Influence of an external pacemaker on bispectral index

Abstract

EDITOR: Bispectral index monitoring is one technique to measure level of consciousness. The concurrent use of a variety of medical devices can lead to inaccuracies in its output. Among the recognized situations in which an external signal may interfere with it is an activated cardiac pacing device [1]. Usually, the improved BIS-XP® platform identifies and filters out the signals emitted from pacemakers as they are of high and more or less stable amplitude and of regular pattern. This kind of interference is not followed by a loss of bispectral index value as it permits sufficient electroencephalogram (EEG) to be available for analysis and calculation. Furthermore, the usual location of implanted pacemakers in adult patients reduces the likelihood of EEG signal contamination. In this report, we present a case where erroneous bispectral index readings were produced from an external pacemaker and clearly dependant on the pacing output. A 72-yr-old man, New York Heart Association class II, with a myocardial infarction 4 months earlier, a history of controlled hypertension and mild diabetes mellitus presented for aortocoronary bypass grafting. Eighteen months before and due to inadequate medical control of repeated episodes of disrhythmias, a pacemaker (type DDD, model κ™; Medtronic Inc., Minneapolis, MN, USA) with transvenous lead-electrode system had been implanted in the area near to his left clavicle. Preoperatively and in the operating room, 5-lead electrocardiography (Solar 8000; Marquette Medical Systems, Milwaukee, USA) revealed normal sinus rhythm without noticeable spikes. Following venous and arterial line placement and after skin preparation, a bispectral index sensor (BIS-XP®; Aspect Medical Systems, Newton, MA, USA) was placed which, when connected to the monitor showed a value of 98. After induction, the patient received continuous infusions of propofol (approximately 5 mg kg−1 h−1) and remifentanil (approximately 20 μg kg−1 h−1). Central venous access and mixed venous oximetry were established via the right internal jugular vein. After field preparation and a small incision, the implanted generator was disconnected and its transvenous electrode was connected to a stand-by single chamber external pacemaker (model: 5348; Medtronic Inc., Minneapolis, MN, USA). Normal sinus rhythm (55-65 beats min−1) and bispectral index values ranging from 33 to 48 were recorded throughout the pre-bypass period. The patient's body temperature was allowed to decrease passively during cardiopulmonary bypass while in the period near the completion of the main surgical procedure, before aortic unclamping and during the rest of bypass, active rewarming aided by the heart-lung machine was applied. In the 55 min of aortic clamping, bispectral index values were 23-44 and the lowest temperature recorded was 34.1°C. After removal of the aortic cross-clamp, the external pacemaker was connected to an epicardial electrode and set at 80 beats min−1 in asynchronous mode with output varying from 5 to 20 mA, in an effort to avoid loss of pacing capture. An increase in the bispectral index value to 69 was immediately noticed. The signal quality index was high. Changes in the pacemaker position, in order to increase the distance from the bispectral index sensor and digital signal converter did not affect the picture. Although the situation was suggestive of arousal, the attending anaesthesiologist noted that electromyographic activity was also high and proceeded to the evaluation of the real-time EEG. The rhythmic emission from the pacemaker was apparent, periodically interrupting the EEG and rarely followed by artefact recognition in the display. The bispectral index decreased to 45 shortly after the decrease in the output of the pacemaker and the displayed waves were also decreased in amplitude. Pacing was disconnected but the heart was still unresponsive and it was restarted under the lowest output for capture (10 mA). Bispectral index value showed marked variability ranging from 45 to 55, with no artefact detection but with high electromyogram intensity. Fifteen minutes later, pacing was discontinued as the patient established adequate sinus rhythm. Bispectral index values returned to around 40 and remained at that level. The remainder of the operation proceeded uneventfully and postoperative questioning elicited no evidence of recall. Estimation of the current anaesthetic effect extends from the absolute bispectral index value to the assessment of the signal quality index, electromyogram and the real-time EEG. Activated pacemakers have been reported to interfere with bispectral index [2] but newer versions are supposed to recognize and eliminate such problems. Performance was quite acceptable in the beginning of pacing, when generator output was high. Nevertheless, the artefactual rise in signal could have been easily misinterpreted if the output had been low from the beginning. Brain temperature is expected to increase rapidly in the period of rewarming and its perfusion/mass ratio is high. Consequently cerebral metabolic rate is expected also to increase. A significant correlation has been found between bispectral index values and brain metabolic activity [3]. Although these changes were drug-induced, the pattern is suggestive of a similar relation between brain temperatures and bispectral index under a stable hypnotic drug blood concentration. A rise during rewarming and at the termination of bypass has been reported by many investigators [4-6]. In our case, this could obscure any moderate increase falsely arising from a pacemaker's low current. During bispectral index monitoring, external interference remains a potential limitation and in similar situations it must be used in an appropriate manner. Observation of the real-time EEG waveform may aid in the diagnosis of this kind of artefact in order to avoid unnecessary interventions. G. Vretzakis C. Dragoumanis H. Ferdi P. Papagiannopoulou 1Department of Anaesthesiology, University Hospital of Alexandroupolis, Alexandroupolis, Greece 2Department of Anaesthesiology, “G. Gennimatas” General Hospital, Thessaloniki, Greece