The Anesthesia Machine and Workstation - Key Concepts
The Anesthesia Machine and Workstation: Comprehensive Notes
The modern system components
Anesthesia gas delivery system comprises: the anesthesia machine, anesthesia vaporizer(s), breathing system, ventilator, and waste gas scavenging system.
The breathing system is the functional center: it connects to all other components and to the patient.
Gas flows back and forth with spontaneous ventilation or manual ventilation through the reservoir bag.
If the lungs are mechanically ventilated, the ventilator bellows or piston acts as a counterlung, exchanging volume with the patient’s lungs via the breathing system.
The anesthesia machine receives gases (O₂, and sometimes air, N₂O, Heliox, CO₂) from storage sources and safely creates a known-mixture gas at a known flow rate, which is delivered to a vaporizer to add a controlled anesthetic agent.
The resultant fresh gas mixture is delivered to the common gas outlet (CGO) and into the patient breathing system (commonly a circle system).
Excess fresh gas must exit the system to prevent rising circuit pressure; excess gas leaves via the adjustable pressure-limiting (APL) valve during spontaneous/manual ventilation or via the ventilator pressure relief valve during mechanical ventilation.
Waste gas exits to a scavenging system (safety interface to the facility waste-gas system).
ISO/IEC standard reference: 80601-2-13 (Part 1 of a broader standard) with updates; older ASTM and ANSI standards historically defined the workstation concept and requirements.
Flow of gas through the machine: high-, intermediate-, and low-pressure systems
Gas pathways are traditionally divided into three pressure systems:
High-pressure: upstream of the first-stage regulator; pressures ~45–2200 psig.
Intermediate-pressure: downstream of the first-stage regulator and upstream of gas-flow control valves; ~16–55 psig.
Low-pressure: downstream of the flow-control valves; typically just above atmospheric.
Some authors group these as: high-pressure (upstream of flow-control valves) and low-pressure (downstream of flow-control valves).
The flow relationship follows Ohm’s law analogue for gases: ext{Flow} = rac{ ext{Pressure}}{ ext{Resistance}}.
Modern machines include safety features to prevent hypoxic mixtures: oxygen supply alarms, fail-safe (pressure sensor shut-off) valves, gas-proportioning (ORC/S-ORC/Link-25) systems, and usability features to reduce errors.
Anesthesia machine components (key parts and functions)
GAS INLETS (pipeline inlets)
DISS (Diameter Index Safety System) connectors ensure correct gas inlet connections and prevent cross-connection between different pipeline gases.
Gas-specific DISS connectors exist for O₂ (CGA 1240), N₂O (CGA 1040), Air (CGA 1160), Suction (CGA 1220), WAGD (CGA 2220), Heliox, CO₂, etc.
Each inlet has a filter (to block particles >100 μm) and a check valve to prevent leakage if pipelines are disconnected while cylinders are in use.
Upstream of the pipeline check valve is a junction leading to a pipeline-pressure gauge; gauge reads zero when the wall pipeline is disconnected.
Oxygen can also be supplied from freestanding E-size cylinders (regulated to 55 psi) via a DISS connector, allowing a backup oxygen source if pipeline fails.
In remote locations, large H-cylinders with regulators can supply via pipeline inlets; back-up E-cylinder yoke (hanger yoke) can be attached.
The medical gas pin-index safety system ensures only oxygen fits a cylinder in an oxygen hanger.
Cylinder mounting steps and safety practices are described (washer placement, T-handle, proper alignment, etc.).
Check-valve in each hanger yoke prevents gas loss when a yoke is unoccupied and prevents cross-transfilling between cylinders.
GAUGES AND SENSORS
Pipeline and cylinder supply pressures are read on gauges, traditionally Bourdon-tube gauges with protective glass (or newer electronic transducers in contemporary machines).
Bourdon-tube gauges have a pressure fuse mechanism to vent if pressure spikes (diaphragm/pressure-relief concerns).
Front-panel gauges are color-coded and labeled to indicate gas type and whether gauge shows cylinder or pipeline pressures.
Some newer machines replace gauges with electronic transducers; readings appear on the information screen and can be alarmed by the workstation.
PRESSURE REGULATORS
First-stage regulators (cylinder regulators) reduce variable cylinder pressures (up to ~2200 psig for O₂, N₂O, etc.) to a constant lower output (typically ~45 psig in many two-O₂-cylinder setups).
A regulator often includes a pressure-relief valve in the low-pressure chamber to vent excess pressure to the atmosphere.
Failure of a regulator (diaphragm rupture, etc.) can cause loud audible cues and potential loss of regulated gas pressure downstream.
