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Awareness detected by auditory evoked potential monitoring
British Journal of Anaesthesia 91 (2): 290±2 (2003)
DOI: 10.1093/bja/aeg144
CASE REPORTS Awareness detected by auditory evoked potential monitoring L. Trillo-Urrutia*, S. FernaÂndez-Galinski and J. CastanÄo-Santa Servicio de Anestesiologia y Reanimacion, Hospital del Mar, Passeig MarõÂtim 25, 08003 Barcelona, Spain *Corresponding author: [email protected] We report a case of awareness detected by the Alaris AEP Monitor, a device that measures anaesthesia by tracking changes of the waveform of the mid-latency auditory evoked potential. Br J Anaesth 2003; 91: 290±2 Accepted for publication: January 13, 2003
Possible awareness during general anaesthesia depends on the type of procedure, the dose of anaesthetics and the anaesthetic technique used.1 Awareness can occur if drug delivery is faulty. Measurements of the cardiovascular system are poor predictors of a patient's hypnotic state2 and should not be relied on to prevent awareness. Auditory evoked responses can be used to measure depth of anaesthesia.3±6 Recently, the Alaris AEP MonitorÔ (Alaris Medical Systems, Basingstoke, UK) has become available. This measures anaesthesia by tracking changes of the waveform of the mid-latency auditory evoked potential. These changes are converted into an A-line ARX index (AAI). An AAI value <30 is considered to be appropriate for surgical anaesthesia and an AAI >60 indicates consciousness. The monitor also measures electromyographic activity. We report a case of awareness detected by this device.
Case report A 39-yr-old male, height 165 cm, weight 52 kg, ASA II, was scheduled for a subtotal thyroidectomy. He had no clinical evidence of hyperthyroidism. However, thyroid function tests were abnormal, with the following values (patient, normal range): thyroid-stimulating hormone <0.1 (0.35±5) mU ml±1; total triiodothyronine 277 (58±159); thyroxine 2.1 (0.89±1.76) ng dl±1. He had volunteered for a research study in which anaesthesia was induced and maintained with propofol and remifentanil i.v. Drug administration was to be adjusted to maintain mean blood pressure and depth of hypnosis within agreed limits. The patient was given diazepam 5 mg and ranitidine 150 mg the night before surgery. The patient was awake and alert on arrival in the operating room. Standard monitoring was started and an
infusion of lactated Ringer's solution was started using a cannula in the back of his left hand. Three electrodes (A-Line AEP; Danmeter, Odense, Denmark) were then positioned at mid-forehead (+), left mastoid (±) and left forehead (reference) together with a pair of small headphones to deliver the auditory click stimulus. These were connected to an Alaris AEP MonitorÔ. The mean (SD) AAI value before induction was 72.5 (7.8). Oxygen (100%) was given via a face-mask and anaesthesia induced by administering boluses of propofol 90 mg and remifentanil 60 mg i.v. Cisatracurium 6 mg i.v. was administered to aid tracheal intubation. A slight delay before connecting the tubing for drug administration caused a brief increase in the AAI just after induction of anaesthesia, and the AAI and EMG reached 65 and 50% respectively (Fig. 1). Ventilation was started using a face-mask, and a cannula was then placed in the left foot to continue propofol and remifentanil delivery through two Perfusorâ fm pumps (Braun, Melsungen, Germany). The trachea was intubated and the patient was placed in the Kocher position. The research plan stipulated that anaesthesia should be maintained with propofol 3 mg kg±1 h±1 and remifentanil 0.5 mg kg±1 min±1 i.v. If needed, these infusion rates could be increased by 25% to keep the AAI <30 and to reduce the mean blood pressure by up to 30% of the baseline value. Two minutes had to elapse before a further increase in the infusion rate could be made. After intubation the AAI value began to ¯uctuate between 10 and 60, but cardiovascular measurements were stable. Immediately after skin incision, the AAI increased sharply from 20 to 70. Three minutes later the systolic blood pressure increased from 100 to 150 mm Hg. The propofol and remifentanil infusions were increased gradually according to the research plan to 6 mg kg±1 h±1 and 1.5
Ó The Board of Management and Trustees of the British Journal of Anaesthesia 2003
Downloaded from http://bja.oxfordjournals.org/ at New York University on May 11, 2015
Keywords: anaesthesia, general; monitoring, auditory evoked potential
Awareness detected by auditory evoked potential monitoring
mg kg±1 min±1 respectively, but the AAI, pulse rate and blood pressure did not change. After 5 min, the EMG activity increased suddenly from 0 to 60% and the blood pressure increased to 180 mm Hg. The tubing on the patient's foot was checked and it was discovered that the insertion point of the cannula had accidentally come out and no drugs were being infused i.v. The tubing was promptly ®xed and the AAI began to decrease immediately. In 5 min the systolic blood pressure returned to the normal value. Both indicators remained within the normal range until the end of the procedure. Drug infusion was then stopped and the patient woke up 9 min later, when he was extubated. When questioned after awakening, the patient recalled being aware during surgery, hearing the doctors' voices in
the operating room and feeling `painful manipulations' in his throat. He remembered being unable to speak or move before falling asleep again. He was not able to recall the intensity of the pain or the doctor's words. A detailed explanation was given to the patient regarding his experience and he reported no further dissatisfaction.
