Relationship between bispectral index values and volatile anesthetic concentrations during the maintenance phase of anesthesia in the B-Unaware trial.
ABSTRACT Hypnotic depth during anesthesia affects electroencephalography waveforms and electroencephalogram-derived indices, such as the bispectral index (BIS). Titrating anesthetic administration against the BIS assumes reliable relationships between BIS values, electroencephalogram waveforms, and effect site concentration, beyond loss of responsiveness. Associations among BIS, end-tidal anesthetic concentrations (ETAC), and patient characteristics were examined during anesthetic maintenance, using B-Unaware trial data.
Pharmacokinetically stable ETAC epochs during intraoperative anesthetic maintenance were analyzed. A generalized estimating equation determined independent relationships among BIS, ETAC (in age-adjusted minimum alveolar concentration equivalents), patient characteristics, and 1-yr mortality. Further individual and population characteristics were explored graphically.
A total of 3,347,523 data points from 1,100 patients were analyzed over an ETAC range from 0.42 to 1.51 age-adjusted minimum alveolar concentration. A generalized estimating equation yielded a best predictive equation: BIS = 62.9-1.6 (if age younger than 60 yr) -1.6 (if female) -2.5 (if American Society of Anesthesiologists physical status more than 3) -2.6 (if deceased at 1 yr) -2.5 (if N2O was not used) -1.4 (if midazolam dose more than 2 mg) -1.3 (if opioid dose more than 50 morphine equivalents) -15.4 × age-adjusted minimum alveolar concentration. Although a population relationship between ETAC and BIS was apparent, interindividual variability in the strength and reliability of this relationship was large. Decreases in BIS with increasing ETAC were not reliably observed. Individual-patient linear regression yielded a median slope of -8 BIS/1 age-adjusted minimum alveolar concentration (interquartile range -30, 0) and a median correlation coefficient of -0.16 (interquartile range -0.031, -0.50).
Independent of pharmacokinetic confounding, BIS frequently correlates poorly with ETAC, is often insensitive to clinically significant changes in ETAC, and is vulnerable to interindividual variability. BIS is therefore incapable of finely guiding volatile anesthetic titration during anesthetic maintenance.
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ABSTRACT: Two defining effects of inhaled anesthetics (immobility in the face of noxious stimulation, and absence of memory) correlate with the end-tidal concentrations of the anesthetics. Such defining effects are characterized as MAC (the concentration producing immobility in 50% of patients subjected to a noxious stimulus) and MAC-Awake (the concentration suppressing appropriate response to command in 50% of patients; memory is usually lost at MAC-Awake). If the concentrations are monitored and corrected for the effects of age and temperature, the concentrations may be displayed as multiples of MAC for a standard age, usually 40 yr. This article provides an algorithm that might be used to produce such a display, including provision of an estimate of the effect of nitrous oxide. IMPLICATIONS: Two defining effects of inhaled anesthetics (immobility in the face of noxious stimulation, and absence of memory) correlate with the end-tidal concentrations of the anesthetics. Thus, these defining effects may be monitored and the results displayed if the concentrations are known and corrected for the effects of age and temperature.Anesthesia & Analgesia 11/2001; 93(4):947-53. · 3.30 Impact Factor
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ABSTRACT: Because of the complexity of raw electroencephalogram (EEG), for the anesthesiologist it is very difficult to evaluate the patient's hypnosis state. Because of this, several depth of anesthesia monitors have been developed, and are in current use at the operating room (OR). These monitors convert the information supplied by the EEG or derived signals into a simple, easy to understand index. Nowadays, general anesthesia is controlled only by the clinician, which decides what is the best drug combination for the patient, regarding all information given by monitors and sensors in the OR. In this work, we collected data from two study groups with auditory evoked potentials (AEP) monitoring, and Entropy (SE) monitoring. A model was fitted to the signals and the Hill equation parameters adjusted, in both study groups. The objective was to predict hypnosis indices, regarding only the drugs administered to a patient, and capture the initial individual patient characteristics that might influence the drugs interaction in the human body. Hypnotic and analgesic drugs interact in different ways throughout the anaesthesia stages. The models obtained captured the different dynamic interaction of drugs, during the induction and maintenance phases, demonstrating that the model must have incorporated all this information in order to perform satisfactorily. Other information like haemodynamic variables might be included in the search for the optimum model.Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 02/2007; 2007:1949-52.
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ABSTRACT: Inhalational anesthetics produce dose-dependent effects on electroencephalogram-derived parameters, such as 95% spectral edge frequency (SEF) and bispectral index (BIS). The authors analyzed the relationship between end-tidal sevoflurane and isoflurane concentrations (FET) and BIS and SEF and determined the speed of onset and offset of effect (t1/2k(e0)). Twenty-four patients with American Society of Anesthesiologists physical status I or II were randomly assigned to receive anesthesia with sevoflurane or isoflurane. Several transitions between 0.5 and 1.5 minimum alveolar concentration were performed. BIS and SEF data were analyzed with a combination of an effect compartment and an inhibitory sigmoid Emax model, characterized by t1/2k(e0), the concentration at which 50% depression of the electroencephalogram parameters occurred (IC50), and shape parameters. Parameter values estimated are mean +/- SD. The model adequately described the FET-BIS relationship. Values for t1/2k(e0), derived from the BIS data, were 3.5 +/- 2.0 and 3.2 +/- 0.7 min for sevoflurane and isoflurane, respectively (NS). Equivalent values derived from SEF were 3.1 +/- 2.4 min (sevoflurane) and 2.3 +/- 1.2 min (isoflurane; NS). Values of t1/2k(e0) derived from the SEF were smaller than those from BIS (P < 0.05). IC50 values derived from the BIS were 1.14 +/- 0.31% (sevoflurane) and 0.60 +/- 0.11% (isoflurane; P < 0.05). The speed of onset and offset of anesthetic effect did not differ between isoflurane and sevoflurane; isoflurane was approximately twice as potent as sevoflurane. The greater values of t1/2k(e0) derived from the BIS data compared with those derived from the SEF data may be related to computational and physiologic delays.Anesthesiology 05/1999; 90(5):1345-53. · 5.16 Impact Factor