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Anesthesia Delivery Systems
THE ANESTHESIA WORKSTATION
In the decades after the first public demonstration of ether anesthesia in 1846 the anesthesia delivery system consisted of handheld devices ranging from ether- or chloroform-soaked cloths to more sophisticated inhalers that could regulate the administered dose of anesthetic. The modern anesthesia workstation remains at its core a device for delivering inhaled anesthesia, but incorporates many additional functions focused on safety and ease of use ( Table 15.1 ). The sheer number of tasks and solutions for which the anesthesia workstation is designed explains its complexity. Many innovations in workstation design aim to enhance patient safety. American Society of Anesthesiologists (ASA) closed claims analysis of adverse anesthetic outcomes related to anesthetic gas delivery equipment shows a decrease in such claims from 4% in the 1970s to approximately 1% in 2000–2011. Further, the severity of the events leading to the claims has decreased, with more reports of awareness under anesthesia and fewer involving death or permanent brain injury.
Table 15.1
Functions of the Modern Anesthesia Workstation
| Function | Description | Safety Feature |
|---|---|---|
| Inhaled anesthetic delivery | Deliver volatile anesthetic gas at precise concentrations. Allow rebreathing of the exhaled anesthetic gases after removing carbon dioxide. |
Continuously display inspired and exhaled anesthetic concentration.Continuously display inspired and exhaled carbon dioxide concentration. |
| Oxygen delivery | Individually meter oxygen and two or more other breathing gases, while continuously enriching the inhaled gas with anesthetic vapors. | Continuously measure and display the inspired oxygen concentration. Prevent hypoxic gas mixtures caused by operator error or gas supply failure. Provide a breathing circuit manual oxygen flush feature. Possess a backup supply of oxygen. Display gas pipeline and backup tank supply pressures. |
| Facilitate ventilation of patient’s lungs | Ventilate the patient manually (“bag” ventilation) with adjustable breathing circuit pressure. Ventilate the patient mechanically, with sophisticated ventilator modes comparable to the ICU. |
Measure and display ventilatory parameters such as respiratory rate, tidal volume, and airway pressure. |
| Remove excess anesthetic gases | Eliminate (“scavenge”) excess gas from the patient’s breathing circuit and remove this gas from the room. | |
| Information display | Provide an integrated platform for displaying anesthetic, hemodynamic, and respiratory parameters and for collecting this data into an electronic medical record. |
ICU, Intensive care unit.
Anesthesia providers must be aware of the operational characteristics and “functional anatomy” of their anesthesia workstations. There is increased variation among anesthesia workstations, with operational and preuse checkout procedures becoming more divergent, thus mandating device-specific familiarity. Contemporary machines have automated preuse checkout procedures, but machines can pass automated checkouts despite the presence of unsafe conditions.
Although providing a detailed description of each component of the anesthesia machine is beyond the scope of this chapter, starting with a generic approach provides a suitable foundation for understanding a specific workstation model. It is worth emphasizing that if there is any doubt about the correct functioning of an anesthesia workstation, and there is difficulty with ventilation or oxygenation, then ventilating the patient from an alternative source of oxygen such as an E-cylinder is often appropriate. Trouble-shooting of the anesthesia machine can commence once the patient is safe.
FUNCTIONAL ANATOMY OF THE ANESTHESIA WORKSTATION
Gas Supply System
Although modern anesthesia workstations are often largely electronically controlled, the interior of the anesthesia machine remains a pneumatic system, a place where breathing gases are delivered from their supply sources, measured and mixed, passed through (or by) an anesthetic vaporizer, and delivered to the patient’s breathing circuit. Although the details of this gas supply system may differ between the various manufacturers’ anesthesia workstations, their overall schematic is similar. The gas supply system of a typical contemporary workstation with electronic controls is depicted in Fig. 15.1 .
High-Pressure Section
Auxiliary E-Cylinder Inlet
During normal operation, the high-pressure section of the anesthesia machine is not active, because the hospital’s central gas supply system serves as the primary gas source for the machine. However, it is a requirement to have at least one attachment for an oxygen cylinder to serve as a backup source of oxygen in case of failure of the hospital supply source. The cylinders are mounted to the anesthesia machine by the hanger yoke assembly. Each hanger yoke is also equipped with the Pin Index Safety System (PISS), which is a safeguard to reduce the risk of a medical gas cylinder error caused by interchanging cylinders ( Fig. 15.2 ). Two metal pins on the yoke assembly are arranged to project precisely into corresponding holes on the cylinder head–valve assembly of the tank. Each gas or combination of gases has a specific pin arrangement.
