The rectus sheath block: accuracy of local anesthetic placement by trainee anesthesiologists using loss of resistance or ultrasound guidance.

BACKGROUND AND OBJECTIVES: The aim of this study was to compare the accuracy of local anesthetic placement in the rectus sheath block when performed by trainee anesthetists using loss of resistance (LOR) or ultrasound guidance. METHODS: Eighty-one patients undergoing laparoscopic surgery were randomly assigned to undergo rectus sheath block by either LOR or ultrasound guidance. Trainee anesthesiologists were also randomly assigned to provide the rectus sheath block by LOR or by using ultrasound. The placement of local anesthetic was recorded using ultrasound. RESULTS: The placement of local anesthetic by LOR was accurate in 45% of attempts but was superficial and deep to the rectus sheath in 34% and 21% of punctures, respectively. Accurate placement of local anesthetic within the rectus sheath decreased significantly as body mass index increased. Ultrasound guidance significantly improved the accuracy of needle placement, with 89% of abdominal punctures being correctly placed at the time of first injection of local anesthetic. An additional fascial plane lying at variable distance above the anterior layer of the rectus sheath was commonly observed. CONCLUSIONS: Ultrasound guidance improves the accuracy of local anesthetic placement when undertaking the rectus sheath block. An additional fascial plane above the anterior layer of the rectus sheath may be wrongly perceived as the anterior layer of the rectus sheath when the block is undertaken without the aid of ultrasound.

Use of target controlled infusion to derive age and gender covariates for propofol clearance.

BACKGROUND AND OBJECTIVE: Attempts to describe the variability of propofol pharmacokinetics in adults and to derive population covariates have been sparse and limited mainly to experiments based on bolus doses or infusions in healthy volunteers. This study aimed to identify age and gender covariates for propofol when given as an infusion in anaesthetized patients. STUDY DESIGN AND SETTING: One hundred and thirteen patients (American Society of Anesthesiologists class I or II and aged 14-92 years) were anaesthetized for elective surgical procedures with propofol using a target controlled infusion (TCI) system and with alfentanil as a baseline analgesic infusion. Frequent venous blood samples were obtained for measurement of propofol plasma concentrations. PHARMACOKINETIC AND