False negative BIS? Maybe, maybe not!

Intraoperative Awareness and Recall: A Comparative Study of Dexmedetomidine and Propofol in Cardiac Surgery

Background Awareness during general anesthesia is undesired and unanticipated patient wakefulness during surgery or recall of intraoperative events. Incidence of awareness in patients undergoing cardiac surgery is significantly higher than the overall incidence of 1% during general surgery. Awareness during cardiac surgery can be prevented by a number of methods. One such method is the supplemental, intraoperative use of sedative agents. Propofol, a bisubstituted phenol, is an intravenous general anesthetic that has been shown to reduce the incidence of awareness. Dexmedetomidine-an alpha₂-adrenergic agonist with anxiolytic, opioid, and general anesthetic-sparing properties-is being considered for maintaining intraoperative depth of anesthesia. The purpose of this study was to evaluate the effect of dexmedetomidine on depth of anesthesia and to compare it with the effect of propofol in cardiac surgery. Methods This was a prospective, randomized, double-blind study conducted in a tertiary-care hospital. Sixty patients with American Society of Anesthesiologists (ASA) physical status I-III planned for elective open heart surgery were randomized into two groups of 30 patients each. Each patient of the dexmedetomidine group received an initial loading dose of dexmedetomidine at 1 mcg kg^-1 over 10 minutes followed by infusion at the rate of 0.2-0.6 mcg kg^-1 hr^-1. Patients of the propofol group received propofol infusion at the rate of 0.25^-1 mg kg^-1 hr^-1. An identical technique-of standard general anesthesia and routine physiological monitoring-was used in both groups. Bispectral scores were recorded at predetermined intervals during surgery and the target bispectral index (BIS) was kept at 50±10. The patients were assessed for awareness and recall 24 hours after tracheal extubation using the Brice Questionnaire. Results Intraoperative BIS scores remained within the target range in both groups; however, the BIS scores showed variable trends between the groups and were significantly lower in the dexmedetomidine group (p < 0.001). None of the patients in either group had recall of intraoperative events. Conclusion Administration of dexmedetomidine was as effective in reducing awareness and recall in cardiac surgery compared to propofol. Thus, dexmedetomidine can be used as an alternative sedative agent to prevent awareness and recall in cardiac surgery.

Monitoring depth of anesthesia

Devices which monitor some aspect of anesthetic drug effects have evolved in the past few years into imperfect, but very useful, clinical tools. With appropriate respect for their limitations these monitors can be used to reduce anesthetic drug utilization and turnover time. The intriguing hypothesis that such monitors will reduce the risk of intraoperative awareness is currently under test.

[Awareness: a problem in paediatric anaesthesia?]

Intraoperative awareness has been reported to occur in 0.8-5.0% of paediatric patients undergoing anaesthesia and, therefore, seems to be more common than in adults (incidence 0.1-0.2%). In adult patients, the consequences of intraoperative awareness are well known and can be severe, in children, however, they have not yet been adequately studied. The causes for intraoperative awareness can be divided into three broad categories: First, no or only a light anaesthetic is given on purpose, second, an insufficient dose of an anaesthetic is given inadvertently, third, there is equipment malfunction or the anaesthesiologist makes an error. Unfortunately, especially in young children, painful interventions are still performed without adequate analgesia, e.g. awake intubation or fracture manipulation under midazolam sedation alone. The key issue is, however, that pharmacokinetics and pharmacodynamics change enormously from the 500 g preterm baby to the adolescent patient. Adequate dosing is much more difficult in paediatric patients compared to standard adult surgical patients. Solid knowledge of the pharmacokinetic and pharmacodynamic characteristics of commonly used drugs in different paediatric age groups, as well as aiming for perfection in daily care will help to reduce the incidence of awareness. Methods for monitoring the depth of hypnosis, e.g. the bispectral index, will be used increasingly, at least in children above 1 year of age. In addition to clinical parameters, they will hopefully help to further reduce the incidence of intraoperative awareness.

