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THE ANESTHESIA MACHINE

Moderator: Prof. Jaishree BograPresented By: Dr Rajesh Raman &

Dr. Hemant Rastogi

7/7/2011

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A device which delivers a precisely known but variable gas mixture

Types of anesthesia machine• Intermittent-Gas flows only during inspiration– Operate on demand system– Used rarely: obstetric & dental analgesia, OPD diagnostic

procedures – Eg.: Entonox apparatus

• Continuous-Gas flows both during inspiration and expiration.

• Eg. :– Boyle’s apparatus– Forregar – Dragger

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Boyle’s apparatus

• Invented in 1917 by Henry Gaskin Boyle• Anaesthesia workstation consists of the

anesthesia machine, vaporizers, ventilator, breathing system, scavenging system, and monitors.

• ASTM F1850-00: Defines the requirements for designing of anesthesia workstations and their components

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BASIC FUNCTION

To receive compressed

gases from their supplies.

To create a safe gas mixture of

known composition and flow rate

To deliver a gas mixture to

patient at a safe pressure.

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Monitor

Ventilator

Data management system

Vaporiser

Flowmwter

Breathing circuit

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System ComponentsA. Electrical Components:

1. Master Switch: activates both pneumatic & electrical functions.2. Power Failure Indicator: audible & visible3. Reserve Power: rechargeable and dual supply4. Electrical Outlets: to power monitors, does not provide electricity

during power failure5. Circuit Breakers: when activated, electrical load should be reduced

before resetting the circuit breaker 6. Data Communication Ports: communicate b/w the machine, monitors

and the data management systemB. Pneumatic System:

1. High pressure circuit2. Medium pressure circuit3. Low pressure circuit

9Simplified Diagram of 2 gas anesthesia machine

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High pressure Intermediate pressure

Low pressure

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The High-pressure CIRCIUT

Receives gases from cylinders at high, variable pressures and reduces those pressures to lower,

more constant pressure suitable for use in the machine.

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COMPONENTS

• Hanger yoke assembly• Cylinder pressure gauge• Pressure reducing device

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Hanger Yoke• Orients and supports the

cylinder, provides a gas-tight seal, and ensures a unidirectional gas flow

• ASTM requires that there must be at least one yoke each for oxygen and nitrous oxide

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7 parts:1. Body: principal framework and the

supporting structure2. Retaining screw: tightens cylinder in yoke3. The nipple: entrance of gas into the machine4. Index pins: prevent accidental placement of

an incorrect cylinder5. The washer (Bodock seal): helps to form a

gas-tight seal6. Filter: to remove particulate matter7. Check valve assembly: prevent retrograde

flow of gases

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CHECK VALVE ASSEMBLY • It is a one way valve located just upstream to cylinder

pressure gauge• Prevents retrograde flow of gas through the yoke• Allows an empty cylinder to be replaced with a full

one without losing gas from the machine• Prevents gas from being transferred from a cylinder

with a higher pressure to another one with lower pressure

• Minimizes leakage (but does not completely stop) from an open cylinder to atmosphere if one cylinder is absent from its yoke: use yoke plug

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• It consists of 2 holes on the cylinder valve positioned in an arc below the outlet port and pins on the yoke or pressure regulator to fit into these holes.

• Pins:– 4 mm diameter– 6 mm long.

• The seven hole positions are on the circumference of a circle of 9/16 inch radius with the outlet port as centre

• The position of pins and corrosponding holes are different for different gases

• To prevent placement of wrong gas cylinder in yoke

Pin Index Safety System

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Nipple

Retaining screw

The wrong cylinder may be fitted by:•Using extra sealing washers or by•Removing the safety pins,•Inverting the gas cylinder

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Cylinder Pressure Indicator (Gauge)• ASTM Standard requires presence of pressure indicator which displays the

cylinder pressure for each gas supplied by cylinders • Most indicators are made of the bourdon tube

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Bourdon pressure gauge

• A Bourdon tube is a hollow metal tube (copper alloy) bent into a curve, then sealed on one side and linked to a clocklike mechanism

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Bourdon Pressure Gauge

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MECHANISM• As gas pressure within the flexible tube increases, the

tube tends to straighten. The motion is translated through the gearing mechanism so that the indicator shows a higher pressure.

