Preservation of humidity and heat of respiratory gases during anaesthesia - a laboratory investigation

Humidification and heating of anaesthetic gases are desirable to prevent respiratory tract damage and a fall in body temperature during operative procedures. Numerous studies on the humidity and temperature of inspiratory gases in different breathing systems for anaesthesia have been carried out, but comparisons are difficult since different methods have been used. In this laboratory set-up we studied a non-rebreathing system with and without humidifiers and a circle absorber system with low (0.5 l/min) or medium (5 l/min) fresh gas flows regarding their ability to heat and humidify anaesthetic gases. The humidity of inspired gases was acceptable in the non-rebreathing system using either a Bennett Cascade humidifier or disposable humidifiers and in the circle absorber system using a fresh gas flow of 5 l/min or less. The temperature of the inspired gases was highest with the Bennett Cascade humidifier, followed by the low-flow circle system. The circle absorber system used with low fresh gas flow gave higher inspiratory gas temperature and humidity than the non-rebreathing system with a good disposable humidifier.     

Kostenaspekte in der Anästhesie

Cost control is no longer an option, but a necessity. Propofol anaesthesia is expensive, however, the true differences in comparison to volatile anaesthetics (isoflurane) are not known. Methods. Sixty patients undergoing either thyroidectomy (n=30) or laparoscopic cholecystectomy (n=30) were randomly divided into 3 groups of 20 patients. In group I propofol and fentanyl were used for anaesthesia, in group II isoflurane (,standard` isoflurane anaesthesia), and in group III isoflurane using a low-flow system (fresh gas flow 2?l/min) was given. All patients were ventilated using 70% N<sub>2</sub>O in oxygen. Vecuronium was used in all cases for muscle relaxation. Isoflurane consumption was measured by weighing the isoflurane vaporiser. Results. Biometric data and time of administration of the anaesthetic were similar in the three groups. Propofol patients stayed significantly shorter than isoflurane patients in the postanaesthesia care unit (PACU). Costs of additional drugs (antiemetics, analgesics) in the PACU were least in the propofol patients. Costs were without differences between the propofol (78.30 DM/patient) and `standard' isoflurane groups (81.69 DM/patient). Patients in group III showed the lowest overall costs (57.46?DM/patient) (P<0.05). Conclusion. A climate of cost-consciousness and cost-containment prevails at the present time. The costs of propofol and `standard' isoflurane anaesthesia were without differences; however, isoflurane used in a low-flow system had the lowest cost in this study. Doubts are justified, however, as to whether the choice of anaesthetic agents may considerably lower the costs of an anaesthesia department.     

Low-flow anaesthesia

An 8-week survey was conducted to determine whether the introduction of low-flow anaesthesia (a fresh gas flow of 4 litres/minute or less) into routine use would be acceptable to members of a representative anaesthetic department and if the consequent reduction in use of volatile anaesthetics would result in financial savings. The hourly consumption of the volatile agents was measured during anaesthesia conducted using either conventional or low fresh gas flows. Anaesthetists' acceptance of low-flow anaesthesia was assessed using a questionnaire. Data were gathered on 286 patients undergoing inhalational anaesthesia for routine operative procedures. A 54.7% reduction in the consumption of isoflurane and a 55.9% reduction in that of enflurane was found. Of the 28 anaesthetists at the hospital, 21 would use low-flow anaesthesia routinely. The routine use of low-flow anaesthesia would therefore be acceptable and could result in annual savings of 26,870 pounds at Northwick Park Hospital.     

