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Anesth Analg 2002;95:639-644
© 2002 International Anesthesia Research Society

TECHNOLOGY, COMPUTING, AND SIMULATION

Bispectral Index in Patients with Target-Controlled or Manually-Controlled Infusion of Propofol

Andreas Lehmann, MD*, Joachim Boldt, MD*, Elfi Thaler, MD*, Swen Piper, MD*, and Udo Weisse, MD{dagger}

Departments of *Anesthesiology and Intensive Care Medicine and {dagger}Cardiac Surgery, Klinikum der Stadt Ludwigshafen, Ludwigshafen, Germany

Address correspondence and reprint requests to Andreas Lehmann, MD, Department of Anesthesiology and Intensive Care Medicine, Klinikum der Stadt Ludwigshafen, Postfach 21 73 52, D-67073 Ludwigshafen, Germany. Address e-mail to Dr.A.Lehmann{at}web.de


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 References
 
In this prospective, randomized study we compared bispectral index (BIS), hemodynamics, time to extubation, and the costs of target-controlled infusion (TCI) and manually-controlled infusion (MCI) of propofol. Forty patients undergoing first-time implantation of a cardioverter-defibrillator were included. Anesthesia was performed with remifentanil (0.2–0.3 µg · kg-1 · min-1) and propofol. Propofol was used as TCI (plasma target concentration, 2.5–3.5 µg/mL; n = 20) or MCI (3.0–4.0 mg · kg-1 · h-1; n = 20). BIS, heart rate, and arterial blood pressure were measured at six data points: T1, before anesthesia; T2, after intubation; T3, after skin incision; T4, after first defibrillation; T5, after third defibrillation; and T6, after extubation. There were no significant hemodynamic differences between the two groups. BIS was significantly lower at T3 and T4 in the TCI group than in the MCI group. The mean dose of propofol was larger in TCI patients (5.8 ± 1.4 mg · kg-1 · h-1) than in the MCI patients (3.7 ± 0.6 mg · kg-1 · h-1) (P < 0.05), whereas doses of remifentanil did not differ. Time to extubation did not differ between the two groups (TCI, 13.7 ± 5.3 min; MCI, 12.3 ± 3.5 min). One patient in the MCI group had signs of intraoperative awareness without explicit memory after first defibrillation (BIS before shock, 49; after shock, 83). Costs were significantly less in the MCI group (US$34.83) than in the TCI group (US$39.73). BIS failed to predict the adequacy of anesthesia for the next painful stimulus.

IMPLICATIONS: In this prospective, randomized study, bispectral index (BIS), hemodynamics, time to extubation, and costs of target-controlled infusion (TCI) and manually-controlled infusion of propofol were compared. TCI increased the amount of propofol used. BIS failed to predict the adequacy of anesthesia for the next painful stimulus.


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 References
 
Propofol is a fast-acting IV drug with a favorable pharmacokinetic profile for inducing and maintaining total IV anesthesia. The development of computer-assisted target-controlled infusion (TCI) systems of propofol provided the anesthesiologist with a convenient method for directly controlling the blood concentration of anesthetics (1). TCI systems for propofol are based on averaged pharmacokinetic models derived from large population samples, specific pharmacokinetic variables for propofol, and infusion-controlled algorithms (2). In a prospective, randomized multicenter study comparing TCI with manually-controlled infusion (MCI) of propofol, most investigators expressed an overall preference (93%) for TCI and found it easier (76%) to handle than conventional propofol application (3). TCI reduced the incidence of an inadequate depth of anesthesia compared with MCI (3). However, use of TCI of propofol increased the amount of drug used (4) and increased the recovery times from anesthesia as compared with MCI (3).

In October 1996, the bispectral index (BIS) was approved by the Food and Drug Administration as the first monitor of anesthetic depth (5). BIS reduces complex electroencephalogram processing to a simple number ranging from 100 to 0. With increasing depth of anesthesia, BIS decreases (5). However, clinical evaluation of BIS is still controversial. Some authors state that anesthesia can be adapted by BIS to a patient’s individual needs (6,7). This optimal adaptation of anesthesia reduces the amount of anesthetics used, time to extubation, stay in the postanesthesia recovery area, and costs (6,7). However, according to O’Connor et al. (8), the assumption that monitoring the depth of anesthesia with BIS reduces the risk of intraoperative awareness is yet not proven. BIS might even increase the risk of intraoperative awareness and costs (8).

