Vaporizer Classification

1. Circuit Location

Out-of-Circuit (Plenum) Vaporizers

• Located outside the patient's breathing circuit
• Controlled output is introduced into the breathing circuit through a fresh gas line
• Gas flows through under positive pressure (plenum = pressurized chamber)
• Virtually all modern vaporizers in well-equipped settings are out-of-circuit
• Examples: GE Tec 5, Tec 7, Tec 850; Dräger Vapor 2000, Vapor 3000; Aladin cassette; Tec 6 (desflurane)

In-Circuit (Draw-Over) Vaporizers

• Located within the breathing circuit
• Gas flow is driven by negative downstream pressure - typically the patient's own respiratory effort, a bellows, or compressible bag
• Low resistance to gas flow is essential for this design
• Used in draw-over anesthesia systems: resource-constrained settings, military field operations, and as an ICU sedation option
• Example: Oxford Miniature Vaporizer (OMV)

2. Method of Vaporization

Flow-Over (Wick-Based)

• Gas flows over a wick system saturated with liquid anesthetic
• Wicks and baffles increase the surface area for vaporization and promote mixing
• Used in all modern variable bypass vaporizers

Bubble-Through

• Gas is bubbled through the liquid anesthetic
• Used in older, now-obsolete measured flow vaporizers (e.g., Copper Kettle, Verni-Trol)
• Output was determined by calculating flow rates rather than a concentration dial

Injection (Liquid Injection)

• Liquid anesthetic is sprayed or injected by a fuel injector into a heated vaporizing chamber where it evaporates rapidly
• Microprocessor-controlled
• Examples: Dräger DIVA vaporizer, Maquet anesthetic vaporizer

3. By Operating Mechanism (Modern Vaporizer Types)

A. Variable Bypass Vaporizer

• Most common type in contemporary practice
• Saturated vapor from the vaporizing chamber is diluted by a bypass stream of fresh gas that does not contact the liquid
• A concentration control dial sets the splitting ratio (bypass vs. vaporizing chamber flow)
• A temperature-compensating device automatically adjusts the splitting ratio to maintain constant output across temperature ranges
• Because each agent's physical properties and clinical concentrations are unique, these vaporizers are agent-specific
• Examples: GE Tec 5, Tec 7, Tec 850; Dräger Vapor 2000, Vapor 3000 (for halothane, enflurane, isoflurane, sevoflurane)
• Approximate splitting ratios at 20°C (most gas goes through bypass, very little through vaporizing chamber)

Desflurane cannot be used in a standard variable bypass vaporizer because:

1. Its very high vapor pressure (669 mmHg at 20°C) would require prohibitively high bypass flow rates (~12 L/min) to dilute to clinical concentrations
2. Massive evaporation would cause excessive cooling of the liquid
3. Its boiling point (22.8°C) is near room temperature, risking uncontrollable boiling within the vaporizer

B. Dual-Circuit (Electrically Heated & Pressurized) Vaporizer

• Designed specifically for desflurane (e.g., Dräger Tec 6, GE D-Vapor)
• The desflurane sump is electrically heated to 39°C, generating a vapor pressure of ~1300 mmHg
• Has two independent parallel gas circuits: a fresh gas circuit and a vapor circuit
• A pressure-regulating valve downregulates desflurane vapor pressure to match the fresh gas circuit pressure
• The operator adjusts the concentration control valve (variable restrictor) to set output
• More accurately described as a dual-gas blender than a traditional vaporizer
• Miller's Anesthesia, 10e

C. Cassette Vaporizer

• A single electronically controlled vaporizer unit accepts interchangeable agent-specific cassettes
• The cassette (vaporizing chamber) contains the liquid anesthetic; the control unit is permanently housed in the workstation
• Functionally similar to a computer-controlled variable bypass vaporizer: bypass chamber + vaporizing chamber + CPU-regulated flow control valve
• CPU receives input from concentration dial, pressure/temperature sensors in the cassette, flow sensors, and carrier gas composition from flowmeters
• Can deliver multiple agents (halothane, enflurane, isoflurane, sevoflurane, desflurane) from one machine by cassette swap
• Cassettes are color-coded and magnetically coded: red (halothane), orange (enflurane), purple (isoflurane), yellow (sevoflurane), blue (desflurane)
• Example: GE Aladin/Aladin2 cassette system (used in Aisys, Avance Carestation)
• Barash Clinical Anesthesia, 9e; Miller's Anesthesia, 10e

D. Injection Vaporizer (DIVA - Direct Injection of Volatile Anesthetic)

• Liquid anesthetic flows by gravity into a dosing chamber, is pressurized, then sprayed by a fuel injector into a heated vaporizing chamber
• Microprocessor controls the dosing valve; feedback control can target a set end-tidal or fresh gas concentration
• Because vapor can be injected directly into the breathing system (not available in the US), it enables rapid depth changes at low or zero fresh gas flow
• Agent-specific interchangeable modules
• Example: Dräger DIVA vaporizer; Maquet anesthetic vaporizer
• Miller's Anesthesia, 10e

4. Temperature Compensation

5. Agent Specificity

Summary Table

• Miller's Anesthesia, 10e - "Modern Vaporizer Types" (p. 2278-2314)
• Barash, Cullen, and Stoelting's Clinical Anesthesia, 9e - "Variable Bypass Vaporizers" (p. 1970-1995)