To the Editor:—

We read with interest the report by Gallagher 1on the influence of pacer artifact on the bispectral index (BIS). We wish to report another situation in which an item of operating room equipment resulted in alteration in the BIS signal.

The patient was a 58-yr-old woman who was anesthetized with isoflurane, sufentanil, and atracurium for a left partial hepatectomy. After induction, the BIS (model A1000 v3.12; Aspect Medical Systems, Natick, MA) was maintained at < 60 with electrodes impedances < 1,000 Ω. Forced-air warming therapy was performed using an upper-body blanket (Bair Hugger; Augustine Medical Inc., Eden Prairie, MN). Just before incision, the BIS was 56. After incision, cautery interrupted the electroencephalographic signal for a few minutes. When the signal returned, it showed a marked increase to > 80. However, hemodynamics were unchanged, and there were no other changes in clinical signs. On investigation, we noted that the forced air blanket, which had been turned on simultaneously with the incision, lay directly on the skin with the warm air blowing on the forehead electrodes. Turning off the heating unit resulted in a decrease in BIS to ≈ 60 (fig. 1).

Fig. 1. Bispectral index trend display showing increases when cover of Bair Hugger is on patient.

Fig. 1. Bispectral index trend display showing increases when cover of Bair Hugger is on patient.

Each time forced-air warming resumed, the BIS increased. When the Bair Hugger unit was on but disconnected from the blanket, the BIS returned to values < 60 (fig. 1). This intervention eliminated any role for electrical interference caused by the device. We speculate that vibration of the head wires caused by the air circulation generated an artifact that was interpreted as a higher BIS value; however, this artifact was not visible on the raw electroencephalographic trace.

Bispectral index monitoring in the operating room is generally straightforward, but new medical devices may interfere with it. Knowledge of potential interference from forced-air warming systems must be taken into account when interpreting BIS.

Gallagher JD: Pacer-induced artifact in the bispectral analysis for the electroencephalogram (letter). A NESTHESIOLOGY 1999; 90:636