Nitrous oxide: use in low-flow systems/economics

The development of rebreathing systems and the history of the rebreathing technique in anaesthesia are closely related to the use of nitrous oxide (N₂O) as an anaesthetic gas. For many years a mixture of oxygen and N₂O has been used as the carrier gas for delivering inhalational agents, with no thought being given to its true value or disadvantages. Several contra-indications and undesirable effects as well as the need to reduce pollution of the workplace and the atmosphere by this gas are strong arguments for consistently omitting N₂O. This has become all the more apparent as clinical experience has revealed that the use of this anaesthetic gas only subtly, if at all, alters the course of inhalation anaesthesia and the development of untoward outcomes. This chapter focuses on the more technical and practical aspects of N₂O-free inhalation anaesthesia. When N₂O is used as a component of the carrier gas, with a fresh gas flow reduced to 0·5 l/min, i.e. with minimal flow anaesthesia, generally the limits of safe performance of conventional anaesthetic machines are reached. Consistent omission of N₂O, however, considerably facilitates the routine use of fresh gas flows that are even lower than 0·5 l/min. The fresh gas flow can be reduced to just that amount of oxygen taken up by the patient, thus making it possible to even perform closed system anaesthesia with conventional anaesthetic machines. The missing anaesthetic and analgesic effects, resulting from the omission of N₂O, necessitates only a minor increase in the anaesthetic agent concentration and in the dose of supplementarily applied opioids. Nevertheless, if consistently judicious use is made of the low-flow anaesthetic technique an increase in costs can be avoided.     

Increase in the use of rebreathing gas flow systems and in the utilization of low fresh gas flows in Finnish anaesthetic practice from 1995 to 2002

BACKGROUND: The use of rebreathing systems together with low fresh gas flows saves anaesthetic gases, reduces the costs of anaesthesia, causes less environmental and ergonomic adverse effects, i.e. less air contamination in the operating room, and has favourable physiological effects. We assessed whether the use of non-rebreathing vs. rebreathing gas flow systems and high vs. lower fresh gas flows has changed during recent years. METHODS: The use of rebreathing and non-rebreathing systems and the utilization of fresh gas flows were evaluated by sending a questionnaire to the heads of anaesthesia departments at all public health care hospitals in Finland in 1996 and 2003. The data was gathered from the previous years 1995 and 2002, respectively. RESULTS: The use of rebreathing systems increased from 62% to 83% of all instances of general anaesthesia (P < 0.001). In rebreathing gas flow systems, there was a significant shift from high fresh gas flows (3 l min(-1) and more) towards lower fresh gas flows (between 1 to 2 l min(-1) and even below 1 l min(-1)) (P < 0.001). CONCLUSIONS: The benefits of low fresh gas flows have now been achieved in most instances of rebreathing system anaesthesia, which was not the case in 1995.     

Low–flow anaesthesia does not increase the risk of microbial contamination through the circle absorber system

In the circle absorber system, a decrease in fresh gas flow means a higher degree of rebreathing, and, consequently, a higher temperature and humidity within the system. With our present hygienic routines, the circle system tubings are changed and decontaminated once daily. Thus, the same circle system is used for several patients each day. In order to evaluate whether the risk of bacterial contamination increased with the introduction of low-flow anaesthesia, 122 patients anaesthetized with either a low-flow technique (less than 1.5 l fresh gas flow/min) or with medium fresh gas flows (3-6 l/min) were studied. Bacterial samples were taken preoperatively from the oropharynx and postoperatively from five locations in the circle system. The patients were studied postoperatively for signs of respiratory tract infection. There were few positive bacteria cultures from the tubings in the circle system, regardless of fresh gas flow. No pathogens were found in the inspiratory tubings and no cases of postoperative respiratory tract infection could be related to bacterial spread from the anaesthesia machine. There were no indications that the present hygienic management was insufficient for the medium- or the low-flow circle system techniques.     

Xenon expenditure and nitrogen accumulation in closed-circuit anaesthesia

The high price of xenon has prevented its use in routine, clinic anaesthetic practice. Xenon therefore has to be delivered by closed-circuit anaesthesia. The accumulation of nitrogen is a significant problem within the closed circuit and necessitates flushing, which in turn increases gas expenditure and costs. In previous investigations, nitrogen concentrations between 12% and 16% have been reported in closed-circuit anaesthesia. In order to avoid such nitrogen accumulation, we denitrogenised seven pigs using a non-rebreathing system and connected the animals to a system primed with a xenon/oxygen mixture. In comparison, seven pigs were anaesthetised with xenon using a standard low-flow anaesthetic procedure. Anaesthesia time was 2 h. Nitrogen concentrations in the closed system ranged from 0.08 to 7.04% and were not significantly different from those observed during low-flow anaesthesia. Closed-circuit anaesthesia reduced the xenon expenditure 10-fold compared with low-flow anaesthesia.     

