Inhaled Anesthetics: Delivery Systems

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 oxygen source in case of failure of the hospital supply. Many machines have up to three and sometimes four E-cylinder attachment points to accommodate oxygen, air, and nitrous oxide. Some machines have attachments for two oxygen tanks, and some rare systems can accommodate carbon dioxide (CO ₂ ) or helium tanks used for special applications. The cylinders are mounted to the anesthesia machine by the hanger yoke assembly, as seen in Fig. 22.3 . The hanger yoke assembly orients and safely supports the cylinder, provides a gas-tight seal, and ensures unidirectional flow of gases into the machine. Each yoke assembly must have a label designating which gas it is intended to accept. 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 error caused by interchanging cylinders. 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. Although infrequent, failures of the PISS have been reported, and like all safety systems, the PISS should be considered partial protection. Conditions in which failure occurred have included the following: excessive seating (jamming) of the pins back into the hanger yoke; the presence of bent or broken pins; and an excessive use of washers between the cylinder and the yoke that can override pin alignment, yet allow for a gas-tight seal. Medical gas cylinder errors can have tragic outcomes, so it is critical to ensure that the proper gas is being connected to the proper inlet by also checking the tank and yoke labels.

Once a gas cylinder valve is opened by the operator, gas flows first through a filter to entrap any particulate matter from the tank inflow. The maximum pressure in full E-cylinders (approximately 750 psig for nitrous oxide, 2200 psig for air, and 2200 psig for oxygen) is much higher than the normal hospital pipeline supply pressure of 50 to 55 psig. A high-pressure regulator reduces the variable high pressure in the E-cylinder to a constant pressure slightly lower than the normal pipeline supply pressure, approximately 40 to 45 psig (depending on the specific anesthesia machine) (see the O ₂ high-pressure section in Fig. 22.1 ). The lower pressure is a safety feature: if both the E-cylinder and the oxygen pipeline are connected and the E-cylinder is open, the anesthesia machine will draw its gas from the pipeline rather than the E-cylinder, thereby preserving the contents of the E-cylinder in case of pipeline failure. Fluctuations in the pipeline pressure below 40 to 45 psig could allow the E-cylinder to be drained, as could silent leaks in the high-pressure system, so E-cylinders should be closed during normal operation. One implication of this design warrants emphasis: in case of known or suspected pipeline contamination or crossover leading to delivery of a hypoxic gas mixture (as might be caused by nitrous oxide in the oxygen pipeline), backup oxygen from the E-cylinder will not flow unless the anesthesia machine is disconnected from the pipeline. Merely turning the backup tank on will not help, if the pipeline pressure remains higher than the high-pressure regulator’s output.

After the high-pressure regulator, cylinder gas flows through a one-way valve called the cylinder check valve, which prevents any backflow of machine gas out through an empty yoke or back into a nearly empty cylinder (see Fig. 22.1 ). If the anesthesia machine allows two oxygen E-cylinders to be mounted on a common manifold, then each mount must have a check valve. On some machines, a single high pressure regulator is downstream from the two check valves; on others, each mount on the manifold has its own high pressure regulator and check valve. In either configuration, transfer of gas from a full tank to an empty tank is prevented, and the system allows for a cylinder to be exchanged while the other cylinder on the manifold continues to supply gas to the anesthesia machine.

As noted on Fig. 22.1 , there are a number of pressure gauges in the system. The pressure in each of the gas pipelines and each of the auxiliary E-cylinder manifolds must be displayed on the front of the anesthesia machine. The E-cylinder pressures are accurate only when the tank is open; in the case of a two-tank manifold, the pressure of the open tank with higher pressure will be displayed. In systems with electronic pressure displays, the pipeline and tank pressures are visible only when the machine is on.

Two points about the safe use of the auxiliary E-cylinder system should be noted. First, checking the E-cylinders is not part of an automatic machine checkout. The practitioner must manually open each cylinder and check the pressure gauges on the front of the machine. In the case of a two-tank oxygen manifold, the tanks must be serially opened and checked. The oxygen flush valve may be used to vent the pressure from the system after closing the first tank, so the pressure in the second tank can be accurately assessed. Second, it is imperative to keep the auxiliary E-cylinders closed during normal operation using pipeline gases because of the possibility of small leaks in the high-pressure system, or fluctuations in pipeline pressures allowing flow from the cylinder to be activated. An open oxygen cylinder may allow the anesthesiologist to be unaware of catastrophic pipeline failure. When the oxygen cylinder is closed, the immediate result of oxygen pipeline failure is a low oxygen pressure alarm. The auxiliary E-cylinder can then be opened, ensuring continued flow of oxygen to the patient while troubleshooting occurs. If the oxygen tank is already open when pipeline failure occurs, there may be only a subtle indication from the anesthesia machine that the oxygen source has switched from pipeline to auxiliary tank. In this case, the low oxygen pressure alarm only occurs once the auxiliary tank has been depleted, nullifying the utility of the backup system.

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. The main supply source of oxygen in a large hospital usually is a large cryogenic bulk oxygen storage system. These are refilled on site from a truck carrying liquid oxygen. Smaller hospitals may use liquid oxygen tanks that can be replaced rather than refilled on site, or even a bank of oxygen H-cylinders connected by a manifold. Oxygen storage systems must have backup supply and alarm systems in place. Most hospitals use compressors to deliver cleaned, dried air to a pressurized reservoir for delivery to the pipeline system. Centrally supplied nitrous oxide arises either from a bank of H-type cylinders, or a bulk liquid storage system similar to that for oxygen.

The gas pipeline terminates in patient care areas of the hospital 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, which helps to minimize the potential for connecting to the wrong gas. DISS connectors (as seen in Fig. 22.4 ) rely on matching diameters in the male and female connections to properly seat and thread the connection. The quick couplers ( Fig. 22.5 ) utilize 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 addition, in both systems the wall plates and hoses are color-coded for ease of identification.

