Vaporizers and Waste Gas Evacuation

Vaporizer Principles^1-3

Overview

• Vaporizers are essential for administering volatile anesthetic agents, ensuring precise, controlled delivery. These devices provide accurate regulation of anesthetic vapor concentration, enhancing patient safety and procedural reliability. Volatile anesthetic agents exist as liquids at room temperature and have vapor pressures that depend on temperature.
• A vaporizer operates by exposing a carrier gas to the liquid anesthetic (or by mixing it with pressurized vapor), allowing the gas to become saturated with anesthetic molecules.
• The output concentration depends on multiple factors, including the agent’s saturated vapor pressure (SVP), temperature (which affects vapor pressure), carrier gas flow rates, and the vaporizer’s internal design.
• Modern vaporizers incorporate temperature-compensation mechanisms and carefully engineered flow pathways to maintain consistent, stable anesthetic delivery across varying clinical conditions.

Definitions and Physics of Vaporizers^1-3

• When a gas is confined within a container, its molecules collide with the walls and exert forces. The pressure within the container is the force per unit area exerted on the walls.
• When a mixture of ideal gases exists in a container, each gas creates its own pressure.
• In the clinical setting, oxygen and anesthetic concentrations are specified as a volume percent. Volume percent is the percentage of volume occupied by the gas relative to the total volume of all gases present, which is also the same as the proportion of an individual gas by the partial pressure as a percentage of the total pressure.
• Volatile liquids have a high propensity to evaporate/vaporize. Therefore, when a volatile anesthetic is present in a space or vaporizer, its molecules will enter the vapor phase once equilibrium is reached.
• Volatile liquids that have a higher tendency to evaporate and expel higher vapor pressures are considered to be “more volatile.”
• Once the anesthetic is at equilibrium, the vapor is considered constant and saturated within the anesthetic, creating a partial pressure within the container known as the SVP.
• SVP is the maximum pressure of vapor in equilibrium with a liquid at a given temperature. The more volatile agents have higher SVPs.
  • This pressure is unique to each agent. At higher temperatures, the SVP increases due to increased kinetic energy and greater conversion to the gas phase.
  • SVP is not affected by changes in atmospheric pressure.

• The boiling point is the temperature at which the vapor pressure equals the ambient atmospheric pressure and the liquid begins to boil. This is not relevant for most inhaled anesthetics, but it is relevant for desflurane. Desflurane boils at 22.8°C (73°F); therefore, it requires a specialized vaporizer to control its delivery.
• Fresh gas flow (FGF): the total flow of gases entering the vaporizer. This affects the response speed and homogeneity of the inhaled anesthetic delivered.
• Output is based on partitioning and the fixed relationship between flow and agent contact to determine concentration.

Vaporizer Designs and Components^1-3,5,6

Copper Kettle Vaporizer³

• No longer in use
• The amount of carrier gas bubbled through the volatile anesthetic was controlled by a dedicated flowmeter. Copper was used as the construction metal because it has relatively high specific heat and high conductivity.