OXYGEN FLUSH
Oxygen flush valve activates a direct path from the O₂ supply (pipeline or first-stage regulator) to the CGO, delivering 35–75 L/min of pure O₂.
The flush bypasses flowmeters and vaporizers, so it can rapidly fill the breathing system or deliver high-flow O₂ in emergencies.
Risk: pressure at the CGO can rise toward the source pressure; safety devices are included (e.g., pressure-limiting features in modern ventilators) to prevent barotrauma if used during inspiratory phase.
Some machines include a recessed, self-closing oxygen flush button to reduce accidental activation.
It can be used for high-pressure oxygen for emergency jet ventilation, or to rapidly fill the breathing system, but must be used with caution during mechanical ventilation.
PRESSURE SENSOR SHUT-OFF VALVES AND ALARM SYSTEM (Fail-Safe)
Fail-safe valves reduce or interrupt flow of the second gas if the oxygen supply pressure falls below a preset threshold.
Older designs were all-or-nothing; newer ones may proportionally reduce the second gas (O₂ may be the last gas to flow to the patient).
A common safety concept: the term “fail-safe” is somewhat a misnomer because it does not guarantee adequate oxygen flow; the oxygen analyzer is essential to detect unintended hypoxic mixtures.
If the oxygen pressure falls below the threshold, a fail-safe valve can close or proportionally reduce gas flows to prevent hypoxic delivery.
An oxygen supply pressure alarm exists on all machines to alert when the oxygen supply is insufficient (typical threshold around 30 psig; alarm is high-priority and cannot be silenced for more than 120 seconds).
OXYGEN RATIO PROPORTIONING SYSTEMS (hypoxic-avoidance devices)
Purpose: prevent delivery of a hypoxic mixture by limiting nitrous oxide (N₂O) flows relative to oxygen.
Dräger ORC (Oxygen Ratio Controller): earlier systems used mechanical flow-control with interlock linking oxygen and N₂O flows; prevents O₂ < 25% in the mixture.
ORC uses two diaphragms with a slave N₂O flow-control valve in series with the N₂O valve; the relative backpressures determine N₂O flow.
In mechanical ORC, the maximum O₂ to N₂O ratio is restricted; O₂ flows less than 25% of N₂O are limited.
Dräger S-ORC (Sensitive Oxygen Ratio Controller): newer design with electronic flowmeters; the system has two connected diaphragms controlling a slave N₂O valve; maximum O₂ flow is limited to maintain safe ratio; a bypass valve allows O₂ up to ~9 L/min in some configurations.
GE Link-25 Proportioning Limiting System: robust proportioning limiting mechanism that restricts N₂O flow in relation to O₂; once O₂ is increased, the system maintains the increased O₂ setting and proportioning continues.
All these systems work only between O₂ and N₂O; they do not interlink O₂ with non-hypoxic gases such as air or helium. If a gas other than O₂ is used downstream of the proportioning system (e.g., a high-concentration anesthetic agent or helium), the hypoxic protection may not apply.
If the fresh gas proportioning system malfunctions, an oxygen analyzer is essential to detect a hypoxic mixture.
FRESH GAS CONTROLLERS AND FLOWMETERS
Two components deliver the intended flow to the CGO: a flow-control device (needle valve or electronic valve) and a flow-measuring device (rotameter or electronic flowmeter).
Mechanical flow-control valves (needle valves):
Gas flow is set by turning knobs; turning left (counterclockwise) opens, right (clockwise) closes.
Flow depends on orifice size and pressure differential; two rotameters in series may be used for each gas (low-flow up to ~1 L/min; high-flow up to ~10–12 L/min).
Oxygen control knob is colored green; N₂O is blue; knobs can be shielded to prevent accidental changes.
Flowmeter tubes are vertical rotameters (Thorpe-tube principle): a float rises with flow; calibrated against standard atmosphere (760 mm Hg) at 20°C.
Flowmeter tubes are gas-specific; flow readings are not interchangeable among different gases. Flowmeter calibration is checked by verifying the resulting gas concentrations with a analyzer.
Oxygen flowmeter position is important: in a common-manifold design, O₂ is delivered downstream of other gases to minimize hypoxic risk if a leak occurs in other tubes.
Rotameter physics (Thorpe tube)
Flow through a tapered glass tube with a float is driven by differential pressure at the float.
For laminar flow (low flows), Poiseuille’s law dominates; historically expressed as:
ext{Flow} = rac{ ext{ΔP} imes ext{π} imes r^4}{8 imes ext{η} imes L}
where ΔP is pressure drop, r is tube radius, η is viscosity, and L is length of the flow path.At higher flows (turbulent or orifice-dominated), density becomes more influential than viscosity; the exact relationship depends on gas properties and geometry (not purely Poiseuille).
