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TECHNOLOGY, COMPUTING, AND SIMULATION

AnaConDa® Reflection Filter: Bench and Patient Evaluation of Safety and Volatile Anesthetic Conservation

From the Anesthesia and Intensive Care Unit, Angers Teaching Hospital, Angers, France.

	Abstract

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BACKGROUND: The AnaConDa® filter permits administration of volatile anesthetic without the use of an anesthesia machine. It is intended for use in the intensive care unit.

METHODS: We studied the AnaConDa® reflection filter on the bench and in anesthetized patients. The bench analysis used a test lung, a gas analyzer, an intensive care ventilator, the AnaConDa® filter, and a syringe pump. We studied a range of tidal volume, respiratory rate, and positive end-expiratory pressure values. We simulated errors during syringe refilling and patient transportation. In 15 anesthetized patients, we used the AnaConDa® with constant ventilation variables, a constant sevoflurane infusion rate (4–5 mL/h), and two consecutive fresh gas flow levels.

RESULTS: In the bench study, the expired volatile anesthetic fraction decreased linearly with respiratory frequency at constant minute ventilation, and decreased markedly in a hyperbolical manner when tidal volume increased at a constant respiratory rate. Changing the positive end-expiratory pressure level and inspiration/expiration ratio did not modify the AnaConDa®’s performance. Several safety failures were observed: refilling caused a transient change in AnaConDa® output because of a pumping effect, and a standard Luer lock made it possible to connect the halogenate syringe on an IV infusion line. In anesthetized patients, reducing fresh gas flow from 8 to 1 L/min led to a median 40% increase in the expired volatile anesthetic fraction.

CONCLUSIONS: This study shows that the device is generally reliable, but that there are several conditions under which it might deliver more anesthetic than intended.

	Introduction

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Inhaled volatile anesthetics are useful drugs in the intensive care unit (ICU) for the treatment of status asthmaticus (1,2) and for providing fast reversible sedation (3). However, their administration traditionally requires a cumbersome closed-circuit anesthesia machine. A novel approach to administration of inhaled volatile anesthetics is the use of a reflection filter in the breathing circuit. The reflection filter is a device connected between the patient and ventilator to allow reinhalation of anesthetics. These filters conserve volatile anesthetics in a manner similar to conservation of heat and moisture by a heat and moisture exchanger. Reflection filters containing zeolite as the absorber were introduced in the 1980s (4). However, studies suggesting that zeolite fibers might induce fibrosis and cancer (5–7) precluded widespread adoption.

A new generation of reflection filter (AnaConDa®, Hudson RCI, Uppsland Väsby, Sweden) with charcoal as the absorber and an imbedded heat and moisture exchanger was recently introduced for use in the ICU (7,8). The device allows infusion of liquid volatile anesthetics by a syringe pump via a porous rod on the patient side of the filter. Unfortunately, the manufacturer provides only limited safety and efficacy data on the AnaConDa® filter. No studies have evaluated the influence of ventilatory settings on the safety and efficacy of the device. The main purpose of our study was to conduct a bench evaluation of AnaConDa® to obtain safety data and to investigate the influence of ventilator settings on performance. In addition, we hypothesized that AnaConDa® used with a low-flow closed-circuit anesthesia machine would reduce volatile anesthetic consumption during anesthesia for surgery. We tested this hypothesis in patients undergoing anesthesia for surgical procedures.

	METHODS

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Bench Study
The bench apparatus consisted of a Servo 900D ICU ventilator (Siemens Elema®, Solna, Sweden). An 8-mm endotracheal tube was used to connect the ventilatory circuit to a lung model made of a plastic jar partly filled with water to produce a compliance of 50 mL/cm H₂O. The AnaConDa® filter (Figs. 1 and 2) was placed between the Y piece and the endotracheal tube. Air pollution by the volatile anesthetic was prevented by a charcoal-containing cartridge connected to the gas outlet of the ventilator. An infrared gas analyzer (Brüel and Kjaer, Bron, France) was used to sample gas via the sampling port of the AnaConDa® filter to measure the expired fraction of volatile anesthetic (F_E). Sampled gas was reinjected from the analyzer outlet into the breathing circuit. F_E was recorded every 15 s on a personal computer. The analyzer was calibrated before the study, as recommended by the manufacturer.