GASEOUS HOMEOSTASIS AND THE CIRCLE SYSTEM: Factors Influencing Anaesthetic Gas Exchange

A mathematical model of a subject breathing from a circle systemhas been used to follow the course of anaesthetic uptake duringthe simulated administration of 60% nitrous oxide, 2% halo-thaneand 2% methoxyflurane, under non-re-breathing conditions andwith fresh gas flows to the circle system of between 8 and 0.25litre min^–. Compared with the non-rebreathing state, theuse of a circle system reduced the initial rate of increaseof alveolar towards fresh gas anaesthetic concentration, andthe rate of increase in body anaesthetic content. The degreeof reduction became more marked as fresh gas flow was reduced,and as agents of increasing blood solubility were used. Theseeffects of a circle system were influenced by the volume ofthe circle system and the composition of gas initially presentwithin the system. When the circle system was in use there wereincreases in the magnitude of both the concentration effectand the second gas effect which were related to the magnitudeof fresh gas flow. The use of a circle system augmented theeffects of changes in cardiac output and reduced the effectsof changes in ventilation on the alveolar concentrations ofthe anaesthetic. These influences of a circle system were alsodependent on the magnitude of fresh gas flow. The degree ofaugmentation of the effects of cardiac output decreased withincreasing blood solubility of the agent in use, whilst thelimitation of the effects of ventilation was greatest with theagent of highest blood solubility. Both under non-rebreathingconditions and with the circle system in use, the effects ofcardiac output and ventilation were greater with 2% nitrousoxide than with 60% nitrous oxide, and were also greater whengases were given separately than when administered in combination.     

Efficiency of a circle system for short surgical cases: comparison of desflurane with isoflurane

Patients undergoing short surgical procedures but requiring ventilation of the lungs were allocated randomly to receive either desflurane or isoflurane by circle absorption system, initially at a high fresh gas flow. The inspired and expired concentrations of the volatile agent were measured and the fresh gas flows reduced to low flow (500 ml min-1 total when FE/FI = 0.8), as measured on a multigas analyser. In patients receiving desflurane (n = 32), the median time at which flows were reduced was 5 min (interquartile range (IQR) 1 min) while with isoflurane (n = 32), the median time was 19 (IQR 12) min. After the reduction in flow, expired concentrations of volatile agent decreased in both groups. In the isoflurane group the concentration continued to decrease during anaesthesia. In the desflurane group the initial decrease was followed by a slow recovery. We conclude that the circle system can be used efficiently for short anaesthetics using desflurane.      

Economics of low-flow anaesthesia in children

We have measured the consumption of isoflurane and fresh gas flows in 77 infants and children during 20 all-day operating sessions using either the enclosed Mapleson A or the circle absorber mode of the Garden'Ventmasta'ventilator. The average consumption (SD) of isoflurane in 37 patients anaesthetised using the A mode of the Garden system with a mean fresh gas flow of 2.6l.min⁻¹ was 11.1 (4.2)g.h⁻¹, while that in 40 patients anaesthetised using the circle absorber mode with a mean fresh gas flow of 1.21.min⁻¹ was 4.7 (1.0)g.h⁻¹. These figures represent an overall saving of 58% in the use of isoflurane (p < 0.0001) and a mean reduction in fresh gas flow of 54% (p < 0.0001) as a result of using low-flow anaesthesia. With the addition of small bore breathing hoses the adult circle absorber system was practical to use in both infants and children. These findings should stimulate interest in the use of low-flow techniques in children.     

