Anesth Analg 2004;98:469-470
© 2004 International Anesthesia Research Society
doi: 10.1213/01.ANE.0000096184.32247.D0
CRITICAL CARE AND TRAUMA
The Presence of Working Memory Without Explicit Recall in a Critically Ill Patient
Mehmet S. Ozcan, MD, and
Dietrich Gravenstein, MD
From the Department of Anesthesiology, University of Florida College of Medicine, Gainesville, Florida
Address correspondence to Mehmet S. Ozcan, MD, Department of Anesthesiology, University of Florida College of Medicine, PO Box 100254, Gainesville, FL 32610–0254. Address email to Ozcanms{at}anest1.anest.ufl.edu
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Abstract
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We describe an intubated patient sedated with propofol who interacted with caregivers, demonstrating intact "working memory." When neuromuscular blockade and bispectral index (BIS) monitoring were instituted, a greatly reduced amount of sedative achieved BIS values less than 60. Neither the sedation that allowed working memory nor the lighter sedation that produced BIS values less than 60 resulted in recall. This experience suggests that working memory demonstrated when BIS values are less than 60 is unlikely to lead to recall.
IMPLICATIONS: The presence of intact working memory during sedation is a poor predictor of explicit recall when bispectral index values are maintained less than 60.
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Introduction
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Sedation of intensive care unit (ICU) patients is traditionally done by titrating sedatives to the clinically observed level of sedation (1). The Bispectral Index® (BIS®) monitor (Aspect Medical Systems, Inc, Newton, MA) is a tool that has been used to guide sedation in the ICU setting (2,3). We present a case in which the use of BIS monitoring in an ICU patient weakened with neuromuscular blockade (NMB) contributed to our clinical decision making and provided an example of intact "working memory" (4) without recall.
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Case Report
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A 15-yr-old girl with depression who suffered severe tracheal stenosis secondary to a suicide attempt with acid ingestion was admitted to our pediatric ICU after bronchoscopy with tracheal dilation. In the preceding 6 mo, she had undergone 44 similar procedures where she was postoperatively kept intubated overnight with a propofol infusion to provide sedation. Her depression was treated with paroxetine. She had a body mass index of 19.6 and no other medical problems before or resulting from the suicide attempt.
The anesthetic for bronchoscopy consisted of no premedication, induction with propofol, mivacurium, and 2 µg/kg of fentanyl followed by a maintenance propofol infusion at 150 to 200 µg · kg-1 · min-1. She was intubated at the end of the procedure, and the propofol infusion was discontinued after verifying the return of adequate muscle strength by sustained tetanus. She was transferred to the ICU intubated, where she was awake and alert on arrival. Propofol infusion at 100 µg · kg-1 · min-1 was started to provide an adequate level of sedation (observer’s assessment of alertness/sedation scale score of 3–4). We chose this relatively large starting dose of propofol based on our experience from her previous ICU stays. She initially remained comfortable but self-extubated 12 h after surgery. She was reintubated on developing stridor, tachypnea, and desaturation.
After reintubation, she continuously signaled for more sedation and thrashed restlessly. An arterial blood gas excluded hypoxia or hypercarbia. Increasing the propofol to 150 µg · kg-1 · min-1 did not produce better sedation. The surgical team wished to keep the patient intubated for several additional days before repeating the bronchoscopy.
A vecuronium infusion was begun on postoperative day 3 and titrated to a 2/4–3/4 train-of-four twitch count to reduce the risk of another self-extubation. With the initiation of NMB a BIS monitor was used to titrate the rate of propofol to a BIS of 50–60. To our surprise, we saw that a propofol infusion rate of only 50 to 80 µg · kg-1 · min-1 was enough to achieve that target range. The nursing staff expressed their concerns regarding the possibility of recall because of the fact that she had followed complex commands earlier at more rapid (100–150 µg · kg-1 · min-1) propofol infusion rates. Nevertheless, we continued to titrate propofol to a BIS number of 50–60.
Our patient remained on vecuronium for 4 days. No sedative or hypnotic medication other than propofol was used during this time. Signal quality index of the electroencephalogram displayed was consistently in the 80%–100% range, and electromyography showed minimal to no activity.
On postoperative day 7, our patient returned to the operating room for another bronchoscopy and tracheal dilation. The procedure was without complication, and she returned to the ICU intubated. She was extubated the next morning.
One week after her discharge from ICU, our patient was specifically questioned about recall of any events in the ICU, with special emphasis on remembering being awake but weak or paralyzed. She denied any recall of her ICU stay, including the 4 days she received vecuronium infusion, with the exception of those several hours before her discharge from the ICU.
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Discussion
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A continuous and objective method of sedation and amnesia measurement is desirable in ICU patients receiving NMB (5). We thought our experience might be of interest to others because use of BIS monitoring changed our clinical decision making: we would have used propofol at faster infusion rates had we relied only on subjective sedation scales.
Our patient demonstrated a high level of interactiveness on rapid propofol infusion rates. This might be partly attributable to tolerance brought by repeated exposure to the drug (6). Therefore, we used BIS monitoring on institution of NMB to prevent explicit recall. We chose a target BIS value of <60 because this is agreed to be an excellent predictor that a patient will not have explicit recall (7,8). In healthy volunteers receiving propofol, Glass et al. (8) demonstrated a range of BIS values (BIS95 = 51–77) where subjects remained conscious (i.e., had intact working memory) but did not have explicit recall. Our experience is in agreement with those results; under propofol infusion, working memory can remain intact at BIS scores of 50–60.
Although explicit recall can be prevented in the range of BIS index chosen for this case, implicit recall is possible even at BIS values less than 60 (9). A dose range of propofol that reliably and consistently prevents both implicit and explicit recall has not been established. Our aim in the care of this patient was to prevent explicit recall.
In short, we would like to emphasize that the level of responsiveness to a given sedative may not necessarily correlate with presence of recall. If NMB drugs are being used in an ICU setting, use of BIS monitoring might be useful to avoid excessive as well as inadequate administration of sedative drugs to provide amnesia.
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References
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Accepted for publication September 2, 2003.
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Abstract
Introduction
