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Understanding Anesthesia

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Neil Harrison

Eric Ogden for Proto

Anesthesia's secret, Neil Harrison thinks, involves inhibitory synapses that block messages on their paths through the brain.

However anesthesia works, a major benefit for patients—in addition, of course, to avoiding pain—is remembering nothing afterward. “What patients want is oblivion,” says Rampil. Sometimes, though, despite anesthesia, patients are aware of what they’re experiencing and may be left with traumatizing memories. And though such cases are rare, the frequent use of general anesthesia means that even a rate of one or two per 1,000 cases results in as many as 40,000 U.S. patients each year who suffer from recall.

Preventing recall is difficult because each patient has a different tolerance for anesthesia. A level of drug that renders one patient unaware (and thus safeguarded against recall) might leave another insufficiently sedated. Most anesthesiologists have tended to err on the side of caution, administering ample doses to ensure forgetfulness. But recently, they’ve been able to employ a more precise approach, using electroencephalograms (EEGs) to monitor the brain’s electrical activity under anesthesia.

The EEG is, medically speaking, an ancient technology, introduced more than 125 years ago by a British physician named Richard Caton. But a device developed in the 1990s by Aspect Medical Systems, a Massachusetts company, translates EEG readings into a single number called a BIS Value. On the bispectral index’s scale of zero to 100, the highest BIS numbers represent degrees of full consciousness. A fully alert patient might have a BIS reading of 94; as the level of anesthetic increases, the number steadily falls. At a BIS of 60 or lower, the probability of a patient being subject to recall is reduced by as much as 80%.

An EEG can also produce graphs showing the amplitude and frequency of electrical activity in the brain. With patients who are alert or even in conventional sleep, the tracings on the graphs (the brain waves) seem to make little sense. The points charted are packed together and dart up and down in no apparent pattern. That’s because a brain’s normal electrical activity is ever changing, rising and falling as the barrage of sensory impulses is processed into thought and action.

Under anesthesia, though, a striking pattern emerges. As more drugs are administered, the frequency of activity drops, and the spikes of the brain waves are higher and spread farther apart. Eventually, as the subject enters a state of deep anesthesia, the scattered waves elongate and reveal a series of long, slow ocean swells. Yet as with so much else associated with anesthesia, it isn’t certain just why the swells occur.

To begin to unlock that secret, Brown and his research team have used both EEGs and functional magnetic resonance imaging (fMRI). Unlike conventional MRIs, which yield detailed pictures of body parts, fMRIs map areas of brain activity by tracking blood flow through thousands of tiny veins and arteries. Oxygenated blood (carrying oxygen to the cells) and deoxygenated blood (returning to the heart and lungs) have different magnetic properties. When captured by fMRI, those characteristics translate into intricate pictures of brain activity.

In Brown’s experiments, subjects are slipped into the tunnel of a machine, wearing nonmagnetic helmets with electrical receptors that feed signals from the brain to an EEG monitor. They are put under anesthesia in stages, with detailed fMRIs taken during each stage. Blood increases and decreases relative to the amount of electrical activity going on in a particular area, showing which parts of the brain are active under anesthesia.

Because fMRI images take several minutes to produce, volunteers must be anesthetized very slowly. That means a breathing tube must be in place while a subject is still fully awake. Because most people would gag on the tube, Brown’s team has sought out volunteers with tracheostomies, which provide a built-in external connection for the tube. In more than a year, studies have been conducted on only four volunteers. Even so, initial results are promising.

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Consciousness Unbound

Some researchers say the key to learning how anesthetics work is to examine a much tougher subject: consciousness.

Number Conscious

The bispectral index uses EEG readings to assign a number to a patient's level of consciousness, allowing doctors to administer more precise doses of anesthesia.

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hed-dossier

1.“A Primer for EEG Signal Processing in Anesthesia,” by Ira J. Rampil, Anesthesia, October 1998. A definitive if highly technical account of BIS readings and other uses of EEG in anesthesiology.

2.“The Effects of Anesthetics on Brain Activity and Cognitive Function,” by Wolfgang Heinke and Stefan Koelsch, Current Opinion in Anaesthesiology, December 2005. An excellent overview of recent work, from EEGs to neuroimaging.

3.“Consciousness Unbound,” by George A. Mashour, Anesthesiology, February 2004. A reflection on the various “cognitive binding” theories of consciousness, and their implications for anesthesia.

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