The bispectral index (BIS) monitor is a noninvasive derivative of the electroencephalogram (EEG) used to assess the depth of anesthesia to help prevent intraoperative awareness – a rare (~ 0.1% of general anesthetics) but serious complication that can lead to post-traumatic stress disorder (PTSD). To avoid this, some providers run their anesthetics “deep” to prevent awareness/movement but have to combat hemodynamic lability with vasopressors (which aren’t benign) and the resulting polypharmacy/metabolites which can prolong emergence and lead to side effects post-operatively.
The BIS monitor is essentially a frontal cerebral cortex EEG consisting of a sensor placed on the patient’s forehead, a signal converter, and a monitor which outputs a single number from 0 to 100. Awake patients tend to have values from 90-100, and a value less than 60 is associated with a low likelihood of awareness. As an anesthesiologist, I place BIS monitors to assess EEG activity prior to deep hypothermic circulatory arrest (DHCA) and burst suppression for cerebral aneurysm clippings. In the ICU, patients receiving neuromuscular blockers will also have a BIS monitor to gauge awareness while paralyzed.
Some studies have shown that while the BIS monitor is fairly good at detecting alertness (ie, in awake patients), the “asleep” state is difficult for it to ascertain. Therefore, patients receiving an adequate depth of anesthesia may register a BIS higher than 60 causing providers to administer more hypnotics. Like all things in medicine, the pursuit of treating a single number is often dangerous and riddled with confounders. BIS monitors are a useful tool, but tailoring the anesthetic’s depth should still be left to the provider based on the patient’s comorbidities, nature of the surgery, pharmacokinetics, anticipated emergence plan, etc.
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