The final medical gas pipeline connection to the anesthesia workstation is always through a DISS connector ( Fig. 22.4 C ). Once the gas enters the machine, it encounters a filter followed by a pipeline check valve. This one-way valve prevents reverse flow of gas from the machine into the medical gas pipeline system or into the atmosphere from an open inlet. Interposed between the DISS inlet and the pipeline check valves is a sample port to measure pipeline oxygen pressure. The pipeline pressure must always be clearly visible on the front of the machine.

Oxygen Flush Valve

The oxygen flush valve is probably one of the oldest safety features on the machine and remains a machine standard today. The oxygen flush valve allows manual delivery of a high flow rate of 100% oxygen directly to the patient’s breathing circuit in order to overcome circuit leaks or to rapidly increase the inhaled oxygen concentration. Flow from the oxygen flush valve bypasses the anesthetic vaporizers (see Fig. 22.1 ). The intermediate-pressure segment of the gas supply system feeds the valve, which remains closed until the operator opens it. The feature is usually available even when the machine is not turned on because the valve is located upstream from the machine’s pneumatic power switch. Flow from the oxygen flush valve enters the low-pressure circuit downstream from the vaporizers at a rate between 35 and 75 L/min, depending on the machine and operating pressure.

Several hazards have been reported with the oxygen flush valve. 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 or overzealous oxygen flushing can dilute the inhaled anesthetic agent concentration, potentially resulting in awareness under anesthesia. 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 controller. Fresh gas decoupling prevents the fresh gas inflow from either the flowmeters or the oxygen flush valve from increasing the delivered ventilator tidal volume presented to the patient’s lungs (see section on fresh gas flow compensation and fresh gas decoupling). With most older anesthesia breathing circuits, excess volume could not be vented during the inspiratory phase of mechanical ventilation because the ventilator relief valve was closed and the breathing circuit adjustable pressure-limiting (APL) valve was either out of circuit or closed.

Although the oxygen flush valve can potentially provide a high-pressure, high-flow oxygen source at the machine’s fresh gas outlet suitable for jet ventilation, it has potential limitations. In some machines, the fresh gas outlet is no longer easy to access, and not all machines are capable of generating pressures at the outlet that are sufficient to deliver jet ventilation. An alternate source of high-flow oxygen should be sought if jet ventilation is needed and cannot be supported by the machine’s oxygen flush function.

Pneumatic Safety Systems

One of the primary safety goals of contemporary anesthesia machines is to guard against the potential of delivering an excessive concentration of nitrous oxide relative to oxygen (hypoxic mixture). ISO standards require delivery of a nonhypoxic gas mixture to the patient, or generation of an alarm condition. Several safety devices discussed below have been introduced to prevent generating a hypoxic mixture.

Auxiliary Oxygen Flowmeter

Although auxiliary oxygen flowmeters are not mandatory, they are commonly encountered. During normal operation, the auxiliary flowmeter is a convenience feature that allows the use of low-flow oxygen for devices independent of the patient’s breathing circuit. Similar to the oxygen flush feature, oxygen flow from the flowmeter is usually accessible even when the machine is not turned on, because the flowmeter is typically fed before the pneumatic power switch in the intermediate-pressure section. As long as oxygen is available from the pipeline inlet or from an attached E-cylinder, the auxiliary oxygen flowmeter can serve as a source of oxygen delivery for use with a manually powered resuscitation bag in the case of a system power failure. The auxiliary oxygen flowmeter may also potentially serve as gas source for a manual jet ventilator; however, not all machines can generate sufficient working pressure. Some auxiliary oxygen flowmeters have a DISS connector that would be a better source for manual jet ventilation.

The operator should be aware that the source of oxygen for the auxiliary flowmeter is the same as for the other oxygen flow control valves. This is an important consideration in cases of suspected hospital oxygen pipeline contamination or crossover. If the pipeline oxygen supply line is connected to the machine and the pressure is sufficient, the gas source will be the pipeline even if the auxiliary oxygen tank valve is opened. In a simulation experiment, a nitrous oxide–oxygen pipeline crossover situation was created whereby the inspired oxygen concentration became alarmingly low, and the “patient” became hypoxemic after turning the nitrous oxide flow off. Researchers noted that many study participants tried to make inappropriate use of the auxiliary oxygen flowmeter and oxygen E-cylinders on the machine as an external source of oxygen without disconnecting the pipeline source. The participant’s suboptimal management was attributed to a lack of knowledge of the anesthesia machine and its gas supply.

Second-Stage Pressure Regulators

Some machines have second-stage regulators located downstream from the gas supply sources in the intermediate-pressure circuit. These regulators supply constant pressure to the flow control valves and the proportioning system regardless of potential fluctuations in hospital pipeline pressures. They are adjusted to lower pressure levels than the pipeline supply, usually between 14 and 35 psig, depending on the workstation.

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 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 fresh gas outlet (see Figs. 22.1 and 22.2 ).

Electronic Flow Sensors

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. Flows can be displayed numerically or sometimes graphically in the form of a virtual, digitalized flowmeter. Numerous types of flow sensor technologies can be applied, such as hot-wire anemometers, a differential pressure transducer method, or mass flow sensors. An example of an electronic mass flow sensor is seen in Fig. 22.6 . The illustrated device relies on the principle of specific heat to measure gas flow. As gas flows through a heated chamber of known volume, a specific amount of electricity is required to maintain the chamber temperature. The amount of energy required to maintain the temperature is proportional to the flow and specific heat of the gas. Regardless of the mechanism of flow measurement, these systems depend on electrical power to provide a display of gas flow. When electrical power is totally interrupted, some backup mechanical means usually exists to control (mechanical flow control) and display (flow tube) oxygen gas flow.

Flow Control Valves

The flow control valve assembly consists of a flow control knob, a tapered needle valve, a valve seat, and a pair of valve stops ( Fig. 22.7 ). The inlet pressure to the assembly is determined by the pressure characteristics of the machine’s intermediate-pressure segment. The location of the needle valve in the valve seat changes to establish different orifices when the flow control valve is adjusted. Gas flow increases when the flow control valve is turned counterclockwise, and it decreases when the valve is turned clockwise. Because their use is frequent and the consequences of damage are significant, the controls must be constructed so extremes of rotation will not cause disassembly or disengagement.