Variable Bypass Vaporizers^1,6

• This is the most common type of vaporizer. Variable bypass vaporizers come in two main types: plenum and draw-over.
• Plenum vaporizers have high internal resistance and therefore require fresh gas supplied at pressures above atmospheric, which the anesthesia machine provides.
• Draw-over vaporizers, on the other hand, rely on ambient air at atmospheric pressure, which is pulled through the vaporizer by the patient’s own breathing. Because they do not require compressed gas, draw-over vaporizers are especially useful in portable or resource-limited settings, such as military or field anesthesia.
• Plenum vaporizers are generally more accurate. Their design helps maintain a consistent anesthetic vapor concentration despite factors that would normally reduce vapor production within the vaporizing chamber.
• Gas flow rate plays an important role in vaporizer performance. At higher flow rates, gas has less time to become fully saturated with anesthetic vapor. To compensate for this, plenum vaporizers are designed to greatly increase the surface area available for vaporization. This is achieved using components such as wicks, metal plates, or internal baffles that spread the liquid anesthetic into thin layers. Fresh gas is then directed through these structures, allowing efficient vapor pickup. As a result, modern plenum vaporizers can deliver reliable anesthetic concentrations across a wide range of flow rates.
• Draw-over vaporizers may also include wicks, but they are less effective because the design must maintain very low resistance. At very low or very high flow rates, more gas tends to bypass the vaporizing chamber, which reduces accuracy. For this reason, draw-over vaporizers are less consistent in their output compared with plenum vaporizers.
• Temperature is another key factor affecting vaporizer output. As the temperature decreases, the vapor pressure of an anesthetic decreases, reducing vapor delivery. Temperature changes can occur due to shifts in room temperature or because vaporization itself removes heat from the liquid anesthetic, particularly at high gas flows.
• Vaporizers address temperature changes in two main ways. First, they are built from materials that efficiently store and conduct heat, helping stabilize the temperature within the vaporizing chamber. Plenum vaporizers typically use dense metals for this purpose, while some draw-over vaporizers use liquids such as glycol to act as a heat reservoir. Second, many modern vaporizers include automatic temperature-compensation systems. These systems adjust gas flow to increase fresh gas entering the vaporizing chamber as temperature decreases. This can be achieved using mechanisms such as bimetallic strips or expandable bellows, which respond to temperature changes by redirecting gas flow to maintain a consistent anesthetic concentration.

Measured Flow Vaporizers^1,3,5

• These vaporizers can inject the agent into the gas stream using injectors or pumps, providing high precision independent of the bypass flow. These are controlled electronically on the anesthesia machines, and this allows for rapid changes of the FGF rate and integration in closed-loop systems.
• A common example is the Ohmeda Tec 6, which is used specifically with desflurane. Desflurane has a very high SVP, and the boiling point for desflurane is near room temperature. The Ohmeda Tec 6 uses an electrical filament to heat desflurane to 39°C, raising the SVP to nearly 2 atm to maintain a constant, predictable output concentration.
• In desflurane vaporizers, the anesthetic is delivered as a gas that is mixed directly into the fresh gas flowing toward the patient. To ensure the correct amount of anesthetic is delivered, the vaporizer automatically adjusts the desflurane addition rate based on the FGF rate.
• As the gas flow increases, pressure builds up inside the vaporizer. This pressure change is detected by an internal sensor, which then reduces the vaporizer’s internal resistance, allowing more desflurane vapor to be released to maintain accurate concentration. When the gas flow decreases, the opposite happens, and less vapor is added.
• The final concentration delivered to the patient is set using the vaporizer dial. The dial normally limits output to 12%, but a safety mechanism requires the user to press a release bar to increase the concentration further, up to a maximum of 18%.
• Because desflurane must be kept warm to vaporize properly, the vaporizer will not function correctly if the temperature drops. For safety, the vaporizer includes electrically controlled locks that prevent desflurane delivery until the vaporizer reaches the correct operating temperature.

