Minimum Alveolar Concentration (MAC) of Desflurane for Effective Tracheal Intubation (MAC-EI)

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Minimum Alveolar Concentration (MAC) of Desflurane for Effective Tracheal Intubation (MAC-EI)

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Photo by Gabriella Clare Marino on Unsplash

Photo by Gabriella Clare Marino on Unsplash

Why is it important?

Introduction: The minimum alveolar concentration (MAC) which prevents movement in response to surgical incision in 50% of patients for halothane and enflurane is less than that which prevents movement in response to laryngoscopy and tracheal intubation. Desflurane may differ from older anaesthetics in its capacity to prevent movement in response to laryngoscopy and tracheal intubation. Aim: To calculate the minimum alveolar concentration of desflurane for effective endo tracheal intubation. Materials and Method: It was a prospective study conducted at Nizam’s Institute of Medical Sciences, between July 2015 and September 2015. The study recruited seventy patients scheduled for general anaesthesia for an elective surgery with age ranging between 18 to 60 years of either gender with ASA physical status I and II. After giving the induction drugs, nitrous oxide in oxygen (50:50), each at 4 litres/minute with desflurane was commenced. Desflurane was started at 2% and increased by 2% every 30 sec, until patient lost consciousness, with entropy less than 60 and permitted manual ventilation. Then the dial setting was changed to achieve predetermined endtidal desflurane concentration within the first 5 min, to start with 6% in the first patient (best guess for MACEI). After establishing and maintaining the target endtidal concentration for 5 more minutes, tracheal intubation was attempted at 11th minute without neuromuscular relaxants and ±0.5% difference in the predetermined/target endtidal desflurane concentration was allowed. Each concentration at which tracheal intubation was attempted was predetermined according to the upanddown method (with 1% as step size). Outcome measure of success/failure (unresponsive/responsive) for intubation was based on a score formulated on parameters like, ease of intubation, vocal cords position & movement , reaction to intubation (in terms of movements)score of 36 being regarded as success and 7 as failure. The Dixon’s methodological principles were applied to determine MACEI in the present study. Values for MACEI were obtained by calculating the midpoint concentration of all independent pairs of patients involving a crossover, i.e., responsive (failure) to unresponsive (success). We also calculated the eighteen crossover pairs, success to failure. Minimum alveolar concentration was defined as the average of the crossover midpoints in each crossover subgroup. Blood pressure (SBP & DBP), heart rate, saturation, response entropy, state entropy and train of four count (neuro muscular junction monitor) were noted at baseline. Along with these, desflurane dial setting, inspired concentration, endtidal concentration, FIO2 and ET N2O were also noted at 5minutes, 10minutes (preintubation) and 1 min, 3 min & 6 min postintubation. Results: The state entropy similar to the response entropy decreased across the time periods with least value noted just before intubation. The post hoc analysis showed significant decrease in all periods when compared to base line. There were no significant changes in oxygen saturation. Post hoc analysis of the heart rate data showed that there was a significant increase in heart rate when compared to base line at the time of intubation and one and 3 minute after intubation but this returned to normal at 6 min after intubation. There was significant decrease in Systolic and diastolic blood pressure across all periods when compared with the base line and was significant. The difference in desflurane dial concentration and inspired desflurane across the time periods was clinically not significant. The end tidal desflurane rose to the level of set concentration at the end of 5min, and was maintained at this level till intubation. The krushkal wallis test of the desflurane dial concentration, inspired concentration and end tidal concentration compared at different time periods revealed significant changes only in time period 1 (5 minutes after starting Desflurane). Although the desflurane decreases the entropy very fast, there is still a difference between failure and success groups and there is a difference also in the end tidal desflurane concentration between the success and failure groups, successful cases had a greater concentration of end tidal desflurane which corresponded to lower entropy values and successful intubation The MACEI for desflurane in 50% N2O was calculated to be 6.37%.

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