Comprehensive Guide to Anesthesia Vaporizers

Learning Outcomes

• Recall basic principles regarding vapor and vapor pressure.

• Identify different classifications of anesthetic vaporizers.

• Describe the basic design of vaporizers and the mechanisms involved in adding anesthetic vapor to gas flows.

• Compare different types of vaporizers based on their internal resistance.

• Describe various methods of temperature compensation used to maintain a constant vaporizer output.

• Identify different methods of filling both old and new types of vaporizers and describe the disadvantages of each system.

Definition of Anesthesia Vaporizer

• A vaporizer, also known as an anesthetic agent or vapor delivery device, is a component that changes a liquid anesthetic agent into its vapor state.

• It adds a controlled amount of that vapor to the fresh gas flow (FGF) entering the breathing system.

Fundamental Physical Principles

• Heat of Vaporization:     * Defined as the number of calories required to vaporize $1\,ml$ of the liquid.

• Latent Heat of Vaporization:     * The number of calories needed to convert $1\,g$ of liquid to vapor without a change in temperature.     * As vaporization occurs, the temperature of the remaining liquid falls, potentially decreasing the rate of vaporization.     * This is termed "latent" because the heat energy transfer cannot be measured by a thermometer during the phase change.

• Vapor Pressure:     * In a closed container, molecules from a volatile liquid escape the liquid phase to become vapor.     * These gaseous molecules strike the walls of the container, exerting what is known as vapor pressure.     * Vapor pressure is directly correlated with temperature: increasing the temperature results in an increase in vapor pressure.

Classification of Vaporizers

• A. Method of Regulating Output Concentration:     1. Concentration calibrated (Variable-bypass).     2. Measured flow (e.g., Copper Kettle).

• B. Method of Vaporization:     1. Flow over.     2. Bubble through.     3. Injection.

• C. Temperature Compensation:     1. Thermocompensation (Automatic adjustment).     2. Supplied heat.

• D. Internal Resistance:     1. High resistance (Plenum).     2. Low resistance (Draw-over).

• E. Specificity:     1. Agent specific.     2. Multiple agent.

• F. Position Relative to Circuit:     1. Vaporizer inside circuit (VIC).     2. Vaporizer outside circuit (VOC).

Basic Design and Mechanism for Adding Vapor

• Primary Design Goal: Volatile anesthetics are extremely potent at their saturated vapor pressures. They must be diluted to a safe concentration before delivery to the patient.

• General Mechanisms:     * Flow over Vaporizers: Carrier gas flows over the surface of the liquid agent and becomes saturated with vapor.     * Bubble through Vaporizers: Carrier gas is bubbled through the liquid agent to achieve saturation.     * Injection Vaporizers: A known amount of liquid agent or pure vapor is injected directly into the gas stream.

• Variable Bypass Mechanism (e.g., Modern Vaporizers):     * The fresh gas flow (FGF) is split into two streams.     * Stream 1: Enters a vaporization chamber and leaves fully saturated with anesthetic vapor.     * Stream 2: Bypasses the chamber entirely.     * The two flows reunite downstream (between the common gas outlet and relevant flowmeters) to produce the final desired concentration.     * Altering the FGF does not typically alter the splitting ratio, ensuring the final concentration remains stable.

• Measured Flow (Injection) Mechanims (e.g., Desflurane/Tec 6):     * Used specifically for Desflurane due to its very high vapor pressure.     * Utilizes a separate heated and pressurized vapor stream injected into the FGF.     * Because increasing FGF would naturally dilute output, an automated mechanism compensates to maintain concentration.

Internal Resistance: Plenum vs. Draw-over Vaporizers

• Plenum Vaporizers:     * Term Origin: "Plenum" is Latin for "full."     * Mechanics: Carrier gas is pushed through the vaporizer at pressures higher than ambient pressure.     * Resistance: High internal resistance.     * Use Case: Used with continuous flow anesthetic machines; relies on pressurized gas flow rather than patient inspiration.     * Saturation: Designed to saturate all gas passing through the vaporization chamber to ensure consistent output even at high FGF levels.     * Location: Used outside the breathing system (VOC).     * Examples: Boyle’s bottle, Copper kettle.

