The effect of a dissociative dose of ketamine on the bispectral index (BIS) during propofol hypnosis.

STUDY OBJECTIVE

To compare the effect of a standardized stimulus during propofol-only hypnosis on the bispectral index (BIS) value with the effect of the injection of local anesthesia for surgery during ketamine plus propofol hypnosis (dissociative monitored anesthesia care). To determine whether ketamine increases the level of propofol hypnosis when used in dissociative doses.

DESIGN

Descriptive case study.

SETTING

Private practice office plastic surgery suites.

PATIENTS

30 nonpremedicated ASA physical status I and II adult female (23) and male (7) patients scheduled for elective cosmetic surgery.

INTERVENTIONS

Hypnosis was induced via slow (60 to 80 drops [gtts]/min), dilute (5 mg/ml) propofol solution. Hypnosis was induced using the BIS monitor as an adjunct to traditional vital signs and verbal contact. Patients were engaged in conversation and note was taken of the BIS value when verbal contact was lost and when BIS appeared to stabilize (BIS1). A standardized stimulus (0.3 ml 1% lidocaine plain via 30-gauge needle) was applied to the area of the supraorbital nerve. Note was taken of the highest BIS value (BIS2) in the patient response. The BIS returned to baseline hypnosis (BIS1) and a 50-mg dissociative dose (independent of body weight) of ketamine was administered. Two minutes were allowed to elapse and then the surgeon was allowed to inject the local anesthesia for the proposed surgery. Note was taken of the BIS value (BIS3) in response to the surgeon's injection.

MEASUREMENTS AND MAIN RESULTS

The average delta (BIS2 - BIS1) was 9.5 + 6.9. Patients did not move in response to the surgeon's injection: BIS3 = BIS1. When movement occurred, the injection was terminated and additional ketamine was given before resuming the injection. Sixteen patients received ketamine 50 mg, 12 received ketamine 100 mg, one received ketamine 150 mg, and one received ketamine 200 mg. Men required an average 19% less propofol than women in this group.

CONCLUSION

This study demonstrated a positive BIS response to a standardized local anesthetic stimulus during propofol-only hypnosis and a zero response during ketamine plus propofol hypnosis (dissociative anesthesia). Ketamine administered in dissociative doses does not deepen the level of propofol hypnosis. Hypnosis alone does not imply general anesthesia. Patients move in response to inadequate local anesthesia. Because the ketamine analgesia is only transitory and the primary analgesia is not given intravenously, propofol-ketamine technique is not a total intravenous anesthetic technique (TIVA). Instead, propofol-ketamine technique may be classified as a form of monitored anesthesia care (MAC).