INTERMEDIATE & LOW PRESSURE SYSTEMModerator: Dr. Srinivas HT

Speaker: Dr. Deepa Sinha

History

One of the First Anaesthetic Apparatus designed in 1912

1889: Invention of pressure regulator; as beer pressure controller and oxygen controller, the pressure regulator laid the foundations of Drägerwerk’s success

1902: Development of first anaesthetic apparatus in Germany: hand-held apparatus 145 N, also known as the Roth-Dräger

1910: The Roth-Dräger Mixed Anaesthetic Apparatus was developed in co-operation with surgeon friend, Dr Otto Roth.

1917: Henry Boyle designs his first anaesthetic machine. Boyle's left-handedness lead to the arrangement of flow meters and vaporisers that is still used today. This original Boyle machine was improved in a stepwise fashion between 1920 and 1965.

• 1921 – Waters to and fro absorption apparatus was introduced.

• 1927 – Flow meter for carbon dioxide was included, the volatile controls were of the lever type and the familiar back bar made its first appearance.

• 1930 – The plunger of the vaporiser appeared in the 1930 model.

• 1930 – Circle absorption system was introduced by Brian Sword.

• 1933 – Dry bobbin flow meters were introduced.• 1952 – Pin index safety system (PISS) by Woodbridge.• 1958 – Introduction of Bodok seal

Function of Anaesthesia Machine

The machine performs four essential functions:

- Provides O2,

- Accurately mixes anaesthetic gases and vapours,

- Enables patient ventilation and

- Minimises anaesthesia related risks to patients and staff.

BASIC DESIGN OF A CONTINUOUS ANAESTHESIA MACHINE

The basic design of an anaesthesia machine consists of pressurised gases supplied by cylinders or pipelines to the anaesthetic machine, which controls the flow of gases before passing them through a vapouriser and delivering the resulting mixture to the patient through the breathing circuit

• The early Boyle's machine had five elements, which are still present in modern machines: (1) A high pressure supply of gases,

(2) pressure gauges on O2 cylinders, with pressure reducing valves,

(3) flow meters

(4) metal and glass vapouriser bottle for ether

(5) a breathing system.

• The basic machine has provision for fixing two O2 cylinders and two N2O cylinders through the yoke assembly with PISS.

• Provision for connecting the pipeline gas source of O2 and N2O (from the wall outlet with quick couplers and yoke blocks at the machine end) instead of one of the cylinders at the yoke assembly.

• A pressure gauge is mounted on to the yoke assembly to read the pressure in the cylinder. Pressure regulators are located downstream of the yoke assembly, which reduce the high pressure in the cylinders to a low and constant pressure of 45-60 PSIG

• From the pressure regulators, there are connections through high pressure tubings constructed of heavy duty materials to the flow meter assembly, which is secured to the back bar of the machine by one or more bolts.

• The back bar supports the flow meter assembly and the vapourisers.

• At the end of the back bar, there is the common gas outlet to which the breathing circuits are connected to provide the anaesthetic vapour containing O2 enriched gases to the patient.

Anaesthesia machine can be conveniently divided into three parts:

1) The high pressure system, which receives gases at cylinder pressure, reduces the pressure and makes it more constant,

2) The intermediate pressure system, which receives gases from the regulator or hospital pipeline and delivers them to the flow meters or O2 flush valve

3) The low pressure system, which takes gases from the flow meters to the machine outlet and also contains the vapourisers

HIGH PRESSURE SYSTEM

High pressure system consists of all parts of the machine, which receive gas at cylinder pressure. These include the following:

(a) The hanger yoke which connects a cylinder to the machine,

(b) The yoke block, used to connect cylinders larger than size E or pipeline hoses to the machine through the yoke,

(c) The cylinder pressure gauge, which indicates the gas pressure in the cylinder

(d) The pressure regulator, which converts a high variable gas pressure into a lower, more constant pressure, suitable for use in the machine.

Intermediate Pressure System

The intermediate pressure system receives gases from the pressure regulator or the pipeline inlet to the anaesthesia machine.

