The ability of bispectral index to detect intra‐operative wakefulness during total intravenous anaesthesia compared with the isolated forearm technique

Abstract

SummaryIt has been suggested that monitoring during total intravenous anaesthesia should include aspects of brain function. The current study used a manually adjusted target-controlled infusion of propofol for anaesthesia, guided to a bispectral index range of 55-60. Intra-operative responsiveness, as assessed by the isolated forearm technique, was compared with whether the bispectral index predicted/identified a patient's appropriate hand movements in responses to commands. Twenty-two women underwent major gynaecological surgery with total intravenous anaesthesia, propofol, remifentanil and atracurium. Sixteen women responded, on 80 occasions, with appropriate hand movements to commands during surgery, of which the bispectral index detected 47 (sensitivity 59%). The bispectral index suggested consciousness 220 times in the absence of movement responses (specificity 85%). The positive predictive value of a bispectral index response was 18%. While two women had vague recall about squeezing fingers, none had recall of surgery. For patients who responded more than once during surgery the bispectral index value associated with a response was not constant. Although there was no difference in the median (IQR [range]) effect site propofol concentration between intra-operative responses (2.0 (1.5-2.3 [1.2-4.0]) lg.ml À1) and eye opening after surgery (2.1 (1.7-2.8 [1.5-3.9]) lg.ml À1 ), the median (IQR [range]) bispectral index value at eye opening after surgery was significantly higher than that associated with responses during surgery: 75 (70-78 [51-93]) vs 61 (52-67 [37-80]) respectively, (p < 0.001). The manual control of propofol intravenous anaesthesia to target a bispectral index range of 55-60 may result in an unacceptable number of patients who are conscious during surgery (albeit without recall).

Monitoring (un)consciousness: the implications of a new definition of ‘anaesthesia’

“…likely to be fully awake) cannot be the same as that of a patient who responds only to the command (who, I have proposed, is in a state of 'dysanaesthesia' [10][11][12][13][14]). These two patients are 'not anaesthetised' in very different ways, and the latter appears an acceptable state for surgery to continue.…”

Bispectral index compared with the isolated forearm technique

“…As the results of this [1] and other [2][3][4][5] studies show, that is not a valid assumption on which to base any argument. Certainly in relation to the use of neuromuscular blocking drugs, the various BIS 'guidelines' and suggested responses to changes in BIS were developed by anaesthetists blind to the actual conscious state of patient, for use by other anaesthetists who are also blind to the conscious state of the patient.…”

“…In the two cases to which they refer (presented in Figs. 2 and 3 of the paper [1]) the patient responses occurred at propofol concentrations varying from around 4 lg.ml À1 (BIS > 60) to < 2 lg.ml À1 (BIS 45-55). These two patients simply reflect the fact that neither a specific BIS value nor a calculated propofol concentration can guarantee unconsciousness.…”

“…In the methodology [1], it was clearly spelled out that BIS > 60 was only allowed if the clinical signs of anaesthesia remained stable. However, I am bemused by the statement of Andrzejowski and Wiles that they would accept a BIS value > 60 if 'anaesthetic concentrations are clinically adequate'.…”

Bispectral index compared with the isolated forearm technique

“…Dr Russell is to be congratulated on conducting a very worthwhile but difficult study on the risk of intraoperative wakefulness during total intravenous anaesthesia [1], but the devil is unfortunately in the lack of detail, particularly when a small number of patients were so intensively studied. Some of the important missing factors in determining depth of anaesthesia are highlighted in the review article on minimum alveolar concentration [2].…”