Rotameters are precision instruments; gas-specific tubes and floats are calibrated for each gas; not interchangeable.
Electronic flow-control valves and electronic flowmeters
Electronic flow-control valves use a computer interface to set flows; feedback from downstream electronic flowmeters maintains target flows; improves safety by preventing hypoxic mixes and maintaining stability under varying pressures.
Electronic flowmeters use mass-flow or differential-pressure transducers; readings appear as digits and as virtual flowmeters on the information screen.
Benefits: can enable closed-loop control, data logging, better resistance to pressure changes; can stop flow if system detects unsafe conditions.
Drawbacks: require power; hence mechanical backup flow-control and rotameters are still present in most models.
In power failures, mechanical flow-control systems or backup modes take over to ensure basic gas delivery.
VAPORIZER MANIFOLDS
Downstream of the flowmeters, gas passes through a vaporizer manifold to add volatile agents.
Dräger and other manufacturers use vaporizer manifolds with one, two, or three vaporizers mounted (e.g., Vapor 2000, Vapor 3000, D-Vapor). Vaporizers may be quick-mounts for easy installation/removal; interlock devices ensure only one vaporizer is active at a time to prevent inadvertent multiple-vapor delivery.
Vaporizer designs vary (mechanical vs electronically controlled). Quick-mounts allow changing vaporizers; however, leaks can occur if not properly seated.
Some departments use vaporizer-free setups (e.g., for malignant hyperthermia-susceptible patients) with the gas path bypassing vaporizers entirely.
Important design note: No gas from the active flowmeters should backflow into a vaporizer that is turned off; this isolates vaporizers when not in use.
COMMON GAS OUTLETS AND OUTLET CHECK VALVES
After mixing with flowmeters, gases (and vapor) exit through the CGO.
Some machines include an outlet check valve and a downstream pressure-relief valve between the vaporizers and CGO to prevent reverse flow or backpressure effects.
Location of CGO check valves varies by model; some machines omit an outlet check valve entirely.
An auxiliary common gas outlet (ACGO) may be provided to deliver gas to non-circle circuits or other devices; the ACGO retains a standard 15-mm internal diameter and 22-mm external diameter to fit standard connectors.
The presence/absence of an outlet check valve and relocation of the relief valve affect leak-testing and low-pressure system behavior.
ELECTRICAL SYSTEMS
Most modern workstations include electrical power and backups: main switch power, backup battery, and sometimes an uninterruptible power supply (UPS) to support critical components.
Battery backup times vary by model (e.g., ranges from roughly 30 minutes to 150 minutes in some systems) and generally do not power patient monitors or auxiliary outlets for extended periods.
Some devices allow automated self-tests or guided pre-use checks powered by the computer, showing the status of various subsystems and documenting pre-use checks.
FLOW THROUGH THE ANESTHESIA MACHINE: O₂, N₂O, Air, and Other Gases
Oxygen (O₂)
Oxygen is a central element in safe anesthesia delivery; it powers the fresh gas flow and is involved in fail-safes, oxygen pressure alarms, and O₂-supply pathways.
Nitrous Oxide (N₂O)
N₂O flows are governed by the proportioning systems and safety devices to avoid hypoxic mixtures.
Air
Compressed air may power certain components or be used in auxiliary gas outlets; it is not typically routed through fail-safe valves in the same way as O₂.
Other medical gases (e.g., Heliox, CO₂)
Heliox or CO₂ can be delivered to the CGO in some machines; they usually have dedicated flowmeters and may or may not be protected by a hypoxic-protection proportional device. When used, these gases require careful monitoring of inspired oxygen concentration and CO₂ concentration.
Oxygen supply safety and alarms
Oxygen supply pressure alarm: active if oxygen pressure falls below the manufacturer-specified threshold (typically around 30 psig); alarm is high-priority and cannot be silenced for more than 120 seconds.
Oxygen analyzer is essential to confirm the actual inspired gas composition; fail-safes may be unable to detect cross-overs or incorrect gas identities in the supply lines (e.g., pipeline crossover, nitrogen in O₂ line).
Fail-safe valves only sense O₂ pressure in the supply line and do not guarantee safe oxygen delivery to the patient if other faults exist; the oxygen analyzer provides a necessary cross-check.
Important numerical references and concepts (highlights)
Pipeline and regulator pressures
High-pressure systems: pressures between 45–2200 psig (varies by gas and cylinder size).
Intermediate-pressure systems: roughly 16–55 psig.
Low-pressure systems: typically just above atmospheric pressure downstream of flow-control valves.
First-stage regulator output
Reduces cylinder pressures to a constant, lower output, typically around 45 psig (for oxygen in dual-cylinder setups).
The regulator often includes a pressure-relief valve.
