LOW PRESSURE SYSTEM

Presented by : Dr Rashmi

Moderator: Dr Gian Chauhan

Starts downstream of flow control devices

Pressures: Slightly above atmospheric pressure (Max upto 6-8psig)

& variable

Components:

• Flowmeters

• Hypoxia prevention safety devices

• Unidirectional valves

• Pressure relief devices

• Common gas outlet

• Vaporizers & Mounting devices including backbar

Flowmeters: (aka flow indicators, flow tubes, rotameters)

Types: Mechanical & Eletronic

Mechanical flowmeters: Based on principle that flow past a

resistance is proportional to pressure

Variable orifice type tubes (Thorpe tube), internally tapered

narrowest at bottom

Indicator: free to move inside the tube

Gas passes through an annular opening b/w the indicator and

the tube towards the outlet

Wt. of bobbin balances the upward force by the gas pressure

Rate of flow depends on:

• Pressure drop across the constriction (frictional resistance

caused to the gas, equals wt. of float/ area), remains constant

throughout (Constant pressure flowmeters)

• Size of the annular opening

• Physical properties of the gas

(for Low flows (Laminar flow) flow is dependent on

viscosity; Hagen Poiseuille equation (Tubular orifice)

(High flows(Turbulent flow) flow is dependent on density;

Graham’s law (orificial orifice)

Calibrated at atmospheric pressure (760 mmHg) &

room temperature (20 Degree)

If pressure decrease as in high altitude, more gas

will flow that what the indicator specified and in

hyperbaric chamber, flow will be less than what is

indicated.

Temperature may alter viscosity and density of gas

but effect on gas flows is less significant.

Flowmeter Assembly:

Tube , Scale , Indicator,

Stop at the top

Lights

Coding (color, touch,

name)

Plastic shield (for

protection)

Tubes:

Glass tubes

Single taper (different tubes for low and high

flows)

Double taper (single tube with gradations for

fine and coarse flows)

Stop (at the top):

Prevents bobbin from plugging the outlet (Damage, hidden

bobbin)

If falls into the tube on the bobbin will increase the wt. & so

show falsely low reading

Scale:

Flow indicator (marked on or immediately adjacent to tube)

L/ min (lower than 1 L as ml/min or Decimals of L)

Lights/ fluorescent background: helps in darkened rooms

Flowmeter Tube Arrangement:

Tubes for various gases

arranged side by side, meet at

the top into a common

manifold (mixing chamber)

Arranged in series or tandem

One flow control valve for one

gas (single/ double tube)

Normal direction of flow:

bottom to top, then left to right

FLOWMETER ARRANGEMENT

Auxiliary Oxygen Flowmeters:

Self contained flowmeter with

its own flow control valve, flow

indicator, outlet

Short tube, max of 10 L/min flow

Usually mounted on the left side

of the machine

Can be used to supply oxygen

even without turning the

machine on

Problems with Flowmeters:

Inaccuracy:

• Inseparable unit (mixing of components not allowed)

• Calibrated for a particular gas (other gas cannot be

used)

• Greatest accuracy in the middle half of the tube (single

taper provides better accuracy)

Indicator problems: (worn, distorted, sudden burst of gas

pushing bobbin to top, stop may dislodge- rest on bobbin)

Leaks:

• leak upstream of the bobbin will result in low FGF but will

not be indicated by the bobbin

• If cylinder/ yoke plug not in place, open flow control valve

will l/t leakage

Using wrong flowmeter:

• Changing the position of the gases (arrangement) l/t errors

as anaesthetists are used to a particular arrangement

• Usually with Nitrous and air

Electronic Flowmeters:

Use electronic flow sensors and conventional flow

control valves

Slow measured is represented digitally or by a

simulated flowmeter

Clockwise movement opens the gas flow

Advantage: Information is available in a form which

can be directly sent to data management system

HYPOXIA PREVENTION SAFETY DEVICES Mandatory Minimum Oxygen Flow: (50 to 250 mL/minute), preset by

the manufacturer (altered on customer specification), activated

when master switch is turned on

Minimum Oxygen Ratio: Prevents delivery of mixture of oxygen and

nitrous oxide having an oxygen concentration below 21% oxygen

Mechanical Linkage: 14-tooth sprocket on the nitrous oxide flow

control valve and a 29-tooth sprocket on the oxygen flow control

valve (maintain a min. of 25% oxygen), if we increase nitrous beyond

this it engages the Oxygen valve.