The aim of this study was to compare hemodynamics, depth of anesthesia as assessed by BIS, postanesthesia recovery, and costs delivered by MCI or TCI of propofol in combination with remifentanil in patients undergoing implantation of a cardioverter-defibrillator (ICD).


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 References
 
Forty patients undergoing the first implantation of an ICD were enrolled in this study. Inclusion criteria were good or only slightly reduced left ventricular function (ejection fraction >40%, left ventricular end-diastolic pressure <16 mm Hg, or both) and an age less than 80 yr. Patients with a history of alcohol or drug abuse and those with severe hepatic or renal insufficiency were excluded. The study was approved by the Ethics Committee of the hospital, and all patients gave written, informed consent.

All ICDs were implanted subpectorally on the left side. Ventricular and dual-chamber atrio-ventricular ICDs were used. A right ventricular electrode was placed transvenously with one-chamber systems. Patients with dual-chamber systems received two electrodes placed transvenously, one in the right ventricle and the other in the right atrium. After surgery, the wound was infiltrated with local anesthetics (20 mL of mepivacaine 1%) for postoperative analgesia.

The patients were premedicated with 1 to 2 mg of lorazepam orally 1 h before the start of anesthesia. Hemodynamic monitoring consisted of a five-lead electrocardiogram and radial artery cannulation. Heart rate, mean arterial blood pressure (MAP), systolic arterial blood pressure, and diastolic arterial blood pressure were continuously recorded. BIS was measured at the frontal lobe of the dominant hemisphere after skin preparation with disinfectant alcohol and slight rubbing by using the Patient Care Monitoring System (SpaceLabs Medical, Redmond, WA) and BIS-sensorTM (Aspect Medical Systems, Natick, MA). When electrode impedance exceeded 10 k{Omega}, the electrode was replaced, and skin preparation was repeated. Electrode impedance was tested repeatedly every 30 min. When BIS increased to >65, the dose of propofol was increased to deepen anesthesia. All patients were carefully monitored for any signs of possible intraoperative awareness (movement, opening of the eyes, and sweating). The following data points were defined: T1, before the induction of anesthesia, while the patient was awake; T2, 3 min after intubation; T3, after skin incision; T4, after the first defibrillation; T5, after the third defibrillation; and T6, 5 min after extubation.

The patients were prospectively randomized into one of the following two groups by using a closed-envelope system. In Group MCI (n = 20; MCI of propofol), anesthesia was induced with a bolus of propofol (0.7–1.0 mg/kg) and a continuous infusion of remifentanil (1 µg · kg-1 · min-1). Tracheal intubation was facilitated by cisatracurium (0.15 mg/kg). No further neuromuscular-blocking drugs were given for the maintenance of anesthesia. Anesthesia was maintained by continuous infusion of propofol (3.0–4.0 mg · kg-1 · h-1) and remifentanil (0.3 µg · kg-1 · min-1) to keep BIS <65 and MAP >60 mm Hg. In Group TCI (n = 20; TCI of propofol), propofol was infused with a TCI device (Graseby 3500 Disoprifusor®; Graseby Medical Ltd., Watford, UK). Anesthesia was induced with a plasma target concentration of propofol of 3 µg/mL. A continuous infusion of 1 µg · kg-1 · min-1 of remifentanil was also added. After intubation, the plasma target concentration of propofol was adjusted to 2.5 to 3.5 µg/mL to keep BIS <65 and MAP >60 mm Hg, and the infusion rate of remifentanil was reduced to 0.3 µg · kg-1 · min-1. Cisatracurium was used as in the other group. In both groups, all patients were ventilated to normocapnia (36–44 mm Hg) with oxygen in air (fraction of inspired oxygen, 0.4). When the mean MAP decreased to <60 mm Hg, colloids (gelatin) were given and, until a clinically adequate volume load was achieved, a vasopressor (norepinephrine) was used to restore MAP immediately. When MAP remained at <60 mm Hg despite volume load, dobutamine (2 µg · kg-1 · min-1) was started.

The continuous infusions of propofol and remifentanil were stopped at the beginning of skin closure. The time from stopping the propofol/remifentanil infusion until the patients opened their eyes and underwent tracheal extubation was also documented.

