Levels of Anesthesia Consciousness

With the widespread advent of diethyl ether, anesthesia became somewhat standardized and it became useful to delineate "stages" in the continuum from the awake state through anesthesia to respiratory paralysis, cardiovascular collapse, and death. In 1847 John Snow published his pioneer monograph, On the Inhalation of the Vapour of Ether in which he described five empirical levels through which an anesthetized patient progressed from consciousness to respiratory paralysis. Surgery could be performed in "stage three" characterized by analgesia (pain relief) and amnesia (lack of memory storage), or in "stage four" characterized by muscular relaxation and regular, automatic breathing. Snow's scheme was expanded in 1920 by Arthur Guedel who published codified stages and signs of anesthesia, later detailed in his 1937 monograph, On Inhalation Anesthesia—A Fundamental Guide. Guedel predicated his stages on obvious physical signs involving muscle tone, respiratory patterns, and eye signs. He enumerated four levels: stage of analgesia, stage of delirium, surgical stage (subdivided into four planes), and stage of respiratory paralysis. Others have added a fifth stage, anesthetic overdose: lack of tissue oxygen, convulsions, and imminent death.

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Figure 7.1: Levels of anesthetic depth from different systems displayed on a common axis. Guedel's (1937) stages and planes serve as a standard. EEG levels (Courtin, Bickford and Faulconer, 1950), jaw and forearm relaxation (Galla, Rocco and Vandam, 1958), Woodbridge's (1957) classification of nothria, and stage one for ether (Artusio, 1955) are also represented. With permission from Grantham and Hameroff (1985).

Figure 7.1: Levels of anesthetic depth from different systems displayed on a common axis. Guedel's (1937) stages and planes serve as a standard. EEG levels (Courtin, Bickford and Faulconer, 1950), jaw and forearm relaxation (Galla, Rocco and Vandam, 1958), Woodbridge's (1957) classification of nothria, and stage one for ether (Artusio, 1955) are also represented. With permission from Grantham and Hameroff (1985).

In Guedel's stage one (Figure 7.1), the patient progresses from alertness and sensibility to pain to total amnesia, analgesia, and sedation. Breathing is slow and regular, with use of both diaphragmatic and rib muscles; the eyelid reflex (twitching of the eyelids in response to gentle brushing) is intact. Stage two has been variously termed the stage of delirium, excitement, unconsciousness, or the "dream state." The patient may pass through this stage sedately or may manifest wild, uninhibited activity. Breathing is irregular and unpredictable, pupils of the eye may be dilated, ocular muscles are active, and eyelid reflex active. In this stage the patient is at risk for unwanted reflex activity such as vomiting, spasm of the airways and cardiac dysrhythmias. The stage two excitement phase caused by anesthetics is somewhat puzzling. Early explanations described inhibition of neocortical "inhibitory" brain circuits, leading to unchecked primitive activity. However, anesthetic effects on single neurons may also show an excitatory phase, thus anesthetic induced "excitation" may be a molecular level effect at low concentrations. Guedel's stage three has four different "planes" of progressively deeper anesthesia during which surgery may be performed. Muscle relaxation is slight in plane one, but progresses to complete abdominal muscle relaxation in planes three and four. Breathing is very irregular and periodic in plane one, as in normal sleep. With plane two, a pause develops between inhalation and exhalation, and inhalation becomes shorter relative to exhalation. Paralysis of rib muscles begins in plane three, and diaphragmatic breathing is prominent. In plane four, paradoxical movement of the rib cage occurs with inhalation, breathing is irregular, and if anesthetic depth is not lightened breathing will stop altogether. Eye muscles are active initially in plane one, however the eyes become immobile when plane two is reached. The eyelid reflex disappears in plane three. The pupils tend to become dilated with progression to plane four. The fourth stage of anesthesia is respiratory arrest and ensuing cardiovascular collapse. Muscles are flaccid, and pupils are widely dilated. Death will occur if anesthetic depth is not decreased from stage four (Grantham and Hameroff, 1985).

Because of the risks of anesthetic overdose and the variable requirements of individual patients, anesthesiologists have sought methods to judge anesthetic depth in addition to the clinical signs enumerated by Snow and Guedel. Since the early 20th century, electroencephalography (EEG) has been recorded from anesthetized patients. EEG waves recorded at the scalp are thought to emanate from summated dendritic synaptic potentials ("dipole fields") generated by pyramidal cells of the cerebral cortex. Scalp potentials generally range from 10 to 200 microvolts, and in frequencies from several Hz to about 50 Hz. The frequency spectrum of the EEG is usually divided into 4 major classifications, delta: less than 4 Hz, theta: 4-8 Hz, alpha: 8-13 Hz, and beta: greater than 13 Hz. Generally, power at higher frequency decreases as anesthetic depth increases. Accordingly, efforts to derive an appropriate index of anesthetic depth have included attempts to quantify an EEG frequency below which most EEG power occurs. This involves a Fourier transform of raw EEG data into a frequency/power spectrum. One example is "spectral edge," the frequency below which 95 percent of EEG power occurs (Rampil, 1981). As anesthetic depth increases, spectral edge roughly decreases. Other attempts to quantitatively assess EEG and anesthetic depth have included the plotting of EEG voltage in "phase space," yielding phase portraits and chaotic attractors. Phase space plotting involves choosing an arbitrary "phase lag," and plotting EEG amplitude at any point in time against the amplitude at that particular time plus the phase lag. This results in geometric phase portraits which may be analyzed for complexity, or "dimensionality" on a continuum of order and chaos. As patients become more anesthetized, the dimensionality of their EEG becomes more ordered, and less chaotic (Figures 7.2 and 7.3, Watt and Hameroff, 1987). Consciousness may thus be described as a manifestation of deterministic chaos somewhere in the brain/mind.

In addition to observing activities of the brain, anesthesiologists must closely monitor other physiological functions of their patients during and immediately after surgery. Cardiovascular, pulmonary, neuromuscular, kidney, blood clotting, and other factors are carefully followed. A variety of technological advances in monitoring permit safer care of sicker patients for more complex procedures. Future monitoring may utilize even more advanced technologies. One possible example is the "biosensor" field effect transistors which are tiny membrane covered chips which can detect a variety of ions, molecules, drugs or hormones. Small enough to fit on the tip of a small catheter harmlessly inserted into a blood vessel or tissue, these biosensors connected to a computer can yield "on-line" monitoring of blood chemistry and many other functions. With the advent of nanotechnology, even tinier, more profoundly sensitive biosensors will materialize. The capability for monitoring nanoscale conformational dynamics of neural proteins (telemetrically or via very small implants) will lead, not only to more sensitive observation of brain function during anesthesia and surgery, but perhaps eventually to the manipulation of consciousness.

Figure 7.2: EEG from awake patient prior to anesthesia. Top: 4 seconds of "conventional" EEG which shows low voltage, high frequency waves. Bottom: same 4 seconds plotted as phase space trajectory with digitization of 300 Hz and phase lag of 5/300 sec. Phase portrait of awake state is densely centered. From Watt and Hameroff (1987).

Figure 7.2: EEG from awake patient prior to anesthesia. Top: 4 seconds of "conventional" EEG which shows low voltage, high frequency waves. Bottom: same 4 seconds plotted as phase space trajectory with digitization of 300 Hz and phase lag of 5/300 sec. Phase portrait of awake state is densely centered. From Watt and Hameroff (1987).

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