[The unexpectedly difficult airway -- a plea for the oxford-non-kinking-tube]. / Der unerwartet schwierige Atemweg -- Ein Plädoyer für den Oxford-Non-Kinking-Tubus.

The skill to safely manage the unexpectedly difficult airway is expected from every anaesthetist. The strategies to safely overcome this severe problem have to be adapted to the given equipment and the individual aptitude and skills of the respective colleague. The algorithms for management of the difficult airway should be as simple as possible, and one cannot assume that devices for fibre-optic intubation are available at every site. Indispensable, however, is the availability of face masks, naso- and oropharyngeal airways and laryngeal mask airways in different sizes at each induction site. This paper is especially devoted to recalling the Oxford non-kinking tube and its specific way of handling, as a lot of cases of unexpectedly difficult airway can be safely managed with this tool. Alternatives to safeguarding the difficult airway are the intubation laryngeal mask airway or the esophago-tracheal combitube. For managing the worst case, the "cannot ventilate - cannot intubate" disaster, instruments for percutaneous punction of the trachea and devices for oxygen insufflation must be readily available in every theatre.

[Nitrous oxide free low-flow anesthesia]. / Lachgasfreie Niedrigflussnarkosen.

The routine use of nitrous oxide as a component of the carrier gas has been unanimously called into question in recent surveys, in fact, its use is now recommended in indicated cases only. Whereas a lot of contraindications are listed in the surveys, precise definitions of justified indications are not given. In clinical routine practice, there are absolutely no problems in carrying out inhalational anaesthesia without nitrous oxide. The missing analgetic effect can be compensated for by moderately increasing the additively used amount of opioids, while the missing hypnotic effect can be achieved by raising the expired concentration of the inhalational anaesthetic by not more than 0.2-0.25 x MAC. Thus, when isoflurane is used, an expired concentration of 1.2 vol% is desired, in the case of sevoflurane of 2.2 vol% and with desflurane of 5.0 vol%. In addition, doing without nitrous oxide facilitates the performance of low flow anaesthetic techniques considerably. Since the patient only inhales oxygen and the volatile anaesthetic, the total gas uptake is reduced significantly. Washing out nitrogen is no longer necessary. This means that the initial phase of low flow anaesthesia, during which high fresh gas flows have to be used, can be kept short. Its duration is now determined by the wash-in of the volatile anaesthetic. Since there is no uptake of nitrous oxide, a considerably greater volume of gas is circulating within the breathing system, minimizing the possibility of accidental gas volume deficiency. Thus, if anaesthesia machines with highly gas-tight breathing systems are used, even the performance of non-quantitative closed system anaesthesia becomes possible in routine clinical practice. The carrier gas flow can be reduced to just that amount of oxygen which is really taken up by the patient. This oxygen volume can be roughly calculated by applying the Brody's formula. Using fresh gas flows as low as 0.25 l/min, however, will result in a significant decrease of the output of conventional vaporizers outside the circuit. Thus, it becomes nearly impossible to maintain an expired isoflurane concentration of 1.2 vol%. With respect to their pharmcokinetic properties, the newer low soluble volatile agents sevoflurane and desflurane are better suited for use with flows corresponding to the basal oxygen uptake. Our own clinical experience, gained in the last six months from a trial involving over 1,800 patients, shows that the increase in opioid consumption resulted in additional costs of about 0.25-0.50 DM per patient. The increased concentration of inhalational agents brought additional costs of 3.00 to 5.00 DM for a two-hour anaesthesia. On the other hand, doing without nitrous oxide saved 2.61 DM per one-hour anaesthesia, whereby our consumption of nitrous oxide is extremely low as minimal flow anaesthesia is performed consistently. Furthermore, these calculations disregard the cost of the technical maintenance fo the central gas piping system and of the regular measurement of workplace contamination with nitrous oxide by a certified institute, which in Germany, ad least, is obligatory. The additional costs of nitrous oxide-free inhalational anaesthesia seem to be balanced by the savings. Given the numerous justified arguments against the routine use of nitrous oxide, the lack of precisely-defined indications and the clinical experience showing that doing without nitrous oxide is uncomplicated, self-financing and ecologically beneficial, the use of nitrous oxide should be given up completely.

[Climatization of anesthetic gases using different breathing hose systems]. / Klimatisierung von Narkosegasen bei Einsatz unterschiedlicher Patientenschlauchsysteme.