All patients were visited the next day. They were asked about their satisfaction with the anesthesia and postoperative pain. They also were asked whether they would like this type of anesthesia again.

The costs for the anesthetic drugs were taken from the actual hospital’s pharmacy list (propofol, {euro}8.30/500 mg, US$7.55/500 mg; remifentanil, {euro}23.00/5 mg, US$20.94/5 mg). Cost analysis did not include costs for oxygen, air, staff, and disposables. Fixed costs for anesthesia machines and monitoring equipment were also not considered. The exchange rate from Euros to US dollars was 0.91.

Data are presented as mean ± SD unless otherwise indicated. For statistical analysis, SPSS/PC+ software (Version 4.0; SPSS Inc., Chicago, IL) was used. Hemodynamics were analyzed with two-factorial analysis of variance for repeated measurements. For significant findings, post hoc Student’s t-tests were applied at the end point of each measurement. In case of multiple comparisons, P values were corrected according to Bonferroni. Fisher’s exact tests, {chi}2 tests, Mann-Whitney U-tests, or nonpaired Student’s t-tests were also used when appropriate. P values <0.05 were considered significant.


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 References
 
There were no significant differences between the two groups with respect to age, sex, height, or weight. Preoperative evaluation of left ventricular function by ejection fraction and left ventricular end-diastolic pressure did not show any differences. In the MCI group, 8 patients were classified as ASA status II and 12 as ASA III, whereas in the TCI group, 9 were ASA II and 11 were ASA III. Time of anesthesia and time of surgery did not differ between the two groups. Also, time of extubation (MCI, 12.3 ± 3.5 min; TCI, 13.7 ± 5.3 min) did not differ significantly between the two groups (Table 1).


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  		Table 1. Table 1. Demographic Data and Data from the Perioperative Period
 
Heart rate, systolic arterial blood pressure, MAP, and diastolic arterial blood pressure (Table 2) did not differ at any time between the two groups during the study period. There were no differences in crystalloid (MCI, 1160 ± 372 mL; TCI, 998 ± 281 mL) or colloid (MCI, 275 ± 243 mL; TCI, 342 ± 288 mL) volume replacement. Three patients in the MCI group needed a continuous infusion of dobutamine (mean dose, 0.5 ± 1.3 µg · kg-1 · min-1), and three of the TCI patients needed dobutamine (mean dose, 0.5 ± 1.1 µg · kg-1 · min-1) (not significant). Bolus administration of norepinephrine was used in five MCI and six TCI patients to stabilize MAP (not significant).


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  		Table 2. Table 2. Hemodynamic Data of the Two Groups
 
The use of remifentanil during anesthesia (T2 through T4) did not differ at any time between the two groups (total mean dose: MCI, 0.30 ± 0.06 µg · kg-1 · min-1; TCI, 0.26 ± 0.03 µg · kg-1 · min-1; not significant; Fig. 1). TCI patients received significantly more propofol during anesthesia (T2 through T4) than patients from the MCI group (Fig. 2). The total dose of propofol used was 5.8 ± 1.4 mg · kg-1 · h-1 in the TCI group and 3.7 ± 0.6 mg · kg-1 · h-1 in MCI patients (P < 0.05).



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  		Figure 1. Infusion rate of remifentanil (µg · kg-1 · min-1), mean ± SD. MCI = manually controlled infusion; TCI = target-controlled infusion; T1 = before the induction of anesthesia, while the patient was awake; T2 = 3 min after intubation; T3 = after skin incision; T4 = after the first defibrillation; T5 = after the third defibrillation; T6 = 5 min after extubation. No significant differences between the two groups.

 


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  		Figure 2. Infusion rate of propofol (mg · kg-1 · h-1), mean ± SD. MCI = manually controlled infusion; TCI = target-controlled infusion; T1 = before the induction of anesthesia, while the patient was awake; T2 = 3 min after intubation; T3 = after skin incision; T4 = after the first defibrillation; T5 = after the third defibrillation; T6 = 5 min after extubation. #P < 0.05 between the two groups.

