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The Datex-Ohmeda Tec 4, Tec 5, and Tec 7 and the North American Dräger Vapor 19.n and 20.n vaporizers are classified as variable-bypass, flow-over, temperature-compensated, agent-specific, out-of-breathing-circuit vaporizers.[54] 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 a wick system saturated with the liquid anesthetic and subsequently becomes saturated with vapor. Flow-over refers to the method of vaporization and is in contrast to a bubble-through system like that of a copper kettle vaporizer. The Tec 4, the Tec 5, Tec 7 and the Vapor 19.n and 20.n 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. Variable-bypass vaporizers are used to deliver halothane, enflurane, isoflurane, and sevoflurane, but not desflurane.
A diagram of a generic variable-bypass vaporizer is shown in Figure 9-14 . 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 fill 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
Flow from the flow meters enters the inlet of the vaporizer. More than 80% of the flow passes straight through the bypass chamber to the vaporizer outlet, and this accounts for the name bypass chamber. Less than 20% of the flow from the flow meters is diverted through the vaporizing chamber. Depending on the temperature and vapor pressure of the particular inhaled anesthetic, the flow through the vaporizing chamber entrains a specific flow of inhaled anesthetic. All three flows—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.[54] [60]
The vapor pressure of an inhaled anesthetic depends on the ambient temperature (see Fig. 9-13 ). 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 Datex-Ohmeda Tec-type vaporizer 4 is shown in Figure 9-15 .[61] 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 movement 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
Figure 9-15
Simplified schematic drawing of the Ohmeda Tec-type vaporizer.
At high temperatures, the vapor pressure inside the vaporizing chamber is high.
To compensate for the increased vapor pressure, the bimetallic strip of the temperature-compensating
valve leans to the right, allowing more flow through the bypass chamber and less
flow 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 the decreased vapor pressure, the bimetallic strip
swings to the left, causing more flow through the vaporizing chamber and less through
the bypass chamber. The net effect is a constant vaporizer output. (Modified
from Andrews JJ, Brockwell RC: Delivery systems for inhaled anesthetics. In
Barash PG, Cullen BF, Stoelting RK [eds]: Clinical Anesthesia, 4th ed. New York,
Lippincott-Raven, 2000, pp 567–594.)
The output of an ideal vaporizer with a fixed dial setting would be constant, regardless of varied flow rates, temperatures, backpressures, 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.[60] Contemporary vaporizers approach the ideal situation but still have some limitations. Several factors discussed in the following sections can influence vaporizer output.
With a fixed dial setting, vaporizer output can vary 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 inhaled volatile anesthetics. Insufficient turbulence is generated at low flow rates in the vaporizing chamber to upwardly advance the vapor molecules. At extremely high flow rates (e.g., 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. The resistance characteristics of the bypass chamber and the vaporizing chamber can vary as flow increases, and these changes can decrease the output concentration.[60]
Because of improvements in design, the output of contemporary temperature-compensated vaporizers is almost
Intermittent backpressure 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, [54] [60] [63] [64] [65] is more pronounced at low flow rates, low dial settings, and low levels of liquid anesthetic in the vaporizing chamber. The pumping effect is increased by rapid respiratory rates, high peak inspired pressures, and rapid drops in pressure during expiration. [61] [62] [63] [64] [65] The Datex-Ohmeda Tec 4, Tec 5, and Tec 7 and the North American Dräger Vapor 19.1 and 20.n systems are relatively immune from the pumping effect.[61] [62] One proposed mechanism for the pumping effect depends on retrograde pressure transmission from the patient's circuit to the vaporizer during the inspiratory phase of positive-pressure ventilation. Gas molecules are compressed in the bypass and vaporizing chambers. When the backpressure is suddenly released during the expiratory phase of positive-pressure ventilation, vapor exits the vaporizing chamber through 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.[60] [63] [64] [65]
Figure 9-16
Simplified schematic drawing of the North American Dräger
Vapor 19.1 vaporizer. Automatic temperature-compensating mechanisms in bypass chambers
maintain a constant vaporizer output with varying temperatures. An expansion element
directs a greater proportion of gas flow through the bypass chamber as temperature
increases. (Modified from Andrews JJ, Brockwell RC: Delivery systems for
inhaled anesthetics. In Barash PG, Cullen BF, Stoelting
RK [eds]: Clinical Anesthesia, 4th ed. New York, Lippincott-Raven, 2000, pp 567–594.
To decrease the pumping effect, the vaporizing chambers of newer systems are smaller than those of older variable-bypass vaporizers such as the Fluotec Mark II (750 mL).[61] [62] [64] 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 and 20.n and the Datex-Ohmeda Tec 5 and Tec 7 (see Fig. 9-16 ) has a long, spiral tube that serves as the inlet to the vaporizing chamber.[62] [64] [66] 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 the tube's length.[64] The Tec 4 (see Fig. 9-15 ) has an extensive baffle system in the vaporizing chamber, and a one-way check valve has been inserted at the common gas outlet to minimize the pumping effect. This check valve attenuates but does not eliminate the pressure increase because gas still flows from the flow meters to the vaporizer during the inspiratory phase of positive-pressure ventilation.[54] [67]
Vaporizer output is influenced by the composition of the carrier gas that flows through the vaporizer.[61] [62] [68] [69] [70] [71] [72] [73] [74] [75] When the carrier gas is quickly switched from 100% oxygen to 100% nitrous oxide, there is a rapid transient decrease in vaporizer output followed by a slow increase to a new steady-state value ( Fig. 9-17B ). [73] [74] The transient decrease in vaporizer output is attributed to nitrous oxide's being more soluble than oxygen in halogenated liquid.[73] The quantity of gas leaving the vaporizing chamber is transiently diminished until the anesthetic liquid is totally saturated with nitrous oxide.
