[Low flow anesthesia]

Cost analysis of two anaesthetic machines: "Primus®" and "Zeus®"

Background

Two anaesthetic machines, the "Primus^®" and the "Zeus^®" (Draeger AG, Lübeck, Germany), were subjected to a cost analysis by evaluating the various expenses that go into using each machine.

Methods

These expenses included the acquisition, maintenance, training and device-specific accessory costs. In addition, oxygen, medical air and volatile anaesthetic consumption were determined for each machine.

Results

Anaesthesia duration was 278 ± 140 and 208 ± 112 minutes in the Primus^® and the Zeus^®, respectively. The purchase cost was €3.28 and €4.58 per hour of operation in the Primus^® and the Zeus^®, respectively. The maintenance cost was €0.90 and €1.20 per hour of operation in the Primus^® and the Zeus^®, respectively. We found that the O₂ cost was €0.015 ± 0.013 and €0.056 ± 0.121 per hour of operation in the Primus^® and the Zeus^®, respectively. The medical air cost was €0.005 ± 0.003 and €0.016 ± 0.027 per hour of operation in the Primus^® and the Zeus^®, respectively. The volatile anaesthetic cost was €2.40 ± 2.40 and €4.80 ± 4.80 per hour of operation in the Primus^® and the Zeus^®, respectively.

Conclusion

This study showed that the "Zeus^®" generates a higher cost per hour of operation compared to the "Primus®".

Desfluran und Isofluran bei Niedrigflussnarkosen

BACKGROUND

In the present investigation we compared the consumption of desflurane (DES) and isoflurane (ISO) using a standardized minimal-flow protocol with a forced reduction of the fresh gas flow (FGF).

METHODS

54 adult women were examined. After induction of anaesthesia a forced reduction of the FGF was started: 5 min 0.5 l/min O(2) + 1 l/min N(2)O, 10 min 0.5 l/min O(2) + 0.5 l/min N(2)O; finally 0.3 l/min O(2) + 0.2 l/min N(2)O up to the end of surgery. The consumption of DES/ISO was determined with a precision balance.

RESULTS

In the DES group the uptake was around 0.3 vol-%, i.e. less than 8% of the target 2/3 MAC value was taken up. For ISO the uptake was around 0.25 vol-%, i.e. the uptake was approximately 30% of the target 2/3-MAC value. The DES consumption was after 60 min 17.0+/-1.1 g, 120 min--27.3+/-1.8 g and 180 min--36.5+/-1.7 g. ISO consumption was significantly lower: 7.6+/-0.8 g, 12.4+/-1.7 g and 15.5+/-1.6 g. The use of DES yielded higher costs, i.e. 2.28 EUR for 60 min, 3.63 EUR for 120 min and 4.97 EUR for 180 min. The consumption of the inhaled anaesthetics can be calculated as: DES (g)=4.84+0.184 x duration (min) (R(2)=0.981), ISO (g)=2.049+0.0826 x duration (R(2)=0.979). The costs are: DES (EUR)=0.85+0.0323 x duration (min); ISO (EUR)=0.19+0.0077 x duration (min).

CONCLUSION

With a forced reduction of the FGF the DES consumption is still higher.

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.

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.

[Low-flow anesthesia with desflurane]

OBJECTIVES

Due to its low solubility and negligible metabolism, desflurane is assumed to be especially suitable for application by low-flow anaesthetic techniques. The aim of this clinical investigation was the development of a standardised dosing scheme for low-flow and minimal-flow desflurane anaesthesia.

METHODS

One hundred six ASA status I-II patient were assigned to six groups according to the duration of the initial high-flow phase, fresh gas flow, and fresh-gas desflurane concentration. The median age, height, body weight, and constitution of the groups was comparable. After an initial high-flow phase using 4.4 l/min, the fresh gas flow was reduced to 0.5 l/min (minimal-flow anaesthesia) or 1.0 1/min (low-flow anaesthesia). Inspired nitrous oxide concentrations were maintained at 60% to 70%. Using different standardised schemes of vaporizer settings, inspired desflurane concentrations were applied in the range from 3.4% to 8.7%, i.e., between 1 and 1.5 MAC. Inspired and expired desflurane concentrations were measured continuously by the side-stream technique and recorded on-line. Venous blood samples were taken immediately prior to induction and 45 min after flow reduction for measurement of carboxyhaemoglobin (COHb) concentration).

RESULTS

In the 10- to 15-min initial phase during which a high fresh gas flow of 4.4 l/min was used, the inspired desflurane concentration reached values in the range of 90%-95% of the fresh gas concentration. In low-flow anaesthesia this concentration could be maintained without any alteration of the vaporizer setting, whereas in minimal-flow anaesthesia with flow reduction the fresh gas concentration had to be increased by 1% to 2%: The quotient calculated by division of the inspired desflurane concentration by its fresh gas concentration (Q = CI/CF) ranges between 0.65 and 0.75 in animal-flow and between 0.80 and 0.85 in low-flow anaesthesia. If use was made of the wide output range of the desflurane vaporizer, the inspired concentration could be increased rapidly by about 5% in 8 min, although the flow was kept constant at 0.5 l/min. Compared with its value prior to induction (2.13 +/- 1.05%), the COHb concentration decreased statistically significantly by about 0.7% during the 1st hour of minimal-flow anaesthesia (1.42 +/- 1.01%). In no case was a COHb concentration observed that exceeded threatening or even toxic values, although the soda lime was changed routinely only once a week.

CONCLUSIONS

The pharmacokinetic properties of desflurane, resulting in especially low individual uptake, and the wide output range of the vaporizer facilitate the use of low-flow anesthetic techniques in routine clinical practice. Even in minimal-flow anesthesia, the duration of the initial high-flow phase can be shortened to min. If the flow is reduced to 1 l/min, the inspired desflurane concentration achieved in the initial high-flow phase can be maintained without any alteration of the vaporizer setting. In minimal-flow anesthesia, however, with flow reduction to 0.5 l/min, the fresh gas concentration has to be increased to a value 1%-2% higher than the inspired nominal value. Due to the wide dialing range of the desflurane vaporizer, the amount of vapour delivered into the breathing system can be increased to about 110 ml/min even at a flow of 0.5 l/min. The large amount of agent that can be delivered into the system even under low-flow conditions, together with the very low individual uptake, results in a time-constant that is sufficient short for the clinically required rapid increase in inspired desflurane concentrations. The short time-constant of low-flow desflurane anaesthesia improves the control of the anaesthetic concentration. If all measures are taken to safely avoid inadvertent drying out of the soda lime, there is no evidence that low-flow anaesthesia with desflurane is liable to increase the risk of accidental carbon monoxide poisoning. (ABSTRACT TRUNCATED)

Low-flow anaesthesia

Although many anaesthesia machines are equipped with circle rebreathing systems, inhalational anaesthesia remains frequently performed using relatively high fresh-gas flows. The major advantages of rebreathing techniques can be achieved only if the fresh-gas flow is reduced to 1 l.min-1 or less. Although there are potential risks associated with low-flow anaesthesia, modern anaesthesia machines meet all the technical requirements for the safe use of low-flow techniques if they are used in conjunction with equipment for monitoring inhaled and exhaled gas concentrations; these monitors are already increasingly available and, in the near future, are likely to become an obligatory safety standard in many countries. For both economic and ecological reasons, the use of new inhalational anaesthetics, with low tissue solubility and low anaesthetic potency, can be justified only if the efficiency of administration is optimised by using low-flow anaesthetic techniques.