The maximum pressure in E-cylinders full of oxygen is approximately 2200 pounds per square inch gauge (psig). (The gauge pressure is the pressure above the ambient atmospheric pressure.) The full tank pressure is 750 psig for nitrous oxide and 2200 psig for air. These pressures are much higher than the normal hospital pipeline supply pressure of 50 to 55 psig. A high-pressure regulator therefore reduces the variable high pressure in the E-cylinder to a lower, nearly constant pressure output to the intermediate-pressure section of the anesthesia machine.
Three points about the safe use of the auxiliary oxygen E-cylinder system should be noted. First, checking the E-cylinders is not part of an automatic machine checkout. The anesthesia provider must manually open each cylinder and check the pressure gauges on the front of the machine. Second, it is imperative to keep the auxiliary E-cylinders closed during normal operation using pipeline gases, as an open oxygen cylinder may allow the anesthesia provider to be unaware of catastrophic pipeline failure. If the oxygen tank is already open when pipeline failure occurs, the “low oxygen pressure” alarm, which is a high-intensity alarm, will not sound until the auxiliary tank is already depleted. Finally, in case of known or suspected pipeline contamination or crossover (e.g., from nitrous oxide in the oxygen pipeline) leading to delivery of a hypoxic gas mixture, backup oxygen from the E-cylinder will not flow unless the anesthesia machine is disconnected from the pipeline, because the system is designed to preferentially draw from the pipeline as long as it is adequately pressured. ,
Calculation of remaining gas volume in an oxygen E-cylinder based on its pressure has practical implications for patient care in the setting of failed pipeline oxygen supply in the operating room or for patient transport within the hospital. A full oxygen E-cylinder contains 650 L of oxygen at 2200 psig. Because oxygen is a compressed gas, the gauge pressure is directly proportional to the amount of oxygen in the tank. For example, at 1100 psig, there is half a tank left (325 L of oxygen) remaining. Because nitrous oxide is a compressed liquid, the relationship between its pressure and volume is different. A full nitrous oxide E-cylinder contains about 1600 L of gas at 750 psig. As the nitrous oxide gas flows out of the cylinder, further liquid is vaporized and the pressure in the tank remains unchanged. Therefore the pressure of a nitrous oxide tank remains at 750 psig until there is no more liquid in the tank, at which point the pressure begins to fall. By this point, at least 75% of the tank has been depleted.
Intermediate-Pressure Section
Gas Pipeline Inlet: Central Gas Supply Source
Three gases are typically piped into the operating room by the hospital’s central gas supply system: oxygen, air, and nitrous oxide, all at 50 to 55 psig. The pipeline supply terminates with one of two types of connector: the Diameter Index Safety System (DISS) connector system or the quick coupler system. Within each type, the connectors for oxygen, air, and nitrous oxide are mutually incompatible. DISS connectors rely on matching diameters in the male and female connections to properly seat and thread the connection. Quick couplers use pins and corresponding slots on the male and female ends, respectively, in order to ensure correct connections. Because these connectors can be plugged together or released with a simple twisting motion, they are especially appealing for equipment that needs to be moved between locations. In both systems the wall plates and hoses are color-coded for ease of identification: green for oxygen, yellow for air, and blue for nitrous oxide.
Oxygen Flush Valve
The oxygen flush valve allows manual delivery of a high flow rate of 100% oxygen directly to the patient’s breathing circuit. The oxygen flush valve may be employed to overcome circuit leaks or to rapidly increase the inhaled oxygen concentration. Flow from the oxygen flush valve bypasses the anesthetic vaporizers and enters the low-pressure circuit downstream at a rate between 35 and 75 L/min.
Oxygen flushing during the inspiratory phase of positive-pressure ventilation can produce barotrauma if the anesthesia machine does not incorporate a fresh gas decoupling feature or an appropriately adjusted inspiratory pressure limiter. A defective or damaged valve can stick in the fully open position and result in barotrauma. Oxygen flow from a valve sticking in a partially open position can dilute the inhaled anesthetic agent concentration, potentially resulting in awareness under anesthesia.
