases. In other words, there is a variable orifice
around the bobbin which depends on the gas flow.

As the bobbin rises from
A to B, the
clearance(annulus)
increases (from X toY)

Laminar flow-
 Flow (Q) through a tube is laminar.
 In order to drive fluid through a tube, a pressure difference (P = P1-P2)
must be present across the ends
• There is a linear relationship so that flow is directly
proportional to pressure under conditions of laminar
flow (Q@P)
• Reducing the diameter (d) in half reduces the flow
to 1/16 of its original value if the pressure drop along
the tube remains the same, i.e. flow is proportional to
the 4th power of the diameter (Q@d4)
• Reducing the length by half, doubles the flow
(Q@1/L)

Summary: Q @ P Q = flow through tube
Q @ D4 P = pressure across tube
Q @1/L D = diameter of tube
Q @1/n L = length of tube
n= viscosity of fluid
All these factors are incorporated in the Hagen-Poiseuille equation:
• Viscosity (n) of fluid affects resistance to laminar flow such that the higher the
viscosity, the slower the flow (Q@1/n)
Substituting radius (r)
for diameter

 Turbulent flow is often present where there is an orifice, a sharp bend or
some other irregularity which may cause local increase in velocity.
 Turbulence is also affected by other factors such as viscosity and density
of the fluid and diameter of the tube.
 The effect of density on onset of turbulent flow can be illustrated by use of
helium in respiratory disorders
 Helium reduces the density of the gas inhaled and so reduces the incidence
of turbulent flow, therefore lower resistance to breathing
 These factors combine to give an index called Reynolds number
V = linear velocity of fluid
P = density
D = diameter of tube
U = viscosity
• Reynolds number > 2000 means turbulent flow likely
• Reynolds number < 2000 means flow likely to be laminar
• For a fixed set of conditions, there is a critical velocity at which Reynolds number has
the value of 2000
When the velocity exceeds this critical value, flow is likely to change from
laminar to turbulent .

As the bobbin rises
increase in the area of the annular orifice
flow resistance decreases
flow rate increase
The rate of flow through the flowmeter tube depends on:
- Pressure drop across the constriction
Weight Of Float/Cross-sectional Area
- Size of annular orifice
- Physical properties of the gas

At low flows:
- gas flow around the bobbin approximates to tubular flow (diameter
of channel less than length)
- gas flow is laminar so viscosity is important
At high flows:
- gas is flowing around the bobbin through an orifice (diameter of
channel greater than length)
- gas flow is turbulent so density is important
Tube = Length >
Diameter
Orifice = Diameter >
Length

 Flowmeter are calibrated in litres per min. For <1 L/min
expressed in ml or decimal fractions of a litre per minute with a
zero before the decimal point.
 Are calibrated at atmospheric pressure
(760 torr) and room temperature(200C) based
on physical properties of individual gases.
 Changes in temperature & pressure affect
density & viscosity of gas and affect flowmeter
accuracy.
 As flow changes from laminar to turbulent within
the flowmeter the flow changes from being directly
proportional to pressure to proportional to the
square root of pressure and hence the graduations
on the flowmeters are not uniform.

 There may be one or two rotameters for each gas
 Single flowmeter layout is the safest but less precise for low flows.
 If two are present for any gas, the first permits accurate
measurement of low flows (usually up to 1 L/min) and the other, of
flows up to 10-12 L/min
 Flow indicator tubes for different gases are grouped side by side
 The various gas flows meet at the common manifold (mixing
chamber) at the top . In such a case, the tubes may be arranged
either in parallel or in series

Parallel arrangement:
•Two complete flow indicator assemblies with a flow control valve for each
assembly
•The total flow of the gas to the common manifold is the sum of the flows on
both flow indicators
•Not presently available because accidental use of low-flow oxygen flow
indicator when a high flow is intended is a hazard whenever two oxygen
flow control knobs are present
Series (tandem) arrangement:
• One flow control valve for the two flow indicator tubes
• Gas from the flow control valve first passes through a tube calibrated up to 1
liter per minute, then passes to a second tube that is calibrated for higher flows
• Total flow is not the sum of the two tubes but that shown in the higher flow
tube.Tandem flow tubes increase accuracy at all flow rates

 The O2 flowmeter is positioned on the right side (most distally) of
the rotameter bank, downstream from the other flowmeters and
closest to the common gas outlet
 In the event of a leak in one of the other flowmeter tubes, this
position is the one least likely to result in a hypoxic mixture.
 In A and B a hypoxic mixture can
result because a substantial portion of
oxygen flow passes through the leak,
and all nitrous oxide is directed to the
common gas outlet.
C and D, The safest configuration
exists when oxygen is located in the
downstream position

 However a leak in the oxygen flowmeter tube can cause a
hypoxic mixture, even when oxygen is located in the
downstream position.

Temperature and Pressure Effects
 Changes in temperature and pressure alter both viscosity and
density of gases, thereby affecting accuracy of the indicator on the
flowmeters.
 Temperature effects are slight and do not cause significant changes
 At high altitude, barometric pressure decreases resulting in
increased flow.
 At low flow rates, flow is laminar and dependent on gas viscosity, a
property not affected by altitude.
 At high flow rates flow becomes turbulent, and flow becomes a
function of density, a property that is influenced by altitude.
 The resulting decreases in density will increase the actual flow rate
so the flowmeter will read lower than the actual flow rate.
 At increased pressure, as in a hyperbaric chamber, the reverse is
seen; the delivered flow rate is slightly less than the actual flow rate.

