Vaporizers in Anesthesia: A Comprehensive Overview
Introduction to Vaporizers
What is a vaporizer and what is its primary function?
A vaporizer is a device that allows the vaporization of a liquid anaesthetic and then mixes it with a carrier gas (like oxygen, air, or nitrous oxide) to be administered to the patient in a controlled manner.
It works alongside the anesthesia machine to deliver the required quantity of vapor to the gas delivery system.
Why is a vaporizer essential for general anesthesia?
It is important because there is no proper general anesthesia without the inhalation liquid being attached. The proper depth of general anesthesia is achieved by combining gas, oxygen, muscle relaxants, and narcotics.
What are some historical examples of vaporizers?
From 1846 onwards, many apparatuses were designed. Mr. Morton started with his "Morton's anesthesia inhaler." Other historical devices include the Boyle's bottle, EMO, and OMV. The last of that old period was the arrival of the Halothane and the Flowtech machines.
Physical Principles of Vaporizers
What are the key physical principles one must know to understand vaporizer function?
To understand the function of a vaporizer, one must be familiar with five basic physical principles:
- Saturated Vapor Pressure (SVP): The pressure exerted by vapor when it is in equilibrium with its liquid in a closed container.
- Boiling Point: The temperature at which vapor pressure equals atmospheric pressure.
- Heat of Vaporization: The number of calories required to convert one gram of liquid into vapor.
- Specific Heat: The quantity of heat required to raise the temperature of 1 gram of a substance by 1 degree centigrade.
- Thermal Conductivity: The measure of speed with which heat flows through a substance.
What is Saturated Vapor Pressure (SVP) and why is temperature important when stating it?
SVP is the pressure exerted by the vapor above a liquid when it is in equilibrium with the liquid in a closed container.
Whenever SVP is mentioned, the temperature must also be stated (e.g., SVP of Halothane is 243 mmHg at 20°C) because SVP is dependent on the temperature of the liquid.
What is the difference between boiling and evaporation?
Evaporation happens on the surface of the liquid. Boiling, where bubbles of the gaseous phase form, happens throughout the body of the liquid. Boiling point is the temperature at which vapor pressure equals atmospheric pressure, causing the liquid to boil.
What is the clinical significance of Heat of Vaporization?
As liquid vaporizes, it cools down due to the consumption of heat energy (heat of vaporization). This cooling reduces the kinetic energy in the remaining liquid, slowing down further vaporization. In a vaporizer, if no measures are taken to keep the temperature stable, the output concentration will gradually decrease.
What is the difference between volume percentage and partial pressure in expressing concentration, and why is partial pressure more clinically significant?
Volume percentage is the fractional concentration of a gas. Partial pressure is the pressure exerted by that individual gas in a mixture.
Partial pressure is more absolute and clinically significant. For example, at a high altitude (low atmospheric pressure), 21% oxygen has a lower partial pressure, which is the cause of hypoxia. The effect of an anaesthetic agent is related to its partial pressure.
According to Dalton's law, how do you calculate the percentage of agent above a liquid in a bottle?
If you know the SVP of the agent and the atmospheric pressure, the percentage can be calculated as (SVP / Atmospheric Pressure) * 100.
For example, with Halothane (SVP 243 mmHg) at sea level (760 mmHg), the percentage above the liquid is (243/760)*100 = 32%.
Classification of Vaporizers
How can vaporizers be classified according to the method of vaporization?
Vaporizers can be classified into four types based on vaporization method:
- Flow over: Carrier gas flows over the surface of the liquid. (e.g., Tec vaporizers).
- Bubble through: Carrier gas bubbles through the liquid. (e.g., Copper Kettle).
- Flow over or bubble through: A device that can function as either, like the Boyle's bottle.
- Injection type: Liquid agent is injected and vaporized. (e.g., Tec 6 for desflurane).
What is a Boyle's bottle and how does it work?
A Boyle's bottle is a historical vaporizer that can function as either a flow-over or bubble-through vaporizer by using a lever and a plunger. When the plunger is up, gas flows over the liquid (flow-over). When the plunger is down, gas is forced to bubble through the liquid (bubble-through).
How are vaporizers classified based on the method of regulating output concentration?
There are two main types:
- Variable bypass: The fresh gas flow is split inside the vaporizer between a bypass chamber and a vaporizing chamber. Most modern vaporizers are of this type.
- Measured flow: The splitting of gas happens outside the vaporizer. A dedicated flow meter sends a measured amount of gas through the vaporizing chamber, which is then diluted by the main fresh gas flow. The Copper Kettle is a classic example.
What is a variable bypass vaporizer and how does a splitting ratio work?