Ökonomische Aspekte beim Einsatz moderner Inhalationsanästhetika am Beispiel des Sevofluran

The economic impact of the new German health care laws requires an awareness of cost-effectiveness when using newer drugs. The main goal in patient care, i.e., effective treatment, must be achieved by the rational use of restricted resources at a maximum degree of effectiveness. Economic aspects of the new inhalational anaesthetics such as sevoflurane are discussed in this article. The cost of inhalational anaesthetic agents accounts for up to 5% of all the running expenses of an anaesthesia department. The consumption and cost of an inhalational agent depend on fresh gas flow, vapour setting, and duration of anaesthesia. Comparing the cost for 1 MAC-h of anaesthesia, desflurane is more expensive at current market prices than sevoflurane and isoflurane. However, at low or minimal fresh-gas flows, the price for one MAC-h is almost the same for these volatile anaesthetics. Total intravenous anaesthesia using propofol is even more expensive, partly due to wastage, i.e., opened ampoules with a remainder of propofol that has to be discarded after each case. When choosing an anaesthetic agent, the price of 1?ml liquid anaesthetic is an important factor. However, the overall cost-effectiveness analysis must balance the cost of the agent with its pharmacodynamic advantages such as more rapid recovery from anaesthesia. Furthermore, the indirect costs of side effects have to be taken into account. For example, nausea and vomiting lead to a prolonged stay in the recovery room after anaesthesia for outpatient surgery, which in turn incurs additional costs for antiemetic drugs and the extra time for nursing care. Therefore, a lower incidence of nausea and vomiting and a more rapid recovery from anaesthesia leading to earlier discharge from the recovery room may compensate for the higher price. Volatile agents account for up to 1% of the total intraoperative costs. In analysing the costs of 1?h of anaesthesia, other products such as plasma substitutes and blood products account for a much higher proportion than anaesthetic agents, and reductions or increases in costs pertaining to these products have a bigger impact on overall costs than do volatile anaesthetics. We conclude that volatile anaesthetics account for only a minor portion of the anaesthesia department budget and the cost of anaesthesia delivery. The higher market price of the new agents may be compensated for by the economic impact of fewer side effects and a shorter post-anaesthesia stay in the hospital. In analysing data for sevoflurane, this agent may be cost-effective, for example, for outpatient anaesthesia.     

Low-flow isoflurane-nitrous oxide anaesthesia offers substantial economic advantages over high-and medium-flow isoflurane-nitrous oxide anaesthesia

Isoflurane consumption was studied for three different fresh gas flows in patients scheduled for major elective abdominal, urological or gynaecological surgery under general anaesthesia with an expected duration of 2 h or more. Thirty patients were randomly assigned to either high-flow anaesthesia using a partial rebreathing system without carbon dioxide absorption (Mapleson D) or medium- or low-flow anaesthesia using a circle system with carbon dioxide absorption. Patients were anaesthetised with isoflurane in 40% oxygen and 60% nitrous oxide. The amount of isoflurane consumed was measured with a precision scale. The total consumption of liquid isoflurane (mean ± s.d.) during the first 2 h was 40.8± 12.2 ml in the high-flow group, 18.5 ± 5.4 ml in the medium-flow group and 7.9 ± 2.2 ml in the low-flow group. The corresponding cost of isoflurane for the three groups was 214 Danish kroner (DKK) (±19.5), 97 DKK (±8.8) and 42 DKK (±3.8), respectively. The calculated total cost of anaesthetics was 286 DKK(±26), 155 DKK (±14.1) and 91 DKK (±8.3), respectively. In conclusion, low-flow isoflurane-nitrous oxide anaesthesia offers substantial economic advantages over high- and medium-flow isoflurane-nitrous oxide anaesthesia.     

