Low flow and closed circuits

Guidelines for reducing the environmental impact of general anaesthesia

OBJECTIVE

To provide guidelines for reducing the environmental impact of general anaesthesia.

DESIGN

A committee of ten experts from SFAR and SF2H and SFPC learned societies was set up. A policy of declaration of competing interests was applied and observed throughout the guideline-writing process. Likewise, it did not benefit from any funding from a company marketing a health product (drug or medical device). The committee followed the GRADE® method (Grading of Recommendations Assessment, Development and Evaluation) to assess the quality of the evidence on which the recommendations were based.

METHODS

We aimed to formulate recommendations according to the GRADE® methodology for three different fields: anaesthesia vapours and gases; intravenous drugs; medical devices and the working environment. Each question was formulated according to the PICO format (Population, Intervention, Comparator, Outcome). The literature review and recommendations were formulated according to the GRADE® methodology.

RESULTS

The experts' work on the synthesis and application of the GRADE® method led to the formulation of 17 recommendations. Since the GRADE® method could not be entirely applied to all of the questions, some of the recommendations were formulated as expert opinions.

CONCLUSION

Based on strong agreement between experts, we produced 17 recommendations designed to guide reducing the environmental impact of general anaesthesia.

Target-controlled inhalational anesthesia-isoflurane consumption with adequacy of anesthesia monitoring in conventional and multimodal analgesia – A comparative study

Background:

In a time of increased concern over the environmental impact of chlorofluorocarbons, there is an impetus to minimize inhalational anesthetic consumption. It is possible with multimodal analgesia (MMA) and the use of end-tidal controlled anesthesia (EtCA) which is a low-flow anesthesia technique with adequacy of anesthesia (AoA) monitoring. In MMA, all four elements of pain processing namely transduction, transmission, modulation, and perception are targeted with drugs having a different mechanism of action. In EtCA, anesthetic gases are automatically adjusted for the set minimal alveolar concentration by newer anesthesia work station (GE Healthcare Aisys CS2). AoA is a derived parameter of entropy and surgical pleth index which measures the depth of anesthesia and analgesia, respectively.

Aim:

The aim is to assess the difference in isoflurane consumption between MMA and conventional groups for a given period of time with EtCA and AoA monitoring.

Setting and Design:

This was a prospective randomized controlled trial involving 60 patients undergoing laparoscopic cholecystectomy. They were divided into MMA group and conventional group.

Materials and Methods:

Both the groups received preemptive intravenous diclofenac sodium 75 g and 2% xyloadrenaline infiltration at entry ports. MMA group in addition received paracetamol 1 g and clonidine 0.75 μg.kg^{− 1}. Intraoperatively, patients were on EtCA with AoA monitoring.

Statistical Analysis:

Mean differences in isoflurane consumption between the two groups were compared using an independent t-test. Postextubation adverse effects of analgesic drugs and awareness under general anesthesia were compared using the Chi-square test and presented as numbers and percentages. P < 0.05 was considered a statistically significant.

Results:

Mean isoflurane consumption in the conventional group was 12.7 ± 5.3 mL which was significantly higher than the MMA group which was 8.9 ± 4.1 mL (P = 0.002). The duration of anesthesia between the groups was not significant clinically (P = 0.931).

Conclusion:

EtCA with MMA significantly reduces isoflurane consumption compared to the conventional group of anesthesia.

Minimal fresh gas flow sevoflurane anesthesia and postoperative acute kidney injury in on-pump cardiac surgery: a randomized comparative trial

BACKGROUND

Compound A is generated by sevoflurane when it reacts with carbon dioxide absorbers with strong bases at minimal fresh gas flow (FGF) and is nephrotoxic in animals. No conclusive data has shown increased risk in humans. The aim of this study was to investigate if minimal FGF promotes an increase in the incidence of acute kidney injury (AKI) when compared to high FGF in patients undergoing on-pump cardiac surgery under sevoflurane anesthesia.