Oxygen flush
Delivers pure O₂ at 35–75 L/min directly to the CGO, bypassing the flowmeters and vaporizers.
Fresh gas oxygen fraction and hypoxic prevention
Proportioning systems aim to ensure that the inspired oxygen fraction does not drop below 25%: O₂ fraction ≥ 0.25 in the gas delivered to the patient.
This implies the N₂O:O₂ flow ratio is limited such that the O₂ fraction remains ≥ 0.25 in the mixture:
ext{O}2 ext{ fraction} ightarrow rac{F{O2}}{F{O2} + F{N2O}} \ge 0.25.
Proportional oxygen/nitrous oxide control
ORC/S-ORC/Link-25 configurations limit the N₂O flow relative to O₂; the exact mechanical or electronic implementation depends on the model.
Oxygen supply failure alarms and detection options
Modern workstations use electronic sensors to detect O₂ pressure and trigger alarms if below threshold; some systems tie alarm logic to the oxygen supply sensor downstream of pipeline and/or regulator.
Fresh gas flow measurements and calibration
Flowmeters (rotameters or electronic flowmeters) are calibrated for specific gases; calibration is verified by measuring resulting gas concentrations at the CGO with a gas analyzer.
Oxygen flowmeter location and arrangement are designed to minimize hypoxic risk if a leak occurs in other gas pathways.
In electronic flow-control systems, readings are displayed as digits and virtual flowmeters, and the computer can adjust flow controllers to maintain the set flows; in power failure, a mechanical backup flow control remains.
Vaporizers and interlocks
Vaporizer manifolds hold one to three vaporizers; many devices use quick-mount vaporizer cartridges and an interlock that prevents more than one vaporizer from being active at a time.
Old vaporizers may be mechanically controlled; newer designs can be electronically controlled vaporizers. If power fails, non-electrically powered vaporizers (e.g., Tec-6, Tec-7) may still operate to deliver inhaled anesthetic, except for desflurane depending on the design.
Vaporizer bypass is designed so no gas from the active flowmeters enters an off vaporizer; a vaporizer must be turned on to participate in the circuit.
Breathing system and CO₂ absorbent
The breathing system is normally a circle system (COSY variants in some Dräger machines) and is connected to the CGO.
A fresh gas decoupler allows the COSY versions to separate the fresh gas flow from tidal volume during inspiration, improving accuracy of tidal volume delivery and removing fresh gas flow from affecting Vt during inspiration.
A quick-release mechanism allows the CO₂ absorbent canister to be exchanged during use.
In some devices, a reservoir bag remains in-line and moves during inspiration/expiration; a contemporary design may have a compact integrated breathing system with a reduced external footprint.
Pre-use/obsolescence and checks
Obsolescence guidelines (ASA, 2004) help determine if machines are outdated; modern workstations include extensive pre-use self-checks.
Pre-use checks typically include: checking for adequate backup oxygen and a manual ventilation bag, ensuring suction and monitors are available, confirming alarm settings, verifying vaporizers and their seals, verifying scavenging function, and ensuring CO₂ absorbent is fresh and replaceable.
Some systems provide an automated startup check, while others require manual checks per manufacturer recommendations; the safest approach is to follow local institution protocols.
The most critical pre-use step: ensure immediate manual ventilation capability (e.g., a functioning bag and oxygen cylinder) before anesthetic care begins.
Contemporary U.S. anesthesia workstations (overview of models and distinctive features)
FABIUS (Dräger Medical, Telford, PA)
Models: Fabius GS Premium, Fabius Ti ro, Fabius MRI.
Cylinder options: Various, including mixed gas configurations; outlets calibrated to 35 psig (typical minimum pipeline pressure in some regions).
First-stage regulators and second-stage regulators provide gas to the fresh gas flow, auxiliary O₂ flowmeter, O₂ flush, and auxiliary O₂ outlets.
ORC/S-ORC (O₂/N₂O proportioning) present in many models to limit hypoxic risk.
COSY breathing systems (COSY II, COSY 2.5, COSY 2.6) with electronic vaporizer control and standard vaporizer manifolds. Battery backup supports operation during power loss.
APOLLO (Dräger Medical, Telford, PA)
Similar architecture to Fabius; includes ORC/S-ORC proportioning to prevent hypoxic mixtures; supports gas modules and vaporizer manifolds; APL and ventilator systems integrated with gas delivery.
Provides an oxygen-powered ventilator in some configurations; liquid injection vaporization in other variants; battery backup with variable duration depending on model.
PERSEUS ASOO (Dräger Medical, Telford, PA)
Gas enters through cylinder regulators and pipeline inlets; second-stage regulators typically set to 35 psig for gas delivery to flow