Section 2: Scientific Principles
Part A: Inhaled Anesthetics
Chapter 7: Inhaled Anesthetic Delivery Systems

Variable-Bypass Vaporizers

The Ohmeda Tec 4, the Ohmeda Tec 5, and the North American Dräger Vapor 19.1 are classified as variable-bypass, flowover, temperature-compensated, agent-specific, out-of-circuit vaporizers. 41  Variable-bypass refers to the method for regulating output concentration. As gas flow enters the vaporizer‘s inlet, the setting of the concentration control dial determines the ratio of flow that goes through the bypass chamber and through the vaporizing chamber. The gas channeled to the vaporizing chamber flows over the liquid anesthetic and becomes saturated with vapor. Thus, flowover refers to the method of vaporization. The Tec 4, the Tec 5, and the Vapor 19.1 are classified as temperature-compensated because they are equipped with an automatic temperature-compensating device that helps maintain a constant vaporizer output over a wide range of temperatures. These vaporizers are agent specific and out of circuit because they are designed to accommodate a single agent and to be located outside the breathing circuit. Variablebypass vaporizers are used to deliver halothane, enflurane, isoflurane, and sevoflurane, but not desflurane (see Electrically Heated, Pressurized Vaporizers).

Basic Operating Principles

A diagram of a generic, variable-bypass vaporizer is shown in Figure 7–14 . Vaporizer components include the concentration control dial, the bypass chamber, the vaporizing chamber, the filler port, and the filler cap. Using the filler port, the operator fills the vaporizing chamber with liquid anesthetic. The maximum safe level is predetermined by the position of the filler port, which is positioned to minimize the chance of overfilling. If a vaporizer is overfilled or tilted, liquid anesthetic can spill into the bypass chamber, causing an overdose. The concentration control dial is a variable restrictor, and it can be located either in the bypass chamber or in the outlet of the vaporizing chamber. The function of the concentration control dial is to regulate the relative flow rates through the bypass and vaporizing chambers.

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FIGURE 7–14 Generic variable bypass vaporizer (see text for details). (Modified from Andrews JJ: Delivery systems for inhaled anesthetics. In Barash PG, Cullen BF, Stoelting RK [eds]: Clinical Anesthesia, 3rd ed. Philadelphia, New York, Lippincott-Raven, 1997, pp 535–572.)

Flow from the flowmeters enters the inlet of the vaporizer. More than 80 percent of the flow passes straight through the bypass chamber to the vaporizer outlet, and this accounts for the name “bypass chamber.” Less than 20 percent of the flow from the flowmeters is diverted through the vaporizing chamber. Depending on the temperature and the vapor pressure of the particular inhaled anesthetic, the flow through the vaporizing chamber entrains a specific flow of inhaled anesthetic. All three flows, that is, flow through the bypass chamber, flow through the vaporizing chamber, and flow of entrained anesthetic, exit the vaporizer at the outlet. The final concentration of inhaled anesthetic is the ratio of the flow of the inhaled anesthetic to the total gas flow. 41, 46 

The vapor pressure of an inhaled anesthetic depends on the ambient temperature (see Fig. 7–13 .). For example, at 20°C, the vapor pressure of isoflurane is 238 mm Hg, whereas at 35°C, the vapor pressure almost doubles (450 mm Hg). Variable-bypass vaporizers have an internal mechanism to compensate for different ambient temperatures. The temperature-compensating valve of the Ohmeda Tec 4 is shown in Figure 7–15 47  At high temperatures, such as those commonly used in pediatric or burn operating rooms, the vapor pressure inside the vaporizing chamber is high. To compensate for this increased vapor pressure, the bimetallic strip of the temperature-compensating valve leans to the right. This allows more flow to pass through the bypass chamber and less flow to pass through the vaporizing chamber. The net effect is a constant vaporizer output. In a cold operating room environment, the vapor pressure inside the vaporizing chamber decreases. To compensate for this decrease in vapor pressure, the bimetallic strip swings to the left, causing more flow to pass through the vaporizing chamber and less to pass through the bypass chamber. The net effect is a constant vaporizer output.

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FIGURE 7–13 Vapor pressure versus temperature curves for desflurane, isoflurane, halothane, enflurane, and sevoflurane. The vapor pressure curve for desflurane is both steeper and shifted to higher vapor pressures when compared with the curves for other contemporary inhaled anesthetics. (From inhaled anesthetic package insert equations and from Susay et al74 )



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FIGURE 7–15 Simplified schematic of the Ohmeda Tec 4 vaporizer (see text for details). (Modified from Andrews JJ: Delivery systems for inhaled anesthetics. In Barash PG, Cullen BF, Stoelting RK [eds]: Clinical Anesthesia, 3rd ed. Philadelphia, New York, Lippincott-Raven, 1997, pp 535–572.)