In a variable bypass vaporizer, incoming fresh gas is split into two streams: a major portion goes through the bypass chamber, and a small portion goes into the vaporizing chamber where it becomes fully saturated with agent (e.g., 32% for halothane). The gas from the bypass chamber then dilutes this saturated vapor to the desired output concentration. The ratio of gas flow between the bypass and the vaporizing chamber is called the splitting ratio. It is determined by the dial setting and the saturated vapor pressure of the agent.
What is the main difference between a variable bypass and a measured flow vaporizer (like the Copper Kettle)?
In a variable bypass vaporizer, the splitting of gas happens inside the vaporizer itself. In a measured flow vaporizer (like the Copper Kettle), the splitting happens outside, requiring the anesthetist to manually calculate and set two different flow rates.
How are vaporizers classified based on temperature compensation?
Methods of temperature compensation include:
- Automatic thermal compensation: Using a bimetallic strip (Tec) or a metal rod (Vapor 19.1).
- Supplied heat: Externally heating the agent, as in the Tec 6 for desflurane.
- Manual compensation: Historical methods like warming with a cloth (Goldman) or using a water jacket (EMO).
- Electronic compensation: Used in modern vaporizers like the Aladdin.
The purpose of temperature compensation is to maintain a constant vaporizer output concentration.
How does a bimetallic strip provide temperature compensation?
A bimetallic strip is placed in the bypass chamber, near the aperture. If the temperature drops (due to vaporization), the strip curves and reduces the size of the bypass aperture. This diverts more gas through the vaporizing chamber, compensating for the reduced vaporization and maintaining output concentration.
What are Plenum and Draw-over vaporizers?
- Plenum vaporizers: These are high-resistance vaporizers that require carrier gas to be pushed through them under pressure from flowmeters. Most modern vaporizers are plenum type.
- Draw-over vaporizers: These are low-resistance vaporizers where the patient's own inspiratory effort draws air (carrier gas) through the vaporizer. Examples include the Goldman and EMO vaporizers.
What is the difference between a Vaporizer Out of Circuit (VOC) and a Vaporizer In Circuit (VIC)?
- VOC (Out of Circuit): The vaporizer is placed outside the circle system. Fresh gas entering it is pure and devoid of vapor. All modern vaporizers are VOC.
- VIC (In Circuit): The vaporizer is placed inside the circle system. The gas entering it contains exhaled vapor, which can lead to unpredictable concentrations. The Goldman vaporizer is an example of a VIC.
Are modern vaporizers agent-specific?
Yes, all modern vaporizers are agent-specific, meaning they are designed and calibrated for a single volatile anesthetic. Filling them with the wrong agent is hazardous and prohibited. Historical vaporizers like the Goldman could be used with multiple agents.
Factors Influencing Vaporizer Output & Special Effects
What factors influence the output concentration of a variable bypass vaporizer?
Factors include:
- Fresh gas flow: Very low or very high flows can reduce output concentration.
- Temperature: Extreme ambient temperatures can affect output.
- Intermittent back pressure: From the reservoir bag or ventilator, leading to pumping or pressurizing effects.
- Carrier gas composition.
- Barometric pressure.
What are the pumping effect and pressurizing effect caused by intermittent back pressure?
- Pumping effect: An increase in vaporizer output concentration above the dial setting. It occurs with low fresh gas flows, low liquid levels, low dial settings, and high peak pressures.
- Pressurizing effect: A decrease in vaporizer output concentration below the dial setting. It occurs when flow rates are very high.
How can the pumping effect be reduced?
Manufacturers reduce the pumping effect by:
- Reducing the size of the vaporizing chamber.
- Increasing the length of the inlet to the vaporizing chamber (as in the Tec 5 with its IPPV assembly).
- Removing leaks and increasing the resistance of the vaporizer.
- Incorporating a one-way check valve downstream of the vaporizer in the anesthesia machine.
The EMO (Epstein, Macintosh, Oxford) Vaporizer
What is the EMO vaporizer and what is its significance?
The EMO (Epstein, Macintosh, Oxford) apparatus is a landmark draw-over vaporizer designed in 1950 at the Oxford department of anaesthetics. It was a breakthrough in achieving precise anesthetic delivery irrespective of atmospheric conditions. It is a classic example of a temperature-compensated, variable bypass, draw-over vaporizer, primarily for ether.
How did the EMO vaporizer achieve thermal stability?
It achieved thermal stability through two mechanisms:
- An ether-filled bellows that expanded or contracted with temperature changes to regulate gas flow and maintain output.
- A water chamber that could be filled with water (or warm water/antifreeze in extreme environments) to act as a heat reservoir and buffer against cooling from vaporization.
