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Anesth Analg 2003;96:1056-1061 © 2003 International Anesthesia Research Society
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Abstract |
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The Anesthetic Conserving Device (ACD) is a high-flow anesthesia system closed to volatile anesthetics only. We compared the ACD with a circle system under different fresh gas flow (FGF) conditions. Eighty-one patients undergoing major surgery were randomly allocated to receive sevoflurane from a circle circuit combined either with the ACD placed at the Y-piece (n = 41) or with a vaporizer (n = 40). The FGF was set to 8 L/min in the ACD system, where the circle circuit served as a nonrebreather. In the conventional circle system without ACD, the vaporizer was supplied with 1-, 1.5-, 3-, and 6-L/min FGFs. We compared the ACD with the circle system under the four FGFs in terms of sevoflurane dosing, sevoflurane consumption, humidification efficiency, and environmental pollution. The ACD and the low-flow circle system (1.5- and 1-L/min FGFs) resulted in the smallest sevoflurane consumption. The increase in inspired sevoflurane concentration was faster with the circle system than with the ACD only with FGFs
3 L/min. The removal of ACD from the circuit allowed the fastest washout of sevoflurane. Respiratory gas humidification was always adequate. Sevoflurane ambient concentration with the ACD was 1–70 ppb. The ACD is a valid and simple alternative to low-flow systems. IMPLICATIONS: The Anesthetic Conserving Device (ACD) is a new device for anesthetic vapor delivery. We demonstrated that the ACD reduces anesthetic consumption and environmental pollution similarly to a low-flow circle system, offering advantages such as simplicity, no toxicity from compounds produced in the absorber, and potential cost savings.
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Introduction |
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The advantages of low-flow anesthesia are indisputable and include the decrease of anesthetic consumption, the decrease of atmospheric pollution, and the reduction of costs (1). However, low-flow anesthesia is not free from problems, which include the risk of hypoventilation from leaks, the large volume of the system, the discrepancy between the delivered fraction (FD) and the inspired fraction (FI) of inhaled gases (2,3) , and the accumulation of toxic compounds, such as carbon monoxide (4,5) . Moreover, carbon dioxide (CO2) absorbents free from strong bases (6) are required to avoid the formation of potentially toxic agents, such as Compound A (7).
As an alternative to low-flow anesthesia, Thomasson et al. (8) first proposed a system that is open in regard to oxygen, nitrogen, and nitrous oxide delivery but closed in regard to volatile anesthetics. This was achieved by means of a zeolite filter that was placed at the Y-piece connector and worked as a reflection filter for anesthetic vapors, reducing isoflurane consumption by more than 50% in bench tests.
More recently, the zeolite filter was inserted between the ventilator and a circle system with a CO2 absorber, and an automatic vapor delivery device was placed in the inspiratory limb of the circuit (9). In this setup, the zeolite filter reduced the isoflurane consumption by almost 80%. However, microscopic particles of filter material may reach the lung, and some zeolites are hazardous because of their fibrogenic and carcinogenic activity (10,11) . One proposed solution is to use a charcoal filter instead of a zeolite filter because the effects of charcoal on the lung are better known than those of zeolites (12). The principle of the charcoal filter has been further developed, leading to the recent introduction of the Anesthetic Conserving Device (ACD; Hudson RCI, Upplands Väsby, Sweden). Whereas all previously described reflection filters for volatile anesthetics require the use of a vaporizer, this is not the case with the ACD. Specifically, a syringe pump delivers the volatile anesthetic in liquid status to the ACD where the anesthetic is vaporized. Moreover, contrary to the previously described zeolite filter system (9), the ACD eliminates any need for a CO2 absorber. In a clinical study, an earlier version of the ACD reduced the isoflurane consumption by 40% compared with a Bain anesthesia system with a high fresh gas flow (FGF) (13). Such results suggest that the ACD may decrease anesthetic gas consumption by the same amount that a low-flow circle system does. However, the ACD has not been previously compared with circle systems.
In the present study, we clinically compared the ACD with a circle system used under conditions of low-flow, medium-flow, and high-flow. The different conditions of the circle system were obtained by adjusting the FGF at 1, 1.5, 3, and 6 L/min. The comparison between the ACD and the circle system under the four FGF conditions includes measurement of sevoflurane dosing, sevoflurane consumption, humidification efficiency, and environmental pollution.
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Methods |
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After hospital ethics committee approval and informed consent, 81 adult patients scheduled for major urologic or abdominal surgery with general anesthesia were studied. Exclusion criteria included the need for total IV anesthesia or for anesthesia with nitrous oxide, hemodynamic instability, and a history of drug reaction to any of the planned medications.