Contemporary flow control valve assemblies have numerous safety features. The oxygen flow control knob is physically distinguishable from the other gas knobs. It is distinctively fluted, may project beyond the control knobs of the other gases, and is larger in diameter than the flow control knobs of other gases. All knobs are color coded for the appropriate gas, and the chemical formula or name of the gas must be permanently marked on each knob. Flow control knobs are recessed or protected with a shield or barrier to minimize inadvertent change from a preset position. If a single gas has two flow tubes, the tubes are arranged in series and are controlled by a single flow control valve.

Flow Tubes

With a traditional flowmeter assembly, the flow control valve regulates the amount of flow that enters a tapered, transparent flow tube known as a variable orifice flowmeter or Thorpe tube . These glass tubes are narrowest at the bottom and widen at the top. A mobile indicator float inside the calibrated flow tube indicates the amount of flow passing through the associated flow control valve. The quantity of flow is indicated on a scale specific to the flow tube. Opening the flow control valve allows gas to travel through the space between the float and the flow tube. This space is known as the annular space , and it varies in size depending on the position in the tube ( Fig. 22.8 ). The indicator float hovers freely in an equilibrium position in the tube where the upward force resulting from gas flow equals the downward gravity force on the float at a given flow rate. The float moves to a new equilibrium position in the tube when flow is changed. These flowmeters are commonly referred to as constant-pressure flowmeters because the decrease in pressure across the float remains constant for all positions in the tube.

Flow through the annular space can be laminar or turbulent, depending on the gas flow rate ( Fig. 22.9 ). The characteristics of a gas that influence its flow rate through a given constriction are viscosity (laminar flow) and density (turbulent flow). Because the annular space behaves as a tube at low flow rates, laminar flow is present, and viscosity determines the gas flow rate. At high flow rates, the annular space behaves like an orifice. Turbulent gas flow is present and gas density predominantly influences the flow. Because the viscosity and density of the gas affect flow through annular space around the float, the calibrated flow tubes are gas specific. The tube, the float, and the scale are inseparable. Although temperature and barometric pressure can influence gas density and viscosity, under normal clinical circumstances, flow tube accuracy is not significantly affected by mild changes in temperature or pressure.

The float or bobbin within the flow tube is usually constructed so that it rotates to indicate that gas is flowing and that the indicator is not stuck in the tube. A stop at the top of the flowmeter tube prevents the float from occluding the outlet. Two flowmeter tubes are sometimes placed in series, with a fine flow tube displaying low flows and a coarse flow tube indicating higher flows.

Problems With Flowmeters

Flow measurement error can occur even when flowmeters are assembled properly. Dirt or static electricity can cause a float to stick and misrepresent actual flow. Sticking of the indicator float is more common in the low-flow ranges because the annular space is smaller. A damaged float can cause inaccurate readings because the precise relationship between the float and the flow tube is altered. Backpressure from the breathing circuit can cause a float to drop so that it reads less than the actual flow. Finally, if flowmeters are not aligned properly in the vertical position (plumb), readings can be inaccurate because tilting distorts the annular space.

The flow tube has historically been a very fragile component of the anesthesia workstation. Subtle cracks and chips may be overlooked and can cause errors in delivered flow. Leaks can also occur at the O-ring junctions between the glass flow tubes and the metal manifold. Flow tube leaks are a potential hazard because the flowmeters are located downstream from all hypoxemia safety devices, except the breathing circuit oxygen analyzer. Fig. 22.10 shows an example where an unused air flow tube develops a large leak. When the nitrous oxide flowmeter is in the downstream position ( Fig. 22.10 A and B ), a hypoxic mixture can occur because a substantial portion of the oxygen flow passes through the leak in the air flow tube, and mainly nitrous oxide is directed to the common gas outlet. Safer configurations are shown in Fig. 22.10 C and D , in which the oxygen flowmeter is located in the downstream position. A portion of the nitrous oxide flow escapes through the leak, and the remainder goes toward the common gas outlet. A hypoxic mixture is less likely because all the oxygen flow is advanced by the nitrous oxide (this principle is known as the Eger flow sequence). It has been an industry standard that oxygen be delivered downstream of all other gases, although current ISO standards require only that oxygen be at either end of a bank of flowmeters. It is important to remember that in the case of a leak in the oxygen flow tube, a hypoxic mixture may result even when oxygen is located in the downstream position.

Proportioning Systems

Anesthesia workstations are equipped with an oxygen failure protection device in the intermediate-pressure section that, in response to reduced oxygen pressure, either proportionally reduces or completely inhibits nitrous oxide. However, this system does not prevent the user from selecting a hypoxic gas mixture for delivery to the fresh gas outlet. 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 mechanically controlled flowmeters, the concern is that a user could mistakenly select oxygen and nitrous oxide flows that would result in a hypoxic mixture. According to the ISO, an alarm condition is insufficient and the machine must have a system to prevent delivery of a hypoxic mixture. This is accomplished by a pneumatic-mechanical interface between the oxygen and nitrous oxide flows or by mechanically linking the oxygen and nitrous oxide flow control valves. This way, no matter how high the operator attempts to turn up the nitrous oxide, or how low the operator tries to turn down the oxygen flow, when nitrous oxide is running, the machine will automatically adjust the ratio of these flows so that a hypoxemic gas mixture cannot be delivered. The specific devices used to accomplish this control vary among manufacturers. Two examples are briefly discussed here.

