vapor.pdf

3 Anesthesia Vaporizers JAMES B. EISENKRAFT CHAPTER OUTLINE substance. Critical temperature is defined as the temperature above which a gas cann...

3 Anesthesia Vaporizers JAMES B. EISENKRAFT CHAPTER OUTLINE substance. Critical temperature is defined as the temperature above which a gas cannot be liquefied by pressure alone. If the General Principles vapor is in contact with a liquid phase, the two phases will be Vapor, Evaporation, and Vapor Pressure in a state of equilibrium, and the gas pressure will equal the Measurement of Vapor Pressure and Saturated Vapor equilibrium vapor pressure of the liquid. !e potent inhaled Pressure volatile anesthetic agents (halothane, enflurane, isoflurane, Boiling Point sevoflurane, and desflurane) are mostly in liquid state at Units of Vapor Concentration normal room temperature (20°C) and atmospheric pressure.1 Dalton’s Law of Partial Pressures Anesthesia vaporizers are devices that facilitate the change of Minimum Alveolar Concentration a liquid anesthetic into its vapor phase and add a controlled Latent Heat of Vaporization amount of this vapor to the flow of gases that will enter the Specific Heat patient’s breathing circuit. Regulating Vaporizer Output !e anesthesia care provider should be familiar with Measured Flow the principles of vaporization of the potent inhaled Variable Bypass volatile anesthetic agents and their application in both the Efficiency and Temperature Compensation construction and use of anesthesia vaporizers designed to be Filling and Misfilling of Vaporizers placed in the low-pressure system of the anesthesia machine Effect of Changes in Fresh Gas Composition (i.e., the fresh gas flow circuit downstream of the gas flow Effects of Changes in Barometric Pressure control valves). !e 1989 and subsequent voluntary consensus Arrangement of Vaporizers standards for anesthesia machines and workstations require Calibration and Checking of Vaporizer Output that all vaporizers located within the fresh gas circuit be Preparation of a Standard Vapor Concentration concentration-calibrated and that control of the vapor concentration be provided by means of calibrated knobs or Effect of Use Variables on Vaporizer Function dials.2–3 Measured flow systems (Copper Kettle, Verni-Trol) Fresh Gas Flow Rate are not mentioned in current standards, and are therefore Fresh Gas Composition considered obsolete, as defined in the American Society of Temperature Anesthesiologists (ASA) 2004 statement on determining Fluctuating Back Pressure anesthesia machine obsolescence (https://www.asahq.org/-/ Contemporary Vaporizers media/sites/asahq/files/secure/resources/asa-committee- Dräger Vapor 19.n work-products-members-only/asa-publications-anesthesia- Dräger Vapor 2000 machine-obsolescence-20041.pdf. Accessed February 27, 2020). Dräger Vapor 3000 (see Chapter 23 Hazards of the Anesthesia Delivery System). GE-Datex-Ohmeda Tec 5 Despite their obsolescent status, the principles of measured GE-Datex-Ohmeda Tec 7 and Tec 850 flow vaporizing systems will be briefly discussed in this Limitations of Earlier Select-a-Tec Systems chapter, because they provide a basis for understanding the GE-Datex-Ohmeda Tec 6 (Desflurane) contemporary concentration-calibrated variable bypass Dräger D-Vapor (Desflurane) vaporizers used to deliver isoflurane, enflurane, halothane, Penlon Sigma Alpha and sevoflurane. GE Aladin Vaporizing System Desflurane has certain physical properties that preclude its Maquet FLOW-i delivery by a conventional variable bypass vaporizer, and its Appendix 3.1 delivery is therefore discussed in a separate section. Appendix 3.2 !e Aladin vaporizing system (GE) is a hybrid of the measured flow and variable bypass designs. !is system can accurately deliver desflurane and the four other less volatile potent anesthetic agents. !e most recently introduced vaporizing system is that used in General Principles the FLOW-i anesthesia workstation. In this vaporizer, measured !e term vapor describes the gaseous phase of a substance at amounts of liquid agent are heated (and thereby vaporized) and a temperature at which the same substance can also exist in added to a measured gas flow to obtain the desired concentration a liquid (or solid) state, below the critical temperature of that in the gas mixture that enters the breathing system.! 66  3 Anesthesia Vaporizers 67 Vaporized agent Vapor pressure SVP Liquid Liquid agent 760 mm 760 mm agent Fig. 3.1 Measurement of vapor pres- 1 atm sures using a simple Fortin barometer. SVP, Saturated vapor pressure. (See text for details.) (From Eisenkraft JB: Vaporiz- ers and vaporization of volatile anesthet- ics. In Progress in anesthesiology, vol 2. San Antonio, 1989, Dannemiller Memorial A B C Educational Foundation.) Vapor, Evaporation, and 1600 Desflurane Vapor Pressure 1400 Isoflurane Vapor pressure (mm Hg) Halothane When placed in a closed container at normal atmospheric 1200 Enflurane pressure (760 mm Hg) and room temperature (usually 20°C), a 1000 Sevoflurane potent inhaled anesthetic such as sevoflurane is in liquid form. Some sevoflurane molecules escape from the surface of the 800 liquid to enter the space above as a gas or vapor. At constant 600 temperature, an equilibrium is established between the molecules in the gas phase and those in the liquid phase. !e molecules in 400 the gas phase are in constant motion, bombarding the walls of the 200 container to exert a vapor pressure. An increase in temperature causes more sevoflurane molecules to enter the gas phase (i.e., to 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 evaporate), which results in an increase in vapor pressure. !e gas phase above the liquid is said to be saturated when it contains all Temperature °C the sevoflurane molecules that it can hold at a given temperature, Fig. 3.2 Vapor pressure curves for enflurane, halothane, isoflurane, at which point the pressure exerted by the sevoflurane vapor is sevoflurane, and desflurane. referred to as its saturated vapor pressure (SVP) at that temperature. vapor, and the pressure now exerted by the vapor is the SVP of isoflurane at that temperature. Adding more liquid isoflurane MEASUREMENT OF VAPOR PRESSURE AND will not a$ect the vapor pressure as long as temperature remains SATURATED VAPOR PRESSURE constant. If this experiment is repeated at di$erent temperatures, !e following description is intended to provide an understanding a graph can be constructed that plots SVP on the y-axis of how, in principle, the SVP of a potent inhaled volatile anesthetic against temperature on the x-axis. Such curves for some of the agent could be measured in a simple laboratory experiment. It will potent inhaled volatile anesthetic agents are shown in Fig. 3.2. also help to conceptualize the pressure that a vapor can exert. Fig. Contemporary technologies for measuring the partial pressures 3.1A shows a simple (Fortin) barometer, which is essentially a long or SVPs of gases and vapors are described in Chapter 8.! glass test tube that is filled with mercury and then inverted to stand vertically with its mouth immersed in a trough containing mercury. BOILING POINT When the barometer tube is first made vertical, the mercury column in the tube falls to a certain level, leaving a vacuum (the !e SVP exerted by the vapor phase of a potent inhaled volatile Torricellian vacuum) above the mercury meniscus. In this system, agent is a physical property of that agent and depends only on the the pressure at the surface of the mercury in the trough is that agent and the ambient temperature. !e temperature at which SVP due to the atmosphere. In a communicating system of liquids, the becomes equal to ambient (atmospheric) pressure and