New and alternative delivery concepts and techniques

The suitability of any method of delivering anaesthetic vapours to the breathing system can be judged only if seen in relation to the fresh gas flow. Due to its advantage in essentially reducing anaesthetic gas and vapour consumption, low-flow anaesthesia has become the acknowledged method of performing inhalational anaesthesia. Conventional plenum vaporizers, connected to the fresh gas supply, meet all technical needs for efficient, safe and simple performance of low-flow and minimal-flow anaesthesia. Omitting the use of nitrous oxide even renders it possible to perform closed-system anaesthesia with conventional anaesthetic machines in routine clinical practice. However, if flows used are as low as the individual oxygen uptake, the performance of these vaporizers reaches its limits. Delivery of anaesthetic vapours independently of fresh gas flow would solve the problems, as only in this way could enough anaesthetic vapour be delivered to establish and maintain the desired concentrations. Manual injection of the fluid anaesthetic directly into the breathing system and the alternative use of vaporizers within the circuit are involved and increase the risks of misdosage. The injection of liquid anaesthetics into the breathing system with the aid of a motor syringe seems most promising; however, such a technique is not approved, and in its simple version contravenes several regulations of the technical norm. Closed-loop feedback control of metering anaesthetic gases and vapours, as realized in the PhysioFlex and ZEUS anaesthetic workstations, allows the realization of 'quantitative closed-system anaesthesia' in clinical practice. If complex anaesthetic gas compositions are used, including for instance nitrous oxide, closed-system anaesthesia can be performed in clinical practice only with such sophisticated machines.     

Uptake of enflurane and isoflurane during spontaneous and controlled ventilation.

The uptake of enflurane and of isoflurane were studied in forty patients during anaesthesia with nitrous oxide using either spontaneous or controlled ventilation. A Douglas bag method was used in combination with low fresh gas flows to a circle system and constant end-tidal anaesthetic concentration. The mean enflurane uptake rates were between 24 and 14 ml.70kg-1.min-1 between 10 and 60 minutes. Corresponding isoflurane uptake rates were between 15 and 8 ml. 70 kg-1.min-1. The initial uptake rates were lower than expected from "the square root of time concept". During spontaneous ventilation, the anaesthetic uptake rates were similar or even higher than corresponding rates during controlled ventilation in spite of lower minute ventilation volumes.     

A comparison of the effects of prolonged (>10 hour) low-flow sevoflurane, high-flow sevoflurane, and low-flow isoflurane anaesthesia on hepatorenal function in orthopaedic patients

This study compared the effects of low-flow sevoflurane, high-flow sevoflurane and low-flow isoflurane on hepatorenal function during and after more than 10 hours of anaesthesia. Twenty-five patients scheduled for elective orthopaedic surgery were categorized into three groups; low-flow sevoflurane (fresh gas flow at 1 litre/min, n = 9), high-flow sevoflurane (5 l/rmin, n = 7), or low-flow isoflurane (1 l/min, n=9). Inspiratory compoundA concentrations were measured. The groups had similar duration of anaesthesia and exposure to anaesthetic agents. The area under the curve of concentration (mean, SD) of compound A in the low-flow sevoflurane group (359.8, 106.1 ppm.h) was greater than that in the high-flow sevoflurane group (61.1, 29.3 ppm.h; P<0.01). All groups showed normal plasma creatinine and creatinine clearance, and transient postoperative increases in plasma alanine aminotransferase and alpha glutathione-S-transferase, as well as urinary glucose and alpha glutathione-S-transferase, with no significant differences between groups. There were no significant relationships between the area under the curve of concentration of compound A and the biomarkers. These findings suggest that prolonged anaesthesia with low-flow sevoflurane has similar effects on hepatorenal function to prolonged anaesthesia with high-flow sevoflurane and low-flow isoflurane.     