METHODS

Two hundred and four adult patients scheduled for on-pump cardiac surgery under sevoflurane anesthesia were randomly allocated to two groups differentiated by FGF: minimal FGF (0.5 L.min^-1) or high FGF (2.0 L.min^-1). Baseline creatinine measured before surgery was compared daily to values assayed on the first five postoperative days, and 24-hour urinary output was monitored, according to the KDIGO (Kidney Disease Improving Global Outcomes) guideline to define postoperative cardiac surgery-associated acute kidney injury (CSA-AKI). Creatinine measurements were also obtained 20 and 120 days after hospital discharge.

RESULTS

Postoperative AKI occurred in 55 patients, 26 patients (29.5%) in the minimal FGF group and 29 patients (31.5%) in the high FGF group (p = 0.774). Twenty days after discharge, 11 patients (6.1%) still had CSA-AKI and 120 days after discharge only 2 patients (1.6%) still had CSA-AKI.

CONCLUSIONS

When compared to high FGF, minimal FGF sevoflurane anesthesia during on-pump cardiac surgery is not associated with increased risk of postoperative AKI in this population at high risk for renal injury.

Feasibility, safety, and economic consequences of using minimal flow anaesthesia by Maquet FLOW-i equipped with automated gas control

Low fresh gas flow rates are recommended because of their benefits, however, its use is limited due to associated risks. The main purpose of this study was to investigate whether 300 mL of fresh gas flow that practised with automated gas control mode is applicable and safe. The second aim is to show that automated mode can provide economic benefits. Sixty hepatectomy cases who suitable criterias were included to cohort study in three groups as prospective, sequential, observational. An operating room were allocated only for this study. 300 mL fresh gas flow with automated mode (groupA3), 600 mL fresh gas flow with automated mode (groupA6) and, 600 mL fresh gas flow with manually (groupM6) was applied. Patients’ respiratory, hemodynamic parameters (safety), number of setting changes, O₂ concentration in the flowmeter that maintained FiO₂:0.4 during the low flow anaesthesia (feasibility) and comsumption data of anaesthetic agent and CO₂ absorber (economical) were collected and compared. p < 0.05 was accepted as statistical significance level. No significant differences were detected between the groups in terms of demographic data and duration of operation. Safety datas (hemodynamic, respiratory, and tissue perfusion parameters) were within normal limits in all patients. O₂ concentration in the flowmeter that maintained FiO₂:0.4 was statistically higher in groupA3 (92%) than other groups (p < 0.001) but it was still within applicable limits (below the 100%). Number of setting changes was statistically higher in groupM6 than other groups (p < 0.001). The anaesthetic agent consumption was statistically less in groupA3 (p = 0.018). We performed fresh gas flow of 300 mL by automated mode without deviating from the safety limits and reduced the consumption of anaesthetic agent. We were able to maintain FiO₂:0.4 in hepatectomies without much setting changes, and we think that the automated mode is better in terms of ease of practise.

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.

Feasibility, safety, and economic consequences of using low flow anesthesia according to body weight

BACKGROUND

Low flow anesthesia (LFA) provides a saving up to 75% and improves the dynamics of inhaled anesthesia gas, increases mucociliary clearance, maintains body temperature, and reduces water loss. LFA has been recommended for anesthesiologists in recent years to avoid high fresh gas flow (FGF). However, LFA use is limited due to associated risks. The main purpose of this study was to investigate whether LFA according to body weight, which is the main determinant of oxygen requirement, is feasible and safe in the normoxia range. The second aim was to show that this method can provide economic benefit.

METHODS

Eighty donor hepatectomy cases were included to study in two groups as prospective, observational. A surgery room and a team were allocated only for this study. Considering the oxygen requirement (approximately 3-3.5 mL/kg/min), for the first 40 cases, 10 mL/kg (group 10) FGF was applied; for the second 40 cases, 20 mL/kg (group 20) was applied. Desflurane (Suprane©) was used as an inhalation agent, and analgesia was achieved with remifentanil infusion. Patients' demographic, respiratory, hemodynamic, and tissue perfusion parameters (SpO₂ and NIRS), and comsumption data (anesthetic agent and CO₂ absorbent) were collected and compared.

