Misinterpretation of minimum alveolar concentration: Importance of entering demographic variables.

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 1a 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 1c 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