What is the Oxford Inflating Bellows (OIB) and its purpose?
The Oxford Inflating Bellows (OIB) is a device that could be interposed between the EMO vaporizer and the patient to allow for positive pressure ventilation. It could be used to assist or control ventilation.
What was the Ram Rao adapter used for with the EMO?
At higher altitudes, the partial pressure of oxygen in air is lower. Brigadier Ram Rao developed an adapter to allow oxygen enrichment at the inlet of the EMO vaporizer to prevent hypoxia.
Modern Vaporizers (Tec, Vapor, Aladdin)
Describe the key features of the Tec 5 vaporizer.
The Tec 5 is a flow-over, variable bypass vaporizer. Its key features include:
- Automatic temperature compensation via a bimetallic strip.
- Agent-specific (identified by color code).
- A plenum (high-resistance) vaporizer, placed out of circuit.
- Features an elongated inlet (IPPV assembly) and helical wicks to maximize vapor saturation and reduce the pumping effect.
- Uses a keyed filling device (Fraser Sweatman pin system) to prevent cross-filling.
How does the gas flow through a Tec 5 vaporizer?
Fresh gas enters and is split. The main portion goes through the bypass chamber, passing an aperture regulated by the bimetallic strip. The smaller portion goes down an elongated inlet (IPPV assembly) to the helical wicks. It flows over the wicks saturated with liquid agent, picks up vapor, rises through the chamber, and then mixes with the bypass gas in the mixing chamber before exiting.
What are the differences between Tec 5 and Tec 7 vaporizers?
The differences are primarily in cosmetic appearance and filling systems. The Tec 7 introduced a faster, easier, and more cost-effective filling process compared to the complex six-step procedure of the Tec 5.
How does a Select-a-Tec or interlock system ensure only one vaporizer is used at a time?
When a vaporizer is turned on, an extension rod extends laterally. This rod blocks the extension rod of an adjacent vaporizer from extending, mechanically preventing it from being turned on.
Describe the key features of the Vapor 19.1 (Drager) vaporizer.
The Vapor 19.1 is similar in principle to the Tec. It is a flow-over, variable bypass vaporizer. Its unique temperature compensation mechanism uses an inner rod made of Invar (a non-expanding metal) within an outer brass jacket that is in contact with the liquid agent. As the liquid cools, the rod moves to restrict the bypass flow, diverting more gas through the vaporizing chamber.
What is unique about the Tec 6 vaporizer for desflurane?
The Tec 6 is an electrically heated, pressurized, injection-type vaporizer specifically for desflurane. It requires an electrical connection and a warm-up time of 5-10 minutes. It works by heating liquid desflurane to a temperature where its vapor pressure is high, then a CPU-controlled valve injects a precise amount of vapor into the fresh gas flow.
Why does desflurane require a heated, pressurized vaporizer?
Desflurane has a high SVP (around 669 mmHg at 20°C) and a boiling point (22.8°C) very close to room temperature. It is highly volatile, and a small temperature drop causes a large drop in SVP, making a simple flow-over mechanism imprecise. The heated, pressurized design ensures a stable and predictable output.
What are the Aladdin cassette vaporizers?
Aladdin vaporizers are electronically controlled vaporizers that consist of two parts: a control unit built into the anesthesia workstation and a color-coded, magnetically coded cassette for each agent. The cassette contains the liquid and has its own temperature and pressure sensors.
How does an Aladdin cassette vaporizer work?
When mounted, the machine identifies the agent via magnetic coding. Fresh gas flow is directed into the cassette, which contains wicks for saturation. Sensors monitor temperature and pressure. A CPU-controlled proportional flow valve regulates the amount of saturated vapor released from the cassette to mix with the bypass gas, ensuring a precise output. Safety features include a pressure relief valve.
Injection Vaporizers and Newer Concepts
What is an injection vaporizer and why was it developed?
An injection vaporizer was developed to achieve very high output concentrations (e.g., 40%) needed for techniques like low-flow anesthesia where the fresh gas flow is minimal. It works by using a driving gas to push liquid agent into an injector, which sprays a calculated amount into a heated vaporizing chamber. The resulting vapor is then added to the carrier gas.
What is the AnaConDa (Anesthetic Conserving Device)?
The AnaConDa is a small device that can be placed between the endotracheal tube and the Y-piece of the breathing circuit. It allows for the delivery of volatile anesthetics (via a syringe pump) in an ICU setting without a traditional anesthesia machine. It features a reflector (active carbon layer) that absorbs exhaled agent and releases it on the next inspiration, conserving the anesthetic.