Anesthetic Vaporiser Classification and Clinical Uses
Modern anesthetic vaporisers are classified into variable bypass concentration-calibrated devices (the standard for volatile agents like sevoflurane and isoflurane), electronically controlled injection systems (for rapid titration), and specialized heated pressurized systems (specifically for desflurane), with clinical applications spanning routine general anesthesia, trigger-free anesthesia for malignant hyperthermia-susceptible patients, and ICU sedation. 1, 2
Primary Classification Systems
By Control Mechanism
Variable Bypass Vaporisers are the traditional standard, functioning as flow and temperature compensated, concentration calibrated, direct reading devices that split fresh gas flow between a bypass channel and a vaporizing chamber. 2 These are agent-specific and designed to sit between the flowmeter and common gas outlet on the anesthesia machine. 2
Electronically Controlled Systems include:
- Aladin cassette vaporisers integrated with Datex Ohmeda S/5 ADU and GE Aisys machines, using detachable agent-specific cassettes with electronic vapor control units that can function as both variable bypass and measured flow vaporisers but require power supply. 1
- Injection vaporisers (Maquet and DIVA™) that achieve set end-tidal concentrations rapidly even at metabolic flow rates, allowing rapid depth changes with minimal wastage and pollution. 1 These are customized for specific machines (Maquet FLOW-i and Drager Zeus respectively). 1
Specialized Desflurane Vaporisers are required due to desflurane's unique physical properties (high vapor pressure at room temperature), necessitating heated pressurized systems distinct from other volatile agents. 2
By Location and Application
Conventional Machine-Mounted Vaporisers are the standard operating room configuration, positioned on the anesthesia workstation with safety interlocks. 3
Portable Reflector Systems like AnaConDa™ combine vaporiser function with humidity/moisture exchange filters, fitting directly in the ventilatory circuit for ICU sedation and out-of-operating-room use. 1
Critical Safety Features
Modern vaporisers incorporate multiple safety mechanisms that directly impact patient outcomes:
Interlock Systems prevent simultaneous operation of multiple vaporisers on modern workstations, as non-interlocked systems can deliver uncontrolled and potentially lethal concentrations when multiple units are activated. 4 This represents a critical safety advancement over basic Boyle's machines. 3
Anti-Spill Mechanisms and select-a-tec systems prevent dangerous vapor delivery, as tilting a vaporiser can result in delivery of dangerously high concentrations. 3, 5
Agent-Specific Filling Devices minimize filling errors and cross-contamination between agents. 2
Automatic Integrity Testing on modern workstations continuously monitors vaporiser function, though manual leak testing should only be performed on basic Boyle's machines as it may harm modern anaesthetic workstations. 3, 5
Clinical Applications
Routine General Anesthesia
Variable bypass vaporisers deliver precise concentrations of sevoflurane, isoflurane, and enflurane for maintenance of general anesthesia, with modern units functioning accurately even at low fresh gas flows. 1, 2 Injection vaporisers offer advantages when rapid changes in anesthetic depth are required, achieving target end-tidal concentrations quickly with minimal agent waste. 1
Malignant Hyperthermia-Susceptible Patients
For trigger-free anesthesia, vaporisers must be removed before flushing the anaesthesia machine, as most vaporisers have significant reservoirs for volatile agents that must be eliminated when preparing workstations for MH-susceptible patients. 3 This removal also prevents accidental misuse. 3
The preparation protocol requires:
- Removing vaporisers before machine flushing at >10 L/min for manufacturer-recommended duration 3, 6
- Replacing breathing circuits and soda lime with uncontaminated equipment 3, 6
- Using activated charcoal filters (if available) to reduce volatile agent concentrations to <5 ppm within 2-3 minutes 3, 6
Intensive Care and Non-Operating Room Settings
AnaConDa™ systems enable volatile agent delivery for ICU sedation without traditional anesthesia machines, functioning as both vaporiser and heat-moisture exchanger in the ventilator circuit. 1 This extends volatile anesthetic use beyond the operating room for prolonged sedation scenarios.
Resource-Limited Settings
In developing nations without agent monitors or agent-specific vaporisers, Boyle's bottle vaporisers can deliver therapeutic concentrations of halothane, isoflurane, and enflurane by avoiding bubbling and manipulating splitting ratios, though this practice requires extreme caution and should only occur when absolutely necessary. 7, 8 Agent monitoring at the patient end becomes essential for safety in such circumstances. 7, 8
Essential Pre-Use Safety Checks
Before each use, verify that vaporisers are correctly fitted with locking mechanisms fully engaged, adequately filled but not overfilled, kept strictly upright, and checked for leaks. 3, 5 Control knobs must rotate fully through their complete range. 3, 5
When changing vaporisers during use, repeat leak testing whenever possible—failure to do so is a common cause of critical incidents. 3, 5 Some modern workstations automatically test vaporiser integrity, eliminating this manual step. 3
Common Pitfalls to Avoid
- Never perform manual leak testing on modern workstations without consulting manufacturer recommendations, as this may cause harm. 3, 5
- Never operate multiple non-interlocked vaporisers simultaneously, which creates unpredictable and dangerous anesthetic concentrations. 4
- Never tilt vaporisers during filling or transport, as this delivers dangerously high vapor concentrations. 3, 5
- Never overfill vaporisers or leave filling ports open, which compromises output accuracy and safety. 3, 5
- Never fail to remove vaporisers when preparing for MH-susceptible patients, as residual volatile agents in the reservoir can trigger MH. 3