- Gases are recived at reduced pressures usually 37-55 PSIG

Component• The pneumatic part of the master switch• Pipeline inlet connections• Pipeline pressure indicators• Piping• The gas power outlet• Oxygen pressure failure devices• The oxygen flush• The flow control valves

Master Switch• Pneumatic component is located in the downstream of the

inlets for cylinder and pipeline supplies• Oxygen may enter the pneumatic components by way of

the master switch. When the master switch is turned OFF, the pressure in the intermediate pressure system will drop to zero.

• Oxygen flush is independent of this switch

Pipeline Inlet Connections

• The pipeline inlet connection is the entry point for gases from the pipelines.

• The anaesthesia workstation standard requires pipeline inlet connections for oxygen and nitrous oxide .

• These inlets are fitted fit threaded non interchangeable Diameter Index Safety System (DISS) fitting

• The American Society for Testing and Materials (ASTM) anaesthesia workstation requires that every anaesthesia machine have a DISS fitting for each pipeline inlet

• Each inlet will have unidirectional (check) valve to prevent reversed gas flow from the machine into the piping system .

• Each pipeline inlet is required to have a filter with a pore size of 100 µm or less.

• The filter may become clogged, resulting in a reduction in gas flow.

DISS

• It was developed to provide non-interchangeable connections for medical gas lines at pressures of 1380 kPa (200 psi) or less

• DISS connector consists of a body, nipple, and nut combination. There are two concentric and specific bores in the body and two concentric and specific shoulders on the nipple.

• The American Society for Testing and Materials (ASTM) anesthesia workstation requires that every anesthesia machine have a DISS fitting for each pipeline inlet

Pipeline Pressure Indicators

• Indicators to monitor the pipeline pressure of each gas• Found on a panel on the front of the machine and may be

color coded for the gases that they monitor• 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 the indicator were on the machine (downstream) side of the check valve, it would not give a true indication of the pipeline supply pressure unless the cylinder valves were closed.

• 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 empty

• The indication of an adequate pressure on the pipeline indicator does not guarantee that gas is not being drawn from a cylinder.

• If for any reason the gas pressure coming from a cylinder via a pressure regulator exceeds the pipeline pressure and a cylinder valve is open, gas will be drawn from the cylinder.

• Therefore, cylinder valves should always remain closed when the pipeline supply is in use.

• Pipeline pressure indicators should always be checked before the machine is used.

• Pressure should be between 50 and 55 psig (345 and 380 kPa).

• Indicators should be scanned repeatedly during use

Piping• Piping is used to connect components inside the machine.

It must be able to withstand four times the intended service pressure without rupturing.

• The anesthesia workstation standard specifies that leaks between the pipeline inlet or cylinder pressure reducing system and the flow control valve not exceed 25 mL/minute.

• If the yoke and pressure reducing system are included, the leakage may not exceed 150 mL/minute.

• Piping cross connections inside the machine have been reported.

• Disconnections in the piping may occur but are rare

Gas Power Outlet

• One or more gas power (auxiliary gas) outlets may be present on an anesthesia machin.

• 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, and if there is a choice, there should be a gas power outlet for each gas.

• Power outlet is not found on many anesthesia machines today.

Oxygen Pressure Failure Devices

Devices that shut off the supply of gases other than oxygen (oxygen failure safety device) or alarm when oxygen pressure has fallen to a dangerous level.

• When oxygen pressure in the machine is normal, it will push the diaphragm and stem downward, opening the valve.

• The anesthetic gas then flows in at A, around the stem, and out at C.

• When the oxygen pressure falls, the stem moves upward, closing the valve.

• The middle chamber is vented to atmosphere to prevent mixing of anesthetic gas and oxygen in the event that the diaphragm ruptures or the packing leaks

Oxygen Supply Failure Alarm• The anaesthesia workstation standard specifies that

whenever the oxygen supply pressure falls below a manufacturer-specified threshold (usually 30 psig (205 kPa)), at least a medium priority alarm shall be enunciated within 5 seconds.

• It shall not be possible to disable this alarm• Drawbacks:

1) Because both the oxygen failure safety device and alarm depend on pressure and not flow, they have limitations that are not always fully appreciated by the user.