Electronic cylinder pressure indicator

• Light emitting diodes(LED)in electronic pressure gauge indicate– Cylinder valve is close :• Dark color

– Cylinder valve is open:• Pressure adequate –Green• Pressure inadequate-Red

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Pressure Regulators• Reduces high and variable pressure of cylinder

to a lower, more constant pressure suitable for use in an anesthesia machine– O2 : 2200 psig→45 psig– N2O: 750 psig →45 psig

• ASTM Standard requires a pressure regulator for each gas supplied from cylinders

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BASIC PRINCIPLE• A higher pressure acting over a small area is balanced by a

smaller pressure acting over a large area

A1 x Pc = A2 x Pr

Cylinder Pressure regulator

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Safety features

• PISS for small cylinders• DISS for large cylinders• Pressure Relief valves• Color coding of cylinder• Pressure regulator

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INTERMEDIATE PRESSURE SYSTEM

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1. Pneumatic part of the master switch,2. Pipeline inlet connections and check valves, 3. Pipeline pressure indicators, 4. Piping,5. The gas power outlet, 6. Oxygen pressure failure devices7. The oxygen flush,8. Additional pressure regulators, and 9. The flow control valves

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Master Switch (Pneumatic Component)

• Downstream of the inlets for the cylinder and pipeline supplies

• The oxygen flush is usually (but not always) independent of this switch

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Pipeline Inlet Connections• Receives medical gases at a pressure of 50-55 psig• Have a filter with a pore size of 100 µm or less• Have unidirectional check valve to prevent retrograde gas flow • Gas flow can get obstructed when check valve is stuck in closed position or

the filter becomes clogged with dirt• These inlets are fitted with threaded non-interchangeable Diameter Index

Safety System (DISS) fittings

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• Each DISS connection consists of a Body Adaptor, Nipple, and Nut • Diameters of the Body Adaptor and nipple increase/decrease

proportionally. In this way, only properly mated and intended parts fit together to permit thread engagement

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Pipeline Pressure Indicators• Similar to cylinder pressure indicator• Located on the pipeline side of the check

valve in the pipeline inlet (upstream of the check valve) to monitor pipeline pressure only.

• Should be b/w 50 and 55 psig (slightly higher than gases received from cylinders)

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Piping• Connects components inside the machine• Must be able to withstand 4 times the

intended service pressure• Leaks between the pipeline inlet or cylinder

pressure reducing system and the flow control valve (intermediate pressure circuit) not exceed 25 ml/minute

• If the yoke and pressure reducing system are included (intermediate + high pressure circuit), the leakage should not exceed 150 ml/minute.

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Gas Power Outlet• 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

• Present only in older machines

• DISS connection is identical to that of oxygen inlet connection: use carefully

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FAIL-SAFE VALVES

Oxygen Pressure Failure Devices

ASTM standard requires that whenever the oxygen supply pressure falls, the delivered oxygen concentration shall not decrease

below 19% at the common gas outlet

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• To prevent decrease of oxygen concentration at common gas outlet

• The pressure from O2 supply is used to keep a valve open through which N2O is allowed to pass through

• When the pressure of O2 falls in the intermediate circuit, the valve closes and flow of N2O stops

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Pressure Sensor Shut-off Valve: Datex Ohmeda• Operates in a threshold manner: either open or closed• O2 pressure moves the piston and pin upward and the valve

opens for N2O• When pressure of O2 falls below preset value, force of the

valve return spring completely closes the valve

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Oxygen failure protection device :Dräger• Based on a proportioning principle rather than a threshold

principle• Pressure of N2O falls in proportion of decrease of O2

pressure• Seat nozzle assembly connected to a spring-loaded conical

tapered piston

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• Do not prevent delivery of hypoxic gas mixture in following situations:– Leaks and defects downstream – Pipeline crossovers & presence of wrong gas in cylinder– Dilution of gas mixture by inhaled anesthetics

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Oxygen Supply Failure Alarm• ASTM standard specifies that whenever the oxygen supply

pressure falls below a certain threshold (usually 30 psig), a alarm must get activated within 5 seconds. It should not be possible to disable this alarm.

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Pneumatic alarm (Bowman’s Whistle): Uses a canister with whistle in the tip. Normally it gets filled with pressurized O2 at 50 psig. When the oxygen pressure falls in the intermediate circuit, the O2 flows back in the circuit through the whistle in its tip which causes whistling alarm.

