1 Department of Anaesthesia, City Hospital, Nottingham, UK. 2 Department of Anaesthetics and Intensive Care Medicine, University of Wales College of Medicine, Cardiff CF14 4XN, UK
Corresponding author. E-mail: dicknickalls{at}compuserve.com This article is accompanied by the Editorial.
Accepted for publication: February 18, 2003
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Abstract |
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Methods. Age-related iso-MAC charts for isoflurane, sevoflurane and desflurane were developed for the clinically useful MAC range (0.61.6), age range 595 yr, and put in a convenient form for use by practising anaesthetists. The charts are based on Maplesons meta-analysis (1996) of the available MAC data and can be used to allow for the contribution of nitrous oxide to the total MAC.
Results. The charts indicate the influence of age on anaesthetic requirements, showing, for example, that a total MAC of 1.2 using isoflurane and nitrous oxide 67% in oxygen requires an end-expired isoflurane concentration of only 0.25% in a patient of 95 yr vs 1% in a 5-yr-old patient. Colleagues found the charts to be helpful and simple to use clinically.
Conclusions. The iso-MAC charts show clearly how patient age can be used to guide the choice of end-expired agent concentration. They also allow a consistent total MAC to be maintained when changing the inspired nitrous oxide concentration, thereby reducing the chance of inadvertent awareness, particularly at the extremes of age.
Br J Anaesth 2003; 91: 1704
Keywords: age factors; anaesthetics, gases; anaesthetics, volatile; potency, anaesthetic, age factors; potency, anaesthetic, MAC
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Introduction |
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which expresses MAC for a given age (MACage) as a function of that at 40 yr (MAC40). It is noteworthy that the more recent MAC data of Fragen and Dunn,6 Nakahara and colleagues7 and Eger8 are in accord with the Mapleson data.
However, as a nitrous oxideoxygen mixture is used in the majority of anaesthetics, it was felt that a simple chart that allows for the presence of nitrous oxide and is convenient to use would be a useful guide, particularly when anaesthetizing patients at the extremes of age. Maplesons Equation 1 (see above)5 was used as the basis for developing a series of age-related iso-MAC charts for clinical use, the motivation being the lack of an age-related MAC display on anaesthesia monitors.
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Methods |
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To show how FE' varies with age for any given value of k, Equation 1 was substituted in Equation 2:
Equation 3 was then used to plot graphs of FE' against age for a range of MAC multiples: 0.61.6 in steps of 0.2. This covers the range most likely to be used in clinical practice, the lower value being similar to that being used increasingly to supplement remifentanil infusions.911 Because the charts are for clinical use, the FE' scale was made linear rather than logarithmic.5
Additional scales for FE' when the volatile anaesthetic is carried in nitrous oxide 50% or 67% in oxygen were included in the graph (see Appendix). These scales were shown on additional ordinates. The charts were generated using mathsPIC,1214 as this allows the accurate placement of additional non-standard axes. The charts were tested by colleagues for practicality in clinical use.
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Results |
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The charts can also be used in reverse. Suppose it is required to achieve a MAC of 1.2 in a 25-yr-old using isoflurane in nitrous oxide 50%. In Fig. 1, draw a vertical line through the age 25 position and note where it cuts the 1.2 iso-MAC curve. Follow the horizontal broken line to the right-hand ordinate associated with nitrous oxide 50% and read the end-expired isoflurane concentration: 1.0%.
Colleagues found the charts to be helpful and simple to use clinically. An electronic form of the iso-MAC charts giving a real-time digital output of age-related total MAC has been used uneventfully since 1997 as part of an automated anaesthesia information system developed by one of the authors (RWDN).
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Discussion |
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More specifically, the practical value of the iso-MAC charts is twofold. First, they show clearly and easily how patient age can be used to guide the choice of end-expired agent concentration when it is carried in pure oxygen (or an oxygenair mixture) or in nitrous oxide 50 or 67% in oxygen. Secondly, they allow a consistent total MAC to be maintained when making changes to the fractional inspired nitrous oxide concentration (FIN2O).