2) Do not offer total protection against a hypoxic mixture being delivered, because they do not prevent anaesthetic gas from flowing if there is no flow of oxygen

3) Equipment problems or operator

4) Do not guard against accidents from crossovers in the pipeline system or a cylinder containing the wrong gas.

The Ritchie whistle

• Introduced in the mid 1960’s and form the basis for most current oxygen failure devices. It was the first device to rely exclusively on the failing oxygen supply for its power.

• It senses pressure that has become significantly reduced below 4 bar in the oxygen supply.

• It is designed to be used as an oxygen failure alarm on an anaesthetic machine where it is connected to the 4 bar oxygen circuitry.

• Low pressure causes the remaining gas to be directed through a whistle.

• This alarms momentarily every time a machine is disconnected from the pipeline supply.

• If it heralds the emptying of a cylinder, the whistle is prolonged.

• It cannot be disabled and has no other function. • It is powered by the pressure that it senses. Once that

pressure has fallen to the point that the whistle no longer sounds, no further alarm will occur.

Current oxygen failure warning devices

• BS EN 740:1999 and EN ISO 60601-2-1-13, Are gas cut off devices

Oxygen Failure Safety Device

Anesthesia workstation standard requires that whenever the oxygen supply pressure is reduced below the manufacturer-specified minimum, the delivered oxygen concentration shall not decrease below 19% at the common gas outlet• It shuts off or proportionally decreases and ultimately

interrupts the supply of nitrous oxide if the oxygen supply pressure decreases

• When the pneumatic system is activated, oxygen pressure reaches the oxygen failure safety device, allowing other gases to flow.

• Turning OFF the pneumatic system causes oxygen in the machine to be vented to atmosphere.

• Resulting decrease in oxygen pressure causes the oxygen failure safety device to interrupt the supply of other gases to their flow control valves

• When oxygen pressure is normal, the plunger and seal assembly are depressed so that anesthetic gas can flow through the valve.

• When the oxygen pressure decreases, the spring forces the plunger and seal assembly upward, narrowing the valve opening in proportion to oxygen supply pressure loss

• oxygen supply pressure fails completely, the valve closes. • To determine if a machine has a properly functioning

oxygen failure safety device, the flows of oxygen and the other gas (usually nitrous oxide) are turned ON. The source of oxygen pressure is then removed. The fall in oxygen pressure is noted on the cylinder or pipeline pressure gauge.

• If the oxygen failure safety device is functioning properly, the flow indicator for the other gas will fall to the bottom of the tube just before the oxygen indicator falls to the bottom of its tube.

Oxygen Supply Failure Alarm• The anesthesia workstation standard specifies that

whenever the oxygen supply pressure falls below a manufacturer-specified threshold (usually 30 psig (205 kPa)), at least a medium priority alarm shall be enunciated within 5 seconds (7,33,34,35). It shall not be possible to disable this alarm.

Oxygen flush

• 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.

• Labeled as “O2+.” • On most anesthesia machines, the oxygen flush can be

activated regardless of whether the master switch is turned ON or OFF.

• Anesthesia workstation requires that the oxygen flush be a single-purpose, self-closing device operable with one hand and designed to minimize unintentional activation.

• A flow between 35 and 75 L/minute must be delivered

Drawbacks:• Reported hazards associated with the oxygen flush include

accidental activation• internal leakage, which resulted in an oxygen-enriched

mixture being delivered. • The flush valve may stick in the ON position.• There is a report of a flush valve sticking and obstructing

the flow of the gases from the flowmeters. • Barotrauma and awareness during anesthesia have resulted

from its activation. • Oxygen flush activation during inspiration delivered by the

anesthesia ventilator will result in delivery of high tidal volumes and possible barotrauma

Flow Adjustment Control

• The flow adjustment controls regulate the flow of oxygen, air, and other gases to the flow indicators.

• There are two types of flow adjustment controls: mechanical and electronic.

• Anesthesia workstation standard requires that there be only one flow control for each gas

• Turning the stem creates a leak between the pin and seat so that gas flows to the outlet. The stop collar prevents overtightening of the pin in the seat

• The greater the space between the pin and the seat, the greater the volume of gas that can flow. To eliminate any looseness in the threads, the valve may be spring loaded. This also minimizes flow fluctuations from lateral or axial pressure applied to the flow control knob.

