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© 2001 International Anesthesia Research Society
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Abstract |
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We determined the performance of the vaporizer of the ADU machine (Anesthesia Delivery Unit; Datex-Ohmeda, Helsinki, Finland). The effects of carrier gas composition (oxygen, oxygen/N2O mixture, and air) and fresh gas flow (0.2 to 10 L/min) on vaporizer performance were examined with variable concentrations of isoflurane, sevoflurane, and desflurane across the whole range of each vaporizer’s output. In addition, the effects of sudden changes in fresh gas flow and carrier gas composition, back pressure, flushing, and tipping were assessed. Vaporizer output depended on fresh gas flow, carrier gas composition, dial settings, and the drug used. Vaporizer output remained within 10% of dial setting with fresh gas flows of 0.3–10 L/min for isoflurane, within 10% of dial setting with fresh gas flows of 0.5–5 L/min for sevoflurane, and within 13% of dial setting with fresh gas flows of 0.5 to 1 L/min for desflurane. Outside these fresh gas flow ranges, output deviated more. The effect of sudden changes in fresh gas flow or carrier gas composition, back pressure, flushing, and tipping was minimal. We conclude that the ADU vaporizer performs well under most clinical conditions. Despite a different design and the use of complex algorithms to improve accuracy, the same physical factors affecting the performance of conventional vaporizers also affect the ADU vaporizer.
IMPLICATIONS: The ADU vaporizer performs well under most clinical conditions. Despite a different design and the use of complex algorithms to improve accuracy, the same physical factors affecting the performance of conventional vaporizers also affect the ADU vaporizer.
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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The ADU anesthesia machine (Anesthesia Delivery Unit; Datex-Ohmeda, Helsinki, Finland) recently became available in the United States (1). The vaporizing unit of this anesthesia machine differs significantly from conventional vaporizers, and its accuracy has not been determined independently. Good clinical practice demands a thorough understanding of the major working principles of an anesthesia machine and the vaporizer and the knowledge of their accuracy. Therefore, in the first part of this article, we describe the working principles of the vaporizing unit of the ADU. In the second part, we tested vaporizer performance with three different anesthetics over a wide range of vaporizer settings, fresh gas flows, and carrier gas compositions, and we determined the effect of back pressure, flushing, and tipping.
The vaporizing unit in the ADU can be categorized as an electronically controlled, flow-over, variable bypass, and measured flow vaporizer. Accuracy is stated as ±10% of setting or ±3% of maximum dial setting (whichever is more) within a fresh gas flow range of 0.2–8 L/min. (1) Output is claimed to be independent of carrier gas composition.
The vaporizing unit consists of two parts: an anesthetic-specific vaporizing chamber, the so-called Aladin cassette, and the concentration-control hardware and software (Fig. 1). The liquid anesthetic vaporizes freely in the cassette. The ratio of flow leaving the vaporizing chamber to bypass flow, which ultimately determines the delivered anesthetic concentration, is controlled by a throttle valve. A mixing chamber stabilizes anesthetic concentration output and reduces the effect of oscillations resulting from back pressure fluctuations caused by the ventilator.
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The central processing unit (CPU) algorithm controlling the throttle valve regulates cassette flow leaving the vaporizing chamber on the basis of 1) the dial setting, 2) temperature in the cassette, 3) total pressure in the cassette, 4) bypass flow and cassette flow, and 5) carrier gas composition. The temperature is used to calculate the vapor pressure (the electronic temperature compensation). However, to calculate vapor concentration in the cassette (vapor partial pressure/total pressure inside the cassette), total pressure in the cassette needs to be known as well. Total pressure may exceed ambient pressure, e.g., if the temperature in a desflurane cassette increases to >23.5°C, the boiling point for desflurane. As soon as the cassette pressure is higher than the pressure distal to the outflow channel of the cassette, the vaporizer starts to work as an injector. Oxygen flow, N2O and/or air flow, bypass flow, and cassette outflow are measured with constant orifice anemometers in which a laminar flow restrictor (fixed resistance) causes a pressure decrease proportional to gas flow. Information about flow rates as well as fresh gas flow selection (oxygen with or without either air or N2O, depending on the position of an N2O/air selector switch) is used by the CPU algorithm to modify the cassette flow with the intent of making vaporizer output carrier gas independent.
