[Nitrous oxide free low-flow anesthesia].
ABSTRACT 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.
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ABSTRACT: Simple vaporiser setting (F(D)) 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 F(D) sequence that maintains the end-expired sevoflurane concentration (F(A)sevo) at 1.3% using the fewest possible number of F(D) 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 IH patients, F(D) was determined to maintain F(A)sevo 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 F(D) schedule that approached the 1.3% F(A)sevo pattern with the fewest possible adjustments was prospectively tested in another 18 patients. The following F(D) sequence approximated the F(D) 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 F(D) changes at three and 15 minutes maintained predictable anaesthetic gas concentrations during the first 45 minutes in all but one patient in our study.Anaesthesia and intensive care 11/2009; 37(6):911-7. · 1.40 Impact Factor