Awareness: Monitoring versus remembering what happened

Abstract

Background: Awareness during anesthesia is foremost assessed with postoperative interviews, which may underestimate its incidence. On-line monitors such as the Bispectral Index and patient response to verbal command are not necessarily commonly used. This study investigated response to command during deep sedation (Bispectral Index 60-70) and the ability of prevailing monitoring techniques to indicate awareness and predict recall.

Methods: The authors systematically assessed the response to command using the isolated forearm technique while monitoring electroencephalographic and hemodynamic variables. Fifty-six elective surgical patients were repeatedly given verbal instructions to squeeze the observer's hand during target-controlled infusion with propofol and alfentanil. After recovery, conscious recall was assessed with a short structured interview.

Results: Overall, 1,082 commands were given. No response was observed to 887 (82%) commands, an equivocal response was observed to 56 (5%) commands, and an unequivocal response was observed to 139 (13%) commands. Of the 37 patients (66%) with an unequivocal response to command ("awareness"), nine (25%) reported conscious recall after recovery. Their reports provided valuable insights as to how awareness may be adequately addressed. Hemodynamic variables poorly predicted awareness, whereas parameters derived from the encephalogram, especially the Bispectral Index, were highly significant predictors (P < 0.0001). Electroencephalographic parameters did not discriminate between patients with or without conscious recall, whereas heart rate and responsiveness to command did.

Conclusions: The incidence of awareness is underestimated when conscious recall is taken as evidence. Awareness can be monitored on-line with behavioral and modern neurophysiologic measures. Providing feedback during intra-anesthetic awareness helps patients to cope with a potentially stressful situation.