Gent 1998
|
|
|
Gent 1998 |
|
| Page 3of 3 |
Dr. Eisje Vanderheyden, R&D
Frank Beake, Marketing
Drager Medizintechnik GmbH
Moislinger Allee 5
D-23542 Lubeck
The different factors which affect the performance of soda lime, i.e. chemicals, the production process and devices, will be detailed and the significance of a combined quality assurance to cover both the soda lime and the anaesthesia machine itself underlined.
Application-related tests of various types of soda firm with regard to undesirable products of reaction with anaesthetic agents (especially Sevoflurane) will be described.
In addition, it will be explained how the results obtained from these tests can be used to create a marketable product.
Reactions of dry soda lime
H. Forster, Frankfurt, RFG
The exothermic decomposition of sevoflurane flowing through dried CO2 absorbents and the identification of few products of this reaction by gas chromatography /mass spectrometry (GCIMS) has been described earlier. The main reason for the decomposition of sevoflurane seems to be a high concentrations of potassiun, hydroxide (KOH), the most reactive hydroxide with sevoflurane, and of less active sodium hydroxide (NaOH) in the soda lime used. Stability of sevoflurane in various dried CO2 absorbents used for anaesthesia or diving were examined and the toxic decomposition products compound A, methanol and dimethoxymethane (MOM) were quantified (table). MOM is a reaction product of formaldehyde and methanol and will be hydrolysed by water to form formaldehyde. Six different C02 absorbents (with different content of KOH and NaOH) were dried with an oxygen flow of 6- 1 0 ~ for about 48h hours until their weight remained constant. During the experiment a flow of 5% sevoflurane in oxygen or Carbogen (2 l/min) passed through the absorber. The gas stream was condensed into a cooling trap (- 196C) which was exchanged every 15 min over a period of 1-2 hours. After thawing, the condensates were analysed by GCIMS. The table presents the amounts of condensed organic phases collected within 1 hour and the concentration of compound A, methanol and MOM. Depending on high KOH and NaOH content in lime, a great loss of sevoflurane and high anounts of degradation products were determined. Comparing the loss of sevoflurane with the concentrations of compound A, Compound A seems to be an intermediate product by decomposition of sevoflurane to other compounds. It can also be concluded that the reaction of a soda lime with additional potassium hydroxide differs from the reaction profile of soda lime without potassium hydroxide. In additional experiments with wet soda lime preparation it was found that the formation of compound A was reduced when the content of KOH as well as of NaOH was reduced. Also a dry lime without addition of any alkali hydroxides shows reduced formation of compound A and, surprisingly, the characteristic decomposition of sevoflurane was not found with this special lime preparation after drying. Comparative results (i.e. no decomposition of sevoflurane) could be found also in investigations with original dry Baralyme (20%Ba(OH)2 + 80% Ca(OH)2, no alkali hydroxides). It is concluded that according to our former in vitro experiments the compound A production is enhanced by addition of sodium hydroxide and especially of potassium hydroxide. The destruction of sevoflurane is merely caused by dry lime preparation with addition of alkali hydroxides. Therefore, a lime without alkali hydroxides might be better suited for anaesthesia with halogenated volatile anaesthetics.
Table 1:
| CO2-absorbents | hydroxides
contents (mg) |
Condensed org. | Compound A | Methanol (mg) | MOM (mg) |
| Dragersorb 800 | KOH 2.9%
NaOH 2.1 % |
17 | 50.0 | 701 | 0.5 |
| Baralyme | KOH 4.7%
Ba(OH)2 7.4% |
7 | 64.6 | 373 | 1.7 |
| Sodasorb | KOH 2.9%
NaOH 1.4% |
15 | 56.4 | 606 | 2.8 |
| Sofnolime | NaOH 2.6% | 25 | 2.2 | 91 | 0.2 |
| Drager-Dive | Na4P2O7 5.0% | 31 | 26.0 | 169 | 0.2 |
| Drager-Dive Pro | NaOH 2.4% | 27 | 1.5 | 15 | - |
FORMATION OF COMPOUND A WITH SEVOFLURANE
G. Rolly, L. Versichelen
University Hospital, Department of Anaesthesia, De Pintelaan 185, B-9000 Gent (Belgium).