Discussion Accidental interruption of drug delivery is a common cause of awareness during general anaesthesia.1 We noted a rapid change in the AAI when infusion was stopped. From the charts it can be inferred that the `painful manipulations' felt by the patient corresponded to the period after skin incision. Although the precise moment of interruption is not known,
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Downloaded from http://bja.oxfordjournals.org/ at New York University on May 11, 2015
Fig 1 Charts showing changes in the AAI (A), EMG (B) and arterial blood pressure (BP) (mm Hg) and heart rate (HR) (beats min±1) (C). The moments of intubation, incision and ®xing of the i.v. tubing are indicated.
Nouraei et al.
important information that could help prevent awareness. A prompt response to this information is essential, as awareness can precede haemodynamic changes by several minutes.
References 1 Ghoneim MM. Awareness during anesthesia. Anesthesiology 2000; 92: 597±602 2 Struys M, Jensen EW, Smith W, et al. Performance of the ARXderived auditory evoked potential index as an indicator of anesthetic depth. Anesthesiology 2002; 96: 803±16 3 Thornton C. Evoked responses in anaesthesia. Eur J Anaesthesiol 1991; 8: 89±107 4 Jensen EW, Lindholm P, Henneberg SW. Autoregressive modeling with exogenous input of auditory evoked potentials to produce an on-line depth of anaesthesia index. Methods Inf Med 1996; 35: 256±60 5 Kochs E, Stockmanns G, Thornton C, Nahm W, Kalkman C. Wavelet analysis of middle latency auditory evoked responses. Anesthesiology 2001; 95: 1141±50 6 Litvan H, Jensen EW, Revuelta M, et al. Comparison of auditory evoked potentials and the A-line ARX index for monitoring the hypnotic level during sevo¯urane and propofol infusion. Acta Anaesthesiol Scand 2002; 46: 245±51 7 LuginbuÈhl M, Schnider T. Detection of awareness with the bispectral index: two case reports. Anesthesiology 2002; 96: 241±3
Ó The Board of Management and Trustees of the British Journal of Anaesthesia 2003
Downloaded from http://bja.oxfordjournals.org/ at New York University on May 11, 2015
the patient may already have been regaining consciousness in the time between intubation and skin incision. The ¯uctuations of the AAI value may have been caused by changes in the amounts of drugs delivered (the tubing was bent before completely coming out from its insertion point). The mean (SD) AAI value varied from 49.6 (12) for the `aware' period (between intubation and tube ®xing) to 20.3 (7.2) for the period from 5 min after tube ®xing and the end of drug delivery. An increase in the AAI, particularly when there had been no changes in drug dosage, should have prompted immediate checking of the i.v. delivery system. Because both infusions were outside the vein, the pump alarms were not activated. It was not until the blood pressure had reached 180 mm Hg that a problem with the infusion was suspected. Measurements of pulse and blood pressure are poor predictors of the hypnotic state during propofol anaesthesia, but the AAI and the bispectral index are useful measures.2 Our observations support this. Each increase in the AAI preceded changes in blood pressure by at least 5 min. The sudden increase in the EMG signal was not accompanied by other clinical signs of lack of muscular relaxation (swallowing, limb movement or airway pressure increase). Other monitors of depth of anaesthesia are useful when an accidental lack of anaesthetics causes awareness during i.v. and inhalation anaesthesia.7 We also show that monitoring the depth of hypnosis during general anaesthesia adds
DOI: 10.1093/bja/aeg144
CASE REPORTS Awareness detected by auditory evoked potential monitoring L. Trillo-Urrutia*, S. FernaÂndez-Galinski and J. CastanÄo-Santa Servicio de Anestesiologia y Reanimacion, Hospital del Mar, Passeig MarõÂtim 25, 08003 Barcelona, Spain *Corresponding author: [email protected] We report a case of awareness detected by the Alaris AEP Monitor, a device that measures anaesthesia by tracking changes of the waveform of the mid-latency auditory evoked potential. Br J Anaesth 2003; 91: 290±2 Accepted for publication: January 13, 2003
