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How it works

Anaesthesia delivery workstation

A gas delivery system, vaporiser and rebreathing circle with a ventilator, built so that the patient always receives oxygen and the anaesthetist always knows what is being delivered.

Course: Anaesthesia Workstation Service — Level 1

Principle

The workstation receives oxygen, nitrous oxide (where used) and medical air from pipeline or back-up cylinders, reduces them to stable pressures and meters them into a fresh gas flow, either with mechanical flowmeters or electronic flow control. Safety devices such as an oxygen failure protection device and a hypoxic guard (mechanical or electronic link between O2 and N2O) prevent delivery of a hypoxic mixture. The fresh gas passes through a vaporiser that adds a controlled concentration of volatile agent, then enters a circle breathing system: exhaled gas passes through a CO2 absorber and is rebreathed, which saves agent and conserves heat and moisture, while an adjustable pressure-limiting (APL) valve and the scavenging system remove excess gas. A built-in ventilator (bellows, piston or turbine-driven) replaces the reservoir bag in mechanical mode. Monitoring of inspired O2, agent, CO2, airway pressure and volumes, plus a leak and compliance check before each case, closes the loop. Because the device sits between high-pressure gas and an unconscious patient, leak tightness and correct gas identity matter more than any electronic feature.

The chain, block by block

Block	What it does	How it fails
Gas supplies: pipeline and back-up cylinders	Non-interchangeable pipeline connectors and pin-indexed (or equivalent) cylinder yokes with regulators; gauges show supply pressure.	Low supply pressure alarms, leaking yoke seals or missing washers, cylinder regulators drifting, wrong or damaged connectors.
Pressure regulation and O2 failure protection	Second-stage regulators stabilise pressure; O2 failure device shuts off or proportionally reduces other gases if O2 pressure falls; O2 flush bypasses the flowmeters.	Regulator drift causing unstable flows, failure protection not activating (detected only by testing), O2 flush valve leaking or sticking.
Flow control and hypoxic guard	Needle valves with flowtubes or electronic mixers deliver the set fresh gas flow; the guard maintains a minimum O2 proportion with N2O.	Inaccurate flows, cracked flowtubes, sticking floats, electronic flow sensor errors, hypoxic guard out of adjustment.
Vaporiser and mount	Agent-specific vaporiser delivers a set concentration, compensated for temperature and flow; interlocks prevent two vaporisers being on at once.	Output out of tolerance, leaks at the mount or filler, wrong agent filled, interlock failure; desflurane vaporisers are heated and electronic.
Circle breathing system and CO2 absorber	Unidirectional valves, absorber canister, reservoir bag, APL valve, and connections to the patient circuit.	Leaks at canister seals and connections (most common pre-use test failure), sticking unidirectional valves causing rebreathing, exhausted absorbent (inspired CO2 rising), APL valve faults.
Ventilator (bellows, piston or turbine)	Delivers mechanical breaths with volume or pressure control and PEEP.	Bellows leaks or holes, piston seal wear, drive gas leaks, inaccurate volumes, PEEP valve faults.
Scavenging (AGSS) interface	Removes waste anaesthetic gas from APL and ventilator relief valves to the hospital extraction system with protection against positive and negative pressure.	Pollution of the theatre when extraction fails or is disconnected, negative pressure pulling gas from the circuit when the interface is misadjusted.
Gas and patient monitoring	Sidestream gas analysis (O2, CO2, agents, often N2O), airway pressure, volumes, alarms.	Water traps full or occluded sample lines, calibration drift, O2 sensor expiry, alarm settings disabled.
Power, battery and control electronics	Supplies electronics, flow control and ventilator; battery supports operation on mains failure.	Short battery autonomy, display or control failures, self-test failures at start-up.

Block

What it does

How it fails

Gas supplies: pipeline and back-up cylinders

Non-interchangeable pipeline connectors and pin-indexed (or equivalent) cylinder yokes with regulators; gauges show supply pressure.

Low supply pressure alarms, leaking yoke seals or missing washers, cylinder regulators drifting, wrong or damaged connectors.

Pressure regulation and O2 failure protection

Second-stage regulators stabilise pressure; O2 failure device shuts off or proportionally reduces other gases if O2 pressure falls; O2 flush bypasses the flowmeters.

Regulator drift causing unstable flows, failure protection not activating (detected only by testing), O2 flush valve leaking or sticking.

Flow control and hypoxic guard

Needle valves with flowtubes or electronic mixers deliver the set fresh gas flow; the guard maintains a minimum O2 proportion with N2O.

Inaccurate flows, cracked flowtubes, sticking floats, electronic flow sensor errors, hypoxic guard out of adjustment.

Vaporiser and mount

Agent-specific vaporiser delivers a set concentration, compensated for temperature and flow; interlocks prevent two vaporisers being on at once.

Output out of tolerance, leaks at the mount or filler, wrong agent filled, interlock failure; desflurane vaporisers are heated and electronic.

Circle breathing system and CO2 absorber

Unidirectional valves, absorber canister, reservoir bag, APL valve, and connections to the patient circuit.

Leaks at canister seals and connections (most common pre-use test failure), sticking unidirectional valves causing rebreathing, exhausted absorbent (inspired CO2 rising), APL valve faults.