The North American Dräger sensitive oxygen ratio controller system (SORC) is a pneumatic-mechanical, oxygen–nitrous oxide interlock system designed to maintain a ratio of no less than 25% oxygen to 75% nitrous oxide flow into the breathing circuit by limiting the nitrous oxide flow when necessary. The SORC, located after the flow control valves, consists of an oxygen chamber with a diaphragm, a nitrous oxide chamber with a diaphragm, and a nitrous oxide proportioning valve ( Fig. 22.11 ). All are interconnected by a mobile horizontal shaft. As oxygen flows out of the SORC, it encounters a resistor that creates backpressure in the oxygen chamber, which causes the diaphragm to move to the right, thereby opening the nitrous oxide proportioning valve. As the oxygen flow is increased, so too is the backpressure and the rightward motion of the shaft. If the nitrous oxide flow is now turned on, it will also flow into the SORC, through the proportioning valve, and past its resistor to create backpressure that will press on the diaphragm in its respective chamber. The counterbalance between the two gas flows (backpressures) determines the positioning of the nitrous oxide proportioning valve. If the oxygen is turned down too low (<1/3 of the nitrous oxide flow), the shaft will move to the left and thus limit the nitrous oxide flow. If the operator tries to turn up the nitrous oxide too high relative to the oxygen flow, the SORC will limit the nitrous oxide flow because of the nitrous oxide backpressure and leftward movement of the valve. If the oxygen flow is decreased to less than 200 mL/min, the proportioning valve will close completely.

A mechanical proportioning system that remains in use today on many anesthesia machines is the GE/Datex-Ohmeda Link-25 system. The system provides mechanical integration of the nitrous oxide and oxygen flow control valves to maintain a minimum oxygen concentration with a maximum nitrous oxide:oxygen flow ratio of 3:1. Independent adjustment of either valve is allowed as long as the minimum threshold is met. The Link-25 automatically increases oxygen flow when the nitrous oxide flow is increased above the 3:1 ratio. It also will lower nitrous oxide flow if oxygen flow is decreased below that ratio. Fig. 22.12 shows the Link-25 system. A 15-tooth sprocket is attached to the nitrous oxide flow control valve, a 29-tooth sprocket is attached to the oxygen flow control valve and a chain physically links the sprockets. When the nitrous oxide flow control valve is turned through two revolutions, the oxygen flow control valve will revolve once because of the 2:1 gear ratio. The final 3:1 flow ratio results because the flow control valve needle for nitrous oxide has a faster taper than does the oxygen valve needle. The Link-25 system uses a stop tab on each valve stem to allow for independent adjustment of oxygen or nitrous oxide as long as the mixture is at least 25% oxygen; attempting to turn the valve controller past that point will engage the chain and effect a change in the other gas. In addition, the system is designed so that nitrous oxide cannot flow unless the oxygen flow is at least 200 mL/min.

Although both proportioning systems are designed to prevent delivery of a hypoxic gas mixture to the common gas outlet, their effect on the output may be different. If the operator turns down oxygen flow below 25% oxygen, both the Link-25 and SORC systems will respond by decreasing the flow of nitrous oxide. If the operator subsequently increases the set oxygen flow, the nitrous oxide flow will remain at the new, lower value with the Link-25 system, because the mechanical linkage will have physically changed the nitrous oxide control valve setting. With the SORC system, on the other hand, the nitrous oxide flow will return to the higher, previously set value when adequate oxygen flow is restored. If the operator increases nitrous oxide flow beyond the set safe range, the Link-25 system will increase the oxygen flow by changing the setting on the oxygen control valve. The SORC system will instead prevent the increase in nitrous oxide flow from occurring. If the operator subsequently reduces the nitrous oxide flow setting, the oxygen flow will remain at the new, higher level with the Link-25, and will remain unchanged with the SORC.

Proportioning System Malfunction

Proportioning systems are not immune from failure, and workstations equipped with proportioning systems can still deliver a hypoxic mixture under certain conditions. Many case reports have described proportioning system malfunction. Other situations that may defeat the purpose of the proportioning system require operator vigilance. Both mechanical and pneumatic proportioning systems can be defeated if a gas other than oxygen is present in the oxygen pipeline. Proportioning systems such as the Link-25 function at the level of the flow control valves. A leak downstream from these devices, such as a broken oxygen flow tube, could result in delivery of a hypoxic mixture to the common gas outlet. In this situation, oxygen escapes through the leak, and the predominant gas delivered is nitrous oxide. Finally, volatile inhaled anesthetic agents are added to the mixed gases downstream from both the flowmeters and the proportioning system. Concentrations of less potent inhaled anesthetic agents such as desflurane may account for a larger percentage of the total fresh gas composition than is the case with more potent agents. Because significant percentages of these inhaled anesthetic agents may be added downstream of the proportioning system, the resulting gas-vapor mixture may contain an inspired oxygen concentration less than 21% despite a functional proportioning system. The additional complexity of the circle system (discussed below) means that the oxygen concentration of the fresh gas flow delivered to the breathing circuit may be very different from the patient’s actual fraction of inspired oxygen (FiO ₂ ). In each case, the presence of a functioning oxygen analyzer in the patient’s breathing circuit is the last protection against a hypoxic gas mixture.

Vaporizer Mount and Interlock System

Outlet Check Valve

Many older Datex-Ohmeda anesthesia machines and a few contemporary workstations (e.g., GE/Datex-Ohmeda Aestiva and Aespire) have a one-way check valve located between the vaporizer and the common gas outlet in the mixed-gas pipeline (see Fig. 22.1 ). The purpose of this valve is to prevent backflow into the vaporizer during positive-pressure ventilation, thereby minimizing the effects of intermittent fluctuations in downstream pressure on the concentration of inhaled anesthetics (see the discussion of intermittent backpressure in the section on anesthetic vaporizers). The presence or absence of this check valve historically influenced which manual leak test of the low-pressure system was indicated because it precluded positive-pressure tests to detect for leaks upstream of the valve (see the section on checking your anesthesia workstation).

The Ideal Gas Law

When sealed in a container, gas molecules collide with the walls and exert a force or pressure. This pressure is directly proportional to the number of molecules or moles (n) of gas present within the container and to the temperature (T) in degrees kelvin, and inversely proportional to the volume (V) that confines the gas. (One mole of a substance is equal to 6.022 × 10 ²³ [Avogadro’s number] molecules of that substance.) The ideal gas law is:

The ideal gas law provides an important framework for understanding the behavior of anesthetic gases within vaporizers, anesthesia delivery equipment, and the pulmonary alveolus. Key assumptions of this law are that gas molecules (1) behave as points in space and (2) undergo perfectly elastic collisions without attracting or repelling one another or the walls of the container. These assumptions are valid for dilute anesthetic gases at normal operating conditions.