RESULTS

No significant differences were detected between the groups in terms of demographic data, duration of surgery, and hemodynamic, respiratory, and tissue perfusion parameters. These parameters were within normal limits in all patients at all times. The maximum O₂ concentration in the FGF that maintained FiO₂:0.4 and provided adequate oxygenation during the LFA was 61% (min 56%; max 67%) in group 10, and 47% (min 43%; max 51%) in group 20. The hourly anesthetic agent consumption was significantly different in group 10 than in group 20 (12.4 ± 4 mL vs. 21.5 ± 8 mL/h, respectively (p < 0.001).

CONCLUSIONS

We performed 10 mL/kg FGF speed without deviating from the safety limits to be FiO₂:0.4 in donor hepatectomies, reducing the total costs 38% compared with 20 mL/kg FGF.

Evaluation of emergence agitation after general anaesthesia in rhinoplasty patients: Inhalation anaesthesia versus total intravenous anaesthesia

BACKGROUND

Emergence agitation (EA) is a clinical condition that occurs early in recovery from general anaesthesia, and reduces patient comfort. The aim of this study was to compare the effects of low-flow sevoflurane anaesthesia and total intravenous anaesthesia (TIVA) on agitation in rhinoplasty patients, and to determine the frequency of EA in low flow sevoflurane anaesthesia after rhinoplasty.

MATERIAL AND METHODS

A total of 90 rhinoplasty patients, under general anaesthesia were included in this prospective randomised study. After induction of anaesthesia, propofol infusion was initiated in the TIVA group (n = 45), and sevoflurane was administered in the SEVO group with a fresh gas flow of 1 l/min and MAC (minimum alveolar concentration) 1-1.1 (n = 45). Early emergence times, Richmond agitation-sedation scale (RASS), Boezaart scale, Likert scale and incidences of nausea/vomiting were recorded at the end of surgery.

RESULTS

Early emergence time was significantly shorter in the TIVA group, than in the SEVO group (p < 0.001). İntraoperative bleeding was significantly lower in the TIVA group, than in the SEVO group (p = 0.005), and surgical field image quality and surgeon satisfaction were better in the TIVA group (p = 0.016, p < 0.001). The ratio of patients with RASS > +1 for all patients was 35.6% at 0 min, postoperatively. This rate was 12.2% (n = 11) in the TIVA group, and 23.3% (n = 21) in the SEVO group (p = 0.028).

CONCLUSIONS

In rhinoplasty, TIVA caused shorter early emergence times, less bleeding, high surgeon satisfaction, and lower EA scores when compared with low flow sevoflurane anaesthesia.

Cost-effectiveness of Basal Flow Sevoflurane Anaesthesia Using the Komesaroff Vaporizer inside the Circle System

After ethics committee approval, 51 consenting ASA physical status 1 or 2 adult patients were given basal flow sevoflurane anaesthesia using fresh gas flows of 150 to 300 ml.min-1 oxygen. A Komesaroff vaporizer was placed on the inspiratory limb of the circle system. Basal flows were introduced immediately following intravenous induction of anaesthesia. The vaporizer was set to deliver the maximum concentration until the inspired sevoflurane concentration (FSI) reached 3%. The dial was then adjusted to maintain the FSI at 3%. After every 60 minutes, the circuit was washed out with 100% oxygen at a flow rate of 10 l.min-1 for one minute.

The FSI reached 3% after an average of 8.5 (3.8) [mean (SD)] minutes. The trends in FSI and the expired sevoflurane concentrations were significantly different (P<0.05) between the mechanically ventilated patients (n=21) and the spontaneously ventilating patients (n=30) and demonstrated a more gradual build-up in the former group. The consumption of sevoflurane was found to be 9.2 (2.8) ml.h-1. This represented a 52.5% cost saving over the clinical application of the Mapleson's ideal fresh gas flow sequence for low-flow anaesthesia.