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Gas Selector Switch• Prevents air and nitrous oxide from being used

together

In the N2O position, only oxygen and nitrous oxide can be used. In the air position, only oxygen and air can be administered

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•Just upstream of the flow indicators.•Eliminates the fluctuations occurring in the intermediate pressure

circuit•Also plays a role in O2-N2O interlock

•Reduce gas pressures further to around 26 psig for nitrous oxide and 14 psig for oxygen

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Oxygen Flush (O2+)• Directs a high Oxygen flow directly to the common gas outlet• It must 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 is delivered at 45-55 psig• On most anesthesia machines, the oxygen flush can be activated

regardless of whether the master switch is turned on or off• Hazards

– Accidental activation– Barotrauma– Leakage/excessive use – Awareness under anesthesia

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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 depressed

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Flow Control Valve

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• The needle Valve: A finely threaded rod with tapered end which fits into cylindrical tube permitting gas flow to occur between the needle shaft and the seat

• When the rod is fully screwed down, it occludes the flow completely

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Safety features• DISS for pipeline connection• Color coded pipelines• Gauges of manometer• Low O2 pressure cut off system• Alarms• Fail safe valves• Shut off valves

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LOW-PRESSURE CIRCUIT

• Flow meters, • Hypoxia prevention safety

devices,• Pressure relief devices• Vaporizers • Unidirectional valves• Common gas outlet.

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Flow meters• Indicates the rate of flow of a gas passing

through them• Can be electronic or mechanical• Constant pressure/variable orifice• Flowmetres are calibrated at atmospheric

pressure and 20°C

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Flow meter assembly

• Thorpe’s tubes.• Indicators.• Indicator Stop.• Scale.

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• Glass: conical tapering– Single / double taper– Dual tubes: fine and coarse flow.

Thorpe’s Tube

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• There is a decrease in cross sectional area at the float which leads to a drop of pressure across the float

• Pressure exerted on the float depends on 2 factors:– Gas flow rate– Cross sectional area of annular space b/w

float and walls of the tube• The float settles in position where upward

force represented by product of pressure difference across the float and cross sectional area of float = the downward force exerted by gravity

• When the flow through the tube increases, the difference in pressure b/w the two ends of float increases and hence the float raises to a height where cross sectional area b/w the tube and float increases sufficiently to restore the pressure drop to its former value

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• Stop – upper and lower indicator stop– To prevent float from ascending to top of tube and plugging the outlet– Ensures that float will be visible even at max. flow instead of getting hidden

in the manifold.– Bottom stop provides a central foundation for the float when flow control

valve is in off position

• Scale :a. Marked on (Drager) / Right side (Datex Ohmeda) of the tube.b. In L /min.

• Greatest accuracy is in the middle half of tube.• Having one series for low flow and one for high flow results in

higher accuracy.

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FLOW METER SEQUENCE

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Problems with flow meters• Inaccuracy

– Back pressure from breathing circuit– Dirt– Static electricity– Tilting of machine– Damaged float/tube

• Leak.

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Electronic flow meter• Real/ virtual flow control knobs• Dependent on electrical supply• Advantage is that information can be sent

to the data management system

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Hypoxia prevention safety devices• Manadatory Minimum oxygen flow.• Minimum oxygen ratio– Ohmeda link 25 proportioning system– Drager Sensitive-oxygen ratio controller

• Electronic linkage• Alarms

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1. Manadatory minimum oxygen flow :• 50 – 250 ml / min of oxygen flows before the

other gases start.2. Minimum o₂ ratio : Device to prevent gas mixture having low oxygen

conc. in inspired gas.

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• It is a system that increases the oxygen flow when necessary to prevent delivery of gas mixture with low O2 conc.

•The Datex Ohmeda has Link 25 system which uses a chain to connect sprockets having 28 and 14 teeth on the identical flow control valves of O2 and N2O respectively•Revolution of N2O flow control valve by two rounds results in revolution of O2 flow control valve by 1 round.•The N2O Flow control valve is supplied by gas at 26 psig whereas the O2 flow control valve is supplied by gas at 14 psig•This combination of mechanical and pneumatic aspects of the system result in a minimum 25% O2 concentration•Either gas can be adjusted independently, but the mechanical linkage maintains a minimum ratio of 1:3 of O2 to N2O

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• Maintain O2 conc. substantially higher than 25% at O2 flow rates of less than 1 L/min.

• Limit the flow of N2O to prevent delivery of hypoxic mixture.

•Drager machines have OXYGEN RATIO MOTION CONTROLLER(ORMC) and similar SENSITIVE OXYGEN RATIO CONTROLLER(S-ORC, in newer machines) as proportioning system•These are pneumatic O2-N2O interlock system using linear resistors designed to maintain a fresh gas oxygen concentration of 25±3%•The resistors are located downstream to flow control valves•The value of O2 resistor is 3-4 times the value of N2O resistor

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3. Electronic linkage Computer continuously calculates the minimum

allowable O2 to be given 4. Alarms Alerts if the ratio falls below the preset value.

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Backpressure safe guard devices• Pressure relief valve :a. Near the common outlet to prevent from high pressuresb. May limit adequate pressure for jet ventilation• Unidirectional check valve : b/w vaporizer and common gas outlet

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Common fresh gas outlet• Accepts gas and vapours

– 15mm female slip– One / two outlets .