The mechanism for the new steady-state output value is less well understood.[75] 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 (see Fig. 9-17B ).[61] [62] Conversely, the output of some older vaporizers is enhanced when nitrous oxide is the carrier
Figure 9-17
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/min (A).
When the carrier gas is quickly switched to 100% nitrous oxide (B),
the halothane concentration decreases to 3% within 8 to 10 seconds. Then, a new
steady-state concentration of approximately 3.5% is attained within 1 minute (C).
(Modified from Gould DB, Lampert BA, MacKrell TN: Effect of nitrous oxide
solubility on vaporizer aberrance. Anesth Analg 61:939, 1982.)
The North American Dräger 19.n and 20.n and the Datex-Ohmeda Tec 4, Tec 5, and Tec 7 have safety features that have minimized or eliminated many hazards once associated with variable-bypass vaporizers. Agent-specific, keyed filling devices help prevent filling a vaporizer with the wrong agent. Overfilling of these vaporizers is minimized because the filler port is located at the maximum safe liquid level. Modern vaporizers are firmly secured to the vaporizer manifold, and there is little need to move them. Problems associated with tipping are minimized. Contemporary interlock systems prevent administration of more than one inhaled anesthetic.[61] [62] [76]
Despite many safety features, some hazards are still associated with contemporary variable-bypass vaporizers.
Vaporizers not equipped with keyed fillers occasionally have been misfilled with the wrong anesthetic liquid.[77] A potential for misfilling exists even with contemporary vaporizers equipped with keyed fillers.[78] [79] [80]
Contamination of anesthetic vaporizer contents has occurred by filling an isoflurane vaporizer 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.[81]
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 cause output with extremely high concentration of the agent.[82] 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.[54] The Dräger Vapor 2000 series vaporizers have a transport (T) dial setting that helps prevent tipping related problems. When the dial is placed in this position, the vaporizer sump is isolated from the bypass chamber, thereby reducing the likelihood of tipping and a resulting accidental overdose.[83] When one of these vaporizers is moved separate from the anesthesia workstation, the control dial should be placed in the T position. The design of the Aladin cassette system has practically eliminated the possibility of tipping for the Datex-Ohmeda ADU. Because the vaporizer's bypass chamber is physically separated from the cassette and permanently resides in the anesthesia workstation, the possibility of tipping is virtually eliminated barring overturning the entire anesthesia workstation.
Improper filling procedures combined with failure of the vaporizer sight glass can cause overfilling and overdose the patient. Liquid anesthetic enters the bypass chamber, and up to 10 times the intended vapor concentration can be delivered to the common gas outlet.[84]
Just as with overfilling, underfilling of anesthetic vaporizers may be problematic. When a Tec 5 sevoflurane vaporizer is in a low fill state and used under conditions of high fresh gas flow rates (>7.5 L/min) and a high dial setting (e.g., during inhalational inductions), the vaporizer output may abruptly decrease to less than 2%. The causes of this problem are most likely multifactorial, and the exact mechanisms at work are unclear. However, the combination of low vaporizer fill state (<25% full) in combination with the high vaporizing chamber flow can result in a clinically significant and reproducible fall in vaporizer output.[85] Anesthesia care providers must be aware of this phenomenon to avoid problems in their clinical practice.
Two inhaled anesthetics can be administered simultaneously when the center vaporizer is removed from Datex-Ohmeda machines equipped with the older-style Select-a-Tec 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 warning label). The interlock system can then function properly because the two remaining vaporizers are adjacent.[11] [12] [13] Later Select-a-Tec vaporizer manifolds have a built-in device that prevents this problem. On these newer systems, a U-shaped plastic device links the vaporizer extension rods even when the vaporizers are not adjacent on the manifold. The manifold plus the vaporizers themselves comprise the vapor interlock or vapor exclusion system. This system reduces the chances of accidental simultaneous administration of two volatile anesthetic agents.
Leaks occur often with vaporizers, and vaporizer leaks can cause awareness by the patient during anesthesia.[54] [86] [87] [88] A loose filler cap is the most common source of vaporizer leaks. With some key-filled Penlon and Dräger vaporizers, a loose filler screw clamp allows escape of saturated anesthetic vapor.[88] Leaks can occur at the O-ring junction between the vaporizer and its manifold. A vaporizer must be in the on position to detect an internal vaporizer leak. Even though vaporizer leaks in Dräger systems can be detected with a conventional positive-pressure leak test (because of the absence of an outlet check valve), a negative-pressure leak test is more sensitive and allows the user to detect even small leaks. Datex-Ohmeda recommends a negative-pressure leak-testing device (i.e., suction bulb) to detect vaporizer leaks in the Modulus I, Modulus II, Excel, and Aestiva workstations because of the check valve located just upstream of each machine's fresh gas outlet[10] [11] [12] [14] [52] (see "Checking Anesthesia Machines").
Many of the newer anesthesia workstations are capable of performing self-testing procedures, which in some cases may eliminate the need for the conventional negative-pressure leak test. Anesthesia care providers must understand that these machine self-tests cannot detect internal vaporizer leaks on systems with add-on vaporizers. For the self-tests to determine if an internal vaporizer leak is present, the leak test must be repeated with each vaporizer sequentially while its concentration control dial is turned to the on position. When a vaporizer's concentration control dial is set in the off position, it may not be possible to detect even major internal leaks.
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