Pneumatic Safety Systems
Within the intermediate-pressure section of the machine there are two safety systems that minimize the risk of delivery of a hypoxic gas mixture in case the oxygen pressure decreases significantly. The oxygen supply failure alarm sensor provides an audible and visual warning to the clinician if the oxygen pressure drops below a manufacturer-specified minimum. This alarm cannot be silenced until the pressure is restored to the minimum value (e.g., by opening the oxygen E-cylinder on the machine). In addition, the oxygen supply failure protection device, sometimes called the “fail-safe valve,” affects the flow of other gases when oxygen supply pressure is low. The fail-safe valve either shuts off (binary valve) or reduces (proportional valve) the flow of other gases such as nitrous oxide or air. The term fail-safe as it pertains to these valves is a misnomer: only the inspired oxygen concentration monitor in the breathing circuit and clinical acumen can protect the patient from hypoxic gas delivery.
Auxiliary Oxygen Flowmeter
Auxiliary oxygen flowmeters are commonly encountered on anesthesia workstations, serving as a convenience feature for low-flow oxygen. Because the flowmeter is usually operational even when the machine is off, the auxiliary oxygen flowmeter can also serve as a safety feature because it allows the use of an oxygen delivery source (e.g., a manual resuscitation bag) in the case of a system power failure. The source of oxygen for the auxiliary flowmeter is the same as for the other oxygen flow control valves. In cases of suspected hospital oxygen pipeline contamination or crossover switching to an E-cylinder that is not part of the anesthesia machine is imperative.
Low-Pressure Section
The purpose of the high- and intermediate-pressure sections of the anesthesia workstation is to deliver a reliable source of breathing gases at a stable and known working pressure to the low-pressure section of the gas supply system. The clinician interfaces with the low-pressure section of the gas supply system, shaping its output to deliver a known composition of gas and anesthetic to the breathing circuit. The low-pressure section of the gas supply system begins at the flow control valves and ends at the outlet of the fresh gas line (see Fig. 15.1 ). Key components include the flow control valves, the flowmeters or flow sensors, the vaporizer manifold, and the anesthetic vaporizers. The breathing circuit, including the circle system, breathing bag, and ventilator, will be treated separately.
Flow Control Assemblies
The flow control valves on the anesthesia workstation allow the operator to select a total fresh gas flow of known composition that enters the low-pressure section of the anesthesia workstation. These valves are therefore an important anatomic landmark: they separate the intermediate-pressure section from the low-pressure section. After leaving the flowmeters, the mixture of gases travels through a common manifold and may be directed through an anesthetic vaporizer if selected. The total fresh gas flow and the anesthetic vapor then travel toward the common gas outlet, or fresh gas outlet (see Fig. 15.1 ).
Newer anesthesia workstations are increasingly equipped with electronic flow sensors instead of flow tubes. These systems may employ conventional control knobs or an entirely electronic interface to control gas flow. Numerous flow sensor technologies can be applied, such as hot-wire anemometers, a differential pressure transducer method, or mass flow sensors. Regardless of the mechanism of flow measurement, these systems depend on electrical power to display the gas flow. When system electrical power is totally interrupted, some backup mechanical means usually exists.
Mechanical flow control and flow display remain common, even on some newer workstations, either as primary or especially as backup systems. The flow control valve assembly consists of a flow control knob, a tapered needle valve, a valve seat, and a pair of valve stops. For safety, the oxygen flow control knob is typically distinctively fluted, larger in diameter, and may project beyond the control knobs of the other gases.
The flow control valve regulates the amount of flow that enters a tapered glass flow tube. A mobile indicator float inside the calibrated flow tube indicates the amount of flow passing through the annular space between the float and the flow tube. The indicator float hovers freely in an equilibrium position in the tube, where the upward force resulting from gas flow equals the downward force on the float resulting from gravity. The viscosity and density of the particular gas determine its behavior in the glass tube, and therefore the calibration on these flowmeters is gas-specific. The flow tube has historically been a very fragile component of the anesthesia workstation. Flow tube leaks are a potential hazard because the flowmeters are located downstream from hypoxia-preventing safety devices, except the breathing circuit oxygen analyzer.