View larger version (79K): [in this window] [in a new window]   Figure 1. (a)The AnaConDa® filter. 1 = gas sampling port; 2 = evaporating rod; 3 = infusion line; 4 = charcoal layer; 5 = heat and moisture exchanger. (b) Picture of the AnaConDa® filter shown in a clinical setting.

View larger version (11K): [in this window] [in a new window]   Figure 2. Plot of infusion rate versus expired fraction of isoflurane (FE) in the bench study (tidal volume, Vt = 500 mL; respiratory rate, RR = 12 min–1; inspiratory time/total respiratory cycle time, Ti/Ttot = 33%; no positive end-expiratory pressure; and constant flow during inspiration).

Assessment of the AnaConDa® Efficacy and the Influence of Respiratory Frequency Under Three Different Minute Ventilation Regimen.
Isoflurane was infused at a fixed rates of 5 mL/h. We studied the nine combinations of 3-minute ventilation levels (5.9, 6.9, 7.9 L/min) and three breathing frequencies (12, 16, 20 min^–1); with Ti/Ttot: 33%, constant inspiratory flow, and no PEEP. Then, the same nine combinations were tested with a Ti/Ttot of 25%, and then again with a Ti/Ttot of 50%, both without PEEP. Finally, we tested each of these nine combinations with four PEEP levels (5, 7.5, 10, or 15 cm H₂O). Ti/Ttot was set at 33%.

To assess the F_E without the reflecting filter, we also tested an AnaConDa® device from which we had removed the reflecting filter under the conditions described earlier.

Assessment of the Influence of V_T.
As F_E seemed to vary nonlinearly with V_t, we performed an extended study of the effect of V_t on the AnaConDa® filter output. Therefore, we systematically combined the same three respiratory frequencies (12, 16, 20 min^–1) with V_t ranging from 400 to 1300 mL, by 100 mL increments. Ti/Ttot was set at 33% and no PEEP was used.

The measurement error did not exceed ±2%, as assessed by performing runs in triplicate.

Test of Two Scenarios that Might Lead to Anesthetic Overdose
Effect of Gravity.
Given the effect of gravity on the volatile anesthetic column leading down to the AnaConDa®, we expected refilling of the isoflurane syringe to induce changes in F_E. Therefore, we recorded F_E during refilling with the device in line with the breathing circuit.

Risk of Halogenate Accumulation.
We also simulated a possible event that might occur when an ICU patient is transported outside the ICU for a short period (e.g., 1 h for computed tomography). We stopped the ventilator, disconnected the AnaConDa® from the endotracheal tube, and capped its patient end as recommended by the manufacturer to avoid environmental pollution. However, we continued running the syringe pump infusing volatile anesthetic into the filter. After 1 h, the filter was reconnected to test the apparatus and the ventilator restarted, thus simulating reconnection of the patient to the ventilator. F_E was recorded from the time preceding disconnection until achievement of a new steady-state after reconnection.

Patient Study
After IRB approval and obtaining written informed patient consent, we studied 15 ASA I–II patients undergoing venous stripping. The AnaConDa® filter was connected to the circuit of a closed-circuit anesthesia machine (Advance®, Datex-Ohmeda). General anesthesia was induced via a combination of propofol and sufentanil. Tracheal intubation was facilitated by atracurium sulfate. Patients’ lungs were ventilated in a controlled volume mode with a RR of 12 min^–1 and a V_t providing an end-tidal CO₂ level of 30–35 mm Hg. Anesthesia was maintained with 50% N₂O in O₂. Sevoflurane was infused into the AnaConDa® at a rate of 5 mL/h via a syringe pump (Fresenius Vial, Pilot C®). The F_E of sevoflurane was recorded continuously on a computer. Additional doses of sufentanil were given as needed to ensure optimal operating conditions and patient stability. Fresh gas flow (FGF) was set at 8 L/min during this part of the study, corresponding to an open-circuit configuration. Once F_E achieved steady-state, FGF was reduced to 1 L/min, then left unchanged until the end of surgery.