at which all pressures at any given depth are equal, therefore the pressure at the of the liquid agent changes to the gas phase (i.e., evaporates) is the surface of the mercury in the trough is equal to the pressure exerted boiling point of that liquid. Water boils at 100°C at one atmosphere by the column of mercury in the vertical tube. In this example, pressure because at 100°C the SVP of water is 760 mm Hg. !e atmospheric pressure is said to be equivalent to 760 mm Hg because most volatile of the anesthetic agents are those with the highest this is the height of the column of mercury in the barometer tube. SVPs at room temperature. At any given temperature, these agents In Fig. 3.1B, isoflurane liquid is introduced at the bottom of also have the lowest boiling points (e.g., desflurane and diethyl the mercury column and (being less dense than mercury) rises ether boil at 22.9°C and 35°C, respectively, at an ambient pressure to the top, where it then evaporates into the space created by of 760 mm Hg). Boiling point decreases with decreasing ambient the Torricellian vacuum. !e isoflurane vapor exerts a pressure, barometric pressure, such as occurs at high altitude.! causing a decrease in the height of the mercury column by an amount equal to the vapor pressure that it exerts. If one UNITS OF VAPOR CONCENTRATION continues to add liquid isoflurane until a small amount of liquid remains unevaporated on the top of the mercury meniscus (Fig. !e presence of anesthetic vapor may be quantified either in 3.1C), the space above the column must be fully saturated with (1) absolute terms, expressed in mm Hg [or kilopascals (kPa)], 68 PART 1 Gases and Ventilation or in mg per liter; or (2) volumes percent (vol%) of the total TABLE Expression of MAC as a Partial Pressure atmosphere (i.e., volumes of vapor per 100 volumes of total 3.1 gas). From Dalton’s law of partial pressures (see the following Agent MAC (vol%) PMAC1 (mm Hg) section), volumes percent can be calculated as the fractional Halothane 0.75 × 760 5.7 partial pressure of the agent; that is, Enflurane 1.68 × 760 12.8 P i Isoflurane 1.15 × 760 8.7 = % Methoxyflurane 0.16 × 760 1.2 i ! Sevoflurane 2.10 × 760 16 Desflurane 7.25 × 760 55 DALTON’S LAW OF PARTIAL PRESSURES Assumes an ambient pressure of 760 mm Hg. MAC, Minimum alveolar concentration; PMAC1, partial pressure of a potent inhaled agent at a Dalton’s law states that the pressure exerted by a mixture of concentration of 1 MAC. gases (or gases and vapors) enclosed in a given space (such as a container) is equal to the sum of the pressures that each gas Hg. Anesthesiologists should learn to think of MAC in terms of or vapor would exert if it alone occupied that given space (or partial pressure rather than in terms of volumes percent because container).4 A gas or vapor exerts its pressure independently it is the partial pressure (tension) or concentration (mg/liter) of the pressure of the other gases present. For example, in a of the anesthetic in the central nervous system that determines container of dry air at one atmosphere pressure (760 mm Hg) the depth of anesthesia. !is concept has been advocated by with oxygen representing 21% of all gases present, the pressure Fink,8 who proposed the term minimum alveolar pressure exerted by the oxygen (i.e., its partial pressure) is 21% % 760, (MAP), and James and White,9 who suggested minimum or 159.6 mm Hg. Consider the same air at a pressure of 760 alveolar partial pressure (MAPP). In a 2015 editorial, James et al. mm Hg but now fully saturated with water vapor at 37°C proposed that “all anesthesia systems—including gas/vapour (normal body temperature). Because vapor pressure depends analyzers—should be designed to display partial pressure and on temperature, the SVP of water at 37°C is 47 mm Hg. !e not concentration.”10 In this chapter, the term PMAC1 (see Table pressure due to oxygen is therefore now 21% of 713 (i.e., 3.1) is used to express the partial pressure of a potent inhaled 760−47) mm Hg. !e partial pressure of oxygen is therefore anesthetic agent at a concentration of 1 MAC. !us, 1 MAC of 149.7 mm Hg. isoflurane is equivalent to a PMAC1 of 8.7 mm Hg (see Table 3.1).! If the concentration of an anesthetic agent in a gas mixture is known in volumes percent, it may be converted to the mg/ LATENT HEAT OF VAPORIZATION liter equivalent using the following formula (see appendix for derivation)5: Vaporization requires energy to transform molecules from the liquid phase to the gas phase. !is energy is called the latent heat of = (C ) / (F 2 2) /i vaporization and is defined as the amount of heat (calories) required where W = concentration in mg/liter to convert a unit mass (grams) of liquid into vapor. For example, at C = concentration in volumes percent 20°C the latent heat of vaporization of isoflurane is 41 cal/g. !e MW = molecular weight of the anesthetic agent heat of vaporization is inversely related to ambient temperature in F = temperature-barometric factor such a way that at lower temperatures, more heat is required for Where F = (760/p) % [1+(t−20)/273] vaporization. !e heat required to vaporize an anesthetic agent is where t = temperature in °C drawn from the remaining liquid agent and from the surroundings. p = barometric pressure in mm Hg. As vapor is generated and heat energy is lost, the temperatures of For example, to express 2% sevoflurane in units of mg per the vaporizer and the liquid agent decrease. !is causes the vapor liter at 1 atmosphere pressure and a temperature of 20°C: pressure of the anesthetic to decrease and, if no compensatory = (2 2 )/( 2 2) = /i mechanism is provided, will result in decreased output of vapor. !e temperature compensation mechanisms used in vaporizers are Note that volumes percent expresses the relative ratio or described in a later section.! proportion (%) of gas molecules in a mixture, whereas partial pressure (mm Hg or kPa) or mg per liter represents an absolute SPECIFIC HEAT value. Anesthetic uptake and potency are related directly to partial pressure or mg per liter and only indirectly to volumes Specific heat is the quantity of heat (calories) required to raise the percent. !is distinction will become more apparent when temperature of a unit mass (grams) of a substance by one degree of hyperbaric and hypobaric conditions are considered.! temperature (1°C).4 Heat must be supplied to the liquid anesthetic in the vaporizer in order to maintain the liquid’s temperature during the evaporation process, when heat is being lost. MINIMUM ALVEOLAR CONCENTRATION Specific heat is also important when it comes to vaporizer !e minimum alveolar concentration (MAC) of a potent construction material. Temperature changes are more gradual inhaled anesthetic agent is the concentration that produces for materials with a high specific heat than for those with a immobility in 50% of patients undergoing a standard surgical low specific heat, for the same amount of heat lost through stimulus.6,7 Used as a measure of anesthetic potency or depth, vaporization. !ermal capacity, defined as the product of MAC is commonly expressed as volumes percent of alveolar specific heat and mass, represents the quantity of heat stored in (end-tidal) gas at one atmosphere pressure at sea level (i.e., the vaporizer body. 760 mm Hg). Table 3.1 shows how MAC expressed in familiar Also of importance is the material that is used in the construction volumes percent can be expressed as a partial pressure in mm of the vaporizer. Choosing the proper material allows the vaporizer  3 Anesthesia Vaporizers 69 TABLE 3.2 Physical Properties of Potent Inhaled Volatile Agents Agents Halothane Enflurane Isoflurane Methoxyflurane Sevoflurane Desflurane