Proportioning Systems
On anesthesia workstations with electronically controlled gas flow, the machine is programmed to prevent the user from selecting a hypoxic gas mixture for delivery to the fresh gas outlet. For manually controlled flowmeters, the concern is that a user could mistakenly select oxygen and nitrous oxide flows that result in a hypoxic mixture. A proportioning system prevents this by means of a pneumatic-mechanical interface between the oxygen and nitrous oxide flows or a mechanical link between the oxygen and nitrous oxide flow control valves. Proportioning systems can override flows set by the anesthesia provider, either decreasing nitrous oxide flow or increasing oxygen flow in order to maintain a nonhypoxic fresh gas flow. The specific devices and designs used to accomplish this control vary among manufacturers. As always, the presence of a functioning oxygen analyzer in the patient’s breathing circuit is the best and final protection against delivery of a hypoxic gas mixture.
Vaporizer Mount and Interlock System
The vaporizer mounts on modern anesthesia workstations allow for detachment and replacement of the anesthetic vaporizers by the workstation operator. The benefits of detachable vaporizer mountings include ease of maintenance, the need for fewer vaporizer positions on the workstation, and the ability to remove the vaporizer in the setting of malignant hyperthermia. This ability to remove and reintroduce an element to the pneumatic structure of the low-pressure section of the anesthesia machine brings an increased potential for leaks or fresh gas flow obstruction as a result of an inappropriately seated vaporizer or other connection-related failures. After adding or changing a vaporizer on the anesthesia machine, the operator should make sure it is properly seated.
The vaporizer interlock device ensures that fresh gas cannot flow through more than one vaporizer at a time. The design of vaporizer interlock devices varies significantly. These devices are not immune from failure, with anesthetic overdose as a potential consequence.
Anesthetic Vaporizers
The inhaler used by William T. G. Morton in the first public demonstration of ether anesthesia was effective, but there was no way to regulate its output concentration or compensate for changes in temperature caused by the vaporization of the liquid anesthetic. Modern variable bypass–type vaporizers are temperature compensated and can maintain precise outputs accurately over a wide range of input gas flow rates. The introduction of desflurane to the clinical setting required an even more sophisticated vaporizer design to handle the unique physical properties of this agent (also see Chapter 7 ). Vaporizers incorporating computerized control technology emerged in “cassette” vaporizer systems. An injection-type vaporizer has also been introduced, spraying precise amounts of liquid anesthetic agent into the fresh gas stream. Before discussing any of these systems, certain physical chemical principles are briefly reviewed to facilitate an understanding of the operating principles, construction, and design of contemporary anesthetic vaporizers.
Physics
When a gas exists within a container, the gas molecules collide with the walls and exert a force. The pressure within the container is the force per unit area exerted on the walls. According to the ideal gas law, that pressure is directly proportional to the number of molecules of gas present within the space, directly proportional to the temperature, and inversely proportional to the volume of the container that confines a gas. When a mixture of ideal gases exists in a container, each gas creates its own pressure, which is the same pressure as if the individual gas occupied the container alone. The total pressure may be calculated by simply adding together the pressures of each gas—Dalton’s law of partial pressures. At sea level, the ambient pressure is 760 mm Hg, which equals 1 atmosphere (atm) or 101.325 kilopascals (kPa).
In the clinical setting oxygen and anesthetic concentrations are typically specified as a volume percent. Volume percent is simply the percentage of volume occupied by the gas relative to the sum of all gases present, which is the same as the proportion of an individual gas by its partial pressure (mm Hg) as a percentage of the total pressure (under idealized conditions encountered in the operating room). Thus although the partial pressure of “room air” oxygen is lower at elevation (129 mm Hg in Denver, Colorado) than it is at sea level (160 mm Hg), it remains approximately 21% at both altitudes.
Volatile liquids such as inhaled anesthetic agents are characterized by a high propensity to evaporate, or vaporize . When a volatile liquid is exposed to air or other gases, molecules at the liquid surface with sufficient kinetic energy escape and enter the vapor phase. If a volatile anesthetic is placed within a contained space, such as an anesthetic vaporizer, molecules will continue to escape into the vapor phase until the rate at which molecules evaporate is equal to the rate of return to the liquid phase (a process known as condensation ). When this equilibrium is reached, the composition of the vapor remains constant and it is said to be “saturated” with anesthetic, and the anesthetic molecules in the gas phase bombard the walls of the container, creating a partial pressure known as the saturated vapor pressure, or simply vapor pressure. Liquids with a higher tendency to evaporate and generate higher vapor pressures are described as “more volatile.”