GE-Datex-Ohmeda Aladin Cassette Vaporizer^1,2

• The Aladin cassette vaporizer is designed for use with GE Aisys™️ and Avance™️ Carestations™️. The vaporizer is unique in its design for delivering five inhaled volatile agents: halothane, isoflurane, sevoflurane, enflurane, and desflurane. The vaporizer has a permanent internal control within the workstation and has an interchangeable Aladin agent-specific cassette that contains the anesthetic liquid. Each volatile agent is color-coded and magnetically coded, enabling the workstation to distinguish among them.
• Internally, it consists of a bypass chamber and a vaporizing chamber. The center of the Aladin vaporizer is the electronically regulated flow-control valve at the vaporizing chamber outlet. The valve is controlled by the central processing unit (CPU). The CPU receives input from pressure sensors within the cassette, temperature sensors within the vaporizing chamber, the concentration dial, and the flow measurement unit within the bypass chamber, and the flow measurement in the bypass chamber. The CPU controls the throttle valve and regulates the gas flow leaving the vaporizing chamber. The carrier gas saturation and concentration is regulated from the CPU data collected from the various sources of input.
• The fixed restrictor within the bypass chamber separates the flow from the vaporizer inlet into two streams. A portion streams past the bypass chamber, and another portion enters the vaporizing chamber through a one-way check valve. This is unique to the Aladin cassette system. This is critical when administering desflurane and preventing backflow.
• Risks of high FGF rates and high dial settings can lead to rapid vapor evaporation and decreased temperatures within the vaporizer itself. The GE Aisys™️ has a warming fan to raise the temperature to prevent this.

Typical Components of a Vaporizer^1,2

• Agent sump: reservoir that holds the liquid anesthetic
• Vaporizing chamber: increases surface area for vaporization
• Bypass and mixing channels: splits up the channels
• Filler port: the keyed filling system that has a specific key and color to prevent wrong agent filling
• Mount and interlock: responsible for ensuring one vaporizer provides one agent at a time. The benefits of detachable vaporizer mounts include easier maintenance, fewer vaporizer positions on the workstation, and the ability to remove the vaporizer, which is incredibly helpful in the setting of malignant hypothermia. The vaporizer interlock device ensures that fresh gas cannot flow through more than one vaporizer at a time
• Concentration control dial and indexer: links the valve system and interlocks

Vaporizer Safety Features²

• Agent-specific filling systems and color coding that reduce the likelihood of the incorrect inhaled anesthetic filling the incorrect vaporizer chambers.
• Interlock systems to prevent more than one vaporizer from being turned on simultaneously.
• Alarms and diagnostics used to indicate low agent, warm up or failure
• Functional checks include leak tests, output verification with gas analyzers and inspection of seals and filler ports

Anesthetic Considerations with Vaporizers^1,2,5

• Effect of altitude on vaporizer output
  • As altitude increases and barometric pressure decreases, the volume percent concentration increases, while the partial pressure remains constant.
  • Anesthetic effect is solely determined by the partial pressure in the brain, so no adjustments are required with variable-bypass vaporizers.
  • With heated vaporizers, such as with desflurane (Tec 6), a constant volume is delivered. Given the lower partial pressure at altitude, this constant volume results in underdosing of the anesthetic. Therefore, one must set the dial higher to deliver the desired level of anesthetic depth.
  • Please see the OA summary on Anesthetic Management at High Altitude for more details. Link
• Carrier Gas Composition
  • The physical property of gas viscosity influences anesthetic vapor output. Compared with oxygen, both air and nitrous oxide exhibit lower viscosities, with nitrous oxide having the lowest viscosity of the three. Gases with lower viscosity encounter less resistance as they pass through components of the anesthesia delivery system, which can subtly influence vaporizer function.
  • Variable-bypass vaporizers: In variable-bypass vaporizers, fresh gas is split between a bypass pathway and a vaporizing chamber by a flow-splitting mechanism. When carrier gases with lower viscosity are used, system resistance decreases. As a consequence of the design characteristics of the splitting valve, a slightly smaller proportion of the total gas flow is directed through the vaporizing chamber. This reduction in chamber flow results in a modest decrease in anesthetic vapor output when air or nitrous oxide is used instead of 100% oxygen. The magnitude of this effect is minimal and does not result in clinically meaningful changes in anesthetic delivery.
  • Tec 6 vaporizer: The Tec 6 vaporizer, designed specifically for desflurane, uses a heated, pressurized reservoir and controlled vapor injection rather than a traditional variable-bypass design. In this system, a flow restrictor generates backpressure, contributing to accurate vapor delivery. When lower-viscosity carrier gases such as air or nitrous oxide are used, reduced resistance across the flow restrictor results in slightly lower backpressure. This can cause a small decrease in anesthetic agent delivery. However, the Tec 6 incorporates active temperature and pressure regulation, which greatly limits the impact of carrier gas viscosity, rendering the observed differences negligible in clinical practice.
• Misfilled Vaporizer
  • Agent specific vaporizers are calibrated based on the SVP of each agent as well as the temperature and atmospheric pressure. When a vaporizer has been filled with an alternative agent, two factors need to be considered: the SVP of the agent in the vaporizer and the splitting ratio. Agent-specific vaporizers use different splitting ratios determined by the properties of each agent.⁷
  • Vaporizer output = (Carrier gas – SVP) / (P_atm – SVP)
  • When using an agent with a lower SVP than the agent for which the vaporizer is calibrated (e.g., sevoflurane (SVP of 157 mmHg) in isoflurane (SVP of 240 mmHg) vaporizer), a concentration less than that of the dial setting will be achieved.
  • Conversely, using an agent with a greater SVP than that of the agent for which the vaporizer is calibrated (e.g., isoflurane in sevoflurane vaporizer), a higher concentration that that set on the dial will be achieved.⁷