Narkosegasbelastung des Personals in der Kinderanästhesie

Methods. To assess the occupational exposure of the anaesthetist to anaesthetic gases, a total of 1 German and 25 Swiss hospitals were investigated. A Brüel & Kjær Type 1302 multi-gas monitor was used to measure concentrations of nitrous oxide and halogenated anaesthetic agents in the anaesthetist's breathing zone. Measurements were performed during 114 general anaesthetic, 55 of which were in patients under 11 years of age. In these 55 patients, the influence of various factors on the exposure (time-weighted average concentrations) was estimated by comparing different data groups. The efficiency of the applied scavenging equipment was examined by surveying the exhalation valve with a leak detector (type TIF 5600, TIF Instruments, Miami). Results. Sessions with patients under 11 years of age revealed much higher anaesthetic gas exposures compared to older patients. The concentrations of nitrous oxide were on average threefold (Fig. 1), those of the halogenated anaesthetics fivefold higher (Fig. 2) for the younger patients. In 11- to 16-year-old patients the exposure level was the same as in adult patients. The measurements showed a reduction of 85% in exposure if an efficient scavenging system (i.e., no waste gas discharge to room air through the exhalation valve) or lower fresh gas flow were used (Fig. 4); 42% of the inspected scavengers were inefficient, and reduced the exposure on average by only 30%. In operating theatres with a ventilation rate of at least ten air changes per h, the measured concentrations of anaesthetic gases in the inhalation zone of the anaesthetists were reduced more than 50% compared to poorly ventilated rooms (Figs. 4 and 5). The use of tracheal intubation or laryngeal mask airway (LMA) anaesthesia resulted in a reduction of 80% in exposure compared to standard face masks if efficient scavenging was used. The exposures during sessions with inefficiently scavenged Bain coaxial systems or unscavenged semi-open delivery systems of the Jackson-Rees type were tenfold higher than with scavenged rebreathing circuit systems (Fig. 6). During anaesthesia with IV or double-mask induction, the average levels of inhalation anaesthetics were reduced by about 80% compared to inhalational induction with standard masks (Fig. 7). The anaesthetist's working technique is a very important factor that strongly influences the concentrations. Poor work practices, like lifting off the face mask with anaesthetic gas flow turned on, increased the exposure of the anaesthetist and other operating room personnel drastically, even if the other conditions (scavenger and room ventilation) were good. Discussion. The exposure levels of anaesthetic gases are generally higher during anaesthesia in children up to 10 years of age than in older patients. Nevertheless, the measurements showed that exposure during paediatric anaesthesia can be kept below the recommended limit (8-h TWA in Switzerland) of 100 ppm nitrous oxide and 5 ppm halothane or 10 ppm enflurane or isoflurane. Causes of high exposures were particularly high fresh gas flows often applied without scavenging or together with inefficient scavenging devices and the high part of mask anaesthesia and inhalation induction with a loosely held mask. To achieve an effective reduction of occupational exposure, well-adjusted and maintained scavenging systems and low-leakage work practices are of primary importance. As leakage can never be completely avoided, a ventilation rate of at least ten air changes per h should be maintained in operating rooms and rooms where anaesthesia is induced to keep down concentrations of waste anaesthetic gases. High exposure during mask anaesthesia and inhalation induction can be prevented by further measures. Using a LMA instead of a standard mask reduces the exposure to the same level as endotracheal intubation. The exposure during induction can be reduced remarkably by the use of the double-mask system or IV induction. Applying low fresh gas flows reduces not only the exposure concentrations in the theatres, but also the contribution to the environmental burden (`?greenhouse effect?' and ozone layer destruction).     

Cost-effectiveness and high patient satisfaction in the elderly: sevoflurane versus propofol anaesthesia

BACKGROUND AND OBJECTIVE: The use of propofol compared with isoflurane is associated with improved patient comfort and decreased costs. However, as the cost saving, the quicker recovery time and patient comfort may not be evident if sevoflurane is substituted for isoflurane; these two anaesthetic agents were analysed in elderly patients. METHODS: In a prospective randomized study, 96 patients undergoing elective ophthalmic surgery received either total intravenous anaesthesia with propofol (Group P), propofol for induction and sevoflurane for maintenance (Group P/S) or sevoflurane for inhalation induction and maintenance (Group S). Analyses focussed on haemodynamics, the quality of recovery, and the costs for the anaesthetic and the entire procedure. RESULTS: Bradycardia or hypotension, mainly registered in Groups P and P/S, did not influence patients' recovery. In Group S, postoperative nausea and vomiting occurred frequently, and 50% of patients complained of discomfort during induction. In Group P/S, the costs for anaesthetics and total costs were lower than those in Groups P and S. CONCLUSIONS: Propofol- and sevoflurane-based maintenance of anaesthesia were similar with regard to patient comfort and recovery in the elderly. Cost analysis revealed that it was less expensive to use propofol for induction and sevoflurane for maintenance than to use either propofol or sevoflurane as sole agents for anaesthesia.     