Occupational hazards, DNA damage, and oxidative stress on exposure to waste anesthetic gases

Background and objectives

The waste anesthetic gases (WAGs) present in the ambient air of operating rooms (OR), are associated with various occupational hazards. This paper intends to discuss occupational exposure to WAGs and its impact on exposed professionals, with emphasis on genetic damage and oxidative stress.

Content

Despite the emergence of safer inhaled anesthetics, occupational exposure to WAGs remains a current concern. Factors related to anesthetic techniques and anesthesia workstations, in addition to the absence of a scavenging system in the OR, contribute to anesthetic pollution. In order to minimize the health risks of exposed professionals, several countries have recommended legislation with maximum exposure limits. However, developing countries still require measurement of WAGs and regulation for occupational exposure to WAGs. WAGs are capable of inducing damage to the genetic material, such as DNA damage assessed using the comet assay and increased frequency of micronucleus in professionals with long-term exposure. Oxidative stress is also associated with WAGs exposure, as it induces lipid peroxidation, oxidative damage in DNA, and impairment of the antioxidant defense system in exposed professionals.

Conclusions

The occupational hazards related to WAGs including genotoxicity, mutagenicity and oxidative stress, stand as a public health issue and must be acknowledged by exposed personnel and responsible authorities, especially in developing countries. Thus, it is urgent to stablish maximum safe limits of concentration of WAGs in ORs and educational practices and protocols for exposed professionals.

The closed circuit and the low flow systems

A breathing system is defined as an assembly of components, which delivers gases from the anesthesia machine to the patients’ airways. When the components are arranged as a circle, it is termed a circle system. The flow of exhaled gases is unidirectional in the system. The system contains a component (absorber), which absorbs exhaled carbon dioxide and it is not necessary to give high fresh gas flows as in Mapleson systems. When the adjustable pressure limiting (APL) valve is closed and all the exhaled gases without carbon dioxide are returned to the patient, the system becomes a totally closed one. Such a circle system can be used with flows as low as 250 to 500 mL and clinically can be termed as low-flow systems. The components of the circle system can be arranged in different ways with adherence to basic rules: (1) Unidirectional valve must be present between the reservoir bag and the patient on both inspiratory and expiratory sides; (2) fresh gas must not enter the system between the expiratory unidirectional valve and the patient; and (3) the APL valve must not be placed between the patient and the inspiratory unidirectional valve. The functional analysis is explained in detail. During the function, the arrangement of components is significant only at higher fresh gas flows. With the introduction of low resistance valves, improved soda lime canisters and low dead space connectors, the use of less complicated pediatric circle systems is gaining popularity to anesthetize children. There are bidirectional flow systems with carbon dioxide absorption. The Waters to and fro system, a classic example of bidirectional flow systems with a canister to absorb carbon dioxide, is valveless and portable. It was widely used in the past and now is only of historical importance.

Effects of flow rate on hemodynamic parameters and agent consumption in low-flow desflurane anesthesia: An open-label, prospective study in 90 patients

BACKGROUND

In surgical patients, decreasing the fresh gas flow rate in anesthesia may minimize costs, reduce environmental pollution, and preserve heat and humidity in the respiratory system.

OBJECTIVE

The aim of this study was to investigate the effects of 3 low-flowdesflurane rates on perioperative hemodynamic stability, end-tidal desflurane concentration, emergence and recovery characteristics, and agent consumption.

METHODS

This open-label, prospective study was conducted at the Departmentof Anesthesiology and Reanimation, University of Gaziantep, Gaziantep, Turkey. Nonpremedicated adult patients scheduled to undergo surgery (ureterolithotomy, cholecystectomy, pyelolithotomy, or thyroidectomy) were enrolled. Patients were anesthetized with propofol and fentanyl and intubated after neuromuscular blockade with vecuronium. Patients were randomly allocated to 1 of 3 groups according to the fresh gas flow rate: medium flow (2 L/min), low flow (1 L/min), and minimal flow (0.5 L/min). Intraoperative fentanyl volume was recorded. Heart rate, mean arterial pressure, and end-tidal desflurane concentration were recorded before (baseline) and after anesthesia induction; immediately before incision; and 5, 10, 15, 30, 45, and 60 minutes after incision. Emergence time and desflurane consumption after extubation were recorded. Aldrete scores were recorded at 5, 15, and 30 minutes after extubation.