BACKGROUND: During general anaesthesia gas climate significantly is improved by performance of low flow techniques. Gas climatisation, however, markedly also will be influenced by the temperature loss at, and corresponding water condensation within the hoses, factors which are related to the technical design and material of the patient hose system. The objective of this prospective study was to investigate 1. how anaesthetic gas climatisation during minimal flow anaesthesia is influenced by the technical design of different breathing hose systems in clinical practice. 2. to investigate, whether a sufficient gas climatisation also can be gained with higher fresh gas flows if that hose system is used, proven beforehand to optimally warming and humidifying the anaesthetic gases. METHODS: Three different systems, a conventional two-limb hosing consisting of smooth silicone hoses, a coaxial hosing, and a hosing consisting of actively heated breathing hoses, attached to a Dräger Cicero EM anaesthesia machine, were used during minimal flow anaesthesia with a fresh gas flow of 0.5 l/min. Gas temperature and absolute humidity were measured at the tapered connection between the inspiratory limb and the breathing system as well as at its connection to the endotracheal tube. The best gas climatisation was observed if heated breathing hoses were used. Thus, using this hosing, additionally gas temperature and humidity in the inspiratory limb were taken at fresh gas flow rates of 1.0, 2.0 and 4.4 l/min respectively. Measurements were performed in all groups at all general anaesthesias lasting at least 45 minutes during the lists of eight different days each. RESULTS: In minimal flow anaesthesia, with all hose systems likewise, generally an absolute humidity between 17 to 30 mgH2O/l is reached at the endotracheal tube's connector during the course of the list. Only in the first cases of the day there was a short delay of 15 to 30 minutes before reaching a humidity of at least 17 mgH2O/l. Only with heated hoses, however, humidity frequently even exceeded 30 mgH2O/l. If conventional or coaxial hosings were used, during minimal flow anaesthesia gas temperatures in an acceptable range between 23 to 30 degrees C were measured at the tube connector. With heated hoses, however, warming of the gases was excellent with gas temperatures between 28 to 32 degrees C. In minimal flow anaesthesia climatisation of the anaesthetic gases proved to be best if heated hoses were used. Thus, using heated hose systems another three trials with increasing fresh gas flow rates of 1.0, 2.0 and 4.4 l/min respectively were performed. Whereas climatisation of the anaesthetic gases still was found to be optimal with a fresh gas flow of 1.0 l/min, the humidity dropped drastically to values lower than 17 mgH2O/l at 2.0 l/min and even down to 10 mgH2O/l at a flow rate of 4.4 l/min. Gas temperatures, however, turned out to be independent of the flow and remained at 28-32 degrees C, even at a flow as high as 4.4 l/min. CONCLUSIONS: Using conventional hose systems and coaxial hosings acceptable, but not optimal climatisation of the anaesthetic gases can be gained if minimal flow anaesthesia is performed. The use of a coaxial hose system seems to lead to improved climatisation in long lasting procedures only. In routine clinical practice, however, conventional and coaxial hose systems are similar in respect to the climatisation of breathing gases. Heated breathing hoses performed markedly better in terms of climatisation of the breathing gas than the coaxial and the conventional hose system. With this hosing not only sufficient but optimal moisture and temperature values are realized. Optimal climatisation, however, only can be gained if low flow anesthetic techniques with fresh gas flows equal or less than 1 l/min are performed. With higher fresh gas flow rates the humidity decreases markedly while high gas temperatures are maintained. (ABSTRACT TRUNCATED)

[Low-flow and minimal-flow anesthesia with sevoflurane]. / Low Flow- und Minimal Flow-Anästhesie mit Sevofluran.

Due to its low solubility and the high maximum concentration delivered by the vaporizer sevoflurane is especially suitable for the performance of low flow anaesthetic techniques. High flow phases for wash-in or wash-out of anaesthetic gases can be kept short, the difference between the volatile's concentration in the fresh gas and within the breathing system is comparatively small, and the time constants are short even during low flow anaesthesia. The monitoring, required to sufficiently ensure the safety of the patients, corresponds to the current obliging technical safety standards. As compound A may accumulate in the breathing system, sevoflurane should not be administered with fresh gas flows lower than 1.0 l/min, until the scientific discussion on nephrotoxicity of this substance in humans is solved. Low flow anaesthesia guarantees a sufficient and continuous wash-out of trace gases. Thoroughly the use of sevoflurane with dry soda lime must be avoided, as this volatile in an extreme exothermic reaction is absorbed nearly totally and degraded to a considerable degree by dry carbon dioxide absorbent. The gaseous degradation products are pungent and possibly may be harmful to the patients. Only by low flow anaesthesia the use of sevoflurane will gain an economically and ecologically acceptable range of efficiency.

