Anaesthesia machine 2

The Anaesthesia Machine- 2
Presented by: Dr. Vineet Chowdhary
Moderated by: Dr. Avnish Bharadwaj

LOWPRESSURE SYSTEM
The low-pressure system is downstream of the flow
control devices. Pressure in this section is only
slightly above atmospheric and variable.
Consists of:
1. Flowmeters
2. Hypoxia prevention safety devices
3. Unidirectional valve
4. Pressure relief devices
5. The common gas outlet
6. Vaporizers and their mounting devices

Flowmeters (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.

Mechanical Flowmeters
 Measuring gas flow in a mechanical flowmeter is
based on the principle that flow past a
resistance is proportional to pressure.
 Mechanical flowmeters measure the drop in
pressure that occurs when a gas passes through a
resistance.

• Flowmeters on modern anaesthetic machines consist of a
tapered glass tube containing a bobbin or ball, which
floats on stream of moving gas
• As gas flow rate increases,float is carried further up
tube, thereby indicating flow rate
• Flowmeters are specifically constructed for each gas,
since flow rate depends on both viscosity and density
of gas
• Only correct tube and bobbin or ball can be used to
repair broken flowmeters
• Since bobbin floats in gas stream, flowmeters will
only function correctly if tube is vertical
• Flowmeters will not function correctly if the tube
cracked

Physical Principle of Gas Flow
 Traditional mechanical flow indicators used in
anesthesia machines have been of the variable orifice
(variable area, Thorpe tube) type.
 A vertical glass tube is internally tapered with its
smallest diameter at the bottom. It contains an
indicator that is free to move up and down inside the
tube.
 When there is no gas flow, the indicator rests at the
bottom of the tube.
 When the flow control valve is opened, gas enters at
the bottom and flows up the tube, elevating the
indicator.
 Gas passes through the annular opening between the
indicator and the tube and on to the outlet at the
top of the tube.

The indicator floats freely 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.
Because the tube is tapered, the size of the annular
opening around the indicator increases with height,
and more gas flows.
A scale marked on or beside the tube indicates the gas
flowrate

The rate of flow through the tube will depend
on three factors:



Pressure drop across
the constriction
Physical properties of
the gas.
Size of the annular
opening

Pressure Drop across the Constriction
 As gas flows around the indicator, it encounters
frictional resistance between the indicator and
the tube wall. There is a resultant loss of
energy reflected in a pressure drop.
 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.

Size of the Annular Opening
Annular cross-sectional area varies while the pressure
drop across the indicator remains constant for all
positions in the tube.
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 elevation of the indicator is a measure of the
annular area for flow and, therefore, of the flow
itself

Physical Characteristics of the Gas
At (low flows) flow is laminar and is 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 on gas
density (Graham's law).
Q is flow; n is viscosity; p/l
is the pressure gradient; r
is the radius of the tube

Temperature and Pressure Effects
 Flowmeters are calibrated at atmospheric pressure
(760 torr) and room temperature 20°C
 Temperature and pressure changes will affect both
the viscosity and the density of a gas and so
influence the accuracy
Variations in temperature- no significant effect
Hyperbaric( high pressure)- less gas delivered
than intended
Low pressure(increased altitude)- More gas
delivered

Flow indicator indicators. The plumb bob and skirted floats are kept
centered in the tube by constant rotation. The reading is taken at the top.
The ball indicator is kept centered by rib guides. The reading is taken at the
center. The nonrotating float does not rotate and is kept centered by gas
flow

Stop
 At the top of the flowmeter tube.
 Prevents the indicator from plugging the outlet which
could lead to damage to the tube
 Prevents the indicator from ascending to a point in
the tube where it cannot be seen.
 Flowmeter with the indicator hidden at the top looks
much like one that is turned OFF
Scale
Flow indicator scale either be marked on or
immediately adjacent to the tube
 Flowmeters are calibrated in liters per minute.
 For flows below 1 L/minute, the flow may be expressed
either in milliliters or in decimal fractions of a
liter per minute with a zero before the decimal point.
Lights
• Flow indicator lights are offered as an option on most
modern anesthesia machines. These are useful when the
machine is used in a darkened room.

Flowmeter Tube Arrangement
Flowmeter tubes for different gases are grouped 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.
The tubes are arranged in series (tandem)
There is one flow control valve for the both flowmeter
tubes. Gas from the flow control valve first passes
through a tube calibrated up to 1 L/minute, then
passes to a second tube that is calibrated for higher
flows.
Series flowmeter tubes offer increased accuracy .

Flow indicator sequence
A,B: Potentially
dangerous arrangements,
with the oxygen flow
indicator upstream.
If a leak occurs, oxygen
will be selectively
lost.
C,D: Oxygen is
downstream from other
gases, which is a safer
situation because
anesthetic gas rather
than oxygen will be
lost.
Arrows represent flows
of gases.

Auxiliary Oxygen Flowmeter
It is a self-contained flowmeter with its own flow
control valve, flow indicator, and outlet.
It usually has a short tube with a maximum flow of 10
L/minute and a barbed fitting on the outlet.
It is usually mounted on the left side of the machine.
This can be used to supply oxygen to the patient
without turning ON the anesthesia machine.