 
BIS decreased from >90 in both groups after the induction of anesthesia to 47.5 ± 7.7 in MCI patients and to 41.5 ± 7.1 in TCI patients. BIS was significantly lower (P < 0.05) in the TCI group at T3 and T4 compared with MCI of propofol (Fig. 3). At the end of anesthesia, after the third defibrillation, BIS did not differ between the two groups. After extubation, BIS was 87.7 ± 6.3 in MCI patients and 90.0 ± 7.6 in TCI patients (not significant). In one patient in the MCI group, BIS increased from 49 to 83 immediately after the first defibrillation. He also opened his eyes at this time. After anesthesia was deepened with propofol, the further anesthetic course was uneventful. This patient had no explicit memory of this incident after anesthesia. No other patients showed any signs of possible intraoperative awareness, such as movement, sweating, and opening their eyes. At the end of anesthesia, another patient in the MCI group fulfilled the criteria for extubation (sufficient spontaneous breathing, laryngeal reflexes, and responsiveness to verbal command) at a BIS of 47. Immediately after the extubation, the BIS increased to 92.



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  		Figure 3. Changes in bispectral index (BIS), mean ± SD. MCI = manually controlled infusion; TCI = target-controlled infusion; T1 = before the induction of anesthesia, while the patient was awake; T2 = 3 min after intubation; T3 = after skin incision; T4 = after the first defibrillation; T5 = after the third defibrillation; T6 = 5 min after extubation. #P < 0.05 between the two groups.

 
Postoperative analgesia was adequately provided by infiltrating the wound with local anesthetics. In both groups, only five patients needed additional piritramide up to 15 mg (analgesic potency in comparison to morphine [1] was 0.7) during the first postoperative night. No patient complained about pain after the first night. In both groups, no patient reported explicit intraoperative recall. No patient in either group refused this type of anesthesia for future surgery.

TCI anesthesia ({euro}43.65; US$39.73) was more expensive per patient than MCI-based anesthesia ({euro}39.73; US$34.83) (Table 3; P < 0.05). The costs for inotropic drugs, neuromuscular-blocking drugs, and fluid replacement were not analyzed because they did not differ between the two groups.


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  		Table 3. Table 3. Cost Analysis
 

   Discussion
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 Abstract
 Introduction
 Methods
 Results
 Discussion
References
 
Both general and local anesthesia are used for implantation of ICDs (8,9). There are a number of possible complications, including pneumothorax, hemothorax, rupture of the subclavian vein, hematoma, and failure of successful defibrillation by the ICD, requiring external defibrillation (9). Thus general anesthesia is commonly used for implantation of ICDs.

TCI uses averaged pharmacokinetic models to control the infusion rate of a pump. TCI of propofol provides the anesthesiologist with direct control of calculated plasma concentration, rather than indirect control by adjusting the infusion rate (1). In Germany, TCI systems for propofol have been commercially available since 1997 (4). Fechner et al. (4) found a precision of 27.5% of plasma concentration calculated by the TCI device in comparison to the measured plasma concentration of propofol. Divergence per hour was -5.4% (4). These data are in good agreement with a study of 46 patients divided into 3 age groups from 18 to 80 years (10). In this study, measured concentrations tended to be slightly larger than calculated concentrations (10). Both authors concluded that the TCI device had an acceptable predictive performance for clinical purposes (4,10). The accuracy of the calculated plasma concentration depends on the selected pharmacokinetic variable set. However, these differences in accuracy do not appear to make a difference in the clinical application of TCI (2).

In cardiac surgery, total IV anesthesia with propofol combined with an opioid is often used, especially for fast-track procedures (11,12). The combination with an opioid (e.g., remifentanil) allows dose reduction of propofol, providing better hemodynamic stability (8,11). Recommended calculated plasma concentrations of propofol vary from 4 to 6 µg/mL for the induction of anesthesia in healthy, unpremedicated patients scheduled for minor orthopedic surgery (13,14). For maintenance of anesthesia, the adequate target plasma concentration was reported to vary from 3.5 to 4.5 µg/mL in patients undergoing major surgery (10) and, in orthopedic surgery patients, from 4 to 6 µg/mL (14). Elderly patients needed smaller propofol target concentrations compared with patients in the younger age groups (3,10). Olivier et al. (11) reported the use of TCI of propofol combined with remifentanil in 50 patients undergoing cardiac surgery with cardiopulmonary bypass. Because of the combination with remifentanil (0.25–1.0 µg · kg-1 · min-1), they reduced the calculated plasma concentration of propofol to 1.5–2.0 µg/mL (10), which is less than the dose used in our patients (2.5–3.5 µg/mL), most likely because of the larger doses of remifentanil.