After premedication with 2–3 mg of midazolam IV, anesthesia was induced with propofol, which was titrated to loss of consciousness, remifentanil 0.5 µg/kg (more than 60 s), followed by an infusion of 0.25 ± 0.05 µg · kg-1 · min-1, and atracurium 0.5 mg/kg. After orotracheal intubation, the patients were connected to a Cicero respirator (Drägerwerke, Luebeck, Germany); anesthesia was maintained with sevoflurane, which was titrated to an end-tidal concentration of 0.5 ± 0.05 minimum alveolar anesthetic concentration (MAC) (14,15) . A muscle relaxant was infused, and neuromuscular block was measured with a peripheral transcutaneous nerve stimulator. To check the adequacy of the anesthesia level obtained with the anesthesia technique used in this study, 27 randomly chosen patients were submitted to bispectral index monitoring. Mechanical ventilation was adjusted to achieve an end-tidal CO2 (ETCO2) between 32 and 36 mm Hg. The anesthesia circuit was a conventional circle circuit with two unidirectional ventilatory hoses.
Patients were randomly allocated to receive sevoflurane either by the ACD (n = 41) or by a calibrated Vapor 19.1 vaporizer (Drägerwerke) (n = 40). As shown in Figure 1, the ACD was connected between the circle circuit and the endotracheal tube and was supplied with liquid sevoflurane via a syringe pump (Pilot C, Fresenius, Sevres, France), with settings for a 50-mL syringe (Plastipak, Becton Dickinson, Shannon, Ireland).
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In the ACD setup, the circle circuit served as the nonrebreather, and this was achieved by setting 8 L/min of FGF on each patient. In the conventional circle system, the vaporizer was supplied with 1-, 1.5-, 3-, and 6-L/min FGFs, which were applied on four different subgroups of 10 patients each. In the ACD group, immediately before sevoflurane delivery, the pump infusion rate was set at 60 mL/h for 80 s to fill the line connecting the syringe pump to the ACD. Thereafter, the infusion rate of sevoflurane was set at 15 mL/h to reach 1.1% ± 0.05% of sevoflurane expiratory concentration (FE), and this concentration was then maintained by decreasing the infusion rate based on the measured FE sevoflurane values.
In the control group, the FD was set to 3% sevoflurane until 1.1% ± 0.05% FE sevoflurane was reached; thereafter, the target FE of 1.1% was maintained by decreasing FD depending on the FGF. The FGF, minute ventilation, and fraction of inspired oxygen were kept constant through all sevoflurane deliveries. Respiratory gas humidification was performed by the heat and moisture exchanger (HME) component of the ACD in the ACD group and by means of an HME-filter (Hygroster, Mallinckrodt-DAR, Mirandola, Italy) in the control group.
At recovery from anesthesia, the syringe pump was stopped, and the ACD was removed from the circuit. In the control group, the vaporizer was switched off, and the FGF was increased to 8 L/min.
The ACD device, which is still experimental, has been previously described in detail (13). Briefly, the ACD consists of an anesthetic gas exchanger, which absorbs some of the expired anesthetic vapor and desorbs some of it in the next inspiration. This is accomplished by means of an activated carbon filter, which is more efficient in anesthetic gas exchanging compared with the earlier version used by Enlund et al (13). The anesthetic is supplied in liquid status via a syringe pump and is instantaneously vaporized inside the ACD after the contact with a porous rod (evaporator), which diffuses the anesthetic over a large surface. The ACD also contains a conventional HME and an antimicrobial filter.
On each patient, the following measurements were obtained:
- The FI and FE sevoflurane concentrations were measured at the proximal end of the tracheal tube using a Datex AS3 monitor (Datex Instrumentation, Helsinki, Finland). The sampling flow (100 mL/min) was returned to the anesthetic system after analysis. The measurements of FI and FE sevoflurane concentrations were taken every 5 min during the maintenance of anesthesia and every 15 s at the start and at the stop of sevoflurane delivery. We measured the rate of increase of FI sevoflurane from 0% to 1.1% (t-rise) and the rate of decrease of FI sevoflurane from 1.1% to 0.5% (t-fall) at the start and at the stop of sevoflurane delivery, respectively.
- The sevoflurane consumption was measured after anesthesia as the total volume of sevoflurane delivered by the syringe pump to the ACD. In the control group, the vaporizer was weighted before and after anesthesia with a calibrated scale (AND EK-12KG; Tamro Med-Lab, Gothenburg, Sweden; stated precision, ±1 g), and the difference was used to calculate the volume of liquid sevoflurane consumed, assuming a sevoflurane density of 1.53 g/mL.