Problems with Flowmeters
Inaccuracy
The flowmeter scale, tube, and indicator must be
regarded as an inseparable unit.
Should any one of the components need replacement, a
complete new set must be obtained.
The tube assembly calibrated for one gas cannot be
used for a different gas.
The degree of inaccuracy that will occur if flowmeters
are transposed depends on the gases for which the
flowmeters were intended
The greatest accuracy in flowmeter tubes is in the
middle half of the tube.
Having one tube in the series arrangement for low flows
(less than 1 L/minute) and one for high flows (1 to 10
L/minute) will result in greater accuracy.

Indicator Problems
Damage to the flow indicator can result from a sudden
projection to the top of the tube due to a sudden rise
in pressure
Flow indicators can become worn or distorted by
handling
The stop at the top of the flowmeter tube can become
dislodged and rest on top of the indicator.
Leaks
A leak in a flowmeter downstream of the indicator but
upstream of the common manifold will result in a lower
than expected concentration of that gas in the fresh
gas
A leak may occur if a flow control valve is left open and
there is no cylinder or yoke plug in the yoke

Using the Wrong Flowmeter
The problem is most likely to occur between air and
nitrous oxide, as the position of oxygen is generally
fixed by national custom or standards.

Care of flowmeters
includes ensuring that:
Floats spin freely,
Qualified service personnel regularly maintain gas
machines,
An oxygen analyzer used always
One never adjusts a flowmeter without looking at it,
One includes flowmeters in visual monitoring sweeps
One turns flowmeters off before opening cylinders,
connecting pipelines, or turning machine "on".

HYPOXIAPREVENTIONSAFETY DEVICE

Mandatory Minimum Oxygen
 Anesthesia machines require a minimum (50 to 250
mL/minute) flow of oxygen before other gases will
flow.
 This is preset by the manufacturer (sometimes to the
customer's specification).
 Some machines activate an alarm if the oxygen flow
goes below a certain level even if no gases are being
used.
 The minimum flow is activated when the master switch
is turned ON.
 On some machines, it is disabled when air is used.
The minimum oxygen flow does not in itself prevent a
hypoxic gas concentration from being delivered. A
hypoxic gas mixture can be delivered with only
modest anesthetic gas flows

MINIMUM OXYGEN RATIO
The anesthesia workstation standard requires that an
anesthesia machine be provided with a device to
protect against an operator-selected delivery of a
mixture of oxygen and nitrous oxide having an oxygen
concentration below 21% oxygen (V/V) in the fresh gas
or the inspiratory gas

Mechanical Linkage
• There is a 14-tooth sprocket on the nitrous oxide
flow control valve and a 28-tooth sprocket on the
oxygen flow control valve.
• If the flow control valves are adjusted so that a 25%
concentration of oxygen is reached, a pin on the
oxygen sprocket engages a pin on the oxygen flow
control knob.
• This causes the oxygen and nitrous oxide flow control
valves to turn together to maintain a minimum of 25%
oxygen.
• This minimum oxygen ratio device (proportioning
system) permits independent control of each gas as
long as the percentage of oxygen is above the
minimum.

If the operator attempts to increase the nitrous
oxide flow beyond that ratio, the oxygen flow is
automatically increased. If the operator attempts to
lower the oxygen flow too much, the nitrous oxide
flow is lowered proportionally
It should be noted that these devices only link two
gases, normally nitrous oxide and oxygen.
Administering a third gas such as helium can result
in a hypoxic mixture.

In contrast to the Datex-Ohmeda Link-
25 system, which actively increases
oxygen flow to maintain a fresh gas-
oxygen concentration greater than 25%,
the Dräger ORMC and S-ORC systems
limit nitrous oxide flow to prevent
delivery of a fresh gas mixture with
an oxygen concentration of less than
25%

Limitations of Proportioning System
Anti hypoxic device may also deliver hypoxic
mixture under following conditions-
Wrong supply of gas.
Defective pneumatics/mechanics.
Inert gas administration(3rd gas-He,N2,CO2).
Leaks downstream.

UNIDIRECTIONAL CHECK VALVES
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.
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.
These check valves are of great importance when
checking the machine for leaks. Testing the breathing
system for leaks will not detect a leak upstream of
the check valve in a machine equipped with a check
valve.

Some new anesthesia machines provide two common gas
outlets.
A hazard with this arrangement is that the fresh gas
may be directed to the wrong outlet
The anesthesia workstation standard states that there
shall be only one functional common gas outlet at a
time
Many new anesthesia machines have internal connections
to the breathing system. These machines may not have
the conventional common gas outlet described
previously.

CAUTION!!
The common gas outlet should not be used to administer
supplemental oxygen to a patient
This will delay use of the breathing system if an
emergency arises.
Another potential problem is that a vaporizer may be
accidentally left ON, leading to undesired
administration of inhalational agent.
Either the auxiliary oxygen flowmeter or a separate
flowmeter should be used to supply oxygen.

Alternative Oxygen Control
When using an anesthesia machine, there is always the
possibility that the electronics will fail.
Some machines provide an alternative means to
administer oxygen.
This is separate from the auxiliary (courtesy)
flowmeter. If there is a mechanical total flow
flowmeter, it can be used to measure the delivered
oxygen.

Anaesthesia machine 2

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