Dalton’s Law of Partial Pressures

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. This is known as Dalton’s law of partial pressures, where the individual pressures (P _i ) exerted by each of the constituent gases are referred to as partial pressures :

Another useful expression, which can be derived by combining Dalton’s law with the ideal gas law, is:

which states that the partial pressure of gas A can be calculated by multiplying the total pressure of the mixture by the mole fraction ( n _A / n _total ), or the volume percent ( v / v %), of gas A. The volume percent tends to be more useful in day-to-day anesthesia practice (see below).

As a first step to understanding vaporizer function, it is useful to look at an example of Dalton’s law of partial pressures. In Fig. 22.14 A , pure oxygen fills a theoretical container that is open to the environment through a very small hole. The pressure in the container is equal to the ambient pressure, which at sea level is 760 mm Hg or 1 atm or 101.325 kPa, and generated entirely by the oxygen molecules. In Fig. 22.14 B , the container is filled with air, and the total pressure is generated by the additive partial pressures of oxygen, nitrogen, and trace amounts of rare gases.

Evaporation and Vapor Pressure

Volatile liquids, such as inhaled anesthetic agents, are characterized by a high propensity to enter the gas phase, 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. This process is known as evaporation , which is purely a surface phenomenon (in contrast to boiling , which occurs throughout the liquid). If liquid volatile anesthetic is placed within a contained space, such as a vaporizer, molecules will escape into the vapor phase until the rate of evaporation equals the rate of return to the liquid phase (a process known as condensation ). When this equilibrium is reached, the gas above the liquid is said to be “saturated” with anesthetic ( Fig. 22.15 ). The anesthetic molecules in the gas phase create a partial pressure known as the saturated vapor pressure , or simply vapor pressure. Liquids with a greater tendency to evaporate and generate higher vapor pressures are described as “more volatile.”

Vapor pressure is an unique physical property of a substance at any given temperature ( Fig. 22.16 ). Vapor pressure is not affected by changes in atmospheric pressure. As illustrated in Fig. 22.17 for the isoflurane, 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. Although operating room (ambient) temperature can raise or lower liquid anesthetic vapor pressure, the cooling influence of evaporation (the latent heat of vaporization, see below ) has a far more pronounced and dynamic effect. The impact of evaporative temperature change on vaporizer and anesthetic inhaler output has been recognized since the mid-1800s, and addressing this phenomenon has been one of the principal factors influencing design of anesthetic vaporizers.

Because vapor pressures are unique to each liquid anesthetic agent, vaporizers must be constructed in an agent-specific manner. If a vaporizer is inadvertently filled with the incorrect liquid anesthetic agent, the vaporizer output will change (see the discussion of misfilling in the section on variable bypass vaporizers).

Expressing Gas Concentrations and Minimum Alveolar Concentration

When describing a mixture of gases, we can quantify the proportion of an individual gas by either its partial pressure (mm Hg), or by the percentage of volume occupied by the gas relative to the sum of all gases present, which is known as volume percent or volume-volume percent ( v / v %) :

The volume that an ideal gas occupies at a given temperature and pressure is related to the number of molecules of gas present, but not the size or identity of the molecules. This statement is known as the Avogadro Hypothesis . Using the ideal gas law, it is easy to calculate that at 1 atm (760 mm Hg) of pressure and 20°C (68°F or 293°K), conditions that might be found in a typical operating room, 1 mole of an ideal gas occupies a volume of about 24 L. The same is true for any mixture of ideal gases containing a total of 1 mole of gas molecules. Therefore, because partial pressure is directly proportional to the number of molecules of a gas present in the mixture, we can also use partial pressures to calculate the volume percent of any constituent gas :

Using air at sea level ( P _total = P _atm = 760 mm Hg) as an example:

Knowing the partial pressures of the constituent gases of air…

... we can then calculate the volume percent ( v / v %) of oxygen…

When anesthesiologists describe inhaled and exhaled anesthetic concentrations, they typically use volume percent . One percent isoflurane is equal to 7.6 mm Hg isoflurane at sea level. The amount of oxygen and nitrous oxide in the breathing gas is also typically described in terms of volume percent. However, CO ₂ content (i.e., end-tidal carbon dioxide [ET co ₂ ]) is usually displayed as a partial pressure (mm Hg). This was probably adopted because of the relatively close correlation between ET co ₂ and arterial partial pressure of carbon dioxide (Pa co ₂ ), and the latter’s common expression as a partial pressure. Fig. 22.18 illustrates a typical composition of the breathing gases during anesthesia in terms of concentration ( v / v %) and partial pressures.

The minimum alveolar concentration (MAC) is described in terms of volume percent. MAC is the concentration of anesthetic that prevents movement from surgical stimulus in 50% of individuals. MAC is an age-dependent phenomenon, and it can also be affected by other variables. MAC is a clinically useful value given that vaporizer control knobs are marked and calibrated in terms of anesthetic concentration. However, it is actually the anesthetic partial pressure (mm Hg) value in the brain that is responsible for anesthetic depth. The corresponding partial pressure for each MAC value is known as the minimal alveolar partial pressure (MAPP), as listed in Table 22.1 . When discussing anesthetic vaporizers, it is useful to think about their output in terms of partial pressure and how it relates to volume percent and MAC, especially when considering changes in ambient pressure.

Latent Heat of Vaporization

When a liquid such as a volatile anesthetic evaporates into the gas phase, energy is required to overcome the attractive intermolecular forces between molecules in the liquid phase (a property known as cohesion ). The needed energy is absorbed from the surroundings in the form of heat , and is the reason why the human body is cooled by the evaporation of sweat. The amount of energy absorbed by a specific liquid during evaporation is referred to as the latent heat of vaporization . It is more precisely defined as the amount of energy in joules or calories (1 calorie = 4.184 joules) required to change 1 g of liquid into vapor at a constant temperature. In a well-insulated container, the energy for vaporization must come from the liquid itself. In the absence of an outside heat source, the remaining liquid cools as vaporization progresses. This leads to significant reductions in vapor pressure (see Fig. 22.16 ) and therefore the number of volatile anesthetic molecules in the gas phase (see Fig. 22.17 ). If vaporizer design does not mitigate and compensate for evaporative cooling, output will decrease.