The cassette has to be inserted into a slot in the anesthesia machine, where it is automatically connected to the in- and outflow channels and to a temperature sensor. The desired delivered vapor concentration can be increased with 0.1% increments (0.5% for desflurane) by turning a wheel located next to the Aladin cassette. Because there is only one slot, only one cassette (and consequently only one anesthetic) can be used at the time. A cassette is made anesthetic-specific by labeling it with a certain sequence of magnets in the back of the cassette that is recognized by the CPU. In addition, it is color coded and uses anesthetic-specific adaptors (the Quik Fil for sevoflurane, the Saf-T-Fil for desflurane, and the key-fill for the other anesthetics). Inside the vaporizing chamber, several metal plates and lamellas serve to increase heat capacity, heat conductivity, and vaporizing surface, all of which improve temperature stability and complete vaporization and therefore the efficiency of the vaporizer. In addition, a fan located below the Aladin cassette is activated any time the cassette temperature decreases to <18°C, adding heat generated by the electronic circuitry.
Separating the vaporizing chamber from the rest of the vaporizing unit and electronically controlling the cassette flow as described previously has allowed the manufacturer to use a cassette design for desflurane similar to that for the other anesthetics and to eliminate the need for individual calibration of each cassette. The desflurane cassette differs only in that a level detector has been added because relatively large amounts of desflurane are vaporized as compared with the other anesthetics and because (as a result of its low blood/gas solubility) the patient would wake up rapidly should the vaporizing chamber run empty. When only 10% of desflurane remains in the cassette, an alarm message appears. Liquid capacity is 250 mL for all anesthetics.
The vaporizing unit has several safety features. First, by using the gas and anesthetic delivery data, an electronic ratio control guarantees an oxygen concentration of at least 25% at the common gas outlet, even when large concentrations of anesthetics are used (e.g., 18% desflurane). The oxygen, N2O, and air flows are adjusted by means of mechanical needle valves and measured by anemometers. The N2O flow is controlled by two valves—a needle valve (controlled by the anesthesiologist) and a slave valve distal to the needle valve. The latter is controlled by the CPU; if the calculated oxygen concentration at the common gas outlet decreases to <25%, the slave valve is activated and N2O flow decreases. Second, whenever the Aladin cassette is removed from the vaporizing unit, two spring-loaded valves (inflow and outflow close valve) automatically shut off the channels to and from the vaporizer. Third, a safety relief valve opens if the pressure inside the cassette increases to >2.5 bar (1899 mm Hg). Fourth, the block where the proportional valve is mounted and the cassette connector block are both equipped with heating elements to prevent condensation of anesthetic vapor. Fifth, a valve prevents liquid from the cassette from entering the fresh gas line. Sixth, tipping, a problem with conventional vaporizers, should not affect the output of the Aladin cassette because the cassette does not include any flow channels for the bypass flow. Finally, an air lock mechanism prevents overfilling, and a valve (metal ball) closes the anesthetic channel whenever the cassette is tilted by more than 7 degrees during filling.
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Methods |
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The study was performed in Aalst, Belgium, were the average atmospheric pressure is 762 mm Hg. Vaporizer output was measured at the common gas outlet using a single multigas analyzer of the Datex-Ohmeda AS/3 ADU. The accuracy of the analyzer is ±0.2%, except for desflurane concentrations of 5%–10% and 10%–18%, for which it is ±0.5% and ±1%, respectively. Before each experiment, the gas analyzer was calibrated according to the specifications of the manufacturer. Vaporizer performance of five Aladin cassettes for three anesthetics each (isoflurane, sevoflurane, and desflurane) was assessed in two parts.
Part 1
The dial setting for isoflurane, sevoflurane, and desflurane was increased from 0 to the maximum settings of 5%, 8%, and 18%, respectively, by using 1%, 2%, and 3% increments, respectively, with each combination of the following carrier gas compositions and fresh gas flows: 100% oxygen, a 40%/60% oxygen/N2O mixture and air, and fresh gas flow of 0.2, 0.3, 0.5, 0.8, 1, 5, 8, and 10 L/min. The 0.2 and 0.3 L/min groups had to be omitted in the oxygen/N2O groups because the oxygen ratio monitor control mechanism prevents the use of a relatively large proportion of N2O at these low fresh gas flows. All data were collected after the concentration had remained stable for at least 15 s. To facilitate comparison of the performance of the cassettes for different anesthetics, the output is presented as the difference (in percentage of target value) of the dialed value (mean and SD). In the figures, however, absolute values (mean and SD) are used, and only the results for fresh gas flows of 0.2, 0.5, 1, 5, and 8 L/min are presented, to improve legibility.
