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Suppose it is required to achieve a MAC of 1.2 in a 25‐yr‐old using isoflurane in nitrous oxide 50%.
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The total age‐related MAC is the sum of the MAC fractions for each inhalational agent, where the MAC fraction for a given agent is F e′ agent/MAC age,agent and is given by Equation 2. This result can be confirmed by calculation. The two lines intersect on the 1.2 iso‐MAC curve. Next, select the right‐hand scale corresponding to the use of nitrous oxide 67%, and draw a horizontal line through the sevoflurane 0.5% position. 2) and first draw a vertical line corresponding to age 85 yr. To determine the total age‐related MAC, use the sevoflurane chart (Fig. To illustrate the use of the charts, consider an 85‐yr‐old patient for whom the end‐expired concentrations of sevoflurane and nitrous oxide are 0.5 and 67% respectively. The left‐hand scale is for use with oxygen 100% the two right‐hand scales are for use with nitrous oxide 50 and 67% in oxygen. The ordinates (left and right) are scaled as end‐expired agent concentration (%) and the abscissa is scaled as years of age. Dots on the curves are to help alignment with age. Iso‐MAC charts for the inhalational agents isoflurane, sevoflurane and desflurane are shown in Figs 1– 3. The charts were tested by colleagues for practicality in clinical use. The charts were generated using mathsPIC, 12 – 14 as this allows the accurate placement of additional non‐standard axes. These scales were shown on additional ordinates. 5Īdditional scales for F e′ when the volatile anaesthetic is carried in nitrous oxide 50% or 67% in oxygen were included in the graph (see Appendix). 9 – 11 Because the charts are for clinical use, the F e′ scale was made linear rather than logarithmic. 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. To show how F e′ varies with age for any given value of k, Equation 1 was substituted in Equation 2:Įquation 3 was then used to plot graphs of F e′ against age for a range of MAC multiples: 0.6–1.6 in steps of 0.2. When a volatile anaesthetic is administered at an end‐expired concentration ( F e′) in oxygen, this can be described as a multiple (or submultiple) k of MAC:
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Mapleson’s 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. However, as a nitrous oxide–oxygen 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. It is noteworthy that the more recent MAC data of Fragen and Dunn, 6 Nakahara and colleagues 7 and Eger 8 are in accord with the Mapleson data. Which expresses MAC for a given age (MAC age) as a function of that at 40 yr (MAC 40). 1 4 In 1996, Mapleson 5 reported a meta‐analysis of the available data (Table 1) and found that (i) semi‐logarithmic plots of MAC against age (age ≥1 yr) for all inhalational agents are linear and parallel, and (ii) the available data can be represented by an equation, It has long been known that MAC, the minimum alveolar concentration at 1 atmosphere that prevents movement in 50% of patients exposed to a surgical incision, 1 – 3 decreases with age. 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.Īccepted for publication: February 18, 2003 The iso‐MAC charts show clearly how patient age can be used to guide the choice of end‐expired agent concentration.
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Colleagues found the charts to be helpful and simple to use clinically.Ĭonclusions. 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. The charts are based on Mapleson’s meta‐analysis (1996) of the available MAC data and can be used to allow for the contribution of nitrous oxide to the total MAC. Age‐related iso‐MAC charts for isoflurane, sevoflurane and desflurane were developed for the clinically useful MAC range (0.6–1.6), age range 5–95 yr, and put in a convenient form for use by practising anaesthetists. The motivation for this study was the current difficulty in estimating the total age‐related MAC for a patient in a clinical setting.