J. Bruhn

Electromyographic activity falsely elevates the bispectral index.

THE bispectral index (BIS, Aspect Medical Systems, Framingham, MA) is a complex electroencephalographic (EEG) parameter that integrates several disparate descriptors of the EEG into a single variable and correlates behavioral assessments of sedation and hypnosis. We report two cases in which the bispectral index failed to measure depth of anesthesia but instead was a measure of electromyographic (EMG) activity. In the first case bispectral index paradoxically increased after increasing propofol concentration, correlating with increasing EMG activity. In the second case the administration of a nondepolarizing muscle relaxant decreased the bispectral index value at constant anesthetic drug concentrations.

A model of the ventilatory depressant potency of remifentanil in the non-steady state.

Background The C50 of remifentanil for ventilatory depression has been previously determined using inspired carbon dioxide and stimulated ventilation, which may not describe the clinically relevant situation in which ventilatory depression occurs in the absence of inspired carbon dioxide. The authors applied indirect effect modeling to non–steady state Paco2 data in the absence of inspired carbon dioxide during and after administration of remifentanil. Methods Ten volunteers underwent determination of carbon dioxide responsiveness using a rebreathing design, and a model was fit to the end-expiratory carbon dioxide and minute ventilation. Afterwards, the volunteers received remifentanil in a stepwise ascending pattern using a computer-controlled infusion pump until significant ventilatory depression occurred (end-tidal carbon dioxide [Peco2] > 65 mmHg and/or imminent apnea). Thereafter, the concentration was reduced to 1 ng/ml. Remifentanil pharmacokinetics and Paco2 were determined from frequent arterial blood samples. An indirect response model was used to describe the Paco2 time course as a function of remifentanil concentration. Results The time course of hypercarbia after administration of remifentanil was well described by the following pharmacodynamic parameters: F (gain of the carbon dioxide response), 4.30; ke0 carbon dioxide, 0.92 min−1; baseline Paco2, 42.4 mmHg; baseline minute ventilation, 7.06 l/min; kel,CO2, 0.08 min−1; C50 for ventilatory depression, 0.92 ng/ml; Hill coefficient, 1.25. Conclusion Remifentanil is a potent ventilatory depressant. Simulations demonstrated that remifentanil concentrations well tolerated in the steady state will cause a clinically significant hypoventilation following bolus administration, confirming the acute risk of bolus administration of fast-acting opioids in spontaneously breathing patients.

Non–steady State Analysis of the Pharmacokinetic Interaction between Propofol and Remifentanil

Background The pharmacokinetics of both propofol and remifentanil have been described extensively. Although they are commonly administered together for clinical anesthesia, their pharmacokinetic interaction has not been investigated so far. The purpose of the current investigation was to elucidate the nature and extent of pharmacokinetic interactions between propofol and remifentanil. Methods Twenty healthy volunteers aged 20–43 yr initially received either propofol or remifentanil alone in a stepwise incremental and decremental fashion via a target controlled infusion. Thereafter, the respective second drug was infused to a fixed target concentration in the clinical range (0–4 &mgr;g/ml and 0–4 ng/ml for propofol and remifentanil, respectively) and the stepwise incremental pattern repeated. Frequent blood samples were drawn for up to 6 h for propofol and 40 min for remifentanil after the end of administration and assayed for the respective drug concentrations with gas chromatography–mass spectrometry. The time courses of the measured concentrations were fitted to standard compartmental models. Calculations were performed with NONMEM. After having established the individual population models for both drugs and an exploratory analysis for hypothesis generation, pharmacokinetic interaction was identified by including an interaction term into the population model and comparing the value of the objective function in the presence and absence of the respective term. Results The concentration–time courses of propofol and remifentanil were described best by a three- and two-compartment model, respectively. In the concentration range examined, remifentanil does not alter propofol pharmacokinetics. Coadministration of propofol decreases the central volume of distribution and distributional clearance of remifentanil by 41% and elimination clearance by 15%. This effect was not concentration-dependent in the examined concentration range of propofol. Conclusions Coadministration of propofol decreases the bolus dose of remifentanil needed to achieve a certain plasma–effect compartment concentration but does not alter the respective maintenance infusion rates and recovery times to a clinically significant degree.