Waste Gas Evacuation^8-10

Purpose and Importance

• Gas evacuation systems, also referred to as scavenging systems, are designed to capture and remove waste anesthetic gases (WAGs) from the operating room environment.
• WAGs are anesthetic gases and vapors that escape into the operating room during anesthesia delivery, including nitrous oxide and volatile agents such as sevoflurane, isoflurane, desflurane, and halothane. Common sources include excess FGF beyond patient uptake, leaks from anesthesia circuits or machine components, gas released via the adjustable pressure-limiting or ventilator relief valves, and exhaled anesthetic gases from the patient.
• Their primary objective is to capture excess anesthetic gases before they contaminate operating room air, thereby minimizing occupational exposure and improving workplace safety.

Safety Considerations

• Occupational Health Risks: Exposure to WAGs has been associated with acute symptoms such as headache, dizziness, nausea, and fatigue.
• Chronic exposure to trace anesthetic gases has been associated with potential health risks for operating room staff, including neurological and teratogenic reproductive effects.
• Maintaining waste gas concentrations below the recommended exposure limits through effective scavenging and ventilation systems is critical for a safe operating room environment.
• Integration with operating room ventilation further enhances safety by maintaining air quality within regulatory standards.

System Components and Functionality

• A scavenging system typically consists of four key components:
  • Gas collection assembly: Interfaces with the anesthesia machine to capture excess gases.
  • Transfer tubing: Conveys collected gases to the disposal system.
  • Interface: Regulates pressure to prevent interference with ventilation or patient safety.
  • Disposal system: Removes gases to a safe location, often through active suction or passive venting.
  • A complete scavenging system consists of a gas collection assembly, transfer tubing, an interface that protects the patient from excessive positive or negative pressure, disposal tubing, and a gas disposal assembly connected to a ventilation exhaust or vacuum system.
• Proper design and maintenance are critical to avoid pressure buildup or leaks that could compromise patient ventilation.

Types of Scavenging Systems

• Active systems: Utilize suction to actively remove gases, offering greater efficiency in high-flow environments.
• Passive systems: Rely on pressure gradients and ventilation ducts for gas removal. They are considered less complex but potentially less effective.

Efficacy and Best Practices

• Engineering and environmental controls scavenging systems are most effective when combined with appropriate operating room ventilation. Recommendations include a minimum of 15 air changes per hour, including outdoor air exchanges, to dilute residual anesthetic gases.
• Studies have demonstrated that local scavenging systems significantly reduce ambient anesthetic gas concentrations during general anesthesia.⁹
• Regular inspection and calibration of scavenging systems are essential to ensure optimal performance.
• Staff training on proper connection and monitoring procedures reduces the risk of malfunction and occupational exposure.