Adequacy of Anesthesia

Illustrating the modern anesthesia concept
Framework for the components of anesthesia

Leena Pesu
Development Manager, Datex-Ohmeda

Email: leena.pesu@datex-ohmeda.com

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This paper presents a graphical framework, which illustrates the modern anesthesia concept, and has been developed according to published scientific theories. It is aimed as an educational tool to visualize what aspects we need to bear in mind when touching the topic of adequacy of anesthesia. Such a framework may be helpful in understanding various components of anesthesia, effects of anesthetics, as well as drug interactions.

The traditional way to visualize anesthesia was to draw a triangle, and then place the terms hypnosis, analgesia, and relaxation in its three corners. However, such a picture was rather simplified, and new theories have further developed the anesthesia concept. In the clinical review articles of this web journal, there is a more detailed information of the evolution of the modern anesthesia concept.

Kindly see the attached figure and note the conceptual circle which has been divided in two.

On the left side, there are the two cortical anesthesia components (unconsciousness and amnesia). The right side semi-circle displays the three subcortical components of anesthesia (antinociception, motor stability, and autonomic stability).

In the following text, each of the five components will be addressed in more in detail.

CORTICAL COMPONENTS

The cortical components of anesthesia refer to the effects of the anesthetic agents in the brain, particularly in the cerebral cortex, where cognitive processes take place.

Unconsciousness

In this conceptual framework, the term unconsciousness has been taken to denote a component of anesthesia. In this context, many traditional terms have been tried, though not always quite correctly. Some of these terms include "awareness", "wakefulness", "hypnosis", and "depth of anesthesia". In the context of anesthesia, the term consciousness has been defined as a state of awareness of the outside world [1]. Therefore, unconsciousness refers to the lack of this awareness.

Many researches regard unconsciousness as the key component of general anesthesia and hence, much scientific effort has been focused on qualifying anesthesia according to that component. As unconsciousness is a cortical component, obvious indicators for levels of unconsciousness are some forms of neurophysiological measurements of the cortical activity of the brain. Typically, processed EEG and auditory evoked potentials (AEP) have been suggested for this purpose, and there are numerous studies which link them to clinical measures of unconsciousness.

Selected scientific articles have been included in the bibliography listings of the Clinical Window WEB Journal (CWWJ), and more insight into the topic will be given in its clinical articles.

Amnesia

Scientific articles have appeared [2] showing that the sedative and amnesic effects of anesthetic agents actually are two separate phenomena. This makes it clear that amnesia usually occurs at lower drug concentrations than loss of consciousness. [3]

Concerning amnesia, it is important to realize that there are two types of memory: explicit and implicit.

Explicit memory refers to the conscious recollection of actual events. Hence, when the patient is considered to be aware during anesthesia, explicit memory plays a role.
Implicit memory, on the other hand, is not accessible to conscious mind. However, it has an important influence on human behavior. Moreover, Lubke has shown that some memory formation can occur even during adequate anesthesia [4,5].

SUBCORTICAL COMPONENTS

The subcortical anesthesia components also play an important role. They maintain autonomic and motor stability, and also ensure antinociception.

Antinociception

Antinociception refers to inhibition of the nociceptive processing in the nervous system. Analgesia is the treatment to provide antinociception. In adequate anesthesia, the role of antinociception is crucial; it makes surgical operations possible, and it reduces immediate and long-term negative consequences related to those procedures. With inappropriate analgesia, nociception might cause unfavorable responses of the autonomic nervous system, and involuntary movements in the patient.

Incidentally, in the context of general anesthesia, it is actually not quite ‘correct’ to talk about pain. Merely, the sensation of pain means conscious perception of noxious stimulation. Hence, by that definition, without consciousness, there is no pain.

Motor stability

One of the objectives of general anesthesia is to ensure that the patient does not move involuntarily. In fact, in some procedures like in ophthalmic or neurosurgery, unexpected movements might even compromise patient safety.

Hence, muscle relaxation can be considered as important component of adequate anesthesia. Most often, neuromuscular blocking agents can achieve motor stability, but there are other anesthesia drugs, e.g. inhalation agents, which work to the same direction.

Whenever neuromuscular blocking agents are used, modern practice is to monitor level of neuromuscular block by a peripheral nerve stimulator. Today’s technology allows for an objective assessment of an evoked response, making it possible to maintain steady surgical block and ensure safe restoration of muscle strength at emergence of general anesthesia.

As Rampil has pointed out, movement response of an anesthetized patient seems not to be a cortical phenomenon, but is initiated at the level of the spinal cord [6]. Hence, if the anesthetist takes a patient’s unexpected movement as indication of inadequate hypnosis, that interpretation may not necessarily be true.

Autonomic stability

Autonomic stability is an important component of the adequate anesthesia. In that context, stability of blood circulation is crucial, as sudden hemodynamic changes may indicate nociception, thus being a sign of inadequate analgesia. Concerning an anesthetized patient with a compromised cardiovascular system, it is evident that major hemodynamic variations may no be well tolerated, and all drugs including anesthetics need to be administered by careful titration.

References

1. Ghoneim, M (Ed.). Awareness during anesthesia (textbook). Butterworth-Heinemann Publishers, 2001 (ISBN 0-7506-7201-3).
2. Veselis,R.A.; Reinsel,R.A.; Feshchenko,V.A. Drug-induced amnesia is a separate phenomenon from sedation: electrophysiologic evidence. Anesthesiology 2001; 95: 896-907
3. Glass PS, Bloom M, Kearse L, Rosow C, Sebel P, Manberg P: Bispectral analysis measures sedation and memory effects of propofol, midazolam, isoflurane, and alfentanil in healthy volunteers. Anesthesiology 1997, 86, 836-847.
4. Lubke,G.H.; Kerssens,C.; Phaf,H.; Sebel,P.S. Dependence of explicit and implicit memory on hypnotic state in trauma patients. Anesthesiology 1999; 90: 670-680.
5. Lubke,G.H.; Kerssens,C.; Gershon,R.Y.; Sebel,P.S. Memory formation during general anesthesia for emergency cesarean sections. Anesthesiology 2000; 92: 1029-1034.
6. Rampil,I.J. Anesthetic potency is not altered after hypothermic spinal cord transsection in rats. Anesthesiology 1994; 80: 606-610.
   

Last updated: 1 November 2001Created
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