	RESULTS

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Bench Study
As shown in Figure 2, the relationship between infusion rate and isoflurane F_E was linear when ventilator settings were maintained constant.

When the infusion rate was kept constant (5 mL/h), the output of volatile anesthetic from the AnaConDa® filter varied with the ventilator settings. Increasing RR with a constant minute ventilation caused an increase in F_E (Fig. 3a). The efficacy of the filter is illustrated by the fact that F_E is about eight times lower when charcoal has been removed. Increasing V_t with a constant RR caused a marked nonlinear decrease in F_E (Fig. 3b). In this figure, one can see that V_t seemingly plays a larger role than breathing frequency in the AnaConDa®’s performance.

View larger version (22K): [in this window] [in a new window]   Figure 3. (a) Effect of changing the respiratory rate (RR) at three levels of constant minute ventilation, with (dotted lines) or without (solid lines) charcoal in the filter (squares, minute ventilation, Ve = 5.9 L/min; triangles, Ve = 6.9 L/min; circles, Ve = 7.9 L/min). Measurements performed with an infusion rate of 5 mL/h and the following ventilatory settings: respiratory "duty cycle", (Ti/Ttot) = 33%; no positive end-expiratory pressure; and constant inspiratory flow. Corresponding tidal volume (VT)(mL) are shown on the graph. (b) 3D plot of isoflurane expired fraction (FE) in function of RR and Vt. FE fits an hyperbolic relationship to Vt (r = 0.97). Measurements were performed with an infusion rate of 5 mL/h, Ti/Ttot = 33%, no positive end-expiratory pressure, and constant inspiratory flow.

Isoflurane output did not vary with PEEP or when Ti/Ttot was changed.

During syringe refill, gravity-induced flow of anesthetic liquid caused an increase in F_E when the syringe was disconnected 30 cm above the filter, and reverse flow caused a decrease in F_E when disconnection was done 30 cm below the filter (Fig. 4). The 1 h disconnection while leaving the syringe pump led to a large, transient increase in F_E because of the accumulation of volatile anesthetic in the filter (Fig. 5).

View larger version (14K): [in this window] [in a new window]   Figure 4. Changes in isoflurane expired fraction (FE) during the refill maneuver. When the syringe pump is 30 cm above the AnaConDa® filter ([1]), disconnecting the syringe from the infusion line allows it to drain into the filter, leading to a transient increase in FE. When the syringe pump is 30 cm below the AnaConDa® filter ([2]), there is a transient decrease in FE.

View larger version (16K): [in this window] [in a new window]   Figure 5. High isoflurane expired fraction (FE) after reconnection of the filter to the ventilator and resumption of mechanical ventilation. Before reconnection, the filter was left disconnected for more than 60 min (arrow), with the ventilator stopped and the filter capped, but with the infusion pump left on.

Patient Study
The patients were ASA I or II and had a median age of 58 yr [range, 22–83], median body mass index: 26 [range: 20–36]. There were seven men and eight women. The median V_t was 600 mL [range: 475–800 mL] and median expired CO₂ was 31 mm Hg [range: 30–35 mm Hg]. Reducing the FGF from 8 to 1 L/min without changing the volatile anesthetic infusion rate or the ventilator settings caused a 40% median increase [range: 20–67] in F_E (Fig. 6). The potential anesthetic-conserving effect of the AnaConDa® device in a closed-circuit configuration is illustrated by a typical recording obtained in a patient (Fig. 7).

View larger version (24K): [in this window] [in a new window]   Figure 6. Plot of individual data for the sevoflurane expired fraction (FE) at steady-state after fresh gas flow (FGF) has been reduced from 8 to 1 L/min without changing the other ventilatory parameters. The sevoflurane infusion rate = 5 mL/min. Median increase in FE = 40% [range: 20–67].

View larger version (14K): [in this window] [in a new window]   Figure 7. Plot of data from a typical patient: increase in sevoflurane expired fraction (FE) when fresh gas flow (FGF) is reduced from 8 to 1 L/min without changing the other ventilatory parameters. The sevoflurane infusion rate = 5 mL/min.