aka ‘Link 25’ system, drawback: links only 2 gases

MECHANICAL LINKAGE

Electronic linkage: electronic proportioning

valve controls the oxygen concentration in the

fresh gas, supply a minimum of 25% oxygen

Alarms: To alert the operator that % of Oxygen

has fallen in the gas mixture

VAPORIZER MOUNTING SYSTEMS: (BACK BAR)Permanent mounting (Cage mount):

tools are required to remove or install a vaporizer on the

anesthesia machine, back bar is a component of vaporizer

• Adv: Less physical damage to vaporizers and fewer leaks

• Disadv: May not have enough mounting locations to

accommodate all of the vaporizers, malfunctioning vaporizer

cannot easily be exchanged

Penlon “off-line” system: A modified cage mount arrangement,

Flexible back bar hoses with connections which can be separated

and connected to a vaporizer (which will then be included in the

backbar)

Penlon “Back entry” system:

• Vaporizer is attached to the back bar by a fixing bolt protruding

through the middle of the rear panel of the machine

• Protruding pegs and seals on the back bar automatically locate

and seal the inlet and outlet connections to vaporizer

• Fixing bolt is tightened using a cylinder key

• 3 entry systems can be mounted in series

• Each system has a cover (blanking plate),should be fitted

properly when a vaporizer is not connected (or else leak)

Detachable mounting: Are standard on most new anesthesia machines, allow the vaporizer to be mounted & removed without use of tools.

Select-a-tec system: A pair of port valves for each vaporizer, Vaporizer has special mounting brackets with 2 plungers (spindles) which fit over the port valves. Seal created by wt. of vaporizer and O-ring around the post valve. Locking lever on the back of vaporizer.

Before mounting the vaporizer, control dial should be in off

position

Vaporizer is fitted onto the mounting system and locked in

position

When the vaporizer is turned on, the two plungers move

downward, opening the valve ports and connecting the

vaporizer into the fresh gas stream

When the vaporizer is turned off, the gases may still flow

through the head of the vaporizer (even when not locked

properly)

Drager Mounting System:

Vapor 2000 vaporizer must be in the “T” (travel) position before

it can be unlocked from the machine

Teats to be performed before operation of vaporizers:

• Looking at the level of each vaporizer (at same level)

• Attempt to lift each off the manifold without unlocking it

(shouldn’t be able to)

• It should be possible to turn ON only one vaporizer at a time

• Perform leak tests

Advantages of detachable mounting:

• Machine can have fewer mounting locations, allowing a more

compact machine

• Vaporizers can be easily removed and replaced, even during a

case

• If malignant hyperthermia is a potential problem, the vaporizers

can be removed: better results than if the vaporizers remain in

the off position

Disadvantages:

Partial or complete obstruction to gas flow from problems with

the mounting system

Leaks may occur

• an absent or damaged O-ring

• leaving the locking lever in the unlocked position

• something is pushed under the vaporizer enough to lift it

slightly off of the O-ring

Differences among vaporizers and interlocks from different

manufacturers can pose problems of compatibility

Interlocking Devices:

Prevent more than one vaporizer from being turned on at a time

Checking the interlock device should be done

For Datex-Ohmeda vaporizers, operating the dial release activates two

extension rods that prevent operation of any other vaporizer installed

on the manifold

Older versions of the Select-a-tec system: provide mounting positions

for three vaporizers, if only two vaporizers are fitted, the center

position must be occupied. If the center position is not occupied, the

interlock is ineffective

Newer ones: interlocking irrespective of center position occupancy

Unidirectional valves (check valves):

Backpressure from:

• Breathing system during IPPV

• Oxygen flush

• Partial switching from open to closed circuit

Can affect flowmeter readings and conc. of volatile agents

Located b/w the vaporizer and common gas outlet, upstream of

where the oxygen flush joins the fresh gas flow

Disadvantage: while checking for leaks we cannot detect a leak

upstream of the check valve

Pressure relief device:

Near common gas outlet to protect the machine from high

pressures

Opens to atmosphere

Disadv: May limit the ability of anaesthesia machine to provide

adequate pressure for jet ventilation (separate ports

recommended for jet ventilation)

Low pressure piping:

Has large number of connections so prone for breakages and

leaks

Common Gas Outlet:

Delivers the gas mixture to the breathing system

Common site for disconnections so machine standards

mandate that it should be difficult to accidentally

disengage the system

Have a 15mm female slip joint fitting (accepts a tracheal

tube connector) and a co-axial 22mm male connector

Fresh gas supply tube of the breathing system connects

to this outlet

Some machines may have 2 such outlets (may l/t

confusion, gas may be directed to the wrong outlet)

According to standards there should be only one CGO.

Some may not have even one (have internal breathing

system connections)

This outlet should not be used to administer supplemental

oxygen (delays use of circuit if emergency arises, also if

accidentally vaporizer remains on inhalational agent will be

delivered unknowingly)- prefer auxiliary Oxygen flowmeter

Tests for Low pressure system:

Check vaporizer: Level of agent, cap to be tight, should not be

lifted off.

Leak test: performed without the basal flow so machine master

switch to be turned off. To be performed with vaporizer off first

and then on.

• Positive pressure test

Oxygen flush test

Pressure gauge test

• Negative pressure test

THANKYOU