The issue of awareness under anesthesia is of great concern for obvious reasons. The BIS monitor was developed with the intention of ensuring patients are adequately anesthetized. However, given the expense of BIS electrodes and the low incidence of awareness the routine use of the BIS for awareness detection may not be economically efficient. With that in mind I asked the questions: “What else can it do? Can the BIS monitor be used to help fine-tune an anesthetic to achieve the best possible care for an individual patient?” To answer these questions I began using the BIS monitor on as many cases as possible to see if it changed my typical management. The types of cases included craniotomies for tumors and aneurysms, laminectomies, VP shunts, orthopedics, ENT, and an A-fib ablation. Several observations quickly became apparent. First, the BIS revealed that I was frequently overdosing the volatile agent in response to increased heart rate and blood pressure. This observation was consistent with several studies that have shown that hemodynamic indicators do not always correlate with state of consciousness. In these situations the BIS guided me towards using additional narcotic or anti-hypertensive medications rather than simply turning up the anesthetic vapor.
This change in management had several clinical implications. By using less volatile agent emergence times were consistently reduced and cognitive function returned quicker. This was especially beneficial in craniotomies as patients were able to quickly cooperate with neurologic exams at the end of the case. It was also helpful in appropriately titrating opioid in surgeries such as laminectomies where post-operative analgesia can be challenging. In addition to these important short-term benefits, decreasing the amount of volatile anesthetic may affect long-term outcomes. As we learn more and more about the potential neurodegenerative changes associated with anesthetic drugs it becomes evident that reducing the amount of volatile agent may be beneficial down the road for the patient.
While the reduction in anesthetic use was notable across a broad spectrum of cases the BIS helped in other aspects of management as well. In two patients where thiopental infusions were utilized to achieve EEG burst suppression the BIS allowed easy medication titration. One patient in particular required a significantly higher rate of infusion than we expected. I also found the BIS to be a valuable aid in weaning off the anesthetic agent towards the end of craniotomies with the Mayfield. I was able to comfortably titrate down the agent while maintaining neuromuscular blockade without the fear of awareness. The BIS may also augment patient satisfaction with their anesthetic in regards to post-operative nausea and vomiting. One study has shown a decreased in PONV when using the BIS to titrate sevoflurane in ambulatory surgery. In these select cases I did not observe much difference in PONV although this may be partially explained by my increased use of opiates when using the monitor.
In summary, the information provided by the BIS allowed me to better tailor anesthetic management to provide the best possible care for individual patients. The ability to directly monitor individual brain effect of the anesthetic drug facilitated therapeutic decision making. Using the BIS in conjunction with clinical judgment I was able to achieve a better balance between anesthetic agents and other adjuvant medications. In addition to monitoring awareness during anesthesia the data provided by the BIS can be utilized to deliver a precise and personalized anesthetic.
Jonathan Anson, M.D.
Dr. Anson is a resident in Anesthesia at the University of Pennylvania