Use:• The flow control knob should be turned clockwise only

until the gas flow ceases. Further tightening may result in damage to the pin or seat.

• When a machine is not being used, the gas source (cylinder or pipeline) should be closed or disconnected.

• The flow control valves should be opened until the gas pressure is reduced to zero and then closed.

• 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.

Drawback:• Loose or worn

LOW-PRESSURE SYSTEM

Pressure in this section is only slightly above atmospheric and variable, depending on the flow from the flow control valves, the presence of back pressure devices (check valves), and back pressure from the breathing system

Components• Flowmeters• Hypoxia prevention safety devices• Unidirectional valves • Vaporizers and their mounting devices• Pressure relief devices • Common gas outlet Vaporizers and their mounting

devices

Flowmeter

• Indicate the rate of flow of a gas passing through them.• Types: mechanical or electronic• Components of flow meter assembly

a. Flow control knob

b. Needle valve

c. Valve seat

d. A pair of valve stops

Safety features for O2:

Flow control knob has• Larger diameter• Projects beyond the control knobs of other gases• Colour coding• Naming

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.

Physical Principle:• 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.

• Scale-Gradations corresponding to equal increments in flow rate are closer together at the top of the scale because the annular scale increases more rapidly from bottom to top

• Temperature and pressure changes will affect both the viscosity and the density of a gas and so influence the accuracy of the indicated flow rate

Flowmeter Assembly

Consists of :• Tube through which the gas flows • Indicator• Stop at the top of the tube • Scale that indicates the flow• Lights

Flowmeter assembly empties into a common manifold that delivers the measured amount of gases into the low pressure system

1) Tube: Single or double taper. • Single-taper tubes have a gradual increase in diameter

from the bottom to the top. Use- where there are different tubes for low and high flows.

• Dual-taper flowmeter tubes have two different tapers on the inside of the same tube—one corresponding to fine flows and one for coarse flows. Use- when only one tube is used for a gas.

2) Indicator: (float or bobbin) is a free-moving device within the tube.• The nonrotating float–type indicator is designed so that

gas flow keeps the float in the center of the tube if the tube is kept vertical. The reading is taken at the upper rim.

• Rotating indicators (rotameters) have an upper rim of which the diameter is larger than that of the body. Slanted grooves, or flutes, are cut into the rim. There is often a colored dot on one side of the

Flowmeter Tube Arrangement

Tubes for different gases are grouped side by side

• Placing the oxygen flowmeter nearest the manifold outlet, a leak upstream from the oxygen results in loss of nitrous oxide rather than oxygen.

• In the presence of a flowmeter leak (either at the “O” ring or the glass of the flow tube) a hypoxic mixture is less likely to occur if the O2 flowmeter is downstream of all other flowmeters

Problems with Flowmeters• Inaccuracy• Indicator Problems• Leaks• Using the Wrong Flowmeter

ANTI HYPOXIA DEVICES• Mechanical devices- link 25 system.• Pneumatic system- pneupac ratio system.• Electronically controlled anti-hypoxic devices.

Proportioning Systems• Mechanical

integration of the N2O and O2 flow-control valves

• Automatically intercedes to maintain a minimum 25% concentration of oxygen with a maximum N2O:O2 ratio of 3:1

For every 2.07 rotations of the nitrous oxide spindle, the oxygen gear will rotate once.

Limitations of Proportioning Systems

Machines equipped with proportioning systems can still deliver a hypoxic mixture under the following conditions:• Wrong supply gas• Defective pneumatics or mechanics (e.g.. The Link-25

depends on a properly functioning second stage regulator)

• Leak downstream (e.g.. Broken oxygen flow tube)• Inert gas administration: Proportioning systems generally

link only N2O and O2

Pneumatic devices: This system relies on a ratio mixture valve to ensure that the oxygen concentration leaving the flow meter block never drops below 25% of nitrous oxide concentration

Ventilator Alarms• Disconnect / low circuit pressure• Apnea• High circuit pressure• Power failure• Low drive gas pressure