Back Pressure:
 In machines without an outlet check valve, if pressure at the common
gas outlet increases, this is transmitted back to the flowmeters,
compressing the gas above the float
 Pressure above the indicator rises forcing the float down, causing the
flowmeter to be read lower than the actual gas flow rate
Static Electricity:
Static electricity causes the float to stick to the side of the tube causing
reading inaccuracy.These electrostatic charges are negligible as long as
the float rotates freely
Hidden Floats:
The float may adhere to the stop at the top of the tube even if no gas
is flowing
The float may disappear from view if there is no stop present e.g.
broken float stop

 Protection against hypoxic mixture at the flowmeter level.
 Prevention of delivery of a hypoxic gas mixture is a major
consideration in the design of contemporary anesthesia machines.
Mandotary minimum oxygen flow: Some machines require a
minimum flow (50-250ml/min) of oxygen before other gas will flow.
 Some machine activate an alarm if O2 flow falls below a certain limit.
Minimum oxygen ratio:
 In modern anesthesia machines, N2O and O2 flow controls are
physically interlinked so that a fresh gas mixture containing at least
25% O2 is delivered at the flowmeters when only N2O and O2 are
used .
Ohmeda = mechanical + pneumatic interlink (Link–25)
North American Dräger = pneumatic interlink

 A 14-tooth sprocket is
attached to the N2O flow
control valve, and a 28-tooth
sprocket is attached to the O2
flow control valve. A chain
mechanically links the
sprockets.
 For every 2 revolutions of the
N2O flow control spindle, an
O2 flow control, set to the
lowest O2 flow, rotates once
because of the 14:28 ratio of
the gear teeth.

 Regardless, of the O2 flow set, if the flow of N2O is increased >75%,
the gear on O2 spindle will engage automatically with the O2 flow
control knob causing it to rotate and thereby causing O2 flow to
increase to maintain O2 Conc of 25% with a maximum N2O:O2 ratio
of 3:1.
 If attempt is made to increase N2O flow beyond that ratio, O2 flow is
automatically increased & if O2 flow is lowered too much N2O flow
reduces proportionately.  The final 3:1 flow ratio results because the
N2O flow control valve is supplied by
approximately 26 psig, whereas the O2 flow
control valve is supplied by 14 psig. Thus, the
combination of the mechanical and pneumatic
aspects of the system yields the final oxygen
concentration.

 It is a pneumatic O2-N2O interlock system designed to maintain a
fresh gas oxygen concentration of at least 25% .
 The ORMC limits N2O flow to prevent delivery of a hypoxic mixture,
unlike the Ohmeda Link-25, which actively increases O2 flow .
 composed of an O2 chamber, a N20 chamber, & a N20 slave control valve; all
are interconnected by a mobile horizontal shaft. The pneumatic input into the
device is from the O2 & N20 flowmeters.
These flowmeters have specific resistors located downstream from the flow
control valves which create back pressures directed to the O2 & N2O chambers.
The value of the O2 flowtube resistor is three to four times that of the N2O
flowtube resistor..
The back pressure in the O2 & N2O chambers pushes against rubber
diaphragms attached to the mobile horizontal shaft. Movement of the shaft
regulates the N2O slave control valve, which feeds the N20 flow control valve.

 The back pressure exerted on the O2 diaphragm, in the upper configuration, is greater
than that exerted on the N2O diaphragm. This causes the horizontal shaft to move to the
left, opening the N2O slave control valve. N2O is then able to proceed to its flow control
valve and out through the flowmeter.
In the bottom configuration, the N2O slave control valve is closed because of
inadequate O2 back pressure

 The ORMC also rings alarms (it has an electronic component)
to prevent a hypoxic mixture delivery .
 Dräger S-ORC (sensitive oxygen ratio controller), newest
hypoxic guard system as found on Fabius GS guarantees a
minimum FIO2 of 23%. Its fail-safe component shuts off nitrous
oxide if the oxygen flow is less than 200 mL/min, or if the
oxygen fresh gas valve is closed.

 Machines equipped with proportioning systems can still deliver
a hypoxic mixture under the following conditions
 Wrong Supply Gas in oxygen pipeline or cylinder.
 Defective pneumatic or mechanical components.
 Leaks exist downstream of flow control valves.
 Inert gas administration( He,CO2) : Proportioning systems
generally link only N2O and O2.
Use of an oxygen analyzer is mandatory if the operator uses a
third inert gas.

 Present on some machines (Ohmeda)
between the vaporizers and common
gas outlet, upstream of where oxygen
flush flow joins the fresh gas flow .
 Positive pressure ventilation & use of O2
flush cause back flow of the gas.
 This back flow can cause “pumping
effect”, if not prevented, could cause
increased vaporizer output
concentrations.
• Pressure increase can also increase leaks and cause inaccurate flow
indicator readings.

 The purpose of the outlet check valve, where present, is to
prevent reverse gas flow,
 Newer machines (North American Dräger) are equipped with
vaporizers that incorporate a baffle system and specially
designed manifold to prevent pumping effect, making an outlet
check valve unnecessary.
*Important: Testing the breathing
system for leaks will not detect a leak
in the machine equipped with a check
valve

 Situated on the back bar of the machine
downstream of voporizers or near
common gas outlet.
 Prevents high pressure from transmitted
in to machine & from machine to patient.
 When preset pressure is exceeded valve
opens & gas is vented outside.
 Usually opens when pressure in the
back bar exceeds 35 Kpa.

 Located in between flowmeter device and common gas outlet.
 Permanent mounting.
 Vapourizers and flowmeters are connected to each other and
then bolted with back bar.

 Receives all gases & vapors from the machine & delivers the
mixture to the breathing system.
 Outlet in most of the machines have 15 mm female slip-joint
connection, with coaxial 22 mm male connection•
 Machine standards mandates that it be difficult to accidentaly
dis-engage the delivery hose from the outlet.
 The pressure delivered at outlet is 5-8 psi.

Low pressure system in anaesthesia machine

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