Assessing BIS Monitoring in Critical Care
Use increasing, but study questions efficacy for ICUs
By Julie Crawshaw,
Critical Care Plus Editor
Bispectral index (BIS) monitoring has received a generally favorable reception since its formal introduction at the American Association of Critical Care Nurses National Teaching Institute and Critical Care Exposition last spring. BIS technology works through a sensor placed on the patient’s forehead that gathers information from electrical brain activity and translates it into a single number from 100 (indicating an awake patient) to 0 (indicating the absence of brain electrical activity) to describe the patient’s level of consciousness.
Dai Wai Olson, BSN, CCRN staff nurse at Duke University Hospital in Durham, NC, says that BIS has its limitations, as does any tool. "The more that we learn about using BIS, the more benefits and drawbacks we see," Olson says. "We’re finding that by knowing its limitations we’re using it on more appropriate patients and getting the information we need. Patients’ needs for sedation vary, but the BIS gives us an objective guideline."
Over- and under-sedation of patients is a widely recognized challenge in ICUs, where sedation assessment has been guided primarily by vital signs or subjective sedation assessment scales. Olson says his hospital began BIS monitoring about four years ago as part of a research protocol for anesthesiologists. As more research occurred, Duke incorporated BIS into its critical care practice. The hospital has documented significant cost savings when BIS is used to guide sedation management.
Scores Vary, Appear Unrelated to Patient Age
Olson acknowledges that it’s still pretty early in research life to define the perfect BIS score and says that patient age does not appear to be a factor in BIS score variance. "We haven’t identified any factors for variance, and I don’t think I’ve read any research that has tried," he says. " Patients of different ages require different amounts of sedation, but that doesn’t give us a goal. There are some patients we want to be more deeply sedated, so we go for a lower BIS goal. Generally, we shoot for between 60 and 70, and we think that’s where we’ll stay for most of our patients."
Olson notes that under-or-over-sedation can create events that negatively affect nearly every system in the body, ultimately increasing the risk of complications, time on the ventilator, and length of stay in the ICU. Monitoring with the BIS, he says, can be difficult because of space limitations due to the presence of other, more important monitors that makes the monitor useless in patients with certain brain injuries.
"We’re a neural ICU, so when we get someone with a bilateral frontal injury, the area you need to monitor is mush," he says.
BIS monitoring has been used for monitoring anesthesia effects in the operating room for a number of years. BIS manufacturer Aspect Medical Systems says the technology has been used to assess more than five million patients and has been the subject of more than 900 published articles and abstracts. The company further claims that BIS is "currently in use in the ICUs of more than 60 percent of the best-ranked hospitals with operating rooms in the U.S."
Olson says he doubts that the manufacturer initially realized that ICUs were a potential market, adding that his hospital was using BIS before Aspect began marketing it for critical care.
Study Challenges BIS Usefulness in ICUs
However, a study reported in Critical Care Medicine warns, "there are no published, peer-reviewed studies of the level of consciousness in critically ill patients demonstrating a strong correlation between clinical assessment and objective values measured by the BIS monitor."1 The study population included a general ICU heterogeneous population older than 18 years of age with a decreased level of consciousness in the surgical and medical ICUs of the Tufts-New England Medical Center in Boston. Most of the patients studied were receiving pharmacologic sedation; some had sustained concurrent metabolic encephalopathies or cerebral injury from underlying disease. Patients’ severity of illness was described using the Acute Physiology and Chronic Health Evaluation (APACHE II) Severity of Disease Classification.
That study notes that BIS has been "tuned through successive iterations of software revisions to correlate with the degree of sedation in patients undergoing anesthesia for short periods of time in the operating room." The monitor was not designed using data collected from critically ill patients sedated for prolonged periods of time in an ICU setting.
Additionally, investigators in referenced studies observed that the BIS scores did not correlate with clinical assessment in 12 of the 29 patients studied. Yet the investigators concluded that the "BIS provided a reliable index of neurologic status in the awake, unsedated critically ill patient".
The study also found that patient skeletal muscle movement produced inconsistent BIS readings and concluded that "the inconsistent and sometimes inaccurate measurements provided by the BIS device dampen the hopes of caregivers who are in search of an objective measurement of the level of consciousness in sedated, critically ill patients."
For more information contact Dai Wai Olson at (919) 681-4241, and Aspect Medical Systems at (888) 247-4633.
Reference
1. Nasraway S, et al. How reliable is the bispectral index in critically ill patients? A prospective, comparative, single-blinded observer study. Crit Care Med. 2002;30(7):1483-1487.
Bispectral index (BIS) monitoring has received a generally favorable reception since its formal introduction at the American Association of Critical Care Nurses National Teaching Institute and Critical Care Exposition last spring.
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