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LETTER TO THE EDITOR

Cardiopulmonary Bypass Machine Can Interfere with Accuracy of BIS Monitor

Department of Anaesthesia; Trent cardiac centre; Nottingham University Hospitals NHS Trust; City Hospital Campus; Hucknall Road NG5 1PB; Nottinghamshire, United Kingdom; ptewari123{at}yahoo.co.in

To the Editor:

Bispectral index (BIS) monitoring may be used during cardiopulmonary bypass (CPB) to prevent awareness by signaling the need for additional hypnotic or opioid medication. However, with the increasing use of this modality, falsely high BIS values during cardiac surgery (1) and related to hypothermia (2), interference from pacemakers (3), and forced-air-warming therapy (4) have been reported. We describe a case in which a deep level of anesthesia was maintained during CPB because of reliance on a BIS-XP (Ver 4.0) monitor.

A 78-yr-old, NYHA Status II, man scheduled for coronary bypass surgery was anesthetized with midazolam 3 mg, fentanyl 10 mcg kg^–1 and propofol up to 6 mg kg^–1 h^–1 to maintain the BIS <40. Shortly after beginning CPB the BIS increased to 60. Propofol 30 mg was given and the infusion was increased to 9 mg kg^–1 h^–1. However the BIS value increased to and remained between 78 and 84. Fearing awareness we gave an additional dose of fentanyl 5 mcg kg^–1, midazolam 3 mg, and increased the propofol infusion to 12 mg kg^–1 h^–1 again without discernable effect on the BIS value. The SQI was 100%. The electromyography bar showed a very slight but inconsistent increase. The ECG in lead V5 showed a saw-tooth appearance throughout CPB. Immediately after separation from CPB the BIS value plummeted to 10 and after reducing the propofol infusion rate it remained at 28–30 for the duration of surgery. A similar sequence of events occurred with the second patient suggesting that the high BIS values on CPB were artifactual. Both patients recovered well without any recall.

The saw-tooth appearance on chest EKG likely indicated electrical noise. Artifactual signals superimposed on the electrocardiogram have been well documented (5). Interference from other devices can easily be recognized by switching off the devices—but this is not an option during CPB.

According to the manufacturer, the latest algorithm revision (BIS-XP ver 4.0) is intended to make the BIS more resistant to interference from electrocautery and electromyography. In this case we believe that the BIS monitor could not differentiate between EEG waves and the pump-generated spikes, possibly interpreting them as or ß waves and thus a BIS value around 80.

In an attempt to reduce static electricity and electrical interference the CPB machine was rechecked to insure that the ground connection was intact, we reduced operating room humidity, and the perfusionists started using saline spray at the pump heads and the circuit tubing. We have not noticed further spurious BIS values during CPB.

REFERENCES

1. Dahaba AA. Different conditions that could result in the bispectral index indicating an incorrect hypnotic state. Anesth Analg 2005;101:765–73[Abstract/Free Full Text]
2. Mychaskiw G, Heath BJ, Eichhorn JH. Falsely elevated bispectral index during deep hypothermic circulatory arrest. Br J Anaesth 2000;85:798–800[Abstract/Free Full Text]
3. Gallagher JD. Pacer-induced artifact in the bispectral index during cardiac surgery. Anesthesiology 1999;90:636[Web of Science][Medline]
4. Hemmerling TM, Fortier JD. Falsely increased bispectral index values in a series of patients undergoing cardiac surgery using forced air-warming therapy of head. Anesth Analg 2002;95:322–323[Abstract/Free Full Text]
5. Wald AW, Khambatta HJ, Tone JG. Palstic induced ECG interference on cardiopulmonary bypass: an adventure in clinical engineering. Clin Eng 1990;15:301–7

This article has been cited by other articles:

				T. M. Hemmerling, S. Charabati, and D. Bracco Unusual case of low bispectral index values due to electrocardiographic interferences Br. J. Anaesth., December 1, 2008; 101(6): 877 - 878. [Full Text] [PDF]

				P. J. Manberg and S. D. Kelley Cardiopulmonary Bypass Machine Can Interfere with Accuracy of BIS Monitor Anesth. Analg., August 1, 2007; 105(2): 534 - 535. [Full Text] [PDF]

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