Vapor pressure is a physical property of a substance, with each substance having its own unique value at any given temperature ( Fig. 15.3 ). Vapor pressure is temperature dependent and is not affected by changes in atmospheric pressure. Evaporation is diminished at colder temperatures because fewer molecules possess sufficient kinetic energy to escape into the vapor phase. Conversely, at warmer temperatures, evaporation is enhanced and vapor pressure increases. Because vapor pressure values are unique to each liquid anesthetic agent, anesthetic vaporizers must be constructed in an agent-specific manner.
Energy is required for a molecule of volatile anesthetic to evaporate into the gas phase—that energy is absorbed from the surroundings in the form of heat . The amount of energy required to change 1 gram of a particular liquid into vapor at a constant temperature during evaporation is referred to as the latent heat of vaporization . In a well-insulated container the energy for vaporization must come from the liquid itself. In the absence of an outside heat source, the temperature of the remaining liquid decreases as vaporization progresses. This cooling effect can lead to significant reductions in vapor pressure, and therefore of volatile anesthetic molecules moving into the gas phase. Unless the evaporative cooling effect of the liquid anesthetic agent is mitigated and compensated for, vaporizer output will decrease.
The boiling point of a liquid is defined as the temperature at which vapor pressure equals atmospheric pressure and the liquid begins to boil. The boiling point of most contemporary volatile anesthetic agents is not relevant to vaporizer design under most clinical situations. Desflurane, however, boils at a temperature commonly encountered in clinical settings and has a high saturated vapor pressure ( Table 15.2 ). These properties mandate a special vaporizer design to control agent delivery.
Table 15.2
Physical Properties of Inhaled Volatile Anesthetic Agents
| Property | Halothane | Isoflurane | Sevoflurane | Desflurane |
|---|---|---|---|---|
| SVP * @ 20°C (mm Hg) | 243 | 238 | 157 | 669 |
| SVC † @ 20°C at 1 atm ‡ (v/v%) | 32 | 31 | 21 | 88 |
| MAC § at age 40 yr (v/v%) | 0.75 | 1.2 | 1.9 | 6.0 |
| MAPP ¶ (mm Hg) | 5.7 | 9.1 | 14.4 | 45.6 |
| Boiling point @1 atm (°C) | 50.2 (122.4°F) | 48.5 (119.3°F) | 58.6 (137.3°F) | 22.8 (73°F) |
* SVP, Saturated vapor pressure. From anesthetic prescribing information.
† SVC, Saturated vapor concentration: the percentage of anesthetic agent relative to ambient pressure within an equilibrated (saturated) container (SVP/ambient pressure).
‡ 1 atm, 1 atmosphere = ambient pressure at sea level (760 mm Hg).
§ MAC, Minimum alveolar concentration: the alveolar concentration that produces immobility in response to a noxious stimulus in 50% of subjects. The denominator is approximately sea level pressure (760 mm Hg).
¶ MAPP, Minimum alveolar partial pressure. The alveolar partial pressure that produces immobility in response to a noxious stimulus in 50% of subjects (the numerator in the MAC calculation). Not affected by altitude. Calculated as MAC (fraction) × 760 mm Hg (i.e., for isoflurane = 0.012 × 760 mm Hg). × 760 mm Hg (i.e., for isoflurane = 0.012 × 760 mm Hg). v/v%, Volume percent.
Modern Vaporizer Types
The saturated vapor pressure of volatile anesthetic agents, even at normal operating room temperatures, results in gas concentrations that greatly exceed those used clinically, so these concentrations must be diluted to safe ranges. Virtually all modern vaporizers are out-of-circuit, and their controlled output is introduced into the breathing circuit through a fresh gas line. Specific types of vaporizers currently include the variable bypass vaporizer, the dual-circuit vaporizer, the cassette vaporizer, and the injection vaporizer .
