While some of the factors found to be associated with higher one-year mortality might seem to make sense (depression, age, other medical diseases), the finding about anesthetic depth is a somewhat surprising one. These results lend further support to those who advocate the use of monitors such as the BIS monitor, utilized for monitoring anesthetic depth. The BIS monitor was used in this study to determine depth of anesthesia. There is widespread disagreement in the anesthesia community about the necessity and utility of the BIS monitor.

BIS NUMBER REFLECTS ANESTHETIC DEPTH
The BIS monitor works by monitoring a single channel EEG signal from the patient's frontal lobe. This signal is converted to a number, with 100 being completely awake and with lower numbers representing deeper levels of anesthesia. In the study, three categories were used - anesthetic levels below a BIS of 40, anesthetic levels between a BIS level of 40 and 60, and anesthetic levels of greater than 60. The company recommends that patients be maintained with BIS number of less than 60 to ensure that patients are adequately anesthetized. While some have complained that the BIS monitor is not accurate in all situations, there is at least a general correlation between the depth of general anesthesia and the BIS number. (This has been discussed previously on this site.)

Overall, the researchers found that patients maintained at the deepest level of anesthesia had a one year death rate of 7.8%, patients in the middle group had a one year death rate of 6.8% and those maintained with a BIS higher than 60 had a one year death rate of 3.1%. When isolating their analysis to young patients (age less than 40), this trend disappeared. However, for middle age (age 40 to 59) and elderly (age 60 or more), the association held true.

WHAT EXACTLY DOES THIS MEAN?
Although this study is interesting, it raises more questions than it answers. Is it a specific type of anesthesia that this is true for? What if we used a different set of medications? Is it because the older you get, the higher likelihood you are to have other diseases that affect your death rate? Are there changes in the brain as we age that account for these differences? Do patients having more complex or longer surgeries require more anesthesia as well as having higher death rates?

The exact answers to these questions are unclear and require further study. As a result, it is similarly unclear how we should integrate these findings into our practice of anesthesia. Should we seek to give patients the minimal amount of anesthesia that they need? Clearly patients can be maintained at either higher levels or lower levels of anesthesia with little difference in their safety, comfort and satisfaction. Should we choose one over the other?

More answers will come in the future as research is done on this intersting subject. However, the current results are at least suggestive that anesthetic depth has an influence on how a patient does after anesthesia, even after the anesthetic is long gone. This is something that everyone should be more aware of.