Structure CHBrClCF3 CHFClCF2OCHF2 CF2HOCHClCF3 CHCl2CF2OCH3 CH2FOCH(CF3)2 CF2HOCFHCF3 Molecular weight (AMU) 197.4 184.5 184.5 165.0 200 168 Boiling point at 760 mm 50.2 56.5 48.5 104.7 58.5 22.8 Hg (°C) SVP at 20°C (mm Hg) 243 175 238 20.3 160 664 SVC at 20°C and 1 ATAa 32 23 31 2.7 21 87 (vol%) MAC at 1 ATAa (vol%) 0.75 1.68 1.15 0.16 2.10 6–7.25b PMAC1 (mm Hg) 5.7 12.8 8.7 1.22 16 46–55b Specific gravity of liquid 1.86 1.52 1.50 1.41 1.51 1.45 at 20°C mL vapor per gram 123 130 130 145 120 143 liquid at 20°C mL vapor per mL liquid 226 196 195 204 182 207 at 20°C AMU, Atomic mass units; conc., concentration; MAC, minimum alveolar concentration; PMAC1, partial pressure of a potent inhaled agent at a concentration of 1 MAC; SVC, saturated vapor concentration; SVP, saturated vapor pressure. a1 ATA = one atmosphere absolute pressure (760 mm Hg) bAge related Temperature gauge (discussed earlier). !e saturated vapor concentrations of halothane, sevoflurane, and isoflurane are therefore 32%, 21%, and 31%, respectively. !ese concentrations are far in excess of those required clinically (see Fig. 3.2; Table 3.2): therefore, the vaporizer first creates a saturated vapor in equilibrium with Saturated vapor the liquid agent, and second, the saturated vapor is diluted by a bypass gas flow. !is results in clinically safe and useful concentrations flowing to the patient’s breathing circuit. Without this dilution of saturated vapor, the agent would be delivered in a lethal concentration to the anesthesia circuit. Copper Kettle Main flowmeters In the (now obsolete) measured flow (i.e., non–concentration- O2 flowmeter calibrated) vaporizers such as the Copper Kettle (Foregger/ Puritan Bennett) or Verni-Trol (Ohio Medical Products), a To machine measured flow of oxygen is set on a dedicated oxygen flowmeter common gas outlet to enter the vaporizer, from which vapor emerges at its saturated Fig. 3.3 Schematic of measured flow vaporizing arrangement. These vapor concentration (Fig. 3.3). !is flow is then diluted by an are ‘’bubble-through’’ vaporizers. A separate flowmeter controls oxygen flow to the Copper Kettle where it bubbles through liquid agent and additional measured flow of gases (i.e., oxygen, nitrous oxide, saturated vapor is created above the liquid. A thermometer is provided air, etc.) from the main flowmeters on the anesthesia machine so that the user can adjust for saturated vapor pressure changes with (see Fig. 3.3). With this type of arrangement, calculations are temperature changes (see Fig. 3.2). necessary to determine the anesthetic vapor concentration in the emerging gas mixture that flows to the patient’s breathing circuit. to conduct heat from the environment to the liquid anesthetic. Contemporary anesthesia vaporizers are concentration- !is property, called thermal conductivity, is defined as the rate calibrated and most are of the variable bypass design. In a variable at which heat is transmitted through a substance. In order for the bypass vaporizer (e.g., Tec series from GE, Dräger Vapor 19.1, liquid anesthetic to remain at a relatively constant temperature, the 2000 and 3000 series from Dräger) the total fresh gas flow from traditional variable bypass vaporizer is constructed from materials the anesthesia machine flowmeters (or electronic flow control that have a high specific heat and high thermal conductivity. In this system) enters the vaporizer (Fig. 3.4). !e incoming gas flow is respect, copper comes close to the ideal (hence the Copper Kettle split between two pathways. A smaller flow enters the vaporizing vaporizer). More recently, bronze and stainless steel have been used chamber (or sump) of the vaporizer and emerges with the anesthetic in vaporizer construction.!! agent at its saturated vapor concentration. A larger flow bypasses the vaporizing chamber and eventually mixes with the outflow from the vaporizing chamber to create the desired or ‘’dialed-in’’ Regulating Vaporizer Output concentration (see Fig. 3.4) that flows to the patient circuit. MEASURED FLOW With both types of vaporizing systems, there must be an e(cient method to create a saturated vapor in the vaporizing !e SVPs of halothane, sevoflurane, and isoflurane at room chamber. !is is achieved by having a large surface area for temperature are 243 mm Hg, 160 mm Hg, and 241 mm Hg, evaporation. Flow-over vaporizers (e.g., Dräger Vapor 2000, respectively. Dividing the SVP by ambient pressure (760 mm 3000 series and GE Tec series vaporizers) increase the surface Hg) gives the saturated vapor concentration as a percentage area using wicks and ba)es. In the (now obsolete) measured of one atmosphere. !is is an application of Dalton’s law flow, bubble-through vaporizers (i.e., Copper Kettle and 70 PART 1 Gases and Ventilation Concentration control system Vaporizer outflow Fresh gas inflow at dialed-in concentration Bypass flow Concentration control system Vaporizing chamber flow Fig. 3.4 Schematic of concentration-calibrated variable bypass design of vaporizer. Fresh gas enters the vaporizer where its flow is split between a larger bypass flow and a smaller flow to the vaporizing chamber or sump. In the sump is liquid agent above which is saturated vapor. Saturated va- Liquid agent in vaporizer sump por mixes with the bypass flow which dilutes it to the con- centration dial setting. Verni-Trol), oxygen is bubbled through the liquid agent. In For sevoflurane in the previous example, y = 100 mL/min; order to increase the surface area, tiny bubbles are created therefore, by passing the oxygen through a sintered bronze disc (in the / = / Copper Kettle). !is creates large areas of liquid-gas interface over which evaporation of the liquid agent can rapidly occur. from which x can be calculated to be 27 mL (rounded to the nearest whole number). Calculation of Vaporizer Output Conversely, if x is known, the carrier gas flow y can be calculated. A concept fundamental to understanding vaporizer function is At steady state, the total volume of gas leaving the vaporizing that under steady state conditions, if a certain volume of carrier chamber is greater than the total volume that entered, the additional gas flows into a vaporizing chamber over a certain period of volume being anesthetic vapor at its saturated vapor concentration. time, that same volume of carrier gas exits the chamber over the same period of time. However, due to the addition of vaporized Measured Flow Vaporizers (Copper Kettle, Verni-Trol) anesthetic agent, the total volume exiting the vaporizing chamber Although measured flow vaporizers are not mentioned in is greater than that entering. In the vaporizing chamber, anesthetic anesthesia machine standards published after 1988 and are vapor at its SVP constitutes a mandatory fractional volume of the considered to be obsolete, it is helpful to first review the atmosphere (i.e., 21% in a sevoflurane vaporizer at 20°C and 760 function of one such vaporizer, the Copper Kettle. Suppose mm Hg ambient pressure). !erefore, the volume of carrier gas that one wishes to deliver 1% (vol/vol) isoflurane to the patient will constitute the di$erence between 100% of the atmosphere in circuit at a total fresh-gas flow rate of 5 L/min (Fig. 3.5). !is the vaporizing chamber and that due to the anesthetic vapor. In requires the vaporizer to evolve 50 mL of isoflurane vapor per the case of sevoflurane (at 20°C and 760 mm Hg pressure) the minute (1% % 5000 mL) to be diluted in a total volume of carrier gas represents, at any time, 79% of the atmosphere in 5000 mL. the vaporizing chamber. !us if 100 mL of carrier gas flows per In the Copper Kettle, isoflurane represents 31% of the minute through a vaporizing chamber containing