Already from the early use of sevoflurane, it has been know that this very interesting anaesthetic can be degraded to several breakdown products designated as compounds A, B, C, D en E, in an interaction with carbon dioxide absorbents. Only compound A (and to a much lesser extent compound B) is produced in clinical conditions.
As toxicity in rats has been shown, and particularly renal cortico-medullary tubular necrosis, this can give reason to concern. According to literature data in rats, the lethal dose is situated between 330 - 420 ppm, whereas the threshold for renal impairment is situated between 50114 ppm for 3 h exposure or 150-342 ppm-h as it is now frequently expressed.
The production of compound A and the amount generated during anaesthetic practice is determined by several important factors : 1/ the fresh gas flow in the anaesthetic circuit; 2/ the sevoflurane concentration ; 3/ the absorbent temperature; 4/ the minute ventilation ( and hence C02 Output); 5/ the nature of the carbon dioxide absorbent.
Already some years ago Frink et al (1992)(1) reported that that the inhaled concentration was higher than the exhaled compound A concentration, and that with baralyme higher concentrations were detected (1 8 and 14 ppm respectively), than with sodalime (8 and 3 ppm) during low flow anaesthesia (770 ml/min). The maximal concentrations were 8.2 ± 2.7 ppm with sodalime and 20.3 ± 8.7 ppm with baralyme.
The influence of fresh gas low was shown by Bito and Ikeda in 1995 (2), whereby they showed that the mean maximal concentrations of compound A were 19.7 ± 4.3 , 8.1 ± 2.7 and 2.1 1.0 ppm at 1, 3 and 6 l/min fresh gas flow.
During prolonged exposure up to 18 h , at low fresh gas flows of 1 l/Min, Bito and lkeda (1995) (3) found a mean peak compound A concentration of 23.6 ppm with sodalime and 32.0 ppm with baralyme. Concentrations peaked in the first 2-4 h, then remained rather constant from 4- 1 0 h, and afterwards declined.
In totally closed circuit system, Bito and lkeda (1994)(4) found a concentration of compound A of 19.5 ± 5.4 ppm after I h, that remained constant thereafter for 4 h and subsequently declined slightly.
No difference between compound A concentration found during laparoscopic surgery (with C02 insulation) and during tympanoplasty has been noticed by the same authors.
In recent studies, Kharasch et al (1997)(5) showed during low flow anaesthesia (1 l/min) and baralyme absorbent, maximum inspired compound A concentrations of 27 ± 13 ppm. The influence of perhydration of sodalime was recently studied by Bito et al (1998)(6), showing that compound A concentration at fresh gas flow rates of 1 l/min, was even lower than at fresh gas flow rates of 3 1/min with standard sodalime.
With some modern anaesthesia apparatus like the PhysioFlex apparatus, computer controlled liquid anaesthetic administration can be done in rigid closed circuit conditions. Some preliminary data already generated by Baum et al., showed low compound A values with this apparatus. In a recent experimental set up with a test lung generating 160 ml/min CO2, to obtain a PET CO2 of 40 mmhg, we could also show low compound A values even between 6 and 9 ppm, with the PhysioFlex. The temperature of the sodalime canister was always below 30C, when the mean FET concentration of sevoflurane was 2.1%
References :
1. Frink E.J. et al . Anesthesiology 1992, 77, 1064-1069.
2. Bito H. and lkeda K. Br J Anaesth , 1995, 74, 667-669.
3. Bito H. and lkeda K. Anesthesiology 1995, 91, 340-345.
4. Bito H. and lkeda K. Anesthesiology 1994, 80, 71-76.
5. Kharasch E. et al. Anesthesiology 1997, 86, 1238-1253.
6. Bito H. et al. Anesthesiology 1998, 88, 66-71.
The use of the circle system in paediatrics
Kretz, F. J., Stuttgart
The object of ventilation during anaesthesia is to guarantee the gas exchange both in adults and children and to permit delivery and uptake of volatile anaesthetics. Further, adequate ventilation should contribute to maintain body temperature and to avoid fluid loss. In Great Britain the non-rebreathing and partial-rebreathing systems are used in daily practice. Most German paediatric anaesthetists reject these systems, they prefer to use circle systems like the Uhner circle system for infants and children.
