Using bispectral index and cerebral oximetry to guide hemodynamic therapy in high-risk surgical patients

Abstract

High-risk surgery represents 12.5% of cases but contributes 80% of deaths in the elderly population. Reduction in morbidity and mortality by the use of intervention strategies could result in thousands of lives being saved and savings of up to £400m per annum in the UK. This has resulted in the drive towards goal-directed therapy and intraoperative flow optimization of high-risk surgical patients being advocated by authorities such as the National Institute of Health and Care Excellence and the Association of Anaesthetists of Great Britain and Ireland.Conventional intraoperative monitoring gives little insight into the profound physiological changes occurring as a result of anesthesia and surgery. The build-up of an oxygen debt is associated with a poor outcome and strategies have been developed in the postoperative period to improve outcomes by repayment of this debt. New monitoring technologies such as minimally invasive cardiac output, depth of anesthesia and cerebral oximetry can minimize oxygen debt build-up. This has the potential to reduce complications and lessen the need for postoperative optimization in high-dependency areas.Flow monitoring has thus emerged as essential during intraoperative monitoring in high-risk surgery. However, evidence suggests that current optimization strategies of deliberately increasing flow to meet predefined targets may not reduce mortality.Could the addition of depth of anesthesia and cerebral and tissue oximetry monitoring produce a further improvement in outcomes?Retrospective studies indicate a combination of excessive depth of anesthesia hypotension and low anesthesia requirement results in increased mortality and length of hospital stay.Near infrared technology allows assessment and maintenance of cerebral and tissue oxygenation, a strategy, which has been associated with improved outcomes. The suggestion that the brain is an index organ for tissue oxygenation, especially in the elderly, indicates a role for this technology in the intraoperative period to assess the adequacy of oxygen delivery and reduce the build-up of an oxygen debt.The aim of this article is to make the case for depth of anesthesia and cerebral oximetry alongside flow monitoring as a strategy for reducing oxygen debt during high-risk surgery and further improve outcomes in high-risk surgical patients.

Figure 1

Range of propofol requirements in mg.kg^-1.hr^-1 to maintain BIS in the 40 to 60 range. Note the nearly fivefold variation in propofol requirements. BIS: bispectral index.

Figure 2

BIS values obtained for a series of 90 high-risk patients undergoing major vascular surgery using total intravenous anesthesia with propofol and remifentanil. The average BIS was 45 and only 6 patients had BIS maintained below the normal lower range of 40. BIS: bispectral index.

Figure 3

Possible mechanism of how anesthetic concentration may affect outcomes. Anesthetic concentration increase along the x axis. If the concentration is too low then there is a risk of awareness and lack of neuronal protection (left y axis). If the concentration is too high then there is a risk of cardiovascular depression with hypotension and decreased oxygen delivery (DO₂) and a risk of neuronal toxicity (right y axis). The red arc indicates a low-risk patient where the risk of problems is small despite big changes in anesthetic concentration. The blue curves indicate the various likely responses of a high-risk patient where the response to anesthesia is, and the implications for getting it wrong, are much greater as indicated by the much steeper dose–response curves. CV: cardiovascular; DO₂: oxygen delivery.

Figure 4

Intraoperative fluid management strategy as proposed by the Deltex company (Deltex Medical, Chichester, UK) [[55]]. FTc: flow-time corrected; SD: stroke distance; SV: stroke volume.