Factors That Influence Vaporizer Output

The output of an ideal vaporizer with a fixed dial setting would be constant, regardless of varied flow rates, temperatures, back pressures, and carrier gases. Designing such a vaporizer is difficult because as ambient conditions change, the physical properties of gases and of vaporizers themselves can change. 46  Contemporary vaporizers approach ideal but still have some limitations. Several factors listed below can influence vaporizer output.

Flow Rate.

With a fixed dial setting, vaporizer output varies with the rate of gas flowing through the vaporizer. This variation is particularly notable at extremes of flow rates. The output of all variable-bypass vaporizers is less than the dial setting at low flow rates (<250 mL/min). This results from the relatively high density of volatile inhaled anesthetics. The turbulence generated at low flow rates in the vaporizing chamber is insufficient to upwardly advance the vapor molecules. At extremely high flow rates, such as 15 L/min, the output of most variable-bypass vaporizers is less than the dial setting. This discrepancy is attributed to incomplete mixing and saturation in the vaporizing chamber. Also, the resistance characteristics of the bypass chamber and the vaporizing chamber can vary as flow increases. These changes can result in decreased output concentration. 46 

Temperature.

Because of improvements in design, the output of contemporary temperature-compensated vaporizers is almost linear over a wide range of temperatures. Automatic temperature-compensating mechanisms in bypass chambers maintain a constant vaporizer output with varying temperatures. 8, 47, 48  A bimetallic strip (see Fig. 7–15 ) or an expansion element (Fig. 7–16) directs a greater proportion of gas flow through the bypass chamber as temperature increases. 46  Wicks are placed in direct contact with the metal wall of the vaporizer to help replace heat used for vaporization. Vaporizers are constructed with metals having relatively high specific heat and high thermal conductivity in order to minimize heat loss.

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FIGURE 7–16 Simplified schematic of the North American Dräger Vapor 19.1 vaporizer (see text for details). (Modified from Andrews JJ: Delivery systems for inhaled anesthetics. In Barash PG, Cullen BF, Stoelting RK [eds]: Clinical Anesthesia, 3rd ed. Philadelphia, New York, Lippincott-Raven, 1997, pp 535–572.)

Intermittent Back Pressure.

Intermittent back pressure associated with positive-pressure ventilation or with oxygen flushing can cause higher vaporizer output concentration than the dialed setting. This phenomenon, known as the pumping effect, 41, 46, 49, 50, 51  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 inspired pressures, and rapid drops in pressure during expiration. 47, 48, 49, 50, 51  The Ohmeda Tec 4 and the North American Dräger Vapor 19.1 are relatively immune from the pumping effect. 47, 48  One proposed mechanism for the pumping effect is dependent on retrograde pressure transmission from the patient circuit to the vaporizer during the inspiratory phase of positive-pressure ventilation. Gas molecules are compressed in both the bypass and the vaporizing chambers. When the back pressure is suddenly released during the expiratory phase of positive-pressure ventilation, vapor exits the vaporizing chamber via the vaporizing chamber outlet and retrograde through the vaporizing chamber 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. 46, 49, 50, 51 

To decrease the pumping effect, the vaporizing chambers of the Tec 4 and the Vapor 19.1 are smaller than those of older variable-bypass vaporizers, such as the Fluotec Mark II (750 mL). 47, 48, 50  Therefore, no substantial volumes of vapor can be discharged from the vaporizing chamber into the bypass chamber during the expiratory phase. The North American Dräger Vapor 19.1 (see Fig. 7–16 ) has a patented long spiral tube that serves as the inlet to the vaporizing chamber. 48, 50  When the pressure in the vaporizing chamber is released, some of the vapor enters this tube but does not enter the bypass chamber because of tube length. 50  The Tec 4 (see Fig. 7–15 ) has an extensive baffle system in the vaporizing chamber, and a one-way check valve has been inserted at the common outlet to minimize the pumping effect. This check valve attenuates but does not eliminate the pressure increase, because gas still flows from the flowmeters to the vaporizer during the inspiratory phase of positive-pressure ventilation. 41, 52 

Carrier Gas Composition.

Vaporizer output is influenced by the composition of the carrier gas that flows through the vaporizer. 47, 48, 53, 54, 55, 56, 57, 58, 59, 60  When the carrier gas is quickly switched from 100 percent oxygen to 100 percent nitrous oxide, there is a rapid transient decrease in vaporizer output followed by a slow increase to a new steady-state value ( Fig. 7–17B). 58, 59  The transient decrease in vaporizer output is attributed to nitrous oxide‘s being more soluble than oxygen in halogenated liquid. 58  Therefore, the quantity of gas leaving the vaporizing chamber is transiently diminished until the anesthetic liquid is totally saturated with nitrous oxide.