Possible awareness during general anaesthesia depends on the type of procedure, the dose of anaesthetics and the anaesthetic technique used.1 Awareness can occur if drug delivery is faulty. Measurements of the cardiovascular system are poor predictors of a patient's hypnotic state2 and should not be relied on to prevent awareness. Auditory evoked responses can be used to measure depth of anaesthesia.3±6 Recently, the Alaris AEP MonitorÔ (Alaris Medical Systems, Basingstoke, UK) has become available. This measures anaesthesia by tracking changes of the waveform of the mid-latency auditory evoked potential. These changes are converted into an A-line ARX index (AAI). An AAI value <30 is considered to be appropriate for surgical anaesthesia and an AAI >60 indicates consciousness. The monitor also measures electromyographic activity. We report a case of awareness detected by this device.
Case report A 39-yr-old male, height 165 cm, weight 52 kg, ASA II, was scheduled for a subtotal thyroidectomy. He had no clinical evidence of hyperthyroidism. However, thyroid function tests were abnormal, with the following values (patient, normal range): thyroid-stimulating hormone <0.1 (0.35±5) mU ml±1; total triiodothyronine 277 (58±159); thyroxine 2.1 (0.89±1.76) ng dl±1. He had volunteered for a research study in which anaesthesia was induced and maintained with propofol and remifentanil i.v. Drug administration was to be adjusted to maintain mean blood pressure and depth of hypnosis within agreed limits. The patient was given diazepam 5 mg and ranitidine 150 mg the night before surgery. The patient was awake and alert on arrival in the operating room. Standard monitoring was started and an
infusion of lactated Ringer's solution was started using a cannula in the back of his left hand. Three electrodes (A-Line AEP; Danmeter, Odense, Denmark) were then positioned at mid-forehead (+), left mastoid (±) and left forehead (reference) together with a pair of small headphones to deliver the auditory click stimulus. These were connected to an Alaris AEP MonitorÔ. The mean (SD) AAI value before induction was 72.5 (7.8). Oxygen (100%) was given via a face-mask and anaesthesia induced by administering boluses of propofol 90 mg and remifentanil 60 mg i.v. Cisatracurium 6 mg i.v. was administered to aid tracheal intubation. A slight delay before connecting the tubing for drug administration caused a brief increase in the AAI just after induction of anaesthesia, and the AAI and EMG reached 65 and 50% respectively (Fig. 1). Ventilation was started using a face-mask, and a cannula was then placed in the left foot to continue propofol and remifentanil delivery through two Perfusorâ fm pumps (Braun, Melsungen, Germany). The trachea was intubated and the patient was placed in the Kocher position. The research plan stipulated that anaesthesia should be maintained with propofol 3 mg kg±1 h±1 and remifentanil 0.5 mg kg±1 min±1 i.v. If needed, these infusion rates could be increased by 25% to keep the AAI <30 and to reduce the mean blood pressure by up to 30% of the baseline value. Two minutes had to elapse before a further increase in the infusion rate could be made. After intubation the AAI value began to ¯uctuate between 10 and 60, but cardiovascular measurements were stable. Immediately after skin incision, the AAI increased sharply from 20 to 70. Three minutes later the systolic blood pressure increased from 100 to 150 mm Hg. The propofol and remifentanil infusions were increased gradually according to the research plan to 6 mg kg±1 h±1 and 1.5
Ó The Board of Management and Trustees of the British Journal of Anaesthesia 2003
Downloaded from http://bja.oxfordjournals.org/ at New York University on May 11, 2015
Keywords: anaesthesia, general; monitoring, auditory evoked potential
Awareness detected by auditory evoked potential monitoring
mg kg±1 min±1 respectively, but the AAI, pulse rate and blood pressure did not change. After 5 min, the EMG activity increased suddenly from 0 to 60% and the blood pressure increased to 180 mm Hg. The tubing on the patient's foot was checked and it was discovered that the insertion point of the cannula had accidentally come out and no drugs were being infused i.v. The tubing was promptly ®xed and the AAI began to decrease immediately. In 5 min the systolic blood pressure returned to the normal value. Both indicators remained within the normal range until the end of the procedure. Drug infusion was then stopped and the patient woke up 9 min later, when he was extubated. When questioned after awakening, the patient recalled being aware during surgery, hearing the doctors' voices in
the operating room and feeling `painful manipulations' in his throat. He remembered being unable to speak or move before falling asleep again. He was not able to recall the intensity of the pain or the doctor's words. A detailed explanation was given to the patient regarding his experience and he reported no further dissatisfaction.
