[Low-flow and minimal-flow anesthesia with sevoflurane]

Desflurane consumption with automated vapour control systems in two different anaesthesia machines. A randomized controlled study

BACKGROUND

In general anaesthesia practice a fresh gas flow (FGF) of ≥0.5 L/min is usually applied. Automated gas delivery devices are developed to reduce volatile anaesthetic consumption by limiting gas flow. This study aimed to compare desflurane consumption between automated gas control devices compared to conventional low flow anaesthesia in the Flow-I and Aisys anaesthesia machines, and to compare desflurane consumption between the two automated gas delivery devices. We hypothesised that desflurane consumption would be lower with automated gas delivery compared to conventional low flow anaesthesia, and that desflurane consumption could differ between the different gas delivery devices.

METHODS

We allocated 160 patients undergoing robot-assisted laparoscopic surgery into four groups, Flow-I with automated gas control, Flow-i with conventional low-flow (1 L/min), Aisys with end tidal gas control and Aisys with conventional low flow. Patients were maintained at minimum alveolar concentration (MAC) 0.7-0.8. Desflurane consumption was recorded after 9, 30 and 60 minutes of anaesthesia.

RESULTS

After 60 minutes, compared to conventional low flow anaesthesia, automated gas delivery systems reduced desflurane consumption from 25.8 to 15.2 mL for the Aisys machine (P < .001) and from 22.1 to 16.8 mL for the Flow-I (P < .001). Time to MAC 0.7 and stable FGF was shorter with Aisys endtidal control compared to Flow-I automated gas control.

CONCLUSION

Under clinical conditions, we found a reduction in desflurane consumption when using automated gas delivery devices compared to conventional low flow anaesthesia. Both devices were reliable in use.

End-tidal control vs. manually controlled minimal-flow anesthesia: a prospective comparative trial

BACKGROUND

To ensure safe general anesthesia, manually controlled anesthesia requires constant monitoring and numerous manual adjustments of the gas dosage, especially for low- and minimal-flow anesthesia. Oxygen flow-rate and administration of volatile anesthetics can also be controlled automatically by anesthesia machines using the end-tidal control technique, which ensures constant end-tidal concentrations of oxygen and anesthetic gas via feedback and continuous adjustment mechanisms. We investigated the hypothesis that end-tidal control is superior to manually controlled minimal-flow anesthesia (0.5 l/min).

METHODS

In this prospective trial, we included 64 patients undergoing elective surgery under general anesthesia. We analyzed the precision of maintenance of the sevoflurane concentration (1.2-1.4%) and expiratory oxygen (35-40%) and the number of necessary adjustments.

RESULTS

Target-concentrations of sevoflurane and oxygen were maintained at more stable levels with the use of end-tidal control (during the first 15 min 28% vs. 51% and from 15 to 60 min 1% vs. 19% deviation from sevoflurane target, P < 0.0001; 45% vs. 86% and 5% vs. 15% deviation from O₂ target, P < 0.01, respectively), while manual controlled minimal-flow anesthesia required more interventions to maintain the defined target ranges of sevoflurane (8, IQR 6-12) and end-tidal oxygen (5, IQR 3-6). The target-concentrations were reached earlier with the use of end-tidal compared with manual controlled minimal-flow anesthesia but required slightly greater use of anesthetic agents (6.9 vs. 6.0 ml/h).

CONCLUSIONS

End-tidal control is a superior technique for setting and maintaining oxygen and anesthetic gas concentrations in a stable and rapid manner compared with manual control. Consequently, end-tidal control can effectively support the anesthetist.

The effect of heat-moisture exchanger and closed-circuit technique on airway climate during desflurane anesthesia

PURPOSE

We assessed whether closed-circuit anesthesia (CCA) could provide a more favorable airway climate than semi-closed anesthesia (SCA), and we also determined the beneficial effect of heat moisture exchangers (HMEs) on the preservation of airway climate during desflurane anesthesia.