Depth of Anesthesia Monitoring

Depth-of-anesthesia monitoring with EEG or EEG combined with mLAER is becoming widely used in anesthesia practice. Evidence shows that this monitoring improves outcome by reducing the incidence of intra-operative awareness while reducing the average amount of anesthesia that is administered, resulting in faster wake-up and recovery, and perhaps reduced nausea and vomiting. As with any monitoring device, there are limitations in the use of the monitors and the anesthesiologist must be able to interpret the data accordingly. The limitations include the following. The currently available monitoring algorithms do not account for all anesthetic drugs, including ketamine, nitrous oxide and halothane. EMG and other high-frequency electrical artifacts are common and interfere with EEG interpretation. Data processing time produces a lag in the computation of the depth-of-anesthesia monitoring index. Frequently the EEG effects of anesthetic drugs are not good predictors of movement in response to a surgical stimulus because the main site of action for anesthetic drugs to prevent movement is the spinal cord. The use of depth-of-anesthesia monitoring in children is not as well understood as in adults. Several monitoring devices are commercially available. The BIS monitor is the most thoroughly studied and most widely used, but the amount of information about other monitors is growing. In the future, depth-of-anesthesia monitoring will probably help in further refining and better understanding the process of administering anesthesia.

Onset Time for Sevoflurane/Nitrous Oxide Induction in Adults Is Prolonged with Increasing Age

Inhaled induction of anesthesia is occasionally used in adults. Using a modified vital capacity sevoflurane/nitrous oxide (N2O) inhaled induction, we evaluated the effect of increasing age on the onset time of anesthesia. Twenty patients, aged 26-65 yr, performed a vital capacity breath followed by regular tidal breathing from an anesthesia circuit primed with sevoflurane 8%/N2O/O2. The following values were recorded: time to loss of eyelash reflex (LOER); time to bispectral index < or =60 (BIS < or =60); expired fraction of sevoflurane at the time of induction, LOER and BIS < or =60. The mean times and 95% confidence intervals to LOER and BIS < or =60 were 54 s (37-70 s) and 175 s (143-207 s), respectively, and were significantly prolonged by aging (r = 0.65; P = 0.002). Times to LOER and BIS < or =60, predicted from the regression line, were 3.9 and 2 times longer in a 60-yr-old than in a 30-yr-old patient. The expired fraction of sevoflurane measured at time to LOER and BIS < or =60 decreases with increase in age. We conclude that inhaled induction with sevoflurane/N2O is dramatically prolonged with increased age.

Processed EEG and patient outcome

The era of research evaluating clinical outcomes associated with processed electroencephalogram (EEG) monitoring began with the first randomized trial of bispectral index monitoring (BIS) performed as part of the clearance process for approving routine clinical use of the BIS monitor by the United States Food and Drug Administration. Subsequent to this initial investigation, numerous other clinical investigations have demonstrated that the use of processed EEG monitors as an additional method of patient assessment and an aid to anaesthetic dosing can decrease anaesthetic usage and hasten recovery times. Because of the presumed association between anaesthetic effect and EEG changes, it is not surprising that the additional research has focused on the impact of processed EEG monitoring on postoperative outcomes and perioperative safety especially the prevention of intraoperative awareness.

Interventional neurophysiologic monitoring

PURPOSE OF REVIEW

Intraoperative neurophysiologic monitoring provides useful information on the functional status of the nervous system. This review focuses on recently published data concerning the impact of monitoring on patient outcome.

RECENT FINDINGS

There is level I evidence to support the use of bispectral index monitoring to prevent awareness during anesthesia in high-risk patients. A number of randomized trials have shown that monitoring-guided anesthesia using the bispectral index or other devices will expedite recovery and improve perioperative drug utilization. There are also preliminary reports suggesting that anesthesia dictated by bispectral index monitoring may alter long-term outcome and reduce mortality. In surgical procedures, however, it is less clear whether neurophysiologic monitoring will improve patient outcome. Currently, the majority of data are derived from respective case series. Nonetheless, monitoring with somatosensory evoked potential has been shown to reduce postoperative neurologic deficits after spinal surgery. There is also evidence to suggest that electromyography and motor evoked potential are essential complements to somatosensory evoked potential for monitoring of spinal cord surgery.

SUMMARY

Brain monitoring facilitates anesthetic drug administration. An increasing number of neurosurgical procedures will require some form of intraoperative neurophysiologic monitoring to achieve higher degrees of safety and accuracy. In many instances, the data derived from monitoring will guide and influence surgical decisions. In this context, neurophysiologic monitoring should be regarded as interventional.