Correlation between ultrasound imaging, cross-sectional anatomy, and histology of the brachial plexus: a review.

The anatomy of the brachial plexus is complex. To facilitate the understanding of the ultrasound appearance of the brachial plexus, we present a review of important anatomic considerations. A detailed correlation of reconstructed, cross-sectional gross anatomy and histology with ultrasound sonoanatomy is provided.

Visualization of the course of the sciatic nerve in adult volunteers by ultrasonography

Background: The sciatic nerve block by the posterior approaches represents one of the more difficult ultrasound‐guided nerve blocks. Our clinical experiences with these blocks indicated a point slightly distal to the subgluteal fold as an advantageous position to allow good ultrasonic visibility. In this study, we systematically scanned the sciatic nerve from the subgluteal fold to the popliteal crease, to determine an optimal point for ultrasonographic visualization.

Different Approaches to Ultrasound-guided Thoracic Paravertebral Block An Illustrated Review

Given the fast development and increasing clinical relevance of ultrasound guidance for thoracic paravertebral blockade, this review article strives (1) to provide comprehensive information on thoracic paravertebral space anatomy, tailored to the needs of a regional anesthesia practitioner, (2) to interpret ultrasound images of the thoracic paravertebral space using cross-sectional anatomical images that are matched in location and plane, and (3) to briefly describe and discuss different ultrasound-guided approaches to thoracic paravertebral blockade. To illustrate the pertinent anatomy, high-resolution photographs of anatomical cross-sections are used. By using voxel anatomy, it is possible to visualize the needle pathway of different approaches in the same human specimen. This offers a unique presentation of this complex anatomical region and is inherently more realistic than anatomical drawings.

Comparison between Bispectral Index and Patient State Index as Measures of the Electroencephalographic Effects of Sevoflurane

Background: The Bispectral Index (BIS) and the Patient State Index (PSI) quantify depth of anesthesia by analyzing the electroencephalogram. The authors examined the response of BIS and PSI to sevoflurane anesthesia. Methods: In 22 patients, sevoflurane anesthesia was induced by inhalation with a tight-fitting facemask and was maintained via a laryngeal mask. Sevoflurane concentrations were increased until burst suppression occurred and subsequently decreased until BIS recovered to values above 60. This procedure was repeated twice until patients underwent intubation for subsequent surgery. End-tidal sevoflurane concentrations, BIS, and PSI were recorded simultaneously. The performance of PSI and BIS to predict the estimated sevoflurane effect site concentration, as derived from simultaneous pharmacokinetic and pharmacodynamic modeling, was compared by determination coefficients (&rgr;2) and prediction probabilities (PK). Results: A significant (P < 0.001) correlation between BIS and PSI was found (r2 = 0.75), and a close sigmoid relation between sevoflurane effect site concentration and both BIS (&rgr;2 = 0.84 ± 0.09) and PSI (&rgr;2 = 0.85 ± 0.15) was observed. The maximum sevoflurane electroencephalographic effect resulted in PSI values (1.3 ± 4.3) that were significantly (P = 0.019) lower than BIS values (7.9 ± 12.1), and the effect site efflux constant ke0 was significantly smaller (P = 0.001) for PSI (0.13 ± 0.08 min−1) than for BIS (0.24 ± 0.15 min−1). The probability of BIS (PK = 0.80 ± 0.11) to predict sevoflurane effect site concentration did not differ (P = 0.76) from that of PSI (PK = 0.79 ± 0.09). Conclusions: The BIS reacted faster to changes in sevoflurane concentrations, whereas the PSI made better use of the predefined index range. However, despite major differences in their algorithms and minor differences in their dose–response relations, both PSI and BIS predicted depth of sevoflurane anesthesia equally well.