Only one should be functional at one time Disadvantage – wrongly directed.

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Safety features• Larger, Fluted, color coded O2 flow control knob• Down stream placement of O2 knob• Placement of knobs at a distance• Flowmeters• Back light display• Mandatory Minimum oxygen flow.• Link devices• Back pressure check valve• Audible alarms• Oxygen analyzer

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Essential Features Purpose

Noninterchangeable gas-specific connections to pipeline inlets (DISS) with pressure gauges, filter, and check valve

Prevent incorrect pipeline attachments; detect failure, depletion, or fluctuation

Pin index safety system for cylinders with pressure gauges, and at least one oxygen cylinder

Prevent incorrect cylinder attachments; provide backup gas supply; detect depletion

Low oxygen pressure alarmDetect oxygen supply failure at the common gas inlet

Minimum oxygen/nitrous oxide ratio controller device (hypoxic guard)

Prevent delivery of less than 21% oxygen

Oxygen failure safety device (shut-off or proportioning device)

Prevent administration of nitrous oxide or other gases when the oxygen supply fails

Essential Safety Features on a Modern Anesthesia Workstation

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Entry of O2 downstream to other gases in the common manifold Prevent hypoxia in event of proximal gas leak

Oxygen analyzer and alarm Prevent administration of hypoxic gas mixtures in event of a low-pressure system leak; precisely regulate oxygen concentration

Automatically enabled essential alarms and monitors (eg, oxygen concentration) Prevent use of the machine without essential

monitors

Vaporizer interlock Prevent simultaneous administration of more than one volatile agent

Capnography and anesthetic gas measurement Guide ventilation; prevent anesthetic overdose; help reduce awareness

Oxygen flush mechanism that does not pass through vaporizers

Rapidly refill or flush the breathing circuit

Breathing circuit pressure monitor and alarm Prevent pulmonary barotrauma and detect sustained positive, high peak, and negative airway pressures

Exhaled volume monitor Assess ventilation and prevent hypo- or hyperventilation

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Pulse oximetry, blood pressure, and ECG monitoring Provide minimal standard monitoring

Mechanical ventilator

Control alveolar ventilation more accurately and during muscle paralysis for prolonged periods

Backup battery

Provide temporary electrical power (> 30 min) to monitors and alarms in event of power failure

Scavenger system

Prevent contamination of the operating room with waste anesthetic gases

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Unacceptable/Undesirable Features of Older Anesthesia Machines

Unacceptable features 1. Flowmeter-controlled vaporizer (eg, copper, kettle, Vernitrol)2. More than one flow control valve for a single gas3. Vaporizer with a rotary dial that increases concentration with clockwise rotation4. Connections in the scavenging system that are the same size as breathing circuit

connectionsUndesirable features 1. APL valve that is not isolated during mechanical ventilation2. Oxygen flow control knob that is not fluted or larger than other flow control knobs3. Oxygen flush control that is unprotected from accidental activation4. Lack of main On/Off switch for electrical power to integral monitors and alarms5. Lack of antidisconnect device on the fresh gas hose (common gas outlet)6. Lack of airway pressure alarms

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All To Be Completed Daily

1: The auxiliary oxygen cylinder( separate from the anesthesia machine) and self-inflating manual ventilation device (Ambu bag)

2: Suction adequate to clear the airway

3: AC power availability

4: Presence of required monitors, including alarms

Preanesthesia Checkout Procedures

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Required monitors• Oxygenation:– Inspired gas: O2 analyzer and Low O2 conc. alarm– Blood oxygenation: pulse oximeter

• Ventilation:– EtCO2– Volume of expired gases

• Circulation:– ECG– Non invasive blood pressure

• Temperature monitor

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5: Verify that pressure is adequate on the spare oxygen cylinder mounted on the anesthesia machine

6: Piped gas pressures ≥50 psig

7: Adequately filled vaporizers with filler ports tightly closed

8: Verify that there are no leaks between the flow meters and the common gas outlet

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•The machine's master switch, flow control valves, and vaporizers are turned off•Suction bulb is attached to the common fresh gas outlet and squeezed to collapse it fully•The low pressure circuit is free of leaks if the bulb remains collapsed for at least 10 seconds•Repeat test with each vaporizer in on position

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13: Verify that gas flows properly through the breathing circuit during both inspiration and exhalation

14: Documentation of checkout procedures

15: Confirm ventilator settings and evaluate readiness to deliver anesthesia care (Anesthesia time-out)