METHODS

Forty patients scheduled for colorectal surgery (n = 10 for each group) were randomized to receive a fresh gas flow of 250 or 3000 ml.min(-1) with or without HMEs. Anesthesia was maintained by adjusting the inspired concentration of 6% desflurane. Absolute moisture and temperature of inspired gases were measured as the baseline value first at 5 min after tracheal intubation, and then at 10, 20, 45, 60, 90, and 120 min after the induction of anesthesia.

RESULTS

At 120 min, the inspiratory humidity and temperature were higher in CCA than in SCA. The HME led to major improvements of the humidity (from 22.1 to 35.7 mg H(2)O.l(-1)) and temperature (from 23.6 degrees C to 31.5 degrees C) of anesthetic gases in the CCA group.

CONCLUSION

CCA was much more advantageous than SCA for maintaining the patient's airway climate during the 2-h study. The beneficial effect of HME on the airway climate should be emphasized, especially in patients undergoing general anesthesia.

Low flow and economics of inhalational anaesthesia

Even when anaesthesia does not represent a major part of the expense of a given surgical operation, reducing costs is not negligible because the large number of patients passing through a department of anaesthesia accounts for a huge annual budget. Volatile anaesthetics contribute 20% of the drug expenses in anaesthesia, coming just behind the myorelaxants; however, the cost of halogenated agents has potential for savings because a significant part of the delivered amount is wasted when a non- or partial-rebreathing system is used. The cost of inhaled agents is related to more than the amount taken up; it also depends on their market prices, their relative potencies, the amount of vapour released per millilitre of liquid, and last but not least the fresh-gas flow rate (FGF) delivered to the vaporizer--the most important factor determining the cost of anaesthesia. Poorly soluble agents like desflurane and sevoflurane facilitate the control of low-flow anaesthesia and reduce the duration of temporary high-flow phases to rapidly wash in or adjust the circuit gas concentrations. Modelling low-flow or minimal-flow anaesthesia will help anaesthetists to understand the kinetics of inhaled agents in those circumstances and to design their own clinical protocols. The monitoring facilities present on modern anaesthesia machines should convince clinicians that low- or even minimal-flow anaesthesia would not jeopardize the safety of their patients. Cost containment requires primarily a decrease in FGFs, but it may also be influenced by a rational use of the available halogenated agents. Isoflurane, the cheapest generic agent, might be advantageous for maintenance of anaesthesia of less than 3 hours. Sevoflurane is the agent of choice for inhalational induction and might also be used for maintenance. Desflurane might be preferred for long anaesthetics where rapid recovery will generate savings in the PACU.

Time course of inhaled anaesthetic drug delivery using a new multifunctional closed-circuit anaesthesia ventilator. In vitro comparison with a classical anaesthesia machine †

BACKGROUND

The aim of this study was to detail the time-course, defined as the changes in end-tidal drug concentration with time, and consumption of inhaled anaesthetics when using a multifunctional closed-circuit anaesthesia machine in various drug delivery modes, and to compare it with a classical anaesthesia machine using an out-of-circle vaporizer under high and low fresh gas flow conditions.

METHODS

Using an artificial test lung, sevoflurane and desflurane time-course and consumption were compared when using the Zeus apparatus (Dräger, Lubeck, Germany) with direct injection of inhaled anaesthetics or the Primus apparatus (Dräger, Lubeck, Germany) using a classical out-of-circle vaporizer. Anaesthetics were targeted at 1 and 2 MAC end-tidal during 15 min. For both apparatus, out-of-circle high and low fresh gas control (FGC) and for Zeus, auto-control (AC) modes (fixed fresh gas flow at 6 and 1 litre min(-1) and uptake mode) were compared. Time to reach target, initial overshoot and stability at target, and wash-out times were compared.

RESULTS

In FGC, an initial overshoot in end-tidal drug concentration is seen when using 6 litre min(-1) fresh gas flow and a slower time course is observed when using only 1 litre min(-1) in both apparatus. In auto-control mode, the time course of both sevoflurane and desflurane was very fast and not influenced by the changes in fresh gas flow. No overshoot at target was seen. At all settings, the wash-out times were faster when using Zeus than Primus. Inhaled anaesthetic consumption was lowest with the Zeus ventilator in uptake AC mode.