sevoflurane, atmosphere, assuming a constant temperature of 20°C and carrier gas represents 79% (100%−21%) of the atmosphere, and a constant SVP of 238 mm Hg. If 50 mL of isoflurane vapor the remaining 21% is sevoflurane vapor. By simple proportions, represents 31%, the carrier gas (oxygen) must represent the the volume of sevoflurane vapor exiting can be calculated to be 27 other 69% (100%−31%). !us mL [(100/79) % 21], when rounded to the nearest whole number. In other words, if 100 mL of carrier gas flows into the = vaporizing chamber per minute, the same 100 mL of carrier gas will emerge together with 27 mL of sevoflurane vapor per minute (a total of 127 mL). = = Another way of expressing this is, P ( H ) !erefore, if 111 mL/min of oxygen is bubbled through ( H ) liquid isoflurane in a Copper Kettle vaporizer, 161 mL/min of gas emerges, of which 50 mL is isoflurane vapor and 111 A ( ) mL is the oxygen that flowed into the vaporizer. !is vaporizer C i ( ) A ( ) output of 161 mL/min must be diluted by an additional fresh gas flow of 4839 (i.e., 5000−161) mL/min to create an isoflurane mixture of exactly 1% (because 50 mL of isoflurane vapor diluted in a total volume of 5000 mL gives 1% isoflurane i i by volume).  3 Anesthesia Vaporizers 71 Creating 1% isoflurane in a total flow of 5 L/min with a Copper Kettle at 20°C Temperature gauge Saturated (31%) 161 mL/min of 20°C iso vapor 31% iso in oxygen 50 mL/min saturated iso vapor Copper Kettle Main flowmeters: O2 flowmeter: 4839 mL/min 111 mL/min Liquid isoflurane in sump Fig. 3.5 Preparation of 1% isoflurane (iso) by volume using a measured flow (Copper Kettle (CK)) vaporizing system. 1% iso in a 5 L/min flow requires 50 mL/min iso vapor diluted in 1% = 50 mL iso vapor/5000 mL total flow a total volume of 4950 mL fresh gas + 50 mL isoflurane vapor. In Copper Kettle, iso is 31% by volume at 20ºC Iso saturated vapor concentration is 31%. If 31% = 50 mL, then If 50 mL = 31%, flow to CK must be (50/31) × 69 = 111 mL/min 69% = 111 mL/min is the required inflow, and 4839 mL/min Main gas flow = (5000 – 50 – 111) = 4839 mL/min (4950−111) is the required bypass flow. Final dilution is 1% [50/ (50+4839+111)]. A B Fig. 3.6 (A) Copper Kettle vaporizing system. The oxygen flowmeter knob on the extreme left is marked ‘’C-K’’ to indicate that it controls oxygen flow to the Copper Kettle. (B) Close up of Copper Kettle. Although this situation is highly unlikely to occur in Halothane and isoflurane have similar SVPs at 20°C (see contemporary practice (because of the obsolescence of measured Table 3.2), therefore the gas flows to be set for halothane would flow vaporizers), if one had to use a measured flow system to be essentially the same as those to be set for isoflurane when deliver isoflurane, the anesthesia provider would likely set flows a 1% concentration of isoflurane is to be produced from a of 100 mL/min oxygen to the Copper Kettle and 5 L/min of fresh Copper Kettle. A Copper Kettle arrangement on an older model gas on the main flowmeters, which results in very slightly less anesthesia machine is shown in Fig. 3.6. than 1% isoflurane (actually 44.9/5044.9 = 0.89%). Multiples of Enflurane and sevoflurane have similar vapor pressures at either of the vaporizer oxygen flow and main gas flowmeter flows 20°C (175 mm Hg and 160 mm Hg, respectively), therefore would be used to create other concentrations of isoflurane from similar flow settings could be used to create approximately the the Copper Kettle. !us a 200 mL/min oxygen flow to the Copper same agent concentrations with a measured flow system. In Kettle vaporizer and 5 L/min on the main flow meters would the case of sevoflurane, the measured flow vaporizer would create approximately 1.8% isoflurane. It is important to realize contain 21% sevoflurane vapor (160/760 = 21%) (Fig. 3.7). !e that if there is oxygen flow only to the Copper Kettle vaporizer oxygen flow therefore represents the remaining 79% of the and no bypass gas flow is set on the main machine flowmeters, atmosphere in the Copper Kettle. If precisely 1% sevoflurane lethal concentrations (approaching 31%) of isoflurane would be is required at a 5 L/min total rate of flow, 50 mL/min of delivered to the anesthesia circuit, albeit at low flow rates. sevoflurane vapor needs to be generated. If 50 mL represents 72 PART 1 Gases and Ventilation Creating 1% sevoflurane in a total flow of 5 L/min with a Copper Kettle at 20°C Temperature gauge Saturated (21%) 238 mL/min of 20°C sevo vapor 21% sevo in oxygen 50 mL/min saturated sevo vapor Copper Kettle Main flowmeters: O2 flowmeter: 4762 mL/min 188 mL/min Liquid sevoflurane in sump Fig. 3.7 Preparation of 1% sevoflurane (sevo) by volume us- 5 L/min of ing a measured flow (Copper Kettle) vaporizing system. 1% 1% sevo sevo in a 5 L/min flow requires 50 mL/min sevo vapor diluted in a total volume of 4950 mL fresh gas + 50 mL sevo vapor. 1% = 50 mL sevo vapor/5000 mL total flow Sevo saturated vapor concentration is 21%. If 21% = 50 mL, In Copper Kettle, sevo is 21% by volume at 20ºC then 79% = 188 mL/min is the required inflow, and 4762 mL/ If 50 mL = 21%, flow to CK must be (50/21) × 79 = 188 mL/min min (4950−188) is the required bypass flow. Final dilution is Main gas flow = (5000 – 50 – 188) = 4762 mL/min 1% [50/(50+4762+188)]. 21% of the atmosphere in the vaporizer, the carrier gas flow required splitting ratio may be achieved either (1) by a control required is 188 mL/min [(50/21) % 79]. valve located upstream of the vaporizing chamber (i.e., where !us if 188 mL/min of oxygen are bubbled through liquid fresh gas enters the vaporizer) that controls the gas flow sevoflurane contained in a Copper Kettle vaporizer, 238 mL/ entering the vaporizing chamber (somewhat analogous to min of gas will emerge, 50 mL/min of which is sevoflurane the measured flow vaporizing systems); or (2) by a control vapor. !is must be diluted by a fresh gas flow of 4742 mL/min valve located downstream of the vaporizing chamber that (5000−238) to achieve exactly 1% sevoflurane. proportions the flow of saturated vapor leaving the vaporizing Alternatively, using the formula given previously, chamber to mix with the bypass flow. Some older models of vaporizer were designed to create the flow split upstream of = the vaporizing chamber (e.g., Fig. 3.8). In all contemporary variable bypass vaporizers the flow split is achieved by a control downstream of the vaporizing chamber (Fig. 3.9). For = / i all practical purposes, provided that the vaporizer is used at where y = oxygen flow to the Copper Kettle vaporizer. the ambient pressure at which it was calibrated (e.g., ∼760 mm Setting an oxygen flow of 200 mL/min to the vaporizer and Hg; sea level), it should make no di$erence whether the flow 5 L/min on the main flowmeters would result in a sevoflurane split is achieved by a control upstream or downstream of the concentration of 1.01% [(200/79 % 21)/5253.2]. vaporizing chamber. If, however, a vaporizer calibrated for In the preceding examples, it was necessary to calculate use at ∼760 mm Hg ambient pressure is used under hyperbaric both the oxygen flow to the measured flow vaporizer and or hypobaric conditions, then the upstream location of the the total main gas flow needed to produce the desired output control valve will have a greater e$ect on performance. See concentrations of vapor. This was not only inconvenient, later section on use of vaporizers under hypo- or hyperbaric but also predisposed to errors that might result in serious conditions. overdose or underdose of anesthetic. Because of the obvious Fig. 3.10 depicts a variable bypass sevoflurane vaporizer set potential for error with a measured flow vaporizing system, to deliver 