The non-rebreathing and partial-rebreathing systems
After the era of open systems like ether inhalation anaesthesia T-piece related systems gained consideration in paediatric anaesthesia. The goal of all breathing systems for paediatric use is to minimise deadspace ventilation and reduce resistance of the system. The prototype of the non-rebreathing system was the Ayr's-T-piece. This T-piece has been modified to many well known variations regarding the fresh gas inlet, the length of corrugated tubing, the design of the reservoir bag and its connection to the patient.
Mapleson introduced a classification of the non-rebreathing systems into six subsystems, known as Mapleson A-F. Considerable use was made of the Mapleson modification F, the Jackson-Rees system, and to a smaller account the Bain system. Paediatric anaesthetists in Germany preferred the Kuhn system. Deadspace in this modification was reduced by placing the fresh gas inlet near the mask.
Obvious benefits of T-piece technology, such as easy management and lack of complications, contrast with a number of drawbacks:
- adequate CO2-elimination can only be achieved by fresh gas flow in the range of 2-3 fold minute volume resulting in
- uneconomic high consumption of volatile anaesthetics which in turn bear a high load of near (operation theatre) and far surrounds
- end expiratory CO2, airway pressure, and respiration volumes cannot be monitored
- there is only limited feasibility for assisted or controlled ventilation
The circle system
Large deadspace and high resistance of the circle system were of quite some concern to paediatric anaesthetists in former times. Rebreathing would only become possible with the introduction of valves placed at a distance from the patient. Compressible volume and compliance of the system were a matter of further improvements.
In Germany it was the merit of Altemeyer and his co-workers to develop the so called Ulmer circle system for infants and children. They were the first to use steel spiral reinforced tubing with an ID of only 10.5 mm. The system is easy to handle and compliance is notably reduced. An advantageous feature is the possibility of monitoring expiratory CO2, gas concentration, and ventilatory parameters like air pressure and tidal volume. Due to the rebreathing effect and the exothermic reaction in the CO2-absorber water- and temperature-loss are minimised.
Prof. Warwersik, a well-known paediatric anaesthetist, stated at a meeting in Munster 1993: the circle system is well established in paediatric anaesthesia, you cannot turn back time.
Influence of flow reduction on anaesthetic gas climatisation
H.Wissing and Kuhn
Frankfurt
Germany
The importance of the climatisation of anaestheic gases has been demonstrated in many studies [1,2]. In order to prevent damage to the cilia in the bronchial tree, Kleeman suggests a water content of at least 20 mg H2O in long term anaesthesia [2]. In a conventional circle system, fresh gas flow reduction to inflow rates of less than one liter per minute is one method to improve gas climatisation [1,2]. To quantify this effect of this flow reduction, the heat and moisture profiles of three different absorber systems (GMS, MAS, modified MAS) at different gas flows (3, 1, 0.5 l/min) and the Physioflex under closed system conditions were evaluated using a model lung[3]. Upon ventilation at ambient temperatures of about 20 C this model lung delivered water saturated gas at 33 - 34 C and 200 ml/min CO2 into the breathing system. In all settings the temperature and moisture content in the expiratory limb, close to the check valve, were almost constant.
The highest gradients were found between the expiratory check valve and the CO2 ansorber canister. In
the absorber canister, the highest humidity contentt and the highest temperatures were measured, indicating that the heat and water produced by CO2 absorbtion is the major source for the climatisation of anaesthetic gases in a circle system. The exothermic absorbtion of one mole of CO2 generates one mole of H2O and 13.500 calories. The absorbtion of 200 ml/min CO2 yields 160 mg.min of H2O, much more than is needed for climatisation of the cicrle volume. However, the efficiency of flow reduction for the improvement of gas climatisation was found to depend strongly on the design of the circle system~. In some conventional machines, the humidity in the CO2 absorber increased from 35 mg H2O/l to more than 45 mg H2O/l following the reduction of the fresh gas flow from 3 l/min to 0.5 l/min.