Discussion Accidental interruption of drug delivery is a common cause of awareness during general anaesthesia.1 We noted a rapid change in the AAI when infusion was stopped. From the charts it can be inferred that the `painful manipulations' felt by the patient corresponded to the period after skin incision. Although the precise moment of interruption is not known,
291
Downloaded from http://bja.oxfordjournals.org/ at New York University on May 11, 2015
Fig 1 Charts showing changes in the AAI (A), EMG (B) and arterial blood pressure (BP) (mm Hg) and heart rate (HR) (beats min±1) (C). The moments of intubation, incision and ®xing of the i.v. tubing are indicated.
Nouraei et al.
important information that could help prevent awareness. A prompt response to this information is essential, as awareness can precede haemodynamic changes by several minutes.
References 1 Ghoneim MM. Awareness during anesthesia. Anesthesiology 2000; 92: 597±602 2 Struys M, Jensen EW, Smith W, et al. Performance of the ARXderived auditory evoked potential index as an indicator of anesthetic depth. Anesthesiology 2002; 96: 803±16 3 Thornton C. Evoked responses in anaesthesia. Eur J Anaesthesiol 1991; 8: 89±107 4 Jensen EW, Lindholm P, Henneberg SW. Autoregressive modeling with exogenous input of auditory evoked potentials to produce an on-line depth of anaesthesia index. Methods Inf Med 1996; 35: 256±60 5 Kochs E, Stockmanns G, Thornton C, Nahm W, Kalkman C. Wavelet analysis of middle latency auditory evoked responses. Anesthesiology 2001; 95: 1141±50 6 Litvan H, Jensen EW, Revuelta M, et al. Comparison of auditory evoked potentials and the A-line ARX index for monitoring the hypnotic level during sevo¯urane and propofol infusion. Acta Anaesthesiol Scand 2002; 46: 245±51 7 LuginbuÈhl M, Schnider T. Detection of awareness with the bispectral index: two case reports. Anesthesiology 2002; 96: 241±3
Ó The Board of Management and Trustees of the British Journal of Anaesthesia 2003
Downloaded from http://bja.oxfordjournals.org/ at New York University on May 11, 2015
the patient may already have been regaining consciousness in the time between intubation and skin incision. The ¯uctuations of the AAI value may have been caused by changes in the amounts of drugs delivered (the tubing was bent before completely coming out from its insertion point). The mean (SD) AAI value varied from 49.6 (12) for the `aware' period (between intubation and tube ®xing) to 20.3 (7.2) for the period from 5 min after tube ®xing and the end of drug delivery. An increase in the AAI, particularly when there had been no changes in drug dosage, should have prompted immediate checking of the i.v. delivery system. Because both infusions were outside the vein, the pump alarms were not activated. It was not until the blood pressure had reached 180 mm Hg that a problem with the infusion was suspected. Measurements of pulse and blood pressure are poor predictors of the hypnotic state during propofol anaesthesia, but the AAI and the bispectral index are useful measures.2 Our observations support this. Each increase in the AAI preceded changes in blood pressure by at least 5 min. The sudden increase in the EMG signal was not accompanied by other clinical signs of lack of muscular relaxation (swallowing, limb movement or airway pressure increase). Other monitors of depth of anaesthesia are useful when an accidental lack of anaesthetics causes awareness during i.v. and inhalation anaesthesia.7 We also show that monitoring the depth of hypnosis during general anaesthesia adds