Safety features of an anaesthesia machine

1. Gas supply safety

• Non-interchangeable pipeline connections (DISS): Gas-specific connections prevent attachment of a wrong pipeline gas source.
• Pin Index Safety System (PISS): Prevents fitting an incorrect gas cylinder to the yoke.
• Pressure gauges: Show pipeline and cylinder pressures, helping detect depletion or supply failure.
• Check valves: Prevent reverse flow of gases between pipelines or cylinders.
• Reserve oxygen E-cylinder: Provides a backup oxygen source if central pipeline supply fails.

2. Prevention of hypoxic gas mixture

• Oxygen failure alarm: Audible and visual alarm activates when oxygen supply pressure falls below a preset level.
• Oxygen failure protection device (fail-safe valve): Reduces or stops nitrous oxide and other gas flow when oxygen pressure falls. It is pressure-sensitive, not oxygen-sensitive.
• Hypoxic guard or proportioning system: Links oxygen and nitrous oxide flow controls so that the delivered mixture contains at least about 21% oxygen.
• Oxygen flowmeter placed downstream and on the right: Ensures oxygen is added last to the fresh gas mixture, reducing risk from upstream leaks.
• Inspired oxygen analyser with low-FiO₂ alarm: Measures the actual oxygen concentration delivered to the patient. This is the most important final safeguard against a hypoxic mixture.

A fail-safe device cannot detect pipeline crossover or contamination where a non-oxygen gas enters the oxygen pipeline under pressure. In that situation, the inspired oxygen analyser is the key protection.
Miller's Anesthesia, 10e, p. 2243.

3. Flowmeter safety

• Oxygen flow control knob is usually larger, fluted, and colour-coded, distinguishing it from other gas controls.
• Oxygen flowmeter is positioned downstream of other flowmeters.
• Electronic workstations may have flow sensors and alarms for abnormal gas flow.

4. Vaporizer safety

• Agent-specific filling systems prevent filling a vaporizer with the wrong volatile agent.
• Keyed filling devices reduce spills and misfilling.
• Maximum-fill design prevents overfilling.
• Vaporizer interlock system permits activation of only one vaporizer at a time.
• Secure mounting and anti-spill mechanisms reduce leaks and effects of tipping.

5. Breathing system and pressure safety

• APL valve (adjustable pressure-limiting or pop-off valve) releases excess circuit gas during manual or spontaneous ventilation, limiting high airway pressure.
• Ventilator pressure limit and high-pressure alarm alert the user to obstruction, excessive pressure, or reduced compliance.
• Low-pressure/disconnection alarm detects circuit disconnection, leaks, or failure to generate pressure.
• Positive and negative pressure relief valves in the scavenging interface protect the breathing circuit from excessive positive pressure or excessive suction.

6. Oxygen flush safety

• The oxygen flush valve delivers high-flow oxygen directly to the common gas outlet, bypassing flowmeters and vaporizers.
• It can rapidly fill the circuit or provide oxygen in an emergency.
• It must be used cautiously during mechanical ventilation because it can cause barotrauma if activated when the patient is connected.

7. Monitoring and alarms

• Inspired oxygen concentration monitoring
• Capnography
• Spirometry, including tidal volume and minute ventilation
• Airway-pressure monitoring
• Volatile-agent and MAC measurement
• Apnoea, disconnection, high-pressure, and low-volume alarms
• Automatic pre-use self-tests and leak checks in many machines.

8. Electrical and system safety

• Battery backup allows continued monitoring and ventilation for a limited period during power failure.
• Machines have alarms for mains-power loss and low battery.
• A self-inflating resuscitation bag with an independent oxygen source should always be immediately available.