Boiling Point

The boiling point of a liquid is defined as the temperature at which vapor pressure equals atmospheric pressure and the liquid begins to undergo rapid vaporization. From the definition above, it is important to note that the boiling point changes depending on atmospheric pressure. Whereas evaporation is a surface phenomenon, boiling is a bulk phenomenon that occurs throughout the interior of the liquid. The boiling point of a liquid is inversely related to volatility. For example, water is not particularly volatile (see Fig. 22.16 ) and its boiling point of 100°C (212°F) at sea level is much higher than all of the inhaled anesthetic agents. Table 22.1 lists the normal boiling point (defined as the boiling point at a pressure of 1 atm) of the common volatile anesthetic agents. While most inhaled anesthetics boil in the range of 48° to 59°C (118°-138°F), desflurane has a normal boiling point close to room temperature (22.8°C or 73°F).

The boiling point of contemporary volatile anesthetic agents is not relevant to vaporizer design under most clinical situations. Desflurane, however, has a high saturated vapor pressure and boils at a temperature commonly encountered in clinical settings. These properties mandate a special vaporizer design to control agent delivery (see section on desflurane vaporizer). Isoflurane and halothane could theoretically boil at high altitudes and very high temperatures. At least one vaporizer manufacturer specifies a maximum safe operating temperature for these anesthetic agents.

Specific Heat

The specific heat is the amount of energy required to increase the temperature of 1 g of a substance by 1°C. Water, for example, has a specific heat of exactly 1 calorie g ⁻¹ deg ⁻¹ . The concept of specific heat is important to the design, operation, and construction of vaporizers in two ways. First, the specific heat of a liquid anesthetic determines how much heat must be supplied to maintain a constant temperature due to the latent heat of vaporization. Second, vaporizers are built from materials with a high specific heat in order to better resist temperature changes associated with evaporative cooling.

Thermal Conductivity

Thermal conductivity is a property that describes how well heat flows through a substance. The higher the thermal conductivity, the better a substance conducts heat. Vaporizers are constructed of metals with relatively high thermal conductivity, which helps maintain a uniform internal temperature during evaporation by allowing efficient heat absorption from the environment. By contrast, coffee mugs should be made of materials with a low thermal conductivity to slow heat loss to the environment.

Variable Bypass Vaporizers

When volatile anesthetic agents evaporate, their resultant saturated gas concentrations greatly exceed those used clinically, so these concentrations must be diluted to safe ranges (see Table 22.1 ). Variable bypass refers to the method of diluting gas fully saturated with anesthetic agent with a more voluminous flow of gas. A diagram of a variable bypass vaporizer is shown in Fig. 22.19 . Basic vaporizer components include a vaporizer inlet port (fresh gas inlet), the concentration control dial, the bypass chamber, the vaporizing chamber, the vaporizer outlet port, and the filling assembly. The maximum safe level of the vaporizer corresponds to the filling port, which is positioned to minimize the chance of overfilling. A concentration control dial determines the ratio of gas that flows through the bypass chamber and the vaporizing chamber, and a temperature-compensating device further adjusts that ratio. Vaporizer concentration control dials are labeled to set vaporizer output in terms of volume percent ( v / v %), and the vaporizers are calibrated at sea level.

Fig. 22.20 illustrates volatile anesthetic equilibrium concentrations within a theoretical vaporizing chamber of a variable bypass vaporizer. As can be seen, the saturated vapor concentration of sevoflurane within the chamber (21%) far exceeds the clinical concentration. Fig. 22.20 also depicts the volume of anesthetic vapor that is added to the gas stream as it flows through the chamber. These properties are essential for a quantitative understanding of variable bypass vaporizer function ( Box 22.1 ). Although this example and others in the chapter imply that the gas flowing through the vaporizing chamber becomes fully saturated with anesthetic vapor, this is actually not the case. There is insufficient time to reach evaporative equilibrium due to the constant inflow of fresh gas. As a result, the vaporizing chamber becomes only partially saturated with volatile anesthetic. However, for the purposes of this discussion, it is useful to assume that full saturation occurs.

Fig. 22.21 illustrates a modern variable bypass vaporizer set to deliver 2% sevoflurane. Note how the majority of fresh gas flows straight through the bypass chamber. The bypass flow and vaporizing chamber output combine to create the desired output concentration. The fresh gas that is diverted to the vaporizing chamber becomes saturated with anesthetic gas by flowing through the wicks and over the liquid agent (hence the designation flow-over ). The wicks and baffles serve to increase the surface area available for vaporization and promote mixing of the carrier gas with anesthetic vapor . The specific ratio of fresh gas flow divided between the bypass chamber and the vaporizing chamber is determined by the concentration control dial setting and the temperature compensation device (see the later discussion of temperature compensation). Because the physical properties and clinical concentrations of each agent are unique, the diverting ratios are specific to each agent and dial setting (hence the designation agent-specific ). The approximate variable bypass diverting or “splitting ratios” for the common anesthetic agents at 20°C are shown in Table 22.3 . Variable bypass vaporizers cannot be used to deliver desflurane, because of this agent’s unique physical properties (see section on the desflurane vaporizer).

