End-tidal anesthesia monitors are ubiquitous. Their value has been reinforced from recent studies that show they are as useful in preventing recall as the BIS monitor. Can end-tidal anesthesia monitors provide the whole picture in terms of depth of anesthesia, particularly if anesthesia concentration is changing, as it would be, for example, during induction of anesthesia or emergence from anesthesia?
In the article “Sevoflurane End-Tidal to Effect-Site Equilibration in Women Determined by Response to Laryngeal Mask Airway Insertion,” published in the current issue of Anesthesia & Analgesia, Dr. Ross Kennedy, Department of Anaesthesia, Christchurch Hospital and University of Otago, Christchurch, Christchurch, New Zealand determined effect-site half-time concentration for airway manipulation. This calculation was based on a previous manuscript where they demonstrated that effect-site concentration based on fresh gas glows and vaporizer settings could be determined.
The authors used the up-and-down methodology, similar to what is used to determine MAC. Subjects inhaled oxygen (6 L/min via a circle system) and sevoflurane 6%. For the first subject, the effect-size calculation was set to 2.5 vol% and succeeding values were increased or decreased by 0.2 vol% based on response to cLMA airway insertion. They then used these calculations to determine effect-site concentration as a basis for airway manipulation.
Target time for cLMA insertion was on average a little more than 4 min. Use of effect site-concentration time was more accurate than end-tidal concentration of sevoflurane in determining when the LMA could be inserted. This should come as no surprise, because the end tidal monitor concentrations necessary rise faster than the brain concentrations at the beginning of anesthesia, and then fall faster than brain concentrations at the end of anesthesia. Since the brain is the site of anesthetic action on consciousness, it makes sense that a calculated “window” into the brain concentration would be more accurate than the expired anesthetic concentration.
What is the takeaway message? With a little software wizardry, our anesthesia monitors could display effect-site concentrations alongside end-tidal gas concentrations. Titrating effect-site concentrations might permit us to do a better job managing depth of anesthesia.
Naturally, more study is needed.