In some countries anesthetic agent monitoring is a written Standard of Care while in others it is not even mentioned.1 Most anesthesia care providers use an anesthetic agent monitor to measure agent concentrations in their everyday practice and in much of the world it has become a de facto Standard of Care.2 Thus, although widely adopted in patient care, agent monitoring is not considered to have the same importance as other monitoring modalities that have been adopted as standards, such as those that address the various components of oxygenation, ventilation, and perfusion which are rather consistent across the globe.1 But anesthesia providers induce and manage certain behavioral states; in particular, unconsciousness and immobility. Patients expect to be unconscious, and preventing awareness is an important safety imperative. Even though lack of awareness is an essential component of anesthesia, monitoring the hypnotic state is not consistently addressed in standards due to controversy over the reliability of the technology.3 When inhalation anesthetics are used, we have the ability to monitor inspired and end-tidal expired agent (ETA) concentrations to help ensure lack of awareness. We believe that anesthetic agent concentration monitoring provides sufficient information to allow care providers to prevent awareness and that it should be universally adopted as a Standard of Care.
Three properties of inhaled agents provide the rationale for ETA monitoring: the steep dose response curve of volatile anesthetics4,5; the small effect opioids have on this relationship (only a 10–15% reduction in minimal alveolar concentration (MAC) Awake, the median anesthetic level for patients to respond to verbal command)6; and the ease of continuous measurement of their concentration. The end-tidal agent concentration is a good indicator of how likely it is the patient is unconscious8 after taking into account the short delay for the brain partial pressure to equilibrate with that in the blood and alveoli.8-12 With an ETA concentration of 0.7 MAC, awareness is extremely unlikely.13,14
So, how is it that anesthesia continues to be administered without ETA monitoring? An anesthesia care professional without an ETA monitor could titrate the vaporizer output to maintain stable vital signs like blood pressure and heart rate. Underdosing of the volatile anesthetic is generally recognized by a rising heart rate or blood pressure in response to surgical stimulus, and, in the un-paralyzed patient, movement. But without a measure of the partial pressure of anesthetic agent in the body, which is indicated by the ETA concentration, the etiology of vital sign changes is less obvious and can result in incorrect diagnosis and treatment including unnecessary vasopressor use or excessive fluid administration. In addition, clinical signs per se are unreliable indicators of the hypnotic state especially in patients taking sympatholytic medications.
More compelling reasons exist to use ETA monitoring. Forgetting to turn on the vaporizer or an empty vaporizer going unnoticed can result in unappreciated awareness, especially in the patient given a muscle relaxant. In addition, the concentration selected on the vaporizer may not match the end-tidal agent concentrations, putting the patient at risk for under- or overdosing. Efforts to reduce fresh gas flow to reduce waste and environmental contamination also increase the challenge of managing the relationship between the delivered and actual alveolar concentration. Keeping the ETA concentration and thus anesthetic depth constant can require increasing the vaporizer setting well above the desired inspired and ET concentrations. The lower the fresh gas flow, the greater the difference between the vaporizer setting and the inspired agent concentrations, and that difference is only apparent when using an anesthetic agent monitor.
Given the readily available technology for measuring ETA concentration, as well as the well documented relationship between ETA agent concentration and risk of awareness, we believe the use of ETA concentration monitoring should be an official Standard of Care for all anesthesia-related professional organizations.
Dr. Philip is senior consultant anesthesiologist and director of Anesthesia Clinical Bioengineering, Brigham and Women’s Hospital, and professor of Anaesthesia, Harvard Medical School.
Dr. Hendrickx is a staff anesthesiologist at OLV Hospital, Aais, Belgium.
Dr. Philip has received honoraria from Getinge and GE. Dr. Hendrickx has received lecture support, travel reimbursements, equipment loans, consulting fees, and meeting organizational support from AbbVie, Acertys, Air Liquide, Allied Healthcare, Armstrong Medical, Baxter, Dräger, GE, Getinge, Hospithera, Heinen & Lowenstein, Intersurgical, Maquet, MDMS, MEDEC, Micropore, Molecular, NWS, Philips, Piramal, Quantium Medical.
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- Anesthesia Gas Monitoring: Evolution of a de facto Standard of Care. ProMed Strategies for Masimo Phasein: Sweden. 2009.
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- Dilger JP. From individual to population: the minimum alveolar concentration curve. Curr Opin Anaesthesiol. 2006;19:390–396.
- Katoh T, Ikeda K. The effects of fentanyl on sevoflurane requirements for loss of consciousness and skin incision. Anesthesiology. 1998;88:18–24.
- Sonner JM. Issues in the design and interpretation of minimum alveolar anesthetic concentration (MAC) studies. Anesth Analg. 2002;95:609–614.
- Kety, SS. The physiological and physical factors governing the uptake of anesthetic gases by the body. Anesthesiology. 1950;11:5:517–526.
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- Merkel G, Eger EI II. A comparative study of halothane and halopropane anesthesia including method for determining equipotency. Anesthesiology. 1963;24:3:346–357.
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- Eger EI II. A brief history of the origin of minimum alveolar concentration (MAC). Anesthesiology. 2002;96:238–-239.
- Chortkoff BS, Gonsowski CT, Bennett HL, et al. Subanesthetic concentrations of desflurane and propofol suppress recall of emotionally charged information. Anesth Analg. 1995;81:728–736.
- Eger EI II, Sonner JM. How likely is awareness during anesthesia? Anesth Analg. 2005;100:1544 [letter].