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FIGURE 7–17 Halothane output of a North American Dräger Vapor 19.1 vaporizer with different carrier gases. The initial output concentration is approximately 4% halothane when oxygen is the carrier gas at flows of 6 L·min1  (A). When the carrier gas is quickly switched to 100% nitrous oxide (B), the halothane concentration decreases to 3% within 8–10 seconds. Then, a new steady-state concentration of approximately 3.5% is attained within 1 minute (see text for details). (Modified from Gould et al58 )

The explanation for the new steady-state output value is less well understood. 60  With contemporary vaporizers, such as the North American Dräger Vapor 19.1 and the Ohmeda Tec 4, the steady-state output value is less when nitrous oxide rather than oxygen is the carrier gas ( Fig. 7–17B). 47, 48  Conversely, the output of some older vaporizers is enhanced when nitrous oxide is the carrier gas instead of oxygen. 53, 55  The steady-state plateau is achieved more rapidly with increased flow rates, regardless of the ultimate output value. 59  Factors that contribute to the characteristic steady-state response resulting when various carrier gases are used include the viscosity and density of the carrier gas, the relative solubilities of the carrier gas in the liquid anesthetic, the flow splitting characteristics of the specific vaporizer, and the dial setting. 55, 58, 59, 60 

Safety Features

The North American Dräger 19.1, the Ohmeda Tec 4, and the Ohmeda Tec 5 have many safety features that have minimized or eliminated many hazards that were once associated with variable-bypass vaporizers. Agent-specific, keyed filling devices help prevent a vaporizer from being filled with the wrong agent. Overfilling of these vaporizers is minimized because the filler port is located at the maximum safe liquid level. Today‘s vaporizers are firmly secured to the vaporizer manifold, and there is little need to move them. Thus, problems associated with tipping are minimized. Contemporary interlock systems prevent administration of more than one inhaled anesthetic. 47, 48, 61 

Hazards

Despite many safety features, some hazards are still associated with contemporary variable-bypass vaporizers.

Misfilling.

Vaporizers not equipped with keyed fillers have been occasionally misfilled with the wrong anesthetic liquid. 62  A potential for misfilling exists even on contemporary vaporizers equipped with keyed fillers. 63, 64 

Contamination.

Contamination of anesthetic vaporizer contents occurred when an isoflurane vaporizer was filled with a contaminated bottle of isoflurane. A potentially serious incident was avoided because the operator did not use the contaminated vaporizer after detecting an abnormal acrid odor. 65 

Tipping.

Tipping can occur when vaporizers are incorrectly “switched out” or moved. However, tipping is unlikely when a vaporizer is attached to a manifold in the upright position. Excessive tipping can cause the liquid agent to enter the bypass chamber and can cause a high output concentration. 66  The Tec 4 is slightly more immune to tipping than the Vapor 19.1 because of its extensive baffle system. However, if either vaporizer is tipped, it should not be used until it has been flushed for 20 to 30 minutes at high flow rates with the vaporizer set at a low concentration. 41 

Overfilling.

Improper filling procedures combined with failure of the vaporizer sight glass can cause overfilling and overdose. Liquid anesthetic enters the bypass chamber, and up to 10 times the intended vapor concentration can be delivered. 67 

Simultaneous Inhaled Anesthetic Administration.

Two inhaled anesthetics can be administered simultaneously when the center Tec 4 vaporizer is removed from Ohmeda machines equipped with the older style Selectatec vaporizer manifold. The left or right vaporizer should be moved to the central position if the central vaporizer is removed as indicated by the manifold label. The interlock system then functions properly because the two remaining vaporizers are adjacent. 12, 13, 14 

Leaks.

Leaks occur often with vaporizers, and vaporizer leaks can cause patient awareness. 41, 68, 69  A loose filler cap is the most common source of vaporizer leaks. Leaks can occur at the O-ring junction between the vaporizer and its manifold. A vaporizer must be in the on position to detect a leak within it. Vaporizer leaks in the North American Dräger System can be detected with a conventional positive-pressure leak test because of the absence of check valves. Ohmeda recommends using a negative-pressure leak testing device (suction bulb) to detect vaporizer leaks in the Modulus I, Modulus II, and Excel models because of the check valve at the machine outlet 11, 12, 13, 15  (see Checking Anesthesia Machines).