CONCLUSION

A combination of the fastest time course and lowest consumption of sevoflurane and desflurane was found when using the Zeus apparatus in AC uptake mode.

The quotient end-tidal/inspired concentration of sevoflurane in a low-flow system

STUDY OBJECTIVE

To investigate the effect of two different fresh gas flows on inspired and end-tidal sevoflurane concentration for a given vaporizer setting in a low-flow anesthesia system.

DESIGN

Prospective clinical study.

SETTING

Department of Anesthesiology of a university teaching hospital.

PATIENTS

56 ASA physical status I and II patients without systemic diseases, having elective surgery with an expected anesthesia time of at least 120 minutes.

INTERVENTIONS

Patients were randomly assigned to receive either 1.0 or 2.0 L/min fresh gas flow with the vaporizer setting fixed at 2% sevoflurane. The inspired (In), end-tidal (Et), and Et/In ratio sevoflurane concentrations were estimated.

MEASUREMENTS AND MAIN RESULTS

After 120 minutes of sevoflurane anesthesia the inspired and end-tidal sevoflurane concentration were 1.45 +/- 0.10% versus 1.28 +/- 0.12% (p < 0.001) in the 1.0 L/min group and 1.64 +/- 0.08% versus 1.46 +/- 0.11% (p < 0.001) in the 2.0 L/min group. The ratio end-tidal and inspired concentrations/vaporizer setting was 0.64 +/- 0.06 and 0.73 +/- 0.05 in the 1.0 L/min group versus 0.73 +/- 0.05 and 0.82 +/- 0.04 in the 2.0 L/min group. For the ratio inspired and end-tidal/vaporizer setting there were significant difference between the groups (p < 0.001). The estimated ratio end-tidal/inspired was 0.88 +/- 0.04 in the 1.0 L/min group versus 0.89 +/- 0.04 in the 2.0 L/min group (ns).

CONCLUSION

After 120 minutes of sevoflurane anesthesia at a vaporizer setting of 2% there is a significant difference between fresh gas flow of 1.0 and 2.0 L/min for inspired and end-tidal concentrations, but not for the ratio end-tidal/inspired.

Quantitative Determination of Vapor-Phase Compound A in Sevoflurane Anesthesia Using Gas Chromatography–Mass Spectrometry

Background: During low-flow or closed-circuit anesthesia with the fluorinated inhalation anesthetic sevoflurane, compound A, an olefinic degradation product with known nephrotoxicity in rats, is generated on contact with alkaline CO2 adsorbents. To evaluate compound A formation and thus potential sevoflurane toxicity, a reliable and reproducible assay for quantitative vapor-phase compound A determination was developed.

Methods: Compound A concentrations were measured by fully automated capillary gas chromatography–mass spectrometry with cryofocusing. Calibrators of compound A in the vapor phase were prepared from liquid volumetric dilutions of stock solutions of compound A and sevoflurane in ethyl acetate. 1,1,1-Trifluoro-2-iodoethane was chosen as an internal standard. The resulting quantitative method was fully validated.

Results: A linear response over a clinically useful concentration interval (0.3–75 μL/L) was obtained. Specificity, sensitivity, and accuracy conformed with current analytical requirements. The CVs were 4.1–10%, the limit of detection was 0.1 μL/L, and the limit of quantification was 0.3 μL/L. Analytical recoveries were 100.6% ± 10.1%, 102.5% ± 7.3%, and 99.0% ± 4.1% at 0.5, 10, and 75 μL/L, respectively. The method described was used to determine compound A concentrations during simulated closed-circuit conditions. Some of the resulting data are included, illustrating the practical applicability of the proposed analytical approach.

Conclusions: A simple, fully automated, and reliable quantitative analytical method for determination of compound A in air was developed. A solution was established for sampling, calibration, and chromatographic separation of volatiles in an area complicated by limited availability of sample volume and low concentrations of the analyte.