Virtually all variable bypass vaporizers are equipped with a mechanism that helps maintain constant vaporizer output over a wide range of operating temperatures (hence the designation temperature-compensated ). This mechanism automatically alters the ratio of gas flowing through the bypass and vaporizing chambers. Temperature compensation is accomplished by an expansion-contraction element, as seen in Fig. 22.19 , or a bimetallic strip ( Fig. 22.22 ). At cooler temperatures, the vapor pressure of liquid anesthetic decreases ( Fig. 22.16 ), and it is necessary to reduce the splitting ratio to maintain output. In Fig. 22.19 B , as the liquid anesthetic agent cools, the temperature-compensating cone moves upward, restricts bypass flow, and diverts more gas to the vaporizing chamber, thereby maintaining relatively stable vaporizer output. The inverse is also true: warmer temperatures cause the cone to move downward, increasing bypass flow, and diverting less gas to the vaporizing chamber. The most important function of temperature compensation devices is to correct for the effect of evaporative cooling on the liquid anesthetic.

Variable bypass vaporizers are also constructed from materials with high specific heat, yielding temperature stability, and high thermal conductivity, which allows rapid transfer of ambient heat. Additionally, the vaporizer wick systems are located in contact with the metal walls to facilitate absorption of environmental heat.

Factors that Influence Variable Bypass Vaporizer Output

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. ISO standards state that the average output should not deviate from the dial setting by +30% or −20% or more than +7.5% or −5% of the maximum setting. Although modern vaporizers generally have excellent performance characteristics, it is important to understand how these challenges could potentially influence vaporizer output.

Impact of Gas Flow Rate

This factor is notable only at the extremes of flow rates and at higher concentration control dial settings. At low flow rates (<250 mL/min), the output tends to be slightly less than the dial setting due to the relatively high density of volatile anesthetic agents. Insufficient turbulence is generated in the vaporizing chamber to advance the vapor molecules upward. At high flow rates (such as 15 L/min), the output of most variable bypass vaporizers is somewhat less than the dial setting. This discrepancy is due to: cooling during rapid evaporation, incomplete mixing, and failure to saturate the carrier gas in the vaporizing chamber. In addition, the resistance characteristics of the bypass chamber and the vaporizing chamber can vary as flow increases.

Impact of Temperature Change

Despite the impact of evaporative cooling and variation in ambient conditions, modern vaporizers maintain fairly constant concentration output over a wide range of common working temperatures. However, the linear change in the temperature-compensating mechanisms does not precisely match the shape of the vapor pressure curves. As a result, a slight correlation between vaporizer temperature and delivered concentration remains. This correlation is mainly apparent at higher temperatures and higher concentrations ( Fig. 22.23 ). A dangerous but highly unlikely circumstance could occur if the boiling point of a volatile agent within a variable bypass vaporizer were reached. In this situation, the vaporizer output would be impossible to control by any compensatory mechanism. Although it would be rare to reach ambient temperatures around 50°C at sea level (see Table 22.1 ), at higher altitudes, where boiling points are lower, this is theoretically more likely. In fact, the Dräger Vapor 2000 user’s manual decreases the high-altitude operating specification for the vaporizer from 9880 to 4800 feet if halothane or isoflurane is used at higher ambient temperatures. Manufacturers’ published vaporizer operating temperatures range from 10°C to 40°C (50°F-104°F), although the specific ranges vary.

Impact of Intermittent Backpressure

The intermittent backpressure that results from either positive-pressure ventilation or use of the oxygen flush valve may lead to higher than expected vaporizer output. This phenomenon, known as the pumping effect , is more pronounced at low flow rates, low dial settings, and low levels of liquid anesthetic in the vaporizing chamber. Additionally, the pumping effect is increased by rapid respiratory rates, high peak inspiratory pressures (PIPs), the use of anesthesia machines without fresh gas decoupling, and rapid drops in pressure during expiration. Although contemporary variable bypass vaporizers are not highly vulnerable to the pumping effect, the proposed mechanism and preventative design features should be understood. The pumping effect is caused by retrograde transmission of pressure from the patient circuit to the vaporizer during the inspiratory phase of positive-pressure ventilation or use of the oxygen flush function. Gas molecules are compressed in both the bypass and vaporizing chambers. When the backpressure is suddenly released during the expiratory phase, vapor exits the vaporizing chamber both antegrade through the outlet and retrograde through the inlet. This occurs because the output resistance of the bypass chamber is lower than that of the vaporizing chamber, particularly at low dial settings. The enhanced output concentration results from the increment of vapor that travels in the retrograde direction to the bypass chamber.

To decrease the pumping effect, modern vaporizing chambers are smaller than those of early variable bypass vaporizers so that only a small volume of vapor can be discharged retrograde into the bypass chamber. Additionally, some vaporizers have a long spiral tube or labyrinth that serves as the inlet to the vaporizing chamber (see Fig. 22.19 ). When the pressure in the vaporizing chamber is released, the vapor does not flow back into the bypass chamber because of tube length. This serpentine passage also dampens pressure fluctuations and compensates for fluctuations in gas supply pressure. Other designs may also include an extensive baffle system in the vaporizing chamber. Finally, a one-way check valve can be inserted after the vaporizers and before the breathing circuit inlet to minimize the pumping effect (see the discussion of the gas supply system). This check valve can attenuate but does not eliminate the increase in pressure, because gas still flows from the flowmeters to the vaporizer during the inspiratory phase of positive-pressure ventilation. Although intermittent backpressure can result in transient rises in anesthetic concentration at the common gas outlet, the effects are mitigated by dilution within the much larger anesthetic breathing circuit. The goal of all these pressure-compensating mechanisms is to provide an even flow of gas through the vaporizing chamber despite changes in pressure, giving the vaporizers the additional designation pressure-compensated .

Impact of Carrier Gas Composition

Variable bypass vaporizer output can be influenced by fresh gas composition. This phenomenon is the result of differences in the solubility of carrier gases in volatile anesthetic liquids. This effect is most pronounced when nitrous oxide is introduced or removed as a carrier gas. In the experimental example seen in Fig. 22.24 , a change in carrier gas from 100% oxygen to 100% nitrous oxide results in a sudden decrease in halothane output (expressed as volume percent) followed by a slow increase to a new, lower, steady-state (see Fig. 22.24 , label B ). Because nitrous oxide is more soluble than oxygen in the liquid anesthetic within the vaporizer sump, more of the carrier gas dissolves, and the volume output from the vaporizing chamber is transiently reduced. Once the anesthetic liquid becomes saturated with nitrous oxide, the vaporizing chamber output increases and achieves a new steady state.