1% sevoflurane at 20°C and 1 atmosphere pressure. if one were forced to use such a system, the concurrent A gas flow of 2079 mL/min entering the vaporizer must be continuous use of an anesthetic agent analyzer with high- split so that 79 mL enters the vaporizing chamber (where and low-concentration alarms would be necessary to ensure it constitutes 79% of the atmosphere because sevoflurane patient safety.11! vapor constitutes an obligatory 21%) and 2000 mL enters the bypass. Emerging from the vaporizing chamber will be 21 mL VARIABLE BYPASS sevoflurane and 79 mL carrier gas. The 21 mL of sevoflurane vapor is diluted in 2100 mL (2000+79+21), producing 1% In the concentration-calibrated variable bypass design of sevoflurane. This results in a splitting ratio of 25:1 (2000/79) vaporizer, the total flow of gas arriving from the anesthesia between the flow entering the bypass and the flow entering machine flowmeters is split between a bypass and the the vaporizing chamber. A variable bypass vaporizer (e.g., vaporizing chamber containing the anesthetic agent (see Fig. Dräger Vapor 2000) set to deliver 1% sevoflurane is therefore 3.4). !e ratio of these two flows, the splitting ratio, depends effectively set to a splitting ratio of 25:1 (see Fig. 3.10) for the on the anesthetic agent, temperature, and chosen vapor inflowing fresh gas. (The term effectively is used because the concentration set to be delivered to the patient circuit. !e flow split calculated for the incoming gas is really the result  3 Anesthesia Vaporizers 73 Bypass Check Filter valve valve 1 Temperature- compensating 2 bypass valve 3 Concentration control dial (relocated for illustration purposes) Fig. 3.8 Schematic of Ohio Calibrated Vaporizer. Tem- Vaporizing chamber perature compensation is achieved by a gas-filled tempera- Temperature- ture sensing bellows that controls the size of a temperature sensing bellows compensating bypass valve. Note also the check valve in Agent the vaporizer outlet, designed to protect against the pump- ing effect. Note also that in this older model vaporizer, the flow splitting control is upstream of the vaporizing chamber. (Courtesy GE Healthcare, Madison, WI.) of the concentration control system acting downstream of the flowmeters at 3069 mL/min and is split such that 3000 mL/ vaporizing chamber where it actually proportions the flow of min enters the bypass while 69 mL/min enters the vaporizing saturated vapor exiting the vaporizing chamber to mix with chamber, when the gas flows merge, 1% isoflurane is the the bypass flow.) result. The inflow-splitting ratio is therefore 44:1 (3000/69) An alternative approach is to consider the gas mixture (see Fig. 3.11). exiting the vaporizing chamber. Consider, for example, 100 !e flow split of 44:1 calculated for the inflowing gas is mL/min of gas exiting the vaporizing chamber. !is 100 mL really the result of the concentration control system acting comprises 21 mL of sevoflurane vapor and 79 mL of carrier gas. downstream of the vaporizing chamber where it proportions the To create 1% sevoflurane by volume, the 21 mL of sevoflurane flow of gas exiting the vaporizing chamber that mixes with the vapor must be diluted in a total volume of 2100 mL (because bypass flow. Assume that 100 mL/min of gas exits the vaporizing 21/2100 = 1%). !e vaporizer control system acting downstream chamber. !is 100 mL comprises 31 mL of isoflurane vapor and of the vaporizing chamber is therefore creating a flow ratio of 69 mL of carrier gas. To create 1% isoflurane by volume, 31 mL 2000:100, or 20:1 between the bypass flow and the flow exiting of isoflurane vapor must be diluted in a total of 3100 mL. !e the vaporizing chamber. vaporizer control system has therefore created a flow ratio of To summarize, when a contemporary variable bypass 3000:100, or 30:1 between the bypass flow and the flow exiting vaporizer is set to deliver 1% sevoflurane at 20°C, the control the vaporizing chamber. valve located downstream of the vaporizing chamber is creating To summarize, when a contemporary variable bypass a flow proportion of 20:1 between the bypass flow and the flow isoflurane vaporizer is set to deliver 1% isoflurane at 20°C, the exiting the vaporizing chamber, thereby effectively creating a control valve located downstream of the vaporizing chamber flow split of 25:1 (2000/79) between the bypass flow and the is creating a flow proportion of 30:1 between the bypass flow inflow to the vaporizing chamber. and the flow exiting the vaporizing chamber, thereby effectively As another example, consider a concentration-calibrated, creating a flow proportion of 44:1 (3000/69) between the bypass variable bypass, isoflurane vaporizer set to deliver 1% flow and the inflow to the vaporizing chamber. isoflurane (Fig. 3.11). What splitting ratio for incoming gases Table 3.3A shows the inflowing gas splitting ratios for variable does this vaporizer achieve? The SVP of isoflurane at 20°C is bypass vaporizers used at 20°C. An equation for the calculation 238 mm Hg, therefore the concentration of isoflurane vapor of inflowing gas splitting ratios is provided in the Appendix to in the vaporizing chamber is 31% (238/760). If carrier gas this chapter. enters the vaporizing chamber (where it now constitutes 69% Table 3.3B shows the flow ratios between the bypass gas of the atmosphere by volume, the other 31% being isoflurane flow and the flow of saturated vapor exiting the vaporizing vapor) at a rate of 69 mL/min, isoflurane vapor emerges at chamber. 31 mL/min and must be diluted in 3100 mL/min of total gas !e concentration-calibrated vaporizer is agent-specific flow to produce a 1% concentration (since 31/3100 = 1%). and must be used only with the agent for which the device Thus if carrier gas enters the vaporizer from the machine is designed and calibrated. In order to produce a 1% vapor  Rotary valve Flow control 3 Shut-off 4 7 (vapor 10 open channel) Vaporizing chamber From To common flowmeter gas outlet 1 10 11 a b Thermostat c d e A Rotary valve On Enriched fresh gas out Vapor 11 control channel e 3 a 1 Combined fresh gas and 2 enriched 10 gas out Fresh gas bypass 4 d 9 Fresh gas out 7 5 6 Wick assembly 8 b 2 Vaporizing c chamber B Thermostat Fig. 3.9 (A) Tec 5 vaporizer flow diagram. (B) Schematic of Tec 5 vaporizer. Gas flow enters the vaporizer at 1, where it is split into two streams, the bypass circuit and the vaporizing chamber. Gas flows through the bypass circuit vertically downward from a across the sump base b, through the ther- mostat to c, and back up the gas transfer manifold via d to e. Gas flowing to the vaporizing chamber flows from 1 across the sump cover (2), where it is diverted via 3 through the central cavity of the rotary valve and back through the IPPV assembly via 4, 5, and 6. Gas then flows from the IPPV assembly via 7 down the tubular wick assembly, where vapor is added, and then flows across the base of the vaporizing chamber above the liquid agent to 8. From here the gas-vapor mixture flows via 9 through the sump cover to the proportional radial drug control groove of the rotary valve and back into the sump cover (10), where it merges with gas from the bypass circuit. The total flow then exits the vaporizer into the outlet port of the Select-a-Tec manifold. IPPV, Intermittent positive pressure ventilation. (Courtesy GE Healthcare, Waukesha, WI.)  3 Anesthesia Vaporizers 75 Variable bypass vaporizer delivering 1% sevo at 2 L/min at 20°C Concentration control system Vaporizer outflow Fresh gas inflow mL/min Bypass flow at dialed-in concentration 2079 2000 Concentration Vaporizing control system 79 chamber flow 21% sevo vapor 100 mL/min Sevo vapor Fig. 3.10 Preparation of 