Anaesthetic gas scavenging systems

Clinical approach / how to talk through an AGSS in a viva

• Start with the purpose: reduce occupational exposure to waste anaesthetic gases (WAGs) by removing them from the breathing system and venting safely.
• Define the interface with the anaesthetic breathing system: where waste gas leaves the circuit (APL valve / ventilator spill valve) and enters the scavenging system.
• Describe the 4 core components in order: collecting assembly → transfer tubing → receiving system (interface + reservoir) → disposal system (active/passive).
• Explain pressure safety: the scavenging system must not transmit excessive positive or negative pressure to the patient circuit.
  • Positive pressure hazard: obstruction or excessive suction can increase circuit pressure unless protected by pressure relief and correct design.
  • Negative pressure hazard: suction applied to the receiving system can entrain gas from the breathing system causing sub-atmospheric pressure unless protected by negative-pressure relief/air break.
• Finish with checks and common faults: correct connections, reservoir bag movement, no kinks/occlusion, correct disposal type, and functional pressure relief.

Why scavenging matters (occupational exposure)

• Waste anaesthetic gases arise from: leaks (machine/circuit), exhaled gases, APL/ventilator spill, filling vaporizers, mask induction, and PACU recovery.
• Health concerns historically associated with chronic exposure: headache, fatigue, nausea; reproductive risks and other long-term effects have been debated—current practice is to minimise exposure using engineering controls (AGSS) and good technique.
• Scavenging is one part of control: also ensure good ventilation, low-leak technique, cuffed tracheal tubes where appropriate, and prompt repair of leaks.

Definitions and key terms

• AGSS: system that collects waste anaesthetic gases from the breathing system and disposes of them outside the workplace or renders them safe.
• Active disposal: uses a vacuum/suction source to draw gas away (requires protection against negative pressure).
• Passive disposal: relies on patient/circuit pressure to drive flow to a vent (requires low resistance; no suction source).
• Interface: part of the receiving system that isolates the breathing system from pressure fluctuations in the disposal system (open or closed).

Components (in order) and their functions

• 1) Collecting assembly: receives waste gas from APL/ventilator spill valve; may have separate ports for each source; should be secure and low resistance.
• 2) Transfer tubing: conveys waste gas to the receiving system; should be wide-bore, short as practicable, kink-resistant, and not easily misconnections with breathing circuit.
• 3) Receiving system: interface + reservoir (bag/canister) to buffer flow and protect the patient circuit from pressure changes.
  • Reservoir provides compliance to accommodate intermittent high flows (e.g., ventilator spill) and prevents direct transmission of suction/pressure changes.
• 4) Disposal system: removes gas from the receiving system to outside atmosphere or a central extraction system.
  • Active: pipeline vacuum or fan-driven extraction with flow-limiting device; must include negative pressure relief/air break at interface.
  • Passive: ducting to external vent; depends on circuit pressure and therefore must be low resistance and not prone to blockage/ice/water.

Interfaces: open vs closed (receiving system designs)

• Open interface: communicates with room air (air break). Protects against both positive and negative pressure transmission; commonly used with active disposal.
  • Negative pressure relief is inherent via the open air break (suction entrains room air rather than drawing from patient circuit).
  • Positive pressure relief may be present to vent to atmosphere if disposal is obstructed or flow is excessive.
• Closed interface: no direct communication with room air; requires dedicated positive and negative pressure relief valves to protect the breathing system.
  • Negative pressure relief valve admits room air if suction is excessive to prevent sub-atmospheric pressure being transmitted back to the circuit.
  • Positive pressure relief valve vents to atmosphere if disposal is obstructed/excessive inflow to prevent high circuit pressures.
• Reservoir bag location: typically on receiving system; its movement provides a visual cue of function (inflation/deflation with ventilator spill).

Pressure and flow considerations (exam-relevant physiology/physics)

• Waste gas flow is variable: low during inspiration, higher during expiration and when APL/ventilator spill opens; peak flows can be large during manual ventilation or high FGF.
• If disposal flow is too low or obstructed: receiving system reservoir distends, positive pressure relief may open, and (if protections fail) circuit pressure may rise → barotrauma risk.
• If suction is excessive in active systems: without adequate interface/relief, negative pressure can be transmitted → increased work of breathing, reduced delivered tidal volume, or entrainment of room air affecting agent concentration.
• Resistance matters: long narrow tubing, kinks, water traps, and inappropriate connectors increase resistance and risk of pressure transmission.

Where AGSS connects on common anaesthetic systems

• Circle system: scavenging connection from APL valve during manual/spontaneous modes; from ventilator spill valve during mechanical ventilation (depending on machine design).
• Mapleson systems: scavenging typically from APL valve; high fresh gas flows make effective scavenging particularly important.
• Paediatric/low tidal volume cases: be vigilant for excessive suction effects (loss of volume) and ensure interface functioning correctly.

Checks and troubleshooting (practical)

• Pre-use: ensure scavenging hose connected to correct terminal; transfer tubing not kinked/occluded; reservoir bag present and intact; interface relief valves unobstructed; correct assembly for active vs passive.
• During use: observe reservoir bag movement; listen for relief valve chatter; watch airway pressure and delivered tidal volume; investigate unexpected changes in agent concentration or ventilator volumes.
• If high circuit pressure: check for scavenging obstruction (kinked hose, blocked receiving system, stuck positive relief), then check APL/ventilator and circuit obstruction.
• If low delivered volume/negative pressure signs: suspect excessive suction or faulty interface; reduce suction/ensure correct flow-limiting device; check negative relief valve/air break.

Standards, safety design features, and connectors (UK practice focus)

• Scavenging connectors are designed to reduce misconnections with breathing system components (different sizes/standards); avoid improvised adapters.
• Receiving systems incorporate pressure relief (open interface inherently; closed interface via valves) to prevent transmission of pressure extremes to the patient circuit.
• Active systems should include a flow-limiting device so that pipeline vacuum does not directly apply uncontrolled suction to the scavenging interface.