- Heart rate, blood pressure, ETCO2, O2 saturation, and peak inspiratory pressure were continuously displayed by the Datex AS3 monitor, and the values were recorded as the average data over 5 min.
- Temperature of respiratory gases and humidity output were measured after 1 h of sevoflurane delivery using a temperature probe integrated with the Datex AS3 and the Beydon visual scale (16), respectively.
- The ambient sevoflurane concentration was measured at 70 cm from the ground, close to the Y-piece, after 1 h of sevoflurane delivery with the ACD. The measurements were made by means of infrared spectrophotometry and by mass spectrometry, which have a sensitivity to 0.2 ppm and 0.1 ppb, respectively.
All values were expressed as mean ± SD. Comparisons between groups were undertaken by the two-way analysis of variance. A P < 0.05 level was considered significant. The statistical package used was Statview version 4.5 (Abacus Concepts, Berkeley, CA).
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Results |
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As shown in Table 1, the study groups were similar in terms of age, sex, and ASA grade. The anesthesia characteristics and the vital variables of the patients during anesthesia are summarized in Table 2. There were no differences in mean FE sevoflurane concentration, remifentanil consumption per hour, and duration of anesthesia between groups. In the whole study population, the duration of anesthesia was <2 h in only 10 patients. Maximal duration of anesthesia was found in the ACD group and was 368 min. In no cases was the target sevoflurane concentration modified. All patients remained hemodynamically stable, as assessed by measurements of mean arterial blood pressure and heart rate. There were no differences in respiratory variables among groups. The bispectral index values ranged from 35 to 50, suggesting an adequate depth of anesthesia.
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As shown in Figure 2, the higher the FGF in the control group, the larger was the consumption of sevoflurane. The amount of sevoflurane consumed was 5.2 ± 0.4 mL/h in Group 1 L/min, increased to 13 ± 1.3 mL/h in Group 3 L/min, and reached 24 ± 3 mL/h in Group 6 L/min. There was no difference in sevoflurane consumption between the two low-flow conditions (1 and 1.5 L/min). The consumption of sevoflurane by the ACD was 6.2 ± 1.4 mL/h, comparable to that of the 1 L/min and 1.5 L/min groups.
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Figure 3A and B provide the times to achieve a given increase and a given decrease in FI sevoflurane, respectively. In the control group, given a fixed vaporizer setting of 3% sevoflurane, the higher the FGF, the faster the wash-in of the circuit, as indicated by the values for t-rise of FI sevoflurane from 0% to 1.1% in Figure 3A. The t-rise of FI sevoflurane was 316 ± 40 s in Group 1 L/min, decreased to 226 ± 29 s in Group 1.5 L/min, and was 61 ± 10 s and 16 ± 3 s in Groups 3 L/min and 6 L/min, respectively.
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The t-rise of FI sevoflurane with the ACD (without including the 80 s required to fill the line connecting the syringe pump to the ACD) was 205 ± 37 s, comparable to the t-rise values of Group 1.5 L/min.
Compared with the control group, the washout of the circuit was twice as fast when the syringe pump was stopped and the ACD was removed from the circuit, as indicated by the values of t-fall of FI sevoflurane from 1.1% to 0.5% in Figure 3B. No differences were found in the whole control group, whereas in all cases, the vaporizer was switched off and the FGF increased to 8 L/min. However, the washout of the circuit was faster in the control group than in the ACD group, when in this latter group, the syringe pump was switched off, and the ACD was left in the circuit. In this case, the t-fall of FI sevoflurane in the ACD group was 125 ± 27 s.
During use of the ACD, no sevoflurane was detected in the ambient concentration by the infrared spectrometer, and mass spectrometry confirmed a negligible sevoflurane ambient concentration of 8 ± 14 ppb. In all cases, humidity output was adequate, corresponding to a Beydon score of 3, and respiratory gas temperatures ranged from 33°C to 34°C.
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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This study produced two major findings. First, the ACD reduces the consumption of sevoflurane in an amount similar to a low-flow circle system. Second, the ACD is a simple and reliable technique.
In this study, both the ACD and the circle system under low-flow conditions (1–1.5 L/min) reduced the sevoflurane consumption by almost 75% (12 mL per MAC-hour), compared with the circle system under high-flow conditions (6 L/min). Enlund et al. (13) found a reduction in isoflurane consumption by 40% (15 mL per MAC-hour) using the ACD with high FGF. However, the study of Enlund et al. (13) and the present study differ in terms of minute ventilation, duration of anesthesia, and ACD efficiency in gas exchange.