1% (vol/vol) sevoflurane (sevo) in a variable bypass vaporizer. The 2079 mL/min flow enter- ing the vaporizer is split such that 2000 enters the bypass Sevoflurane liquid in sump and 79 enters the vaporizing chamber. In the vaporizing chamber is 21% sevo vapor at 20°C. Leaving the vaporizing chamber/min is 21 mL sevo vapor and 79 mL carrier gas. 21 In the sump, sevo is 21% by volume at 20ºC. mL sevo vapor is diluted in 2100 mL (2000+79+21) result- If sump inflow is 79 mL/min, then outflow is 21 mL sevo vapor + 79 mL carrier gas. ing in 1% sevo at the vaporizer outlet. To achieve this, a To create 1% sevo, 21 mL vapor is diluted in total volume of 2100 mL, splitting ratio of 2000/79 or 25:1 (bypass flow: vaporizing therefore bypass flow = (2100 – 79 – 21) = 2000 mL. chamber flow) is created for inflowing gas. This is the result 2079 mL gas entering the vaporizer are split 2000:79 between bypass and sump. of the concentration control creating a flow ratio of 20:1 Splitting ratio = 2000/79 = 25:1 between bypass and sump inflow between the bypass flow and the flow exiting the vapor- izing chamber. Variable bypass vaporizer delivering 1% iso at 3 L/min Concentration control system Vaporizer outflow Fresh gas inflow mL/min Bypass flow at dialed-in concentration 3069 3000 Concentration Vaporizing control system chamber flow 69 31% iso vapor 100 mL/min iso vapor Isoflurane liquid in sump In the sump, iso is 31% by volume at 20ºC. If sump inflow is 69 mL/min, then outflow is 31 mL iso vapor + 69 mL carrier gas. Fig. 3.11 Preparation of 1% (vol/vol) isoflurane (iso) by a To create 1% iso, 31 mL vapor is diluted in total volume of 3100 mL, variable bypass vaporizer. To achieve this a splitting ratio of therefore bypass flow = (3100 – 69 – 31) = 3000 mL. 3069:69 or 44:1 (bypass flow: vaporizing chamber flow) is cre- 3069 mL gas entering the vaporizer are split 3000:69 between bypass and sump. ated for incoming gas. This is the result of the concentration Splitting ratio = 3000/69 = 44:1 between bypass and sump inflow control creating a flow ratio of 30:1 between the bypass flow and the flow exiting the vaporizing chamber. TABLE 3.3A Vaporizer Inflowing Gas Splitting Ratios at 20°C Halothane Enflurane Isoflurane Methoxyflurane Sevoflurane 1% 46:1 29:1 44:1 1.7:1 25:1 2% 22:1 14:1 21:1 0.36:1 12:1 3% 14:1 9:1 14:1 –a 7:1 (Ratio of flow entering bypass/flow entering vaporizing chamber) aMaximum possible is 2.7% at 20° C (see Table 3.2). 76 PART 1 Gases and Ventilation TABLE Vaporizer Outflowing Gas Splitting Ratios 3.3B at 20°C Dial Setting Isoflurane Sevoflurane 1% 30:1 20:1 2% 14.5:1 9.5:1 3% 9.33:1 6:1 4% 6.75:1 4.25:1 (Bypass flow/Flow exiting vaporizing chamber) concentration, an isoflurane vaporizer makes an inflowing gas flow split of 44:1, whereas a sevoflurane vaporizer makes a flow split of 25:1 (see Table 3.3A). If an empty sevoflurane vaporizer set to deliver 1% were filled with isoflurane, the concentration of the isoflurane vapor emerging would be in excess of 1%. Understanding splitting ratios enables prediction of the concentration output of an empty agent- specific variable bypass vaporizer that has been erroneously filled with an agent for which it was not designed. !e change in concentration output when one-agent specific vaporizer is filled with a di$erent agent can be calculated as the concentration set on the vaporizer dial multiplied by the Fig. 3.12 Dräger Vapor vaporizer. Temperature compensation is ratio of the exit ratios (which is also the ratio of the SVPs of achieved by reading the temperature from the thermometer and then the two agents).12 In the case of the sevoflurane vaporizer set using the control dial to align the desired output concentration (slant- to deliver 1% sevoflurane (approximately 0.5 MAC) but filled ing) line with the marking for ambient temperature. with isoflurane, the resulting isoflurane concentration will be [1 % (30/20)], or 1.5% isoflurane (approximately 1.3 MAC). the prevailing temperature. Such an arrangement, while !is can be potentially dangerous as the vaporizer is delivering tedious, does ensure the most accurate and rapid temperature 2.6 times the anesthetic potency that the user intended (1.3 compensation. !e original Dräger Vapor vaporizer (Fig. MAC/0.5 MAC). 3.12) (to be distinguished from the more recent Vapor 2000 In summary, if a variable bypass vaporizer that has been and 3000 models fitted to contemporary Dräger anesthesia calibrated for agent A and set to deliver a concentration C% workstations) is a variable bypass vaporizer that incorporates is filled with agent B, the concentration C’ delivered by that a thermometer and a grid of lines on the vaporizer control misfilled vaporizer is calculated as: dial for temperature compensation, whereby the desired output concentration is matched to the temperature of the C = C ( PB / PA )! liquid agent. Turning the control dial changes the size of an orifice in the bypass flow. Most of the contemporary variable bypass vaporizers (e.g., GE-Datex-Ohmeda Tec series, Dräger 2000 and 3000) achieve EFFICIENCY AND TEMPERATURE automatic temperature compensation via a temperature- COMPENSATION sensitive valve in the bypass gas flow. When temperature Agent-specific concentration-calibrated vaporizers must increases, the valve in the bypass opens wider to create a higher be located in the fresh gas path between the flowmeter splitting ratio. More gas flows through the bypass, and less gas manifold outlet and the common gas outlet on the anesthesia enters the vaporizing chamber. A smaller volume of a higher workstation.3 !e vaporizers must be capable of accepting a concentration of vapor emerges from the vaporizing chamber. total gas flow of 15 L/min from the machine flowmeters and !is vapor, when mixed with an increased bypass gas flow, of delivering a predictable concentration of vapor.3 However, maintains the vaporizer’s output at reasonable constancy when as the agent is vaporized and the temperature falls, SVP also temperature changes are not extreme. falls. In the case of a measured flow vaporizer (e.g., Copper Temperature-sensitive valves have evolved in design among Kettle) or an uncompensated variable bypass vaporizer, this the di$erent types of vaporizers. Some older vaporizers (e.g., results in delivery of less anesthetic vapor to the patient circuit. Ohio Calibrated Vaporizer) had, in the vaporizing chamber, a For this reason, all vaporizing systems must be temperature- gas-filled bellows linked to a valve in the bypass gas flow (see compensated, either manually (with a Copper Kettle) or, as in Fig. 3.8).13 As the temperature increases, the bellows expands, contemporary vaporizers, automatically. causing the valve to open wider. Contemporary GE Tec series Measured flow vaporizers (e.g., Copper Kettle, Verni-Trol) vaporizers (see Fig. 3.9) use a bimetallic strip for temperature incorporate a thermometer that measures the temperature compensation.14 !is strip is incorporated into a flap valve in of the liquid agent in the vaporizing chamber (Fig. 3.6B). the bypass gas flow. !e valve is composed of two metals each A higher temperature translates to a higher SVP in this having a di$erent coe(cient of expansion (change in length chamber. Reference to the vapor pressure curves (see Fig. per unit length per unit change in temperature). Nickel and 3.2) enables a resetting of either oxygen flow to the vaporizer, brass have been used in bimetallic strip valves, brass having a or the bypass gas flow, or both, to ensure correct output at greater coe(cient of expansion than nickel. As the temperature  3 Anesthesia Vaporizers 77 3 TABLE Output in Volumes Percent and MAC 3.4 (in Oxygen) of Erroneously Filled Vaporizers 2 at 22°C Output Vaporizers Liquid Setting (%) Output (%) MAC 1 10 Halothane Halothane 1.0 1.00 1.25 Enflurane 1.0 0.62 0.37 Isoflurane 1.0 0.96 0.84 7 Enflurane Enflurane 2.0 2.00 1.19 Isoflurane 2.0 3.09 2.69 9 Halothane 2.0 3.21 4.01 4 Isoflurane Isoflurane 1.5 1.50 1.30 6 Halothane 1.5 1.56 1.95 Enflurane 1.5 0.97 0.57 MAC, Minimum alveolar concentration. From Bruce DL, Linde HW: Vaporization of mixed anesthetic liquids. Anesthesiology 60:342–346, 1984. 