Variable Bypass Vaporizers
Variable bypass refers to the method of carefully regulating the concentration of vaporizer output by diluting gas fully saturated with anesthetic agent with a larger flow of gas. A diagram of a variable bypass vaporizer is shown in Fig. 15.4 . Variable bypass vaporizers are agent-specific, flow-over, temperature-compensated, and pressure-compensated . The concentration control dial determines the ratio of the fresh gas flow that continues through the bypass chamber to the flow diverted into the vaporizing chamber. A temperature-compensating device, either an expansion element or temperature-sensitive bimetallic strip, further adjusts that ratio, for example, by increasing the amount of flow into the vaporizing chamber as the temperature falls. Vaporizer concentration control dials are labeled to set vaporizer output in terms of volume percent, and the vaporizers are calibrated at sea level. Because the physical properties and clinical concentrations of each agent are unique, the diverting ratios are specific to each agent and dial setting. Variable bypass vaporizers can be used to deliver halothane, isoflurane, sevoflurane, and older agents, but not desflurane, because of this agent’s unique physical properties.
![Fig. 15.4, Simplified schematic of a variable bypass vaporizer. Rotation of the concentration control dial diverts a portion of the total fresh gas flow through the vaporizing chamber, where wicks saturated with liquid anesthetic ensure a large gas–liquid interface for efficient vaporization. A temperature-compensating valve diverts more or less fresh gas flow through the vaporizing chamber to offset the effects of changes in temperature on the vapor pressure of the liquid anesthetic. Gases saturated with the vapor of the liquid anesthetic join gases that have passed through the bypass chamber for delivery to the machine outlet check valve (if present). When the concentration control dial is in the off position, no fresh gas inflow enters the vaporizing chamber. (Modified from Chakravarti S, Basu S. Modern anaesthesia vapourisers. Ind J Anaesth. 2013;57[S];464–471.)](https://storage.googleapis.com/dl.dentistrykey.com/clinical/AnesthesiaDeliverySystems/3_3s20B9780323796774000152.jpg "Fig. 15.4, Simplified schematic of a variable bypass vaporizer. Rotation of the concentration control dial diverts a portion of the total fresh gas flow through the vaporizing chamber, where wicks saturated with liquid anesthetic ensure a large gas–liquid interface for efficient vaporization. A temperature-compensating valve diverts more or less fresh gas flow through the vaporizing chamber to offset the effects of changes in temperature on the vapor pressure of the liquid anesthetic. Gases saturated with the vapor of the liquid anesthetic join gases that have passed through the bypass chamber for delivery to the machine outlet check valve (if present). When the concentration control dial is in the off position, no fresh gas inflow enters the vaporizing chamber. (Modified from Chakravarti S, Basu S. Modern anaesthesia vapourisers. Ind J Anaesth. 2013;57[S];464–471.)")
An ideal variable bypass vaporizer would maintain a constant concentration output at a given setting regardless of variables such as the fresh gas flow rate, temperature, intermittent backpressure from the breathing circuit, carrier gas composition, and barometric pressure. Modern vaporizers generally have excellent performance characteristics, with minimal variation in clinical performance in conditions encountered in an operating room.
Understanding the influence of barometric pressure on variable bypass vaporizer output can help to illustrate concepts regarding vaporizer function. Perhaps surprisingly, the “depth of anesthesia” administered at a given dial setting on a variable bypass vaporizer is relatively independent of atmospheric pressure. (This is not necessarily true with other vaporizer types.) A liquid’s vapor pressure is independent of barometric pressure; therefore as altitude increases and barometric pressure declines, the partial pressure of anesthetic agent in the variable bypass vaporizing chamber remains constant despite a decline in the partial pressures of other constituent breathing gases and the total ambient pressure. The vaporizer output’s concentration (volume percent) of anesthetic is significantly greater than the dial setting, but the partial pressure delivered is close to what it would be at that dial setting at sea level. And because anesthetic depth is determined by the partial pressure of volatile agents in the brain , the clinical impact is minor. Although inhaled anesthetics are rarely used in hyperbaric conditions, similar considerations would apply.
Contemporary variable bypass vaporizers incorporate many features to minimize or eliminate hazards once associated with these devices. Misfilling of anesthetic vaporizers with the wrong agent can result in an overdose or underdose of volatile anesthetic. Agent-specific, keyed filling devices help prevent misfilling. Overfilling is minimized by locating the filler port at the maximum safe liquid level. Tipping of a variable bypass vaporizer can allow the liquid agent to enter the bypass chamber and cause an extremely high output. Modern vaporizers are firmly secured to a manifold on the anesthesia workstation to prevent tipping.
