The state of general anesthesia is often presented with its analgesic component included. An example of this can be found in one of the classic definitions of the anesthetic state by Dr. Edmond Eger. He describes analgesia as an “unmeasurable component (the anesthetized patients cannot tell an investigator that he/she hurts or does not hurt)”; in addition, he states that “surrogate measures (increases in breathing, blood pressure and heart rate with surgery) suggest that some pain is perceived.” This description reflects two problems that have became obvious with new developments in our understanding of pain and nociception.
The first is related to the analgesic properties of general anesthetics. Does the blockade of a nocifensive response produced by an anesthetic increment in an unconscious patient represent analgesic action? If the answer is yes, it contradicts the 2020 definition of pain accepted by the International Association for the Study of Pain, which states: “An unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage.” A note associated with this definition indicates that “Pain and nociception are different phenomena. Pain cannot be inferred solely from activity in sensory neurons.” Pain is described as both a sensory and an emotional experience. To fit the definition of pain, the term “analgesic action,” in conjunction with general anesthetics, should be applied only when consciousness is present. An anesthetized, unarousable subject responding to noxious stimuli with movement or an increase in heart rate does not have “unpleasant sensory and emotional experience.” It is impossible to determine precisely when—during the induction of anesthesia—nocifensive responses are already not indicative of pain. In an anesthetized patient, the presence of pain, at least as it is required by the International Association of the Study of Pain definition, is not certain. Therefore, the suppression of a nociceptive response by general anesthetics beyond the state of any degree of unconsciousness probably should be regarded as an antinociceptive effect, not an analgesic one.
The second problem is associated with the previously mentioned indication that some pain may be perceived during the state of general anesthesia. What is the appropriate term for this phenomenon? I believe that such terminology is already in use—for the quantification of nociception in unconscious subjects during anesthesia, such as “nociception monitoring.” Proception is not a subjective feeling, but the physiologic encoding and processing of nociceptive stimuli through the sensory system. It is the reaction to nociception that is being used for the purpose of nociception monitoring. The most frequently utilized response is an increase in sympathetic activity (increased heart rate, peripheral vasoconstriction, pupillary dilatation, an increase in galvanic skin conduction, among others). Various nociception monitors are already available, and others are being developed. They estimate nociception during anesthesia and can be used to optimize the administration of antinociceptive agents. Although some nociception monitors show promising results, no clear conclusions on anesthesia-related outcome can be drawn. Various categories of drugs are used as antinociceptive agents in general anesthesia. Differential antinociceptive effects of drugs used for anesthesia were most clearly illustrated by Prys-Roberts. He showed that the depth of anesthesia can be defined by suppression of the clinically relevant responses to noxious stimuli. He divided autonomic responses into four categories—breathing, hemodynamic, sudomotor, and hormonal—and showed that they can be altered differentially by drug and dose. A strategy for choosing a combination of agents that act at different targets in the nociceptive system to control nociception in anesthetized patients was recently presented by Brown et al.
When discussing general anesthetics, the term “analgesic effect” should be used only when consciousness is present; if not, “antinociceptive effect” is the better term to apply.
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