Studies comparing TCI with MCI of propofol showed that induction doses of TCI-based anesthesia were smaller than with MCI. However, during maintenance of anesthesia, TCI delivers significantly larger doses of propofol than the MCI technique (4,5,15,16). Using computer simulations, Struys et al. (16) showed an overshoot in propofol blood and effect-site concentrations with manual induction and significantly higher maintenance levels with TCI. In this study, TCI significantly increased the dose of propofol used compared with MCI. Whether TCI of propofol improves the quality of anesthesia compared with MCI is controversial. In a multicenter study, most anesthesiologists expressed an overall preference for TCI (93%) and found it easier to use (76%) than MCI (3). In 160 patients breathing spontaneously via laryngeal mask airway, Russell et al. (17) demonstrated that inducing anesthesia and positioning the laryngeal mask were faster and safer in TCI compared with MCI patients. They also demonstrated less movement of TCI patients at skin incision and during surgery. Recovery times were prolonged, most likely because the amount of propofol was significantly larger in TCI patients. Hunt-Smith et al. (15) found no difference in recovery times from anesthesia when they compared TCI- with MCI-based anesthesia. In this study we found no differences in recovery times by the increased amount of propofol in TCI patients. In a previous study in patients with severely reduced left ventricular function, TCI patients required more inotropic support with dobutamine compared with the MCI group. This effect was caused by the increased dose of propofol (8). In this study, in patients with normal left ventricular function, TCI did not alter hemodynamics significantly, nor did TCI increase the need for volume load or use of catecholamines.

Depth of anesthesia was assessed with BIS, which appears to be a very promising tool for evaluating the adequacy of anesthesia (18). In one study, it had a sensitivity of 97.3% and a specificity of 94.4% (19). Compared with 95% spectral edge frequency, median frequency, and auditory evoked potential index, BIS showed an excellent correlation with blood concentrations of propofol (18). BIS was suggested to be a good monitor for assessing the level of sedation for various types of anesthetics (20). An adequate level of anesthesia is proposed if BIS is between 40 and 65 (5). Consciousness was lost in 50% of healthy volunteers when BIS was 67, whereas 95% of the volunteers lost consciousness at a BIS of 50 (20). The addition of alfentanil as an opioid resulted in loss of consciousness at higher BIS values (20). No explicit or implicit memory loss of familiar words presented during propofol anesthesia at BIS levels between 40 and 60 was found in 82 elective surgical patients (21). However, a case report of a patient undergoing cardiac surgery using inhaled anesthesia who had explicit intraoperative recall at a BIS of 47 was recently published (22). There seems to be no exact value of BIS that absolutely safely differentiates the awake from the anesthetized state. In our study, BIS values were always <60 in all patients except one. In one patient, BIS increased from 49 to 83 immediately after the first defibrillation. None of our patients reported explicit memory during anesthesia. At recovery from anesthesia, Sleigh and Donovan (19) showed that 6 of 37 patients could follow verbal commands at BIS <60. Immediately after verbal commands were obeyed, BIS always increased to >95 (19). The same phenomenon was observed in one patient in our study. This phenomenon occurs as a result of the time of 20 to 30 seconds needed to recalculate and actualize BIS (19). BIS is always a monitor of the level of anesthesia 20 to 30 seconds in the past (19,23). However, BIS cannot predict whether the actual level of anesthesia will be sufficient for the next painful stimulus (23). The concept of reducing anesthetic drugs until a BIS level of approximately 60 is reached advocates lighter levels of anesthesia for patients. This concept can reduce costs and length of stay in the recovery area (6,24). However, it might be dangerous in procedures with varying intensity of surgical stimuli. Paradoxically, this concept could lead to an increased incidence of intraoperative awareness by monitoring the depth of anesthesia (23,25).

We conclude that both techniques—MCI and TCI of propofol in combination with remifentanil in patients undergoing implantation of ICDs—showed similar hemodynamics, were well controllable, and allowed early extubation. Reducing the amount of applied anesthetics guided by BIS might endanger patients and increase the incidence of intraoperative awareness if anesthesia is not sufficient for the next painful stimulus.


   References
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 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 

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Accepted for publication May 7, 2002.




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Lippincott, Williams & Wilkins Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins and Stanford University Libraries' HighWire Press®. Copyright 2002 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999 HighWire Press