The explanation for the new steady-state output value is less well understood. Differences in density and viscosity between oxygen and nitrous oxide are likely responsible because these physical properties affect the relative amount of gas flow through the bypass and vaporizing channels. Helium, a gas with far lower density than either oxygen or nitrous oxide, has been shown to have variable effect on vaporizer output, depending on the vaporizer model and study design (although the changes tend to be minimal).

Although the carrier gas composition can be demonstrated experimentally to affect vaporizer output, deviations are often within specified accuracy ranges. Vaporizer user’s manuals usually specify the anticipated response to a change in carrier gas relative to the calibration gas, which may be air or oxygen, depending on the vaporizer model.

Impact of Barometric Pressure Changes

Understanding the influence of barometric pressure on variable bypass vaporizer output is probably more important for comprehending vaporizer function than for actual clinical reasons. With variable bypass vaporizers, the depth of anesthesia at a given dial setting is relatively independent of atmospheric pressure, and no adjustments need to be made ( Table 22.4 ).

Table 22.4

Comparative Performance of an Isoflurane Variable Bypass Vaporizer and the Tec 6 Desflurane Vaporizer During Changes in Barometric Pressure

Modified from Ehrenwerth J, Eisenkraft J. Anesthesia vaporizers. In: Ehrenwerth J, Eisenkraft J, eds. Anesthesia Equipment: Principles and Applications . St. Louis: Mosby; 1993:69–71.

		Isoflurane Variable Bypass Vaporizer With Dial Setting of 0.89% ( v / v )			Tec 6 Desflurane Vaporizer With Dial Setting of 6%
Atm	Ambient Pressure (mm Hg)	mL Isoflurane Vapor Entrained by 100 mL O 2	Vaporizer Isoflurane Output ( v / v %)	Vaporizer Isoflurane Output (mm Hg)	Vaporizer Desflurane Output (mm Hg)
0.66	500 (≈10,000 ft)	91	1.7	8.8	30
0.74	560 (≈8200 ft)	74	1.5	8	33.6
0.8	608 (≈6000 ft)	64	1.2	7.6	36.5
1.0	760 (sea level)	46	0.89	6.8	45.6
1.5 ∗	1140	26	0.5	5.9	68.4
2∗	1520	19	0.36	5.5	91.2
3∗	2280	12	0.23	5.2	136

atm, Atmospheres (1 atm = 760 mm Hg); v / v%, volume percent.

∗ ATA or atmospheres absolute . ATA = atmospheric pressure + water pressure . Hyperbaric oxygen chamber protocols apply ATA. Many protocols use depths from 2.0 to 2.5 ATA, but some conditions such as gas embolus or carbon monoxide poisoning may require depths to 3.0 ATA. 2 ATA ≈ 33 feet of sea water (fsw) ≈ 1520 mm Hg ambient pressure .

Higher Altitude

As previously discussed, vapor pressure is independent of barometric pressure. Therefore as altitude increases and barometric pressure declines, the partial pressure of anesthetic agent in the vaporizing chamber remains constant despite decreases in the partial pressures of other constituent breathing gases and the total ambient pressure. This situation results in significantly increased volume percent concentration of anesthetic agent within the vaporizing chamber and at the outlet of the vaporizer (see Table 22.4 ). However, because anesthetic depth is determined by the partial pressure of volatile agent in the brain, the clinical impact is minor (see MAPP in Table 22.1 ).

Let us consider an example of moving a vaporizer from sea level to higher altitude. With a constant dial setting of 0.89%, at 1 atm, a well-calibrated isoflurane variable bypass vaporizer would deliver 0.89 v / v % isoflurane, and the partial pressure of isoflurane output would be 6.8 mm Hg. Assume that we maintain the same dial setting and lower the atmospheric pressure to 0.66 atm or 502 mm Hg (roughly equivalent to an elevation of 10,000 feet. This results result in an increase in the isoflurane concentration output to 1.75% (a 97% increase), but the partial pressure increases to only 8.8 mm Hg (a 29% increase). A similar change in output partial pressure at sea level, in terms of volume percent, would correspond to an isoflurane concentration increase of only 0.2%. So while the anesthetic concentration ( v / v %) changes significantly in this example, it is the partial pressure of volatile agent in the brain that is ultimately responsible for anesthetic depth, and that change is minimal.

As described earlier, MAC values for contemporary inhalational anesthetic agents were determined at sea level. Similarly, anesthetic vaporizers are calibrated at sea level, thus ensuring that vaporizer output ( v / v %) matches the dial setting. Using the isoflurane data shown in Table 22.4 as an example, one can see how confusion may arise when thinking of MAC in terms of volume percent and considering barometric change.

The MAPP at altitude is the same as at sea level because it is a partial pressure, whereas the MAC increases because it is a simple concentration. Table 22.4 shows that the partial pressure output of a variable bypass vaporizer changes proportionally less than the volume percent concentration as altitude increases. Because the partial pressure of volatile agent determines anesthetic depth, the operator does not need to adjust the dial to a higher setting to compensate for barometric pressure. This holds true for variable bypass vaporizers, but not for the desflurane Tec 6–style vaporizer (see later).

Hyperbaric Conditions

Although anesthesia is sometimes delivered in hyperbaric conditions , intravenous anesthesia is easier to deliver in this setting. Under hyperbaric conditions, the partial pressure of volatile anesthetic in the vaporizing chamber remains constant despite an increase in ambient pressure and the partial pressure of the other gases. The net theoretical effects on variable bypass vaporizers are a significant decrease in anesthetic concentration ( v / v %) and a mild decrease in partial pressure output. However, the partial pressure of halothane was noted to increase slightly with increasing barometric pressure under experimental conditions. Possible explanations for this finding include the effect of increased atmospheric gas density on the flow of gas through the vaporizer and the increased thermal conductivity of air at higher pressure. The clinical significance of these small changes in partial pressure output under hyperbaric conditions is unclear.