8 5 FILLING AND MISFILLING OF VAPORIZERS Contemporary concentration-calibrated variable bypass anesthesia vaporizers are agent-specific. If an empty vaporizer designed for one agent is filled with an agent for which it was not intended, the vaporizer’s output likely will be erroneous. Because at room temperature the vaporizing characteristics of halothane and isoflurane (SVP of 243 and 238 mm Hg, respectively), and enflurane and sevoflurane (SVP of 175 and 160 mm Hg, respectively) are = O2 = N2O = Anesthetic agent almost identical, this problem at present mainly applies when Fig. 3.13 Schematic of Dräger Vapor 19.1 vaporizer. 1 = fresh gas inlet; halothane or isoflurane are interchanged with enflurane or 2 = on/off switch; 3 = concentration knob; 4 = pressure compensator; sevoflurane. 5 = vaporizing chamber; 6 = control cone; 7 = bypass cone; 8 = expan- Previously (see also ref. 12) it was shown that if a variable sion element; 9 = mixing chamber; 10 = fresh gas outlet. For details of operation, see text. (Reproduced by permission of Drägerwerk AG, bypass vaporizer that has been calibrated for agent A and Lübeck, Germany.) set to deliver a concentration C% is filled with agent B, the concentration C’ delivered by that misfilled vaporizer is calculated as increases, one surface of the flap expands more than the other, causing the flap to bend in a manner that opens the valve orifice C = C ( PB / PA ) wider, increasing the bypass flow. !e principle of di$erential expansion of metals is applied similarly in the Dräger Vapor Bruce and Linde17 reported on the outputs of erroneously vaporizers (Fig. 3.13), where an expansion element increases filled vaporizers at 22°C (Table 3.4). Erroneous filling a$ects the bypass flow and reduces gas flow in the vaporizing chamber output concentration and consequently the potency output of as temperature increases.15 When temperature decreases, the the vaporizer. In their study, an enflurane vaporizer set to 2% reverse occurs. (1.19 MAC) but filled with halothane, delivered 3.21% (4.01 !e vapor pressures of the volatile anesthetics vary as a MAC) halothane. !is is 3.3 times the anticipated anesthetic function of temperature in a nonlinear manner (see Fig. 3.2). potency output. At 20°C, the predicted concentration of !e result is that the vapor output concentration at any given halothane would be 2.8% (3.7 MAC). vaporizer dial setting remains constant only within a certain To summarize, if a vaporizer specific for an agent with a range of temperatures. For example, the Dräger Vapor 2000 low SVP (e.g., sevoflurane) is misfilled with an agent that vaporizers are specified as accurate to +0.20 vols% or + 20% of has a high SVP (e.g., isoflurane), the output concentration of the concentration set when they are used within the temperature the agent will be greater than that set on the concentration range of 15°C to 35°C at one atmosphere of pressure.16 !e boiling dial. point of the volatile anesthetic agent must never be reached in Conversely, if a vaporizer specific for an agent with a high the current variable bypass vaporizers designed for halothane, SVP is misfilled with an agent that has a low SVP, the output enflurane, isoflurane, and sevoflurane; otherwise, the vapor output concentration of the agent will be less than that indicated on the concentration would be impossible to control and could be lethal. concentration dial.18,19 One must also recognize that the potency (MAC equivalent) Temperature Compensation Times of the agent concentration has to be considered in a misfilling !e temperature compensating mechanisms of contemporary situation. A sevoflurane vaporizer set to deliver 2% sevoflurane variable bypass vaporizers do not produce instantaneous correction (approximately 1 MAC: see Tables 3.1 and 3.2) misfilled with of output concentration. For example, the Dräger 19.n vaporizer isoflurane would produce an isoflurane concentration of 3% requires a temperature compensation time of 6 minutes/°C.15! (approximately 2.6 MAC) (see Tables 3.1 and 3.2). 78 PART 1 Gases and Ventilation Erroneous filling of vaporizers may be prevented if careful bottles that have agent-specific and color-coded collars (Fig. attention is paid to the specific agent and the vaporizer during 3.14). One end of an agent-specific filling device fits the collar filling. A number of agent-specific filling mechanisms, analogous on the agent bottle, and the other end fits only the vaporizer to the pin-index safety system for medical gases, are used in designed for that liquid agent. Although well intentioned, these modern vaporizers. Liquid anesthetic agents are packaged in filling devices cannot totally prevent misfilling. A number of di$erent filling systems are available (e.g., Quik- Fil [Abbott Laboratories, Abbott Park, IL], Key-Fill [Harvard Apparatus, Holliston, MA]). !e agent-specific filling device for desflurane (Saf-T-Fill) is of particular importance because it is essential that a nondesflurane vaporizer never be filled with desflurane (see Desflurane section). Vaporization of Mixed Anesthetic Agents When funnel-fill vaporizer filling systems (Fig. 3.15) were in common use, a more likely scenario was that an agent-specific vaporizer, partially filled with the correct agent, was topped up with an incorrect agent. !is situation is more complex. It is much more di(cult to predict vaporizer output, and large errors in concentration of delivered vapor could occur. Korman and Ritchie20 reported that, when mixed, halothane, enflurane, and isoflurane do not react chemically but do influence the extent of each other’s ease of vaporization. Halothane facilitates the vaporization of both enflurane and isoflurane and is itself more likely to vaporize in the process. !e clinical consequences depend on the potencies of each of the mixed agents and on the Fig. 3.14 Agent-specific filling devices for sevoflurane (yellow) and delivered vapor concentrations. isoflurane (purple). The devices on the agent bottles are quick-fill; the Bruce and Linde17 reported that if a halothane vaporizer others are key-fill. 25% full is filled to 100% with isoflurane and set to deliver 1%, the halothane output is 0.41% (0.51 MAC) and the isoflurane output is 0.9% (0.78 MAC) (Table 3.5). In this case, the output potency of 1.29 MAC is close to the anticipated 1.25 MAC (1% halothane). On the other hand, an enflurane vaporizer that is 25% full and set to deliver 2% (1.19 MAC) enflurane and is filled to 100% with halothane has an output of 2.43% (3.03 MAC) halothane and 0.96% (0.57 MAC) enflurane. !is represents a total MAC of 3.60, or more than three times that intended. It is important to avoid erroneous filling of vaporizers; if an error is suspected, the vaporizer should be emptied, withdrawn from service, labeled as misfilled, and returned to the manufacturer for servicing.21,22 Filling of Vaporizers Vaporizers should be filled only in accordance with their accompanying manufacturer’s instructions. Overfilling or tilting (either by tilting a freestanding unit or by tilting the whole anesthesia machine) a vaporizer may result in liquid agent Fig. 3.15 Dräger Vapor 19.1 vaporizers. Note the (older) funnel-fill entering parts of the anesthesia delivery system (e.g., vaporizer design. This has been superseded by an agent-specific key-fill design. bypass) designed for gases and vapor only. !is could lead to the TABLE 3.5 Vaporizer Output After Incorrectly Refilling from 25% Full to 100% Full VAPORIZER OUTPUT HALOTHANE ENFLURANE ISOFLURANE Vaporizer Setting (%) Refill Liquid % MAC % MAC % MAC Total MAC Halothane 1.0 Enflurane 0.33 0.41 0.64 0.38 — — 0.79 1.0 Isoflurane 0.41 0.51 — — 0.90 0.78 1.29 Enflurane 2.0 Halothane 2.43 3.03 0.96 0.57 — — 3.60 Isoflurane 1.5 Halothane 1.28 1.60 — — 0.57 0.50 2.10 MAC, Minimum alveolar concentration. From Bruce DL, Linde HW: Vaporization of mixed anesthetic liquids. Anesthesiology 1984;60:342–346.  