Ventilator (bellows, piston or turbine)

Delivers mechanical breaths with volume or pressure control and PEEP.

Bellows leaks or holes, piston seal wear, drive gas leaks, inaccurate volumes, PEEP valve faults.

Scavenging (AGSS) interface

Removes waste anaesthetic gas from APL and ventilator relief valves to the hospital extraction system with protection against positive and negative pressure.

Pollution of the theatre when extraction fails or is disconnected, negative pressure pulling gas from the circuit when the interface is misadjusted.

Gas and patient monitoring

Sidestream gas analysis (O2, CO2, agents, often N2O), airway pressure, volumes, alarms.

Water traps full or occluded sample lines, calibration drift, O2 sensor expiry, alarm settings disabled.

Power, battery and control electronics

Supplies electronics, flow control and ventilator; battery supports operation on mains failure.

Short battery autonomy, display or control failures, self-test failures at start-up.

Symptom → where to look first

Symptom	Suspect first
Pre-use leak test fails	Circle system: absorber canister seals, bag and circuit connections, vaporiser seated correctly and filler caps closed.
Inspired CO2 rising	Exhausted absorbent first, then sticking unidirectional valves.
Delivered agent concentration differs from dial setting	Vaporiser output (out of calibration, wrong agent), then gas analyser calibration.
Low O2 supply alarm with pipeline connected	Pipeline pressure at the outlet, hose and connector, then internal regulator and inlet check valves.
Ventilator volumes lower than set	Leaks in bellows or circuit, fresh gas decoupling settings, flow sensor calibration.
Bag fills or collapses unexpectedly	Scavenging interface pressure balance or APL valve, then fresh gas settings.
Gas monitor readings unstable or 'occlusion'	Water trap and sample line, then monitor calibration.
Start-up self-test fails	Read the failed step: usually leak, sensor calibration or gas supply, rarely electronics.

Symptom

Suspect first

Pre-use leak test fails

Circle system: absorber canister seals, bag and circuit connections, vaporiser seated correctly and filler caps closed.

Inspired CO2 rising

Exhausted absorbent first, then sticking unidirectional valves.

Delivered agent concentration differs from dial setting

Vaporiser output (out of calibration, wrong agent), then gas analyser calibration.

Low O2 supply alarm with pipeline connected

Pipeline pressure at the outlet, hose and connector, then internal regulator and inlet check valves.

Ventilator volumes lower than set

Leaks in bellows or circuit, fresh gas decoupling settings, flow sensor calibration.

Bag fills or collapses unexpectedly

Scavenging interface pressure balance or APL valve, then fresh gas settings.

Gas monitor readings unstable or 'occlusion'

Water trap and sample line, then monitor calibration.

Start-up self-test fails

Read the failed step: usually leak, sensor calibration or gas supply, rarely electronics.

Safety

Site and infrastructure

Medical gas outlets (O2, air, N2O where used) at the pressures specified by the manufacturer, correctly labelled and with non-interchangeable connectors.
Full back-up O2 cylinder on the machine (or immediately available) with a working regulator.
Active anaesthetic gas scavenging system connected and working at the specified flow.
Mains outlets on essential (generator-backed) supply; workstation battery charged.
Theatre ventilation and temperature/humidity within design and manufacturer limits; antistatic and fire safety measures in place.

Preventive maintenance basics

Replace seals, O-rings, flow sensors, O2 cells and other parts in the manufacturer's PM kit at the specified interval.
Perform high- and low-pressure leak tests and circle system leak/compliance tests to specification.
Test O2 failure protection device, alarms and hypoxic guard behaviour.
Verify flow accuracy, ventilator volume and pressure delivery and PEEP with a calibrated analyser.
Check vaporiser output with a calibrated agent analyser (or arrange vaporiser service as required) and check interlocks and mounts.
Calibrate and verify the gas monitor (O2, CO2, agent) and check water traps and sample lines.
Test battery autonomy and switch-over; perform electrical safety testing.
Check scavenging interface operation and pressure relief.

Safety

Never service a workstation in use; after any repair, perform the full pre-use check and performance verification before clinical use.
Oxygen and nitrous oxide both support combustion; no oil or grease on fittings, no ignition sources, use only lubricants approved by the manufacturer.
Depressurise pipeline and cylinder supplies before opening the pneumatic system; open cylinder valves slowly.
Exposure to waste anaesthetic agents: work with scavenging connected and the room ventilated; avoid spilling liquid agent and handle vaporiser filling with care.
Gas identity: never alter, adapt or bypass gas-specific connectors; a crossed gas connection can be fatal.

Model pages

GE HealthCareAvance family (Datex-Ohmeda S/5 Avance, Avance CS2)Datex-Ohmeda / GE anaesthesia workstations with an electronic gas mixer, an integrated bellows ventilator and a compact breathing system, used in operating theatres for general anaesthesia from adults to children.

Manufacturers

Dräger · GE HealthCare (Datex-Ohmeda) · Mindray · Getinge (Maquet) · Penlon · Spacelabs (Blease) · Comen · Aeonmed · Heyer Medical