Exam summary

1. PISS and DISS
2. Oxygen pressure alarm
3. Fail-safe valve
4. Hypoxic guard
5. Oxygen flowmeter downstream
6. Inspired oxygen analyser with alarm

Safety features of anaesthesia machine according to pressure systems

1. High-pressure system

1. Pin Index Safety System (PISS)
   • Prevents attachment of a wrong cylinder to the yoke.
   • Each gas cylinder has a unique arrangement of holes that matches pins on the yoke.
   • Limitation: can fail if pins are damaged, multiple washers are used, or the cylinder itself is incorrectly filled.
2. Bodok seal
   • A washer between cylinder valve and yoke that prevents gas leakage.
3. Pressure-relief device
   • A fusible plug or rupture disc prevents cylinder explosion if pressure rises dangerously, such as during a fire.
4. Cylinder pressure gauge
   • Indicates cylinder contents and detects depletion.
   • Particularly useful for the oxygen E-cylinder backup supply.
5. Pressure regulator
   • Reduces high cylinder pressure to a safe working pressure, approximately 45 psi.

2. Intermediate-pressure system

1. Diameter Index Safety System (DISS)
   • Non-interchangeable, gas-specific pipeline connectors prevent connection of a wrong gas hose to the machine.
2. Pipeline pressure gauge
   • Shows pressure of oxygen, nitrous oxide, and air pipeline supplies.
   • Detects low pressure, depletion, or pipeline failure.
3. Check valves
   • Prevent reverse flow of gas from one pipeline or cylinder into another source.
4. Second-stage pressure regulator
   • Maintains a stable gas pressure despite fluctuations in pipeline or cylinder pressure.
5. Oxygen supply failure alarm
   • Audible and visual warning when oxygen pressure falls below a critical preset level.
   • It warns of oxygen pipeline disconnection, major leak, or nearly empty oxygen cylinder.
6. Oxygen failure protection device, or fail-safe valve
   • When oxygen pressure falls, it proportionately reduces or completely stops nitrous oxide and other gas flow.
   • It is a pressure-sensitive device, not an oxygen concentration monitor.
   • It does not protect against wrong gas entering the oxygen pipeline due to crossover or contamination.
7. Oxygen flush valve
   • Supplies high-flow oxygen, about 35 to 75 L/min, directly to the common gas outlet.
   • Bypasses flowmeters and vaporizers.
   • Must be used cautiously during controlled ventilation because it can cause high airway pressure and barotrauma.

3. Low-pressure system

1. Oxygen flowmeter position
   • Oxygen is the last gas added to the common gas manifold.
   • Its flowmeter is placed downstream, conventionally on the right side, to reduce the chance that an upstream leak produces a hypoxic mixture.
2. Distinct oxygen control knob
   • The oxygen knob is large, fluted, and colour-coded, so it can be identified by touch and differentiated from other gas controls.
3. Hypoxic guard or oxygen-nitrous oxide proportioning system
   • Links oxygen and nitrous oxide flow controls.
   • Prevents delivery of a fresh gas mixture containing less than approximately 21% oxygen.
   • It does not prevent a hypoxic mixture if a third gas is used, if there is a downstream leak, or if the oxygen supply is contaminated.
4. Vaporizer-specific filling system
   • Prevents filling a vaporizer with the wrong volatile agent.
5. Vaporizer interlock
   • Prevents more than one vaporizer from being turned on simultaneously.
6. Vaporizer mounting and anti-spill protection
   • Reduces vaporizer leaks, incorrect mounting, and delivery errors after tipping.
7. Low-pressure system leak test
   • The negative-pressure leak test detects leaks from flowmeters to the common gas outlet, including vaporizer leaks.
   • Test each vaporizer individually in the ON position.
8. Inspired oxygen analyser with low-FiO₂ alarm
   • Measures oxygen concentration actually delivered in the fresh gas mixture.
   • It is the final and most dependable safeguard against delivery of a hypoxic gas mixture.

Key exam point

• Fail-safe valve: responds to a fall in oxygen pressure.
• Hypoxic guard: maintains a minimum oxygen:nitrous oxide proportion.
• Oxygen analyser: detects the actual inspired oxygen concentration and can detect pipeline crossover, contamination, and downstream leaks.