To make the iso-MAC charts usable in the presence of nitrous oxide, it was necessary to invoke the generally accepted principle that the clinical effects of the inhalational agents in current use are additive.58 1519 While additivity has not been proved conclusively, it is the best working hypothesis. A significant body of the literature is consistent with this hypothesis, and no human data contradict it.
As the variation of MAC with age is often underestimated by clinicians, the iso-MAC charts are particularly useful when dealing with patients at the extremes of age. For example, Fig. 1 shows that a total MAC of 1.2 using isoflurane and nitrous oxide 67% in oxygen requires an end-expired isoflurane concentration of only 0.25% in a patient of 95 yr vs 1% in a 5-yr-old patient. This very large difference exists partly because the nitrous oxide 67% provides a smaller fraction of MAC in the 5-yr-old (52 against 91%).
Sometimes it is necessary for surgical reasons to reduce the FIN2O or even to stop using nitrous oxide altogether (e.g. in ear or bowel surgery), in which case the iso-MAC chart serves as a useful guide to the required increase in the end-expired concentration of the volatile agent necessary to maintain a consistent total MAC. For example, in order to maintain a total MAC of 1.2 in an 80-yr-old patient using desflurane and nitrous oxide 67% in oxygen, it is clear from Fig. 3 that an increase in the FIO2 from 33 to 100%, associated with eliminating nitrous oxide, requires a compensatory three-fold increase in the end-expired concentration of desflurane, from 2 to 6.25%. However, in this setting (i.e. decreasing the FIN2O and increasing the inspired concentration of the volatile agent) it is important to appreciate that the greater solubility of the volatile agent in the tissues,20 particularly for sevoflurane and isoflurane, means that the increase in the arterial and brain concentrations of the volatile agent towards equilibrium with the reset end-expired concentration will be slower than the corresponding decline in the end-expired concentration of nitrous oxide.1 Therefore, despite the three-fold increase in end-expired desflurane, there is likely to be temporary lightening of the depth of anaesthesia unless an even greater concentration of desflurane was used for a few minutes.
As the charts make it easy to determine age-related MAC in a clinical setting when using the commonly used inhalational agents, they have the potential for reducing the chance of inadvertent awareness, particularly at the extremes of age.
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Appendix |
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On the basis of the increasingly accepted view that the effects of the inhalational anaesthetic agents are additive in man,58 1519 it follows that the total MAC multiple of two inhalational anaesthetics given together is the sum of the individual MAC multiples. Therefore, when a volatile anaesthetic agent is used with nitrous oxide in oxygen
ktotal = kvolatile + kN2O(5)
Therefore, substituting Equation 4, for both the volatile and the nitrous oxide, in Equation 5 and writing FE'volatile+ for the end-expired concentration of the volatile agent when given with nitrous oxide gives
To find the end-expired concentration of volatile agent needed for a given total MAC multiple in the presence of a given end-expired concentration of nitrous oxide, Equation 6 can be rearranged to give
To achieve the same ktotal with the volatile agent in oxygen 100% requires an end-expired concentration FE'volatile which is given by
Combining Equations 7 and 8 shows that
The finding5 that the regression lines of log(MAC) for the different anaesthetics are parallel implies that the ratio MACage,volatile / MACage,N2O is the same for all ages, e.g. 1.49/133=0.01120, 1.17/104=0.01125 and 0.91/81=0.01123 for isoflurane at 1, 40 and 80 yr respectively, rounding errors in the MAC values in Table 1 accounting for the slight variation between ages. Therefore, with isoflurane, for instance, and using the mean (0.01123) of the three values for the ratio, if a given MAC multiple requires a particular FE'isoflurane in oxygen 100%, the required FE'isoflurane+ with nitrous oxide 50% in oxygen will be 50x0.01123=0.56% less than when using 100% oxygen. Accordingly, an FE' scale for isoflurane with nitrous oxide 50% in oxygen will simply be shifted 0.56% compared with that for the scale for use with 100% oxygen. With nitrous oxide 67% the shift is 0.75%. Corresponding shifts are 0.86 and 1.16% for sevoflurane, and 3.15 and 4.22% for desflurane.
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Acknowledgement |
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References |
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