Part 2
By use of 3% isoflurane, 4% sevoflurane, and 9% desflurane, the effects of sudden changes in fresh gas flow and carrier gas composition were examined by changing fresh gas flow from 0.5 to 5 L/min and by changing the carrier gas composition from air to oxygen to an oxygen/N2O mixture by using a fresh gas flow of 4 L/min. The effect of back pressure was examined by completely occluding the common gas outlet while running air at 4 L/min by using 3% isoflurane, 4% sevoflurane, and 9% desflurane. The same settings were used to study the effect of flushing and tipping (cassette removed from the slot and jiggled around over 360 degrees).
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Results |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Part 1
Isoflurane Aladin Cassette
Vaporizer output remained within 10% of dial setting with fresh gas flows ranging from 0.3 to 10 L/min (Fig. 2). Vaporizer output decreased with a fresh gas flow of 0.2 L/min and with high (8 and 10 L/min) fresh gas flows. At higher fresh gas flows, vaporizer output was lower with the oxygen/N2O mixture.
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) and oxygen/N2O (40%/60%) (
.Sevoflurane Aladin Cassette
Vaporizer output remained within 10% of dial setting with fresh gas flows ranging from 0.5 to 5 L/min (Fig. 3). Vaporizer output decreased with lower and higher fresh gas flows. The effect of the higher fresh gas flows on vaporizer output was highly dependent on the duration of the high fresh gas flow and may have obscured the effects of carrier gas composition. The output of the individual vaporizer varied significantly at the extremes of fresh gas flow and dial settings. With one cassette, output was 46% lower than dialed (8% dial setting, oxygen carrier gas, 10 L/min).
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Desflurane Aladin Cassette
Within a fresh gas flow range of 0.5 to 1 L/min, vaporizer output remained within 13% of the dial setting (Fig. 4). Vaporizer output decreased with fresh gas flows
5 L/min but increased with lower fresh gas flow, especially in the 0.2 L/min groups. Vaporizer output was lower with oxygen/N2O mixtures as compared with oxygen or air. Output of the individual vaporizer did vary significantly at the extremes of fresh gas flow and dial settings.
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Part 2
With sudden changes in fresh gas flow or carrier gas composition, vaporizer output changed to a new and stable level within 40 min. Back pressure, flushing, and tipping had no or only a very transient and small effect on vaporizer output.
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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In this study, isoflurane and sevoflurane output of the ADU machine remained within 10% of the dialed concentration with most commonly used fresh gas flows. The accuracy of the desflurane vaporizer was slightly less, and accuracy with all anesthetics decreased at the extremes of fresh gas flow (<0.4 L/min or >5 L/min). Vaporizer output also depended on carrier gas composition, dial setting, and the anesthetic used. As fresh gas flow increases, vaporizer output decreases despite the use of a fan to facilitate heat transfer to the vaporizer, probably because the amount of heat transfer to the vaporizing chamber becomes insufficient, and hence output decreases. The effect of higher fresh gas flows on vaporizer output was highly dependent on the duration of the high fresh gas flow and may have obscured the effects of carrier gas composition. Despite the use of anemometers and algorithms that electronically control vaporizer output, vaporizer output was not independent of carrier gas composition. Changing carrier gas composition changes the viscosity and hence the flow rate across the anemometer: the viscosities of oxygen, air, and 70% N2O in oxygen are 210, 190, and 171 micropoises, respectively (2). These very same physical principles explain the effect of carrier gas composition on vaporizer output in conventional vaporizers (2–8). The performance of the Aladin cassettes is not the same for different anesthetics. Because the cassette design for the three anesthetics is similar, the effect might in part be related to differences in the relative quantities of liquid anesthetic that need to be vaporized to attain the desired concentration. Vaporizer performance could possibly be improved by adjustments of the algorithms by using our data.
Except for desflurane, the ADU vaporizer performance remained within the 10% claimed by the manufacturer, within the clinically commonly used range of fresh gas flow between 0.5 and 5 L/min and vaporizer dial settings of 1 and 3 and 2 and 6 for isoflurane and sevoflurane, respectively. For desflurane, output deviated slightly more (13%). Decreased performance with air fresh gas flow of 0.2–0.4 L/min is clinically irrelevant. However, some techniques warrant the use of oxygen and N2O fresh gas flows outside the 0.5–5 L/min fresh gas flow range and challenge the equipment. For example, during overpressure induction with sevoflurane (fresh gas flow 6–8 L/min), vaporizer output will decrease over the course of the induction. Under closed-circuit anesthesia conditions (oxygen fresh gas flow 0.15–0.25 L/min), vaporizer output of the individual vaporizer may deviate by more than 20% from the dialed value. Consequently, it becomes important to know the accuracy of the individual vaporizers when using them for pharmacokinetic studies of inhaled anesthetics involving the use of closed-circuit anesthesia. Especially under these conditions, end-tidal gas monitoring is highly recommended, both because of the wide range in vaporizer performance and because anesthetic uptake differs as much as 50% in adult patients (9).