9: Test scavenging system function

10: Calibrate or verify calibration of the oxygen monitor and check the low-oxygen alarm

11: Verify that the carbon dioxide absorbent is not exhausted

12: Check for proper breathing system pressure and leaks

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To Be checked before each case

• Suction• Monitors• Vaporizers• Carbon dioxide absorbent• Breathing system pressure and leaks• Gas flow through the breathing circuit• Ventilator settings

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WORKSTATION SELF TESTS• New workstations do automatic self test when they are

powering on• Components checked vary in different devices• Common components checked are:– Gas supply– Flow control valve– Circle system &– Ventilator

• Vaporizers are to be checked saperately in almost all machines

• Read and follow manufacturer’s recommendations strictly

Anesthesia ventilators

classification

• Power source• Cycling mechanism• Bellows classification

Power source• Compressed gas:– Uses O2 or Air as driving force– Dual circuit

• For anesthetic gases• For power source

• Electrically powered ventilators:– Use electrical power as driving force– Computer controlled motor– Piston driven-like a syringe plunger– More precise– Provide advanced type of ventilatory support: SIMV– Single circuit(patient gas only)– Consume less compressed gas

Cycling Mechanism• Time cycled: inspiratory phase is triggered by a timing device

– Older Pneumatic Vent.: Fluidic (mechanical) timing device– Newer electronic vent.: electronic timing device

• Pressure cycled:– Have adjustable pressure trigger– Pressure sensors provide feedback to computerized vent. control

system– Required for advanced ventilatory modes

Bellows classification

• Direction of movement during expiratory phase• Ascending bellow moves upward during

expiration and will not ascend if patient disconnection

• Descending bellow moves upward during inspiratory phase by driving force of the gas and will continue to ascend even if patient disconnection occurs. So patient circuit disconnection may go unnoticed

PROBLEMS AND HAZARDS

1. Disconnection - at y piece ,– complete or partial– disconnections more readily diagnosed with

ascending bellows.– Resp. volume Monitors

2. Misconnections3. Occlusion of breathing circuits4.ETT kinking5.Blockage of bacterial filter6.Barotrauma – O₂ flushing

7.Bellow assembly problems improper sealing – leaks hole – hyperinflation - barotrauma (high

pressure gas enters the circuit )8. Ventilation relief valve problems: barotrauma9.Power supply problems

SCAVENGING SYSTEM• Scavenging is the collection and removal of

waste anesthetic gases from the operating room

Gas-Collecting Assembly

• Captures excess anesthetic gas and delivers it to the transfer tubing

• From the APL valve, the ventilator relief valve and from the ventilator drive gas

Transfer Means

• Carries excess gas from the gas-collecting assembly to the scavenging interface

• The tubing must be either 19 or 30 mm• The two tubes frequently merge into a single

hose before they enter the scavenging interface

• Its occlusion can increase baseline breathing circuit pressure: barotrauma can occur

Scavenging Interface

• Prevents pressure increase or decrease in the scavenging system from being transmitted to the breathing system

• Limits the pressure immediately downstream from the gas-collecting assembly to between -0.5 and +10 cm H2O

• Positive-pressure relief is mandatory in all scavenging systems– to vent excess gas in case of occlusion downstream from the interface

• If the disposal system is an “active” system(vacuum), negative-pressure relief is necessary to protect the breathing circuit or ventilator from excessive subatmospheric pressure

• Open Interface: contains no valves and is open to the atmosphere, thereby allowing both positive- and negative-pressure relief

• Should only be used with active disposal systems that have a central evacuation system

• Open interfaces require a reservoir

• Closed Interfaces: communicates with the atmosphere through valves

• must have a positive-pressure relief valve to vent excess system pressure if obstruction occurs downstream

• A negative-pressure relief valve is mandatory to protect the breathing system from subatmospheric pressure if an active disposal system is used

GAS-DISPOSAL ASSEMBLY TUBING

• Conducts waste gas from the scavenging interface to the gas disposal assembly

Gas Disposal Assembly

• Ultimately eliminates excess waste gas – active or passive

• Active assembly: vacuum pump serves as the mechanical flow-inducing device that removes the waste gases– Negative &positive pressure relief valve and reservoir

required

•Passive disposal system :the “weight” of the heavier-than-air anaesthetic gases produces flow through the system•Negative-pressure relief and a reservoir are not required

SCAVENGING INTERFACE

Closed (valves present)

+ve pressure

relief valve

+ve & -ve pressure relief

valve

Passive disposal (weight of the gas)

Active disposal(Vaccum)

Reservoir required

Reservoir required

Open (no valves)

Active disposal(Vacuum)

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