At the inspiratory check valve, however, humidity content was quite quite drastically reduced due to heat losses in the circle. One exception was the heated circle of the Drager Cicero, whose humidity content at the inspiratory check valve was merely reduced as expected from the mixing of the rebreathed gas with the dry fresh gas added after the soda lime canister. In that device, after 50 minutes under minimal flow conditions, 20 mg H2O/l were available at the inspiratory check valve, reaching moore than 35 mg H2O/l after 100 min [4].
Although there were pronounced differences at the outlet of the anaesthesia machine, there were only small differences in the moisture content delivered to the patient. After 100 min, about 18 to 20 mg H2O/l were achieved with all devices having, a higher moisture output at the inspiratory check valve. The heat loss to the environment in the inspiratory tube, which causes condensation, was revealed to be the limiting factor for anaesthetic gas climatisation.
In contrast, the lowest moisture content was found in the GMS circle systow in all settings. With its gas flow from the top to the bottom through the absorber, its large absorber volume (3 litres) and the fresh gas added after the CO2 absorbant, anaesthetic gases were rather dried than humidified in that circle even under minimal flow conditions. With this system whose moisture output is much below the dew-point at ambient temperature, no further humidity loss was observed in the tubing.
Only with those devices characterised by a high humidity output moisture delivery to the patient can be improved by reducing the heat loss in the tubing. This could he demonstrated with the Drager Cicero using a heated tube in the inspiratory limb. Moisture delivery was almost equal to the moisture output of the device until the dew point (temperature of the tubing) was reached [5].
A markedly different heat and moisture profile was found in the PhysioFlex. The device is designed for quantitative closed system anaesthesia. All system components are integrated in a thermally insulating case. The small soda lime canister helps to optimise the usage of heat and moisture generated by CO2 absorbtion. The system volume is revolved at 70 l/min which allows a rapid heat transport. This results in the heat and moisture profile which show the smallest gradients of all devices tested [3, 4. 6]. With this device, 20 mg H2O/l were delivered to the patient within 10 min, and 30 mg H2O/lwere achieved after 2 hours.
Hence, fresh gas flow reduction may lead to an improvement of anaesthetic gas climatisation, provided that the design of the anaesthesia machine allows the usage of the heat and moisture generted by the CO2 absorbtion. In conventional anaesthesia machines, besdie circle design, heat loss to the environment in the inspirayory limb is the major problem for anaesthetic gas climatisation. In the PhysioFlex, however, with its optimised heat transport, maximal usage is made of the heat and moisture delivered to the circle, leading to sufficiently climatised anaesthetic gases within the first 10 minutes.
1. Chalon J., Patel C., Ramanathan S., Capan L., Tang C.K., Turndorf H, (1979) Humidity and the anaesthetised patient. Anesthesiology 50. 195-198
2. Kleemann PP. (1990) The climatisation of anaesthetic gases under conditions of high flow to low flow. Acta anaesth Belg 41, 189-200
3 Kuhn I. (1996) Temperatur und Feuchtprofile der Narkosegerate Ohmeda Modulus CD/CV, Drager Cicero und Stimotron Physioflex. Inaugural dissertation Universitat des Narkosegerats
4 Kuhn I., Wissing H. (1998) Atemgasklimatisierung und Klimaprofil des Narkosegerats Drager Cicero. AINS 33: S225
5 Wissing H., Kuhn 1. (1998) Beheizung der Inspiretionsschlauche verbessert die Atemgasklimatisierung beim Drager Cicero AINS 33. S225
6. Wissing H, Kuhn I, Kessler P (1997) Das Warme-Feuchte-Profil des Physiofiex Untersuchungen am Modell. Aaesthetist 46: 201 - 206
7. Wissing H., Kuhn I., Kessler P. (1997) Atemgasklimatisierung mit dem Narkosegerat Physioflex. Anaesthetist 46:613-615
Heat and moisture exchangers versus flow reduction
JP Bengtson, MD, PhD
The nose and upper airwways heat and moisturise the inspired gases so that they are saturated with water vapour and at body temperature when they reach the alveoli. During expiration the upper airways conserve heat and moisture and thereby minimise the losses of heat and water from the airways. In the tracheally intubated patient this counter-current mechanism is partly bypassed and ventilation with dry and cold medical gas leads to loss of water from the respiratory tract. During anaesthesia, humidification is desirable to prevent damage to the respiratory tract and to decrease intra- and postoperative hypothermia. Inspired gas is recommended to contain a minimum absolute humidity of 15-20 mg H2O/l to reduce the risk for dehydration in the respiratory tract and to maintain the function of the tracheobronchial epithelium (1,2).
Inspiratory gas humidity in the circle absorber system emanates from both conserving and supplying sources: expired and rebreathed water vapour, and chemically released water in the neutralisation of carbon dioxide and moisture extracted from the hydration of soda-lime granules. Factors of importance for humidification are fresh gas flow rate, volume of breathing system and soda-lime canister, construction of the circle system (Eger type A, C, H), ambient temperature, the metabolic rate of the patient, and the tidal volume. At a fresh gas flow of <2 l/min low flow anaesthesia has inherent humidifying properties with an absolute humidity of >20 mg/l of the inspired gases (3,4). At higher fresh gas flows the humidifying properties are usually insufficient but the insertion of a heat and moisture exchanger, HME, gives adequate humidification of the inspired gases.
References:
I . Ingelstedt S. Studies on the conditioning of air in the respiratory tract. Acta Otolaryngologica 1956; (suppl 131): 1-80.
2. Kleeman, PP. Humidity of anaesthetic gases with respect to low flow anaesthesia. Anaesthesia and intensive Care 1994; 22: 396-408.
3. Bengtsson JP, Bengtsson A, Stenqvist O. The circle system as a humidifier. British Joumal of Anaesthesia 1989; 63: 453-457.
4. Henriksson BA, Sundling J, Hellman A. The effect of a heat and moisture exchanger on humidity in a low-flow anaesthesia system. Anaesthesia 1997; 52:144-149.
Pharmaco-economic aspects of low flow/closed circuit anaesthesia.
L. Versichelen, M. Struys, G. Rolly
University Hospital, Department of Anaesthesia, De Pintelaan 185, B-9000 Gent (Belgium)
Concerns regarding cost containment in healthcare have led to debate the best methods of economic analysis to save these costs. Cost minimalisation-, cost benefit-, cost effectiveness and cost utility analysis are four commonly used methods and discussed by many researchers.(1,2)
Cost analyses are used to determine relationship between cost and outcome.
Macario et al. (3) noted that half of the intraoperative anesthesia costs could be influenced by the choice of the agents and anaesthetic techniques. Although if cost savings in an individual case are small they can however contribute to the total cost saving because we are performing a large number of cases.
The cost of anaesthetic drugs accounts for only a small fraction of the total cost i.e. 8%-12% (4). Simple and effective cost containment can be done by the use of low flow and closed circuit techniques with inhalation agents. Already years ago, researchers tried to reduce the cost of inhalation anaesthetics by reducing the fresh gas flow. To measure anaesthetic consumption, they used simple techniques such as weighing the vaporiser and using manual techniques to inject liquid anaesthetics.
Today newer low flow/closed circuit anaesthesia machines can measure on-line the consumption of inhalation anaesthetics, which enables us to calculate the cost and to compare it with other anaesthetic techniques.
In our department the consumption and the cost of sevoflurane were evaluated in 10 female patients, aged between 18 and 60 years, ASA I - II, undergoing elective breast surgery. Depth of anaesthesia was monitored by the bispectral index (BIS), a processed EEG variable, which is provenly correlated with hypnotic drug effect. All patients recieved a bolus and continuous infusion of sufentanil. Induction of anaesthesia was performed with propofol. Anaesthesia was maintained with a continuous infusion of sufentanil and sevoflurane. The last one was increased or decreased to maintain the BIS between 40 and 60. All patients were ventilated with a fresh gas flow of 2 litres O2 in air. Sevoflurane consumption was measured using the Record keeper from the ADU apparatus (Datex, Finland). The mean ± SD for the consumption and cost per hour of Sevoflurane are 13.7+ 5.6 ml liquid and 322±130 BEF (8.93± 3.61 US $) respectively.
Also in the PhysioFlex (Drager), another modem anaesthesia machine, data on the consumption of the potent inhalation anaesthetic are available. The results of the consumption of the inhalation anaesthetics are comparable with these of other researchers and are much lower than those recorded with high flow techniques.
References :
I . NW Watcha, PF White. Economics of Anesthetics Practice. Anesthesiology, 1997; 86:1170-96.
2. LM Jolicoeur, AJ Jones-Grizzle, JG Boyer. Guidelines for performing a farmacoecononiic analysis. Am. J. Hosp. Pharrn 1992,49:7,1741-7.
3 . FK Orkin. Meaningful cost reduction: Penny Wise, pound foolish. Anesthesiology 1996;83:113 5-7.
4. NW Drummond. Evaluation of health technology: economic issues for health policy and policy issues for economic appraisal. Soc Sci Med, 1994, 38:12, 1593-600.
Electronic Control of Inspired Gas Composition
Jan Baum
Damme Germany
Using a non-rebreathing system the anaesthetist is enabled to follow a very simple dosing scheme: the gas composition which is set at the gas flow controls and the control knob of the vaporiser is just the gas applied to the patient. The fresh gas composition equals the composition of the inspired anaesthetic gas. Using a rebreathing system however, a mixture of fresh gas and expired gas is delivered to the patient. The lower is the fresh gas flow the greater is the rebreathing volume. Thus, the difference between the inspired gas composition and the fresh gas composition is the greater the lower is the fresh gas flow. In clinical practice this disadvantage, inevitably arising if rebreathing systems are used judiciously, can be overcome by the anaesthetist's professional experience. The lower the fresh gas flow, the the higher must be its oxygen content to maintain a sufficiently high inspired oxygen concentration. Furthermore, using vaporisers outside the circuit the lower the fresh gas flow, the higher must be the volatile's fresh gas concentration to maintain a desired concentration of the anaesthetic within the breathing system. For the fine tuning of the controls the anaesthetist has to take into account for instance, anthropomorphic parameters of the patient pharmacokinetic and pharmacodynamic properties of the chosen anaesthetic agent and the chronology of the anaesthetic procedure. Finally the prolongation of the time constant by flow reduction has to be considered. In low flow anaesthesia alterations of the fresh gas composition will lead to corresponding alterations of the gas composition within the breathing system only with great time delay. All these specific rules, which have to be considered in metering of anaesthetic gases during low flow techniques, up to now, are substantial obstacles against further spread of the judicious use of rebreathing systems.
The aim to develop an anaesthetic machine, featuring electronic feed back control of the inspired gas composition was to enable the anaesthetist to set just that gas composition at the controls of the machine which is delivered to the patient. Thus, even if rebreathing systems are used - independently from the share of rebreathing - the dosing scheme is as simple as in the use of non-rebreathing systems. A few prototypes of such a machine are already working in different European hospitals. The nominal values to be set at the gas metering module are the gas mixture (N2O/O2 or air/O2), the inspired oxigen and the inspired anaesthetic concentration. The fresh gas flow can be chosen deliberately to meet the respective needs, thus, this machine can be used universally in clinical routine practice. Its just this feature which facilitates to manage all accidental respiratory problems, makes it possible to perform anaesthesia in cases, in which low flow techniques are contraindicated, but on the other hand, facilitates to reducing the fresh gas flows to even lowest values. With the same settings at the controls of the gas metering module the rebreathing system can be used in semiopen, semielosed or nearly closed function. Always the patient gets the preset inspired gas composition. The dosing scheme is extremely simple: "What you set is what you get", or more precisely, "What you set your patient gets".
The machine was used in clinical practice successfully, nevertheless, there is the demand for some improvements: For improved performance, the volatile agent should be delivered into the breathing system independantly from the fresh flow. The algorhyth