Our finding of a sevoflurane ambient concentration of a few ppb agrees with the results of Enlund et al. (13) and suggests a negligible risk of ambient pollution from use of the ACD. Although we did not measure the entire amount of sevoflurane wasted through the scavenging system, it is likely that the true reduction of sevoflurane waste was in the same order as the reduction of sevoflurane consumption.
Hence, the ACD reduces volatile anesthetic consumption and environmental pollution in a similar degree to the low-flow circle system. These results are of clinical interest, especially when we consider the potential advantages of the ACD over low-flow circle systems. Specifically, the ACD is a simple technique because it can be applied to any ventilator equipped with a nonrebreathing circuit. This circuit can be represented by either a Bain or a Mapleson circuit, or, as in our case, by a circle circuit with a high FGF. As specified in Methods, the addition of the ACD at the Y-piece connector was the only difference between the ACD setup and the conventional circle system used as the control. In this way, we eliminated the potential influence of any factor other than the ACD on the efficiency of mechanical ventilation.
Another advantage of the ACD is that this technique does not require a CO2 absorber and hence eliminates any production of carbon monoxide or other toxic components in the CO2 absorber (4–6) . Such a risk of toxicity for the patient is of particular concern with low-flow circle circuits because it increases as the FGF is decreased (17–19) .
Finally, whereas with conventional low-flow systems, the control of anesthetic and O2 concentrations depend on the FGF, this is not the case with the ACD. Indeed, the ACD is designed as a simple high-flow system closed to volatile anesthetics only.
In the present study, we also addressed the issue of how to achieve rapid induction with the ACD. Under nonrebreathing conditions, the only user-adjustable control for dosing the anesthetic vapor with the ACD is the rate of the syringe pump. In contrast, the dosing strategy that is generally used with circle systems during the early phases of anesthesia is based on the combination of high FGF and high vaporizer settings (20,21) . Our results show that the wash-in of the circuit to the target 1.1% FI sevoflurane was completed in four minutes when the ACD was used with a sevoflurane infusion rate of 15 mL/h. This time interval is clinically acceptable in most clinical situations, where the preparation of the surgical field imposes some delay between orotracheal intubation and the surgical incision. An increase in sevoflurane infusion rate decreases the time required to reach the desired FI sevoflurane with the ACD. However, infusion rates of anesthetic liquid faster than 15 mL/h or even boluses must be used cautiously because they may result in excessive peaks in anesthetic gas concentration, with consequent need for a rapid and marked reduction in the infusion rate of anesthetic liquid. In this case, the titration of the desired anesthetic gas concentration may become more difficult and time consuming.
Compared with open circle systems, the washout of the expired anesthetic from the circuit is much faster if the syringe pump is stopped and the ACD is removed from the circuit. Another possibility is to stop the syringe pump while leaving the ACD in place. In this case, the washout of the circuit is half as fast as with high-flow circle systems but comparable to or faster than a low-flow system in which we simply switch off the vaporizer without increasing the FGF. The washout of the circuit achieved by removing the ACD is essentially appropriate when a rapid recovery from anesthesia is required. We measured the rapidity of changes in FI sevoflurane to compare the two anesthesia systems in terms of sevoflurane dosing. Nevertheless, monitoring of anesthetic vapor FE concentration is essential to titrate the rate of the syringe pump used to supply the ACD.
Data concerning the effect of time on the ACD efficiency are lacking. In this study, the ACD was used for up to five hours without any significant changes in the stability of FI sevoflurane. Theoretically, the ACD, because of its composition, should not be affected by the duration of use, and the need to replace the ACD seems essentially justified by the presence of an HME-filter component, which limits the use of the ACD to no longer than 24 hours without possibility for reuse.
Respiratory gas conditioning was adequate both with the ACD, which contains an HME, and the circle system. The circle system was provided with a high-efficiency HME-filter that ensured adequate humidification regardless of the FGF. High-efficiency HMEs minimize the differences in humidity and temperature of respiratory gases caused by different FGFs (22,23) . The internal volume of the ACD is 90 mL and differs slightly from the volume of the HME-filter used in the circle system, which makes it likely that the ACD system and a conventional circle system do not significantly differ in terms of dead space.
In conclusion, the ACD is suitable for clinical practice and offers a valid alternative to conventional low-flow anesthesia techniques. Further studies are required to assess the potential advantages of the ACD over low-flow techniques, with particular regard to economic considerations.
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Acknowledgments |
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The authors thank Hudson RCI, Upplands Väsby, Sweden, and BlueMed, Bergamo, Italy, for technical support.
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References |
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