3 Anesthesia Vaporizers 79 delivery of lethal concentrations of agent to the patient circuit. 2.6% at 20 minutes, and then to decrease to 1% by 30 minutes, If a vaporizer has been tilted, liquid agent may have leaked into at which point it stabilizes. At 40 minutes (arrow C) the carrier the gas delivery system. A patient must never be left connected gas is changed back to nitrous oxide and oxygen, and the to such a system. Once the machine has been withdrawn from vaporizer output concentration transiently decreases to 0.75%, clinical service, the proper procedure is to purge the vaporizer and gradually returns to 1% by 55 minutes. At 60 minutes with a high flow rate of oxygen from the anesthesia machine’s/ (arrow D) the carrier gas is changed back to nitrogen and workstation’s flowmeter (not the oxygen flush, which bypasses oxygen, and again the vaporizer output increases as occurred the vaporizer), and with the vaporizer concentration dial set to previously (at arrow B). the maximum concentration.14,16 !e maximum concentration !e e$ect of carrier gas composition on vaporizer output can dial-setting ensures the highest possible oxygen flow through be explained by the solubility of nitrous oxide in a liquid volatile the inflow and outflow paths of the vaporizing chamber and anesthetic agent. !us, when nitrous oxide and oxygen begin to through the bypass. A calibrated anesthetic agent analyzer is enter the vaporizing chamber, some nitrous oxide dissolves in essential to check the e(cacy of the flush procedure before the liquid agent and the vaporizing chamber’s output decreases the vaporizer is returned to clinical service. In contemporary until the liquid agent has become saturated with nitrous oxide. practice, it would be prudent to withdraw the workstation and Conversely, when nitrous oxide is withdrawn as the carrier gas, vaporizer from clinical use until they have been declared safe by the nitrous oxide dissolved in the liquid anesthetic comes out of authorized service personnel. solution and represents, in e$ect, additional nitrous oxide gas Table 3.2 shows that 1 mL of liquid volatile agent produces flow to the vaporizing chamber. approximately 200 mL of vapor at 20°C. !e theoretical !e solubility of nitrous oxide in liquid anesthetics is derivation of this volume is presented later in this chapter (see approximately 4.5 mL per mL of liquid anesthetic,23 therefore Preparation of a Standard Vapor Concentration). !us, it is easy 100 mL of isoflurane liquid, when fully saturated, can dissolve to see how very small volumes of liquid agent entering the gas approximately 450 mL of nitrous oxide. When nitrous oxide is delivery system might produce lethal concentrations of vapor. discontinued, the volume of nitrous oxide, because it is added For example, if 1 mL of liquid isoflurane entered the common (by coming out of solution) to the vaporizing chamber flow over gas tubing, some 20 L of fresh gas would be required to dilute a brief time period, causes the observed increase in vapor output the resulting volume of vapor (195 mL, see Table 3.2) down to a concentration.24–26 !is e$ect was not seen with measured flow 1% (0.87 MAC) concentration!! vaporizers (e.g., Copper Kettle, Verni-Trol) because in these vaporizers the carrier gas was always oxygen. Dräger vaporizers are calibrated using air. When 100% EFFECT OF CHANGES IN FRESH GAS COMPOSITION oxygen is used, the output concentration when compared with air increases by 10% of the set value and by not more than 0.4 !e composition of the fresh gas used to vaporize the volatile volumes percent. When a mixture of 30% oxygen and 70% agent in the vaporizing chamber can also a$ect vaporizer nitrous oxide is used, the concentration falls by 10% of the set output because the viscosity and density of the gas mixture value at most, and by not more than 0.4 volumes percent.15 changes as the mixture changes. Fig. 3.16 shows the output GE Tec series vaporizers are calibrated at 21°C using oxygen concentration from an Ohio enflurane variable bypass as the carrier gas. In these vaporizers, when air or nitrous vaporizer set to deliver 1% enflurane. For the first 10 minutes oxide is the carrier gas, the output concentration is less than (arrow A) the carrier gas is 70% nitrous oxide and 30% oxygen, with oxygen. !e e$ect is greatest when nitrous oxide is the and the vaporizer delivers 1% enflurane. After 10 minutes, carrier gas, but using nitrous oxide decreases the required (arrow B) the carrier gas is changed to nitrogen and oxygen, concentration of volatile agent, thereby mitigating somewhat and the vaporizer output is seen to increase to a peak of about the decrease in output concentration.21! 70% N2O, 30% O2 70% N2O, 30% O2 70% N2, 30% O2 70% N2, 30% O2 A B C D 3 L/min 3 L/min 3 L/min 3 L/min 3 % Enflurane 2 Fig. 3.16 Effect of changing the carrier gas com- 1 position (nitrous oxide versus nitrogen) on vaporizer output in an Ohio variable bypass enflurane vapor- izer. (From Scheller MS, Drummond JC: Solubility of N2O in volatile anesthetics contributes to vapor- 0 izer aberrancy when changing carrier gases. Anesth 0 10 20 30 40 50 60 70 Analg 1986;65:88–90. Reproduced by permission of Time (min) the International Anesthesia Research Society.) 80 PART 1 Gases and Ventilation EFFECTS OF CHANGES IN BAROMETRIC at a temperature of 20°C; however, this represents 32 volumes PRESSURE percent (160/500) of the atmosphere there. Now, 100 mL gas exiting the vaporizing chamber contains 32 mL sevoflurane Vaporizers are most commonly used at an ambient pressure of vapor. Diluting this 32 mL in 1050 mL total volume [(32/ 760 mm Hg (one atmosphere at sea level). !ey may, however, (100+950)] gives a sevoflurane concentration of 3% by volume be used under hypobaric conditions—such as at increased (32/1050) which is 1.5 times the dialed-in concentration in altitude—or under hyperbaric conditions—such as in a terms of volumes percent. hyperbaric chamber. To determine the potency of this 3% sevoflurane Few reports are available concerning the use of vaporizers concentration, however, we must consider the partial under hypobaric conditions. !erefore, the theoretical consid- pressure, because it is the partial pressure of the anesthetic erations that apply to such use are discussed here. Since all con- agent that determines potency (see MAPP in earlier section). temporary variable bypass vaporizers place the proportioning If sevoflurane represents 3% of the gas mixture by volume, its control (i.e., ratio of bypass flow to vaporizing chamber outflow) partial pressure in the emerging mixture is: 3% % 500 mm Hg, downstream of the vaporizing chamber, this arrangement will or 15 mm Hg. In terms of anesthetic potency, this represents be considered first. (15/16) or 0.94 MAC, because the PMAC1 of sevoflurane is 16 mm Hg (see Table 3.1). !us, in theory, a variable bypass Bypass Flow-to-Vaporizing Chamber Flow sevoflurane vaporizer used at an ambient pressure of 500 mm Proportioning Control Located Downstream of the