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LETTER TO EDITOR
Year : 2014  |  Volume : 58  |  Issue : 4  |  Page : 504-505  

Misinterpretation of minimum alveolar concentration: Importance of entering demographic variables


Department of Neuroanaesthesia, National Institute of Mental Health and Neurosciences, Bengaluru, Karnataka, India

Date of Web Publication17-Aug-2014

Correspondence Address:
Dr. Vinay Byrappa
Department of Neuroanaesthesia, National Institute of Mental Health and Neurosciences, Bengaluru - 560 029, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0019-5049.139034

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How to cite this article:
Byrappa V, Kamath S, Venkataramaiah S. Misinterpretation of minimum alveolar concentration: Importance of entering demographic variables. Indian J Anaesth 2014;58:504-5

How to cite this URL:
Byrappa V, Kamath S, Venkataramaiah S. Misinterpretation of minimum alveolar concentration: Importance of entering demographic variables. Indian J Anaesth [serial online] 2014 [cited 2019 Nov 20];58:504-5. Available from: http://www.ijaweb.org/text.asp?2014/58/4/504/139034

Sir,

Minimum alveolar concentration (MAC), defined as minimum alveolar (end-tidal) anaesthetic concentration required to produce immobility in 50% of patients to noxious stimuli, is a standard measure for monitoring potency of inhalational anaesthetics. [1] Age is one of the several factors that affect MAC values, with 6-7% decrease in MAC for each increasing decade. [2] Modern day gas analysers in anaesthesia monitors are capable of incorporating age and concurrently administered nitrous-oxide during calculation and display of MAC value. [3] [Figure 1] demonstrates the influence of age on MAC value on Datex-Ohmeda S/5 monitor (Datex-Ohmeda, Helsinki, Finland) following delivery of similar anaesthetic concentration. With default age of 40 years, and end-tidal nitrous-oxide and end-tidal desflurane concentration of 49% and 4.6% respectively, MAC displayed was 1.3 [Figure 1]a and b. When age correction was applied for this 6-year-old child, at the same anaesthetic concentration, MAC value displayed decreased to 0.9 [Figure 1]c and d. This has important clinical implications when anesthetizing children. Despite the monitor display of adequate depth (MAC of 1.3), patient moved and might have experienced pain, and/or awareness. Had the demographic data been included, age appropriate MAC would have been displayed (0.9), facilitating measures to deepen the plane of anaesthesia and avoid this unsavoury incident. This is especially important when depth of anaesthesia monitor is not used, and MAC is depended upon for ensuring an adequate surgical depth. Though MAC is relied upon by the clinician as a metric with which to titrate the anaesthetic agent - the absolute values of each agent, clinical judgement and patient response are far more valuable and important. However, this report demonstrates that there is still possibility of misinterpretation of anaesthetic depth especially in children when demographic details are skipped or when using monitors which do not provide for age-correction when calculating MAC.
Figure 1: (a) and (b) Snapshot of the monitor showing inspired desflurane concentration of 4.6 and minimum alveolar concentration (MAC) value of 1.3 at default age of 40 years. (c) and (d) Snapshot of the monitor showing inspired desflurane concentration of 4.7 and MAC value of 0.9 when demographic data was entered and age changed to 6 years

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   References Top

1.Eger EI 2 nd , Saidman LJ, Brandstater B. Minimum alveolar anesthetic concentration: A standard of anesthetic potency. Anesthesiology 1965;26:756-63.  Back to cited text no. 1
    
2.Mapleson WW. Effect of age on MAC in humans: A meta-analysis. Br J Anaesth 1996;76:179-85.  Back to cited text no. 2
    
3.Eger EI 2 nd . Age, minimum alveolar anesthetic concentration, and minimum alveolar anesthetic concentration-awake. Anesth Analg 2001;93:947-53.  Back to cited text no. 3
    


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