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Potential Avoidance of Intraoperative Awarene= ss During Paralysis
Andrew Alshab MD MPH, Stéphane Bibian MS, Craig R. Ries MD PhD, Anna Jaromczyk MS, Donald M. Voltz MD

University Hospitals of Cleveland and Case Western Reser= ve University, Cleveland, Ohio

Introduction:

Awareness under anesthesia has been estimated to affect between 0.1-0.2% of patients undergoing general anesthesia1. The psychological and potential physiologic consequences of this are not entirely understood, however, a significant number of patients develop post-traumatic stress disorders following such an event. The role of neuromonitoring in anesthesiology is currently under debate as technologies and our understanding of the complexity of cortical activity increase. = 3;

In patients who are paralyzed and have their auton= omic nervous system pharmacologically blunted, traditional metrics for anesthet= ic depth become difficult to interpret, or are completely nonexistent. The us= e of quantitative electroencephalographic (EEG) measures may provide earlier detection and prevention of intraoperative awareness. The reliability of processed EEG variables such as the BIS2 (Aspect Medical Systems) in situat= ions involving paralyzed patients has been questioned. For instance, NMB administration on awake volunteers has been shown to create an important d= rop in BIS3. Also, conscious paralyzed patients can have BIS values as= low as 604. It has been hypothesized5 that the inability of BIS to keep = or regain a pre-induction baseline value may be due to low electromyographic (EMG) activity. This could potentially limit the use of BIS when NMBs are used.

In light of this potential limitation of processed= EEG variables, we present here a case report of a premature emergence uncovere= d by the WAV Index (Wavelet-based Anesthetic Value), in a paralyzed patient who= se hemodynamic profile was blunted with beta-blocking medications. The WAV is= a novel CNS monitor that processes frontal EEG to estimate a patient’s state of consciousness6. Similar to BIS, the WAV Ind= ex is a unitless value that ranges from 100% (awake) to 0% (isoelectric EEG) sca= le. During this case, the WAV regained its baseline (awake) value despite low = EMG activity.

Case Report:

The patient is a 33 year old, ASA III female who underwent a lower extremity vascular bypass procedure. The majority of the case was uneventful from bo= th a hemodynamic and neuromonitoring point of view. During this case, anesthesia was maintained with isoflurane + N2O, vecurononium, and fentanyl= . This patient received metopropolol at induction for cardiac protection as well = as hemodynamic control.

Nearing the end of the case, the volatile anesthetics were decreased. During closu= re of the incision, there was a short period of increased surgical stimulation eliciting movement of the leg by the patient. This response was treated wi= th an increase in the volatile anesthetic and re-dosing of vecuronium by the anesthesiologist. These interventions lead to a precipitous fall in the WAV Index (Fig. 1). Shortly following a nadir, the patient’s WAV Index climbed to 92, well into the consciousness level, while maintaining low EMG activity (<30dB). This rise in the WAV Index was consistent with the presence of elevated alpha and beta activity, and a marked decrease of del= ta activity in the EEG. This EEG activation was treated with a bolus of propo= fol, returning the WAV Index to a level of 50. During emergence from anesthesia, the patient received neostigmine/glycopyrolate for reversal of the muscular block and morphine was titrated for pain control. Immediately following emergence from anesthesia, while in the post-operative recovery area, and = one day later, the patient reported no intraoperative awareness of this event.=

Discussion:

The use of neuromonitoring for the prediction of consciousness continues to be debated and will continue until we have a better metric for consciousness. Currently, we use cortical activity as a surrogate measure to estimate the likelihood of a patient’s ability to process sensory information and form memories of this information7.

When using wavelet-based analysis of EEG data, this case presents a premature emergence from anesthesia based on the increase of alpha and beta activity= as well as the decrease in the delta spectrum. This change in cortical electr= ical activity was not confounded by motor activity (EMG) and it occurred shortly after dosing of the neuromuscular blocking agent.

The utility of BIS and other depth of anesthesia monitors merit some attention when using NMBs. This case reports on a new EEG processing algorithm, the = WAV, able to respond to an EEG signal consistent with light anesthesia despite = low EMG activity. The significance of the WAV to discriminate EEG activity in = the absence of EMG may have resulted in a timely anesthetic administration and prevented the development of awareness. It is difficult to draw conclusions about prevention of awareness from one case in a non-controlled experiment= al condition; however, the ability to capture this information provides some support to the use of processed EEG in the operating room to assist with anesthetic titration and level of cortical function. More investigation in= to wavelet processing of the EEG with the WAV Index are currently underway to better assess its utility in the clinical setting.

Reference:

1. Myles PS, Leslie K, McNeil J, Forbes A, Chan MT. Bispectral index monitoring to prevent awareness during anaesthesia: the B-Aware randomised controlled trial. Lancet. 2004 = May 29;363(9423):1757-63. = ;2. Dahaba AA. Different conditions that could result in = the bispectral index indicating an incorrect hypnotic state. Anesth Analg. 2005 Se= p; 101(3):765-73. 3. Messn= er M, Beese U, Romstock J, Dinkel M, Tschaikowsky K. The bispectral index declines during neuromuscular= block in fully awake persons. Anesth Analg. 2003 Aug;97(2):488-91. = 4. Flaishon R, Windsor A, Sigl J, Sebel PS. Recovery of consciousness after thiopental or propofol. Bispectral index and isolated forearm technique. Anesthesiology. 1997 Mar;86(3):613-9. 5. Bruhn J, Bouillon TW, Shafer SL. Electromyographic activity falsely elevates the bispectral index. Anesthesiology. 2000 May;92(5):1485-7. 6. Steph= ane Bibian, M.A.Sc., Tatjana Zikov, M.A.Sc., Craig R. Ries, M.D., F.R.C.P.C., = Ph.D., Guy A. Dumont, Ph.D., = Mihai Huzmezan. The Wavelet-Based Anesthetic Value (WAV): A Novel Alternative to= the Bispectral Index (BIS). ASA Abstracts, 2004; A-342. 7. Seth AK, Baars BJ, Edelman DB. Criteria for conscious= ness in humans and other mammals. Conscious Cogn. 2005 Mar;14(1):119-39.

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Nerve stimulator

= Vecuronium 1 mg

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= Neostigmine 4 mg

Figure 1 Potent= ial premature emergence (5h22) during paralysis and autonomic blockade detecte= d by the WAV (note that the data presented in these graphs represent the last h= our and 10 minutes of the case).

Sampling time

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