RESULTS

Ninety patients (46 women, 44 men; mean [SD] age, 39.74 [13.73] years; 30 patients per treatment group) participated in the study. Means of hemodynamic parameters, intraoperative volume of fentanyl, end-tidal desflurane concentration, emergence time, and Aldrete score were statistically similar between the 3 groups. Mean (SD) desflurane consumption was significantly higher in the medium-flow group compared with the low- and minimal-flow groups (110.43 [28.18] g vs 98.40 [23.62] g and 79.80 [17.54] g, respectively; both, P < 0.01). Mean (SD) desflurane consumption was also significantly higher in the low-flow group compared with the minimal-flow group (P < 0.01).

CONCLUSION

The results of the present study in adult surgical patients suggestthat desflurane may be used in low-flow anesthesia, even with the minimal fresh gas flow rate.

Comparison of postoperative complication between Laryngeal Mask Airway and endotracheal tube during low-flow anesthesia with controlled ventilation

OBJECTIVE

To compare the postoperative complications between Laryngeal Mask Airway (LMA) and endotracheal tube (ETT) during low-flow anesthesia with controlled ventilation.

METHODOLOGY

Eighty adult Patients with ASA class I or II were randomly allocated into two forty-patient groups (ETT or LMA). Cuff pressure was monitored during anesthesia. After high uptake period, fresh gas flow (FGF) was decreased to 1 lit/min and isoflurane set to 1%. Monitoring during anesthesia included non-invasive blood pressure, ECG, ETCO2 and pulse oximetry. System leakage (>100 ml/min), rebreathing and any attempt to increase FGF to overcome the leak were monitored during anesthesia. Later, patients were extubated and transferred to Post Anesthesia Care Unit (PACU). In PACU, the incidence of sore throat, cough, difficulty in swallowing and shivering was monitored for all patients.

RESULTS

Leakage was observed in two and three cases in ETT and LMA groups respectively (P>0.05). Postoperative cough, sore throat and difficulty in swallowing were significantly less in LMA than ETT group. No significant difference was observed regarding ETCo2 values between 2 groups.

CONCLUSION

If careful measures regarding insertion techniques, correct LMA position and routine monitoring of LMA cuff pressure are taken, LMA can be used as a safe alternative with lower incidence of post operation complication compared with ETT during low-flow controlled anesthesia with modern anesthetic machines.

Low flow anesthesia: Efficacy and outcome of laryngeal mask airway versus pressure-optimized cuffed-endotracheal tube

BACKGROUND

Low flow anesthesia can lead to reduction of anesthetic gas and vapor consumption. Laryngeal mask airway (LMA) has proved to be an effective and safe airway device. The aim of this study is to assess the feasibility of laryngeal mask airway during controlled ventilation using low fresh gas flow (1.0 L/min) as compared to endotracheal tube (ETT).

PATIENTS AND METHODS

Fifty nine non-smoking adult patients; ASA I or II, being scheduled for elective surgical procedures, with an expected duration of anesthesia 60 minutes or more, were randomly allocated into two groups - Group I (29 patients) had been ventilated using LMA size 4 for females and 5 for males respectively; and Group II (30 patients) were intubated using ETT. After 10 minutes of high fresh gas flow, the flow was reduced to 1 L/min. Patients were monitored for airway leakage, end-tidal CO(2)(ETCO(2)), inspiratory and expiratory isoflurane and nitrous oxide fraction concentrations, and postoperative airway-related complications.