• Draw-over Vaporizers:     * Mechanics: A negative pressure is developed in the gas stream distal to the vaporizer, drawing gas through.     * Resistance: Low internal resistance.     * Use Case: Patient's inspiratory effort is sufficient to draw gas through. Useful in "the field" or environments where pressurized gas is unavailable.     * Performance: Often unpredictable because accuracy-improving mechanisms (like temperature compensation) increase resistance and are usually omitted.     * Location: Used within the breathing system (VIC).     * Examples: Goldman, EMO (Epstein, Macintosh, Oxford) vaporizers.

Drawover Anesthesia: Advantages and Disadvantages

• Advantages:     * Conceptual and assembly simplicity with inherent safety.     * No requirement for pressurized gas supplies, regulators, or flowmeters.     * Minimum $FiO_2$ is $21\%$.     * Robust, reliable, and easily serviced.     * Low purchase and maintenance costs.     * Highly portable for field anesthesia.

• Disadvantages:     * Decreasing clinician familiarity with the technique.     * Filling systems often lack agent specificity.     * Lack of temperature compensation affects performance.     * Difficult to monitor spontaneous ventilation when using a self-inflating bag.     * Not advisable for pediatric use unless specialized lightweight tubing is available.

Temperature Compensation Methods

• Problem: Latent heat of vaporization causes the liquid anesthetic temperature to fall, which reduces the vapor pressure and output concentration.

• A. Supplied Heat:     * Uses external sources like a hot water jacket or a heat sink (e.g., OMV).

• B. Automatic Splitting Ratio Adjustment:     * Bimetallic Strip (e.g., Datex-Ohmeda Tec 7): Consists of two metals welded together that expand/contract differently. At cooler temperatures, the strip moves to increase resistance in the bypass, forcing more gas into the vaporizing chamber to maintain constant output.     * Expansion Rod (e.g., Dr$\text{"a"}$ger Vaporizer): Uses an inner rod of Invar (non-expansile) and an outer brass jacket (expansile). When cooling occurs, the brass contracts, dragging a "choke" into the bypass to increase resistance.     * Bellows: Liquid-filled collapsing bellows that decrease in volume when cooled, adjusting flow.     * Electronic Control: Computerized monitoring and adjustment.

Factors Affecting Vaporizer Output

1. Flow through the vaporizing chamber: Increased flow generally leads to increased output ($\uparrow \rightarrow \uparrow$).

2. Surface area of liquid-gas interface: Increased surface area leads to increased output ($\uparrow \rightarrow \uparrow$).

3. Temperature: Decreased temperature leads to decreased output ($\downarrow \rightarrow \downarrow$).

4. Time: Increased time can lead to increased output ($\uparrow \rightarrow \uparrow$).

Comparison of Location: VIC vs. VOC

Safety and Filling Systems

• Vaporizer Interlock:     * Ensures only one vaporizer is turned on at a time.     * Prevents gas from entering vaporizers that are off.     * Minimizes trace vapor output from "off" units.     * Ensures vaporizers are locked and seated correctly in the gas circuit.

• Filling Systems:     * Screw-fill: Risks filling with the wrong agent.     * Key-Fill System: Agent-specific, prevents spillage if fixed properly, but adapters can be lost.     * Pin Safety System: Specifically the Fraser Sweatman system for added safety.

Evolution of Specific Models

• TEC-2, TEC-3, TEC-4: Plenum type, though concentration is poorly calibrated; flow over, temperature compensated, VOC, and agent-specific.

• TEC-5, TEC-7: Newer plenum types; adequately calibrated concentration, flow over, temperature compensated, VOC, and agent-specific.

• TEC-6 (for Desflurane):     * Electrically heated to $39\,^{\circ}C$.     * Thermostatically controlled for constant temperature.     * Pressurized design.

Questions & Discussion

• Q: Modern vaporizers are?     * A: Both Agent-specific and Temperature-compensated.

• Q: The Tec 6 desflurane vaporizer characteristics include?     * A: Electrically heated to $39\,^{\circ}C$, temperature-compensated, and pressure-compensated.

• Q: Where should variable bypass vaporizers be located?     * A: Between the flowmeters (upstream) and the common gas outlet (downstream).

• Q: In a closed container, molecules escaping the liquid phase and striking the wall exert what?     * A: Vapor Pressure.

• Q: Recent vaporizers share all the following EXCEPT?     * A: Being inside the breathing system (they are typically VOC).