Mixed-effects modeling of the intrinsic ventilatory depressant potency of propofol in the non-steady state.

BackgroundDespite the ubiquitous use of propofol for anesthesia and conscious sedation and numerous publications about its effect, a pharmacodynamic model for propofol-induced ventilatory depression in the non-steady state has not been described. To investigate propofol-induced ventilatory depression in the clinically important range (at and below the metabolic hyperbola while carbon dioxide is accumulating because of drug-induced ventilatory depression), the authors applied indirect effect modeling to Paco2 data at a fraction of inspired carbon dioxide of 0 during and after administration of propofol. MethodsTen volunteers underwent determination of their carbon dioxide responsiveness by a rebreathing design. The parameters of a power function were fitted to the end-expiratory carbon dioxide and minute ventilation data. The volunteers then received propofol in a stepwise ascending pattern with use of a target-controlled infusion pump until significant ventilatory depression occurred (end-tidal pressure of carbon dioxide > 65 mmHg and/or imminent apnea). Thereafter, the concentration was reduced to 1 &mgr;g/ml. Propofol pharmacokinetics and the Paco2 were determined from frequent arterial blood samples. An indirect response model with Bayesian estimates of the pharmacokinetics and carbon dioxide responsiveness in the absence of drug was used to describe the Paco2 time course. Because propofol reduces oxygen requirements and carbon dioxide production, a correction factor for propofol-induced decreasing of carbon dioxide production was included. ResultsThe following pharmacodynamic parameters were found to describe the time course of hypercapnia after administration of propofol (population mean and interindividual variability expressed as coefficients of variation): F (gain of the carbon dioxide response), 4.37 ± 36.7%; ke0, CO2, 0.95 min−1 ± 59.8%; baseline Paco2, 40.9 mmHg ± 12.8%; baseline minute ventilation, 6.45 l/min ± 36.3%; kel, CO2, 0.11 min−1 ± 34.2%; C50,propofol, 1.33 &mgr;g/ml ± 49.6%; &ggr;, 1.68 ± 21.3%. ConclusionPropofol at common clinical concentrations is a potent ventilatory depressant. An indirect response model accurately described the magnitude and time course of propofol-induced ventilatory depression. The indirect response model can be used to optimize propofol administration to reduce the risk of significant ventilatory depression.

A needle guidance device compared to free hand technique in an ultrasound-guided interventional task using a phantom.

In this in vitro study, a needle guidance device and a ‘free hand’ technique for ultrasound guided needle insertion were compared in a simulated ultrasound‐guided interventional task using a porcine phantom. Residents inexperienced in using ultrasonography were asked to insert a needle, using an in‐plane techniques, and to make contact with metal rods at a depth of 2 and 4 cm in the phantom. The transducer made angles of 90°, 60° and 45° with the surface of the phantom. The times to perform the procedures were significantly shorter and the needle visualisation was significantly better when using the needle guidance device. The residents ranked their satisfaction with the needle‐guidance device significantly better than the ‘free‐hand’ technique. This device may be beneficial when performing ultrasound guided peripheral nerve blocks, especially by inexperienced operators.

Soft tissue landmark for ultrasound identification of the sciatic nerve in the infragluteal region: the tendon of the long head of the biceps femoris muscle

Background and objectives: The sciatic nerve block represents one of the more difficult ultrasound‐guided nerve blocks. Easy and reliable internal ultrasound landmarks would be helpful for localization of the sciatic nerve. Earlier, during ultrasound‐guided posterior approaches to the infragluteal sciatic nerve, the authors recognized a hyperechoic structure at the medial border of the long head of biceps femoris muscle (BFL). The present study was performed to determine whether this is a potential internal landmark to identify the infragluteal sciatic nerve.