RESULTS

Two patients in the LMA-group developed initial airway leakage (6.9%) versus no patient in ETT-group. Cough and sore throat were significantly higher in ETT patients. There were no evidences of differences between both groups regarding ETCO(2), uptake of gases, nor difficulty in swallowing.

CONCLUSION

The laryngeal mask airway proved to be effective and safe in establishing an airtight seal during controlled ventilation under low fresh gas flow of 1 L/min, inducing less coughing and sore throat during the immediate postoperative period than did the ETT, with continuous measurement and readjustment of the tube cuff pressure.

Derivation and Prospective Testing of a Two-step Sevoflurane-O2-N2O Low Fresh Gas Flow Sequence

Simple vaporiser setting (FD) and fresh gas flow (FGF) sequences make the practice of low-flow anaesthesia not only possible but also easy to achieve. We sought to derive a sevoflurane FD sequence that maintains the end-expired sevoflurane concentration (FAsevo) at 1.3% using the fewest possible number of FD adjustments with a previously described O2-N2O FGF sequence that allows early FGF reduction to 0.7 l.min−1.

In 18 ASA physical status I to II patients, FD was determined to maintain FAsevo at 1.3% with 2 l.min−1 O2 and 4 l.min−1 N2O FGF for three minutes, and with 0.3 and 0.4 l.min−1 thereafter. Using the same FGF sequence, the FD schedule that approached the 1.3% FAsevo pattern with the fewest possible adjustments was prospectively tested in another 18 patients.

The following FD sequence approximated the FD course well: 2% from zero to three minutes, 2.6% from three to 15 minutes and 2.0% after 15 minutes. When prospectively tested, median (25th; 75th percentile) performance error was 0.8 (-2.9; 5.9)%, absolute performance error 6.7 (3.3; 10.6)%, divergence 18.2 (-5.6; 27.4)%.h−1 and wobble 4.4 (1.7; 8.1)%. In one patient, FGF had to be temporarily increased for four minutes.

One O2/N2O rotameter FGF setting change from 6 to 0.7 l.min−1 at three minutes and two sevoflurane FD changes at three and 15 minutes maintained predictable anaesthetic gas concentrations during the first 45 minutes in all but one patient in our study.

Low-flow anaesthesia. Does it have potential pharmacoeconomic consequences?

Healthcare reform has placed increasing pressure on anaesthetists to consider the costs of current anaesthesia strategies. Although the cost of anaesthesia constitutes only a small proportion of total healthcare costs, anaesthetic drug expenditures have been a focus of cost-containment efforts. Low-flow anaesthesia is a simple method of reducing the fresh gas flow rate for anaesthetic gases during inhalational anaesthesia. A knowledge of the pharmacokinetic behaviour of inhaled anaesthetics and the use of modern equipment and monitoring technology meet the requirements for safe application of this anaesthetic technique. Millions of patients receive general anaesthesia each year, and thus the use of this technique could generate substantial savings in anaesthetic drug expenditure without reducing the patient's comfort or increasing adverse events. The new inhaled anaesthetics desflurane and sevoflurane, which have low tissue solubility, provide promising options when used in low-flow anaesthesia. Apart from the economic advantages, low-flow anaesthesia helps to reduce environmental pollution and is associated with several physiological benefits for the patient. Low-flow anaesthesia is a simple but highly effective method of cost minimisation that can be applied to a large number of patients without any compromise in patient care or safety.

Closed circuit anaesthesia in ventilated patients using the Komesaroff vaporizer within the circle

A study was undertaken to assess the performance of the Komesaroff vaporizer, placed within the circuit, in ventilated patients during maintenance of closed circuit anaesthesia with halothane or isoflurane. Following intravenous induction, anaesthesia was maintained by inhalation. This was achieved using a conventional vaporizer outside the circle for the first 10 minutes to manage the fast uptake phase. The fresh gas flow was then reduced to the basal oxygen requirement with the Komesaroff vaporizer within the circle maintaining inhalational anaesthesia. Complete isolation of the circuit was achieved by returning all anaesthetic gases to the circuit following analysis and using a bag-in-bottle ventilator. The Komesaroff vaporizer dial was positioned at between the first and second division and end-tidal volatile anaesthetic agent levels were measured. This study demonstrated that at dial positions 1 or 1.5 with either agent, the end-tidal volatile concentration plateaued at clinically acceptable levels. The Komesaroff vaporizer can therefore be used safely in ventilated patients to maintain closed circuit anaesthesia provided clinical observation and monitoring are meticulous.