Vaporizers and the vaporizer–machine interface are potential sources of gas leaks that can result in patient awareness during inhaled anesthesia. Loose filler caps, filler plugs, and drain valves are probably the most common sources of leaks. Another common source of gas leak occurs at the junction of the vaporizer and the mounting bracket or manifold, where broken mounting assemblies or foreign bodies can compromise the seal between the vaporizer and its point of attachment. Gas leaks can also occur within the vaporizer itself as a result of mechanical failure. Assessment for low-pressure system leaks, including the vaporizer mount, is addressed in the section “Checking Your Anesthesia Workstation.”
Although rarely reported, contamination of anesthetic vaporizer contents has occurred, including organic contaminants and bacteria.
Desflurane Dual-Gas Blender
Because of its unique physical characteristics, a variable bypass vaporizer cannot be used for desflurane. First, the vapor pressure of desflurane is 669 mm Hg at 20°C (68°F), nearly 1 atm, and significantly higher than all other contemporary inhaled anesthetic agents. If desflurane were placed in a variable bypass vaporizer, prohibitively high bypass chamber flow rates would be required to dilute the vaporizing chamber output to clinical concentrations. Second, desflurane’s moderate potency means that the amount of desflurane vaporized over a given period is considerably greater than for other inhaled anesthetics. The concomitantly greater cooling would be difficult to compensate for without an external heat source. Finally, desflurane’s boiling point of 22.8°C (73°F) at 1 atm can be encountered in normal operating room conditions. If the anesthetic agent were to boil within a variable bypass–type vaporizer, the output would be uncontrollable.
By outward appearance, the desflurane “vaporizer” is similar to variable bypass vaporizers, but its design and operating principles are radically different ( Fig. 15.5 ). The desflurane vaporizers are electrically heated, thermostatically controlled, constant temperature, pressurized, electromechanically coupled, dual-circuit, gas vapor blenders . The desflurane sump is heated above the liquid’s boiling point, providing a source of pressurized desflurane vapor that is the starting point of a second gas circuit parallel to the fresh gas flow. The vapor’s pressure is then downregulated, and a conventional concentration control dial is used to control the flow of vapor to mix with the fresh gas.
The desflurane blender behaves differently from the variable bypass vaporizer at altitude. Because of its design, the delivered desflurane concentration is relatively stable as barometric pressure drops, and thus the delivered partial pressure of desflurane is lower. At altitude, therefore, the anesthesia provider must compensate by increasing the set desflurane concentration by a factor equal to the percentage decrease in barometric pressure compared with sea level. Furthermore, use of nitrous oxide at low flows can decrease the output from the desflurane blender independently of atmospheric pressure.
Cassette Vaporizers
Cassette vaporizer systems use a permanent control unit housed within the workstation and interchangeable cassettes containing anesthetic liquid to serve as vaporizing chambers. A variety of agent-specific cassettes are available; they are color-coded for the operator and also magnetically coded to allow the anesthesia machine to identify which cassette has been inserted. For anesthetics other than desflurane, the cassette vaporizing system acts as a computer-controlled variable bypass vaporizer. The flow through the vaporizing chamber is controlled by a central processing unit, such that compensation for temperature, carrier gas concentration, and flow rates can be carried out precisely. When the pressure inside the cassette’s vaporizing chamber rises above the pressure in the bypass chamber, as when room temperature is above desflurane’s boiling point of 22.8°C (73°F), then the function of the cassette changes to become a vapor injector. A check valve between the bypass chamber and the vaporizing chamber closes so that no fresh gas enters the vaporizing chamber and anesthetic vapor does not travel retrograde uncontrolledly into the fresh gas. Under computer control, pure desflurane vapor is injected to achieve the desired concentration.
Injection-Type Vaporizers
Some anesthesia workstations use injection-type vaporizers. Liquid anesthetic is injected into a heated vaporizing chamber that may also contain a heated evaporative surface. The vaporizing chamber may serve as a source of vapor that is metered into the fresh gas, or the fresh gas flow may transit the chamber to be enriched with anesthetic vapor. Design details differ by manufacturer. In all cases the process is under microprocessor control.
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