It is difficult to compare the performance of the ADU vaporizing unit with that of currently used conventional vaporizers because the available data (mainly published in anesthesia textbooks and provided by companies) lack external validation and detail, especially at low fresh gas flows. Except for the Tec 6 (Datex-Ohmeda), no studies provide detailed, complete information similar to that contained in this study (2). In addition, the 1988 Anesthesia Machine Standard (7) only vaguely describes performance criteria for vaporizers: "A vaporizer must be capable of accepting a total gas flow of 15 L/min from the anesthesia machine, and, in turn, of delivering a gas flow with a predictable concentration of vapor." However, from the available data, it is clear that the same factors that influence vaporizer output of conventional vaporizers affect output of the Aladin cassette: fresh gas flow, carrier gas composition, and dial setting. Output by the Dräger Vapor 19.1 (Dräger, Telford, PA) differs up to 5%–10% if a carrier gas other than air is used. With 8 L/min of air, output decreased by 19% after 20 minutes.1
No good data are available on the Tec 5 series and the Penlon PPV Sigma (St. Louis, MO) vaporizer. The Tec 6 vaporizer basically functions as an injector. With a fresh gas flow of 1 L/min and a dial setting of 10%, the average output deviated from +3% to -19%, depending on carrier gas composition.
In summary, the features of a new vaporizer are described. The ADU vaporizer performs well under all but the most extreme of clinically encountered conditions. Despite a different design and the use of complex algorithms to improve accuracy, the same physical factors affecting the performance of conventional vaporizers also affect the ADU vaporizer.
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Footnotes |
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Presented in part at the annual meeting of the American Society of Anesthesiologists, Dallas, TX, October 9–13, 1999, and the IARS 2000 annual meeting, Honolulu, HI, March 10–14, 2000.
1 Fitzal S, Gilly H, Steinbereithner K. Do modern plenum vaporizers provide accurate anesthetic mixtures irrespective of gas flow [abstract]? Anesthesiology 1986;65:A168. 
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References |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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- AS/3 ADU User’s reference manual ref. 983810-1, specification sheet 8501013. Helsinki: Datex-Ohmeda, 1998.
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Johnston RV Jr, Andrews JJ, Deyo DJ, et al. The effects of carrier gas composition on the performance of the Tec 6 desflurane vaporizer. Anesth Analg 1994; 79: 548–52.
[Abstract/Free Full Text] -
Gould DB, Lampert BA, MacKrell TN. Effect of nitrous oxide solubility on vaporizer aberrance. Anesth Analg 1982; 61: 938–40.
[Abstract/Free Full Text] -
Scheller MS, Drummond JC. Solubility of nitrous oxide in volatile anesthetics contributes to vaporizer aberrancy when changing carrier gases. Anesth Analg 1986; 65: 88–90.
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Lin C-Y. Assessment of vaporizer performance in low-flow and closed-circuit anesthesia. Anesth Analg 1980; 59: 359–66.
[Abstract/Free Full Text] -
Nielsen J, Pedersen FM, Knudsen F, et al. Accuracy of 94 anaesthetic drug vaporizers in clinical use. Br J Anaesth 1993; 71: 453–7.
[Abstract/Free Full Text] - Dorsch JA, Dorsch SE. Vaporizers. In: Dorsch JA, Dorsch SE, eds. Understanding anesthesia equipment. 3rd ed. Baltimore: Williams & Wilkins, 1994: 91–148.
- Eisenkraft JB. Anesthesia vaporizers. In: Ehrenwerth J, Eisenkraft JB, eds. Anesthesia equipment: principles and applications. St. Louis: Mosby, 1993: 57–88.
- Hendrickx JFA, Soetens M, Van der Donck A, et al. Uptake of desflurane and isoflurane during closed-circuit anesthesia with spontaneous and controlled ventilation. Anesth Analg 1997; 84: 413–8.[Abstract]
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