	DISCUSSION

Top Abstract Introduction METHODS RESULTS DISCUSSION REFERENCES

 
Our main findings are that the vapor output of the AnaConDa® varies with the anesthetic infusion rate and also with the ventilator settings, and that, in patients, the device reduced volatile anesthetic consumption in a closed-circuit system. Our study also identified a number of safety concerns.

We observed on bench that ventilation settings influenced the output of the AnaConDa®. V_t especially was the most influencing variable on F_E. The increasing effect of dead space may explain the hyperbolic increase of F_E when V_t decreased. Conversely, at high V_t, one may observe a full washout of the filter with F_E converging to an asymptotic value. We hypothesize that the small dependence of F_E on RR at constant V_t may result from slightly different isoflurane mixing conditions in the filter.

The first risk we identified, although not specifically studied, as it is obvious on inspection of the equipment, is that of an inadvertent connection of the volatile anesthetic syringe to an IV infusion line. The Luer lock-fitted volatile anesthetic infusion line has the same appearance as a standard infusion line even if the syringe is labeled in color with a printed warning "not for IV use". At present, no safeguard is available to avoid such an error.

Our bench study, although its results cannot be directly applied to clinical use, suggests that clinicians should be mindful of the possibility that changing ventilator settings may alter inhaled anesthetic delivery.

Our clinical study should be interpreted in the light of the data obtained by Enlund et al. (8). These authors compared isoflurane consumption in patients receiving isoflurane to maintain anesthesia at a F_E around 0.46%, either via a conventional vaporizer or using the AnaConDa® filter. Ventilation was provided by a Mapleson D system configuration with FGF at about 2.2 L/min. The AnaConDa® reduced isoflurane consumption by 39% via a 55% reduction in the isoflurane escape rate to the atmosphere (8). In our study, the AnaConDa® filter with FGF at 8 L/min required the same amount of sevoflurane as observed by Tempia et al. (around 5 mL/h) (9) in the absence of the filter with the FGF at 1 L/min. The F_E achieved in both studies was similar 0.8% [range: 0.5–1.0] in ours and 1.1% ± 0% in that of Tempia et al. Thus, the AnaConDa® filter performs in an open circle configuration (FGF at 8 L/min) equivalently to a conventional closed circuit configuration with a FGF set at 1 L/min. However, because the median increase of the filter output was 40% [range: 20–67] when FGF was set at 1 L/min, using the AnaConDa® on a closed-circuit machine with a low FGF produced greater volatile anesthetic conservation than reported in previous studies (8,9). Connecting the AnaConDa® to a closed circuit with a FGF of 1 L/min should improve the volatile anesthetic-conserving properties of a closed circuit anesthesia.

The AnaConDa® filter may be useful in ICUs if the safety issues are resolved. Once the risks are addressed, the device may be useful for sedation and bronchodilation in mechanically ventilated patients. A randomized crossover (10) study in ICU patients showed that propofol- or isoflurane-based long-term sedation aiming at a predefined sedation level led, on average, to similar times to awakening after sedative discontinuation. Thus, recovery time is not a major advantage of a volatile anesthetic for sedation. In contrast, bronchodilation may be a more specific goal AnaConDa® may achieve in intensive care.

However, if the device is used for long term administration of volatile anesthetics, the risk of inorganic fluoride accumulation must be addressed, as this does not seem to cause detectable renal toxicity in humans (11–13). A scavenging system will be needed to prevent pollution of ambient air. A charcoal cartridge effectively prevents pollution (14). We believe this is likely useful, although others have not found high levels of pollution when using the AnaConDa® with no scavenging system (15).

In conclusion, the performance of the AnaConDa® varies with ventilator settings in a predictable manner, at least during bench testing. The AnaConDa® adds to the anesthetic-sparing effect of conventional low-FGF closed-circuit anesthesia machines, and may be of use during administration of volatile anesthetics in the operating room. However, there are safety issues that should be addressed with the current device.

	Footnotes

 
Accepted for publication September 21, 2006.

Supported by Pôle d’Anesthésie Réanimation.

Author correspondence and reprint requests to Laurent Beydon, MD, Pôle d’Anesthésie Réanimation, CHU d’Angers, 49933 Angers Cedex 09, France. Address e-mail to lbeydon.angers{at}invivo.edu.

	REFERENCES

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