Reinforcing a "low flow" anaesthesia policy with feedback can produce a sustained reduction in isoflurane consumption

A three-month audit of isoflurane consumption at Palmerston North Hospital in 1994 showed an averaged vapour flow rate of approx 85 ml per minute of anaesthesia, equivalent to 1.4% isoflurane at six litres per minute. After purchasing volatile agent analysers, a program encouraging low flow anaesthesia and providing a report of the previous month's consumption rate was started in July 1996. The isoflurane averaged vapour flow rate was tracked over the following twenty-month period and fell by a sustained 65% to range around 30 +/- 5 ml/min, producing savings of approximately NZ$104,000 over this period.

Low and minimal flow inhalational anaesthesia

PURPOSE

To describe the pharmacokinetic behaviour and practical aspects of low (0.5-1 l.min-1) and minimal (0.25-0.5 l.min-1) flow anaesthesia.

METHODS

A Medline search located articles on low flow anaesthesia, and computer simulated anaesthetic uptake models are used.

PRINCIPAL FINDINGS

Most, 85-90%, of anaesthetists use high fresh gas flow rates during inhalational anaesthesia. Low/minimal flow anaesthesia with a circle circuit may avoid the need for in-circuit humidifiers, raise the temperature of inspired gases by up to 6 degrees C, reduce cost by about 25% by reduction of fresh gas flows to 1.5 l.min-1, and reduce environmental pollution with scavenged gas. Knowledge of volatile anaesthetic pharmacokinetic behaviour facilitates the use of minimal/low flow rates. Small amounts of nitrogen or minute amounts of methane, acetone, carbon monoxide, and inert gases in the circuit are of no concern, but the degradation of desflurane (to carbon monoxide by dry absorbent) and sevoflurane (to compound A by using a fresh gas flow of > 2 l.min-1) must be avoided. With modern gas monitoring technology, safety should be no more of a concern than with high flow techniques.

CONCLUSION

The use of fresh gas flow rates of < 1 l.min-1 for maintenance of anaesthesia has many advantages, and should be encouraged for inhalational anaesthesia with most modern volatile anaesthetics.

Diffusion of anaesthetic gases through different polymers

BACKGROUND

Improvement of working conditions and anaesthesia with closed systems includes reduction of gas leaks during anaesthesia. One source of contamination is the permeation of gases through plastic materials. The volume of gas permeating through a polymer depends on its molecular structure, the solubility and the diffusion coefficient.

METHODS

We designed an experimental set-up to measure the permeation rates of nitrous oxide, enflurane and isoflurane through components of the anaesthetic ventilator made of silicone, latex, rubber and polyvinylchloride (PVC). Reservoir bags, ventilation tubes and endotracheal tubes were investigated.

RESULTS

The highest permeation rates of anaesthetic gases were observed with silicone materials. Permeation through silicone exceeded that of the least permeable material by more than 10.000 times. By summarizing the permeation rates of the single items, the use of silicone increases the anaesthetic system's leakage rate by 4 ml/min, which means an increase of 18% in a modern anaesthetic ventilator and of 31% in a closed system.

CONCLUSIONS

The highest permeation rates were found for nitrous oxide through silicone, although nitrous oxide has a known low solubility in plastic materials. The result demonstrates the dependency of the leakage rate on the diffusibility. The leakage of anaesthetic gases caused by silicone items does not alone lead to unacceptable pollution of operating theatres. To minimize the total leakage rates of minimal-flow-systems, however, plastic materials with low solubility and low diffusion coefficients have to be used.