[Low-flow anesthesia with desflurane]. / 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 patient were assigned to six groups according to the duration of the initial high-flow phase, fresh gas flow, and fresh-gas desflurane concentration. The median age, height, body weight, and constitution of the groups was comparable. After an initial high-flow phase using 4.4 l/min, the fresh gas flow was reduced to 0.5 l/min (minimal-flow anaesthesia) or 1.0 1/min (low-flow anaesthesia). Inspired nitrous oxide concentrations were maintained at 60% to 70%. Using different standardised schemes of vaporizer settings, inspired desflurane concentrations were applied in the range from 3.4% to 8.7%, i.e., between 1 and 1.5 MAC. Inspired and expired desflurane concentrations were measured continuously by the side-stream technique and recorded on-line. Venous blood samples were taken immediately prior to induction and 45 min after flow reduction for measurement of carboxyhaemoglobin (COHb) concentration). RESULTS: In the 10- to 15-min initial phase during which a high fresh gas flow of 4.4 l/min was used, the inspired desflurane concentration reached values in the range of 90%-95% of the fresh gas concentration. In low-flow anaesthesia this concentration could be maintained without any alteration of the vaporizer setting, whereas in minimal-flow anaesthesia with flow reduction the fresh gas concentration had to be increased by 1% to 2%: The quotient calculated by division of the inspired desflurane concentration by its fresh gas concentration (Q = CI/CF) ranges between 0.65 and 0.75 in animal-flow and between 0.80 and 0.85 in low-flow anaesthesia. If use was made of the wide output range of the desflurane vaporizer, the inspired concentration could be increased rapidly by about 5% in 8 min, although the flow was kept constant at 0.5 l/min. Compared with its value prior to induction (2.13 +/- 1.05%), the COHb concentration decreased statistically significantly by about 0.7% during the 1st hour of minimal-flow anaesthesia (1.42 +/- 1.01%). In no case was a COHb concentration observed that exceeded threatening or even toxic values, although the soda lime was changed routinely only once a week. CONCLUSIONS: The pharmacokinetic properties of desflurane, resulting in especially low individual uptake, and the wide output range of the vaporizer facilitate the use of low-flow anesthetic techniques in routine clinical practice. Even in minimal-flow anesthesia, the duration of the initial high-flow phase can be shortened to min. If the flow is reduced to 1 l/min, the inspired desflurane concentration achieved in the initial high-flow phase can be maintained without any alteration of the vaporizer setting. In minimal-flow anesthesia, however, with flow reduction to 0.5 l/min, the fresh gas concentration has to be increased to a value 1%-2% higher than the inspired nominal value. Due to the wide dialing range of the desflurane vaporizer, the amount of vapour delivered into the breathing system can be increased to about 110 ml/min even at a flow of 0.5 l/min. The large amount of agent that can be delivered into the system even under low-flow conditions, together with the very low individual uptake, results in a time-constant that is sufficient short for the clinically required rapid increase in inspired desflurane concentrations. The short time-constant of low-flow desflurane anaesthesia improves the control of the anaesthetic concentration. If all measures are taken to safely avoid inadvertent drying out of the soda lime, there is no evidence that low-flow anaesthesia with desflurane is liable to increase the risk of accidental carbon monoxide poisoning. (ABSTRACT TRUNCATED)

Carbon monoxide generation in carbon dioxide absorbents.

Several cases of unexpected high carboxyhemoglobin (COHb) levels in patients undergoing general anesthesia were observed. To avoid carbon monoxide (CO) intoxication, the use of high fresh gas flows and frequent changes of the absorbent were recommended. However, due to economic and ecologic considerations, low-flow anesthetic techniques have advantages. Thus, the subject urgently needed to be reexamined. In 1001 patients undergoing enflurane or isoflurane anesthesia, blood samples were taken 30 min after fresh gas flow reduction to 0.5 L/min and analyzed for COHb. The absorbent canisters, containing soda lime, were used for several days. The statistical mean and SD of COHb was 1.17% +/- 0.97% in the range of 0%-7.6%. There was no statistical difference between the COHb values when broken down by the duration of use of the absorbent canisters. In no case were dangerously high COHb levels observed. As recently revealed, only dry absorbents produce CO if exposed to volatile anesthetics containing a CHF2-moiety. Thus, all measures must be avoided that dry out the absorbent. Low-flow anesthesia preserves the moisture content of the absorbent and, thus, seems to be a factor protecting from CO generation.