The effect of heat and moisture exchanger on humidity and body temperature in a low-flow anaesthesia system

BACKGROUND: Artificial humidification of dry inspired gases seems to reduce the drop in body temperature during surgery. The aim of this study was to evaluate the humidity and temperature of anaesthetic gases with heat and moisture exchangers (HMEs). The secondary aim was to evaluate if HMEs in combination with low-flow anaesthesia could prevent a decrease in the body temperature during general anaesthesia. METHODS: Ninety patients scheduled for general surgery were randomised to receive a fresh gas flow of 1.0, 3.0 or 6.0 l min-1 with or without HMEs in a circle anaesthesia system. Relative humidity, absolute humidity, temperature of inspired gases and body temperatures were measured during 120 min of anaesthesia. RESULTS: The inspiratory absolute humidity levels with HMEs were 32.7 +/- 3.1, 32.1 +/- 1.1 and 29.2 +/- 1.9 mg H2O l(-1) and 26.6 +/- 2.3, 22.6 +/- 3.0 and 13.0 +/- 2.6 mg H2O l(-1) without HMEs after 120 min of anaesthesia with 1.0, 3.0, or 6.0 l min(-1) fresh gas flows (P < 0.05, between with and without HME). The relative humidity levels with HMEs were 93.8 +/- 3.3, 92.7 +/- 2.2 and 90.7 +/- 3.5%, and without the HMEs 95.2 +/- 4.5, 86.8 +/- 8.0 and 52.8 +/- 9.8% (P < 0.05, between with and without HMEs in the 3.0 and 6.0 l min(-1) groups). The inspiratory gas temperatures with HMEs were 32.5 +/- 2.0, 32.4 +/- 0.5 and 31.0 +/- 1.9 degrees C, and 28.4 +/- 1.5, 27.1 +/- 0.8 and 26.1 +/- 0.6 degrees C without HMEs after 120 min of anaesthesia (P < 0.05, between with and without HME). The tympanic membrane temperatures at 120 min of anaesthesia were 35.8 +/- 0.6, 35.5 +/- 0.6 and 35.4 +/- 0.8 degrees C in the groups with HMEs, and 35.8 +/- 0.6, 35.3 +/- 0.7 and 35.3 +/- 0.9 degrees C in the groups without the HMEs (NS). CONCLUSIONS: The HMEs improved the inspiratory absolute humidity, relative humidity and temperature of the anaesthetic gases during different fresh gas flows. However, the HMEs were not able to prevent a body temperature drop during low-flow anaesthesia.     

Niedrigflußnarkosen mit Desfluran

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 patients 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?l/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=C_I/C_F) ranges between 0.65 and 0.75 in minimal-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 anaesthetic techniques in routine clinical practice. Even in minimal-flow anaesthesia, 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 anaesthesia, 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 dialling 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 sufficiently 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. As the use of desflurane with high-flow anaesthetic techniques becomes wasteful, its routine clinical use from an economic and ecologic standpoint will only be justified if consistently applied with low-flow or minimal-flow anaesthesia.     

Measuring the costs of inhaled anaesthetics

The cost of inhalation anaesthesia has received considerablestudy and is undoubtedly reduced by the use of low fresh gasflows. However, comparison between anaesthetics of the economiesachievable has only been made by computer modelling. We havecomputed anaesthetic usage for MAC-equivalent anaesthesia withisoflurane, desflurane, and sevoflurane in closed and open breathingsystems. We have compared these data with those derived duringclinical anaesthesia administered using a computer-controlledclosed system that measures anaesthetic usage and inspired concentrations.The inspired concentrations allow the usage that would haveoccurred in an open system to be calculated. Our computed predictionslie within the 95% confidence intervals of the measured data.Using prices current in our institution, sevoflurane and desfluranewould cost approximately twice as much as isoflurane in opensystems but only about 50% more than isoflurane in closed systems.Thus computer predictions have been validated by patient measurementsand the cost saving achieved by reducing the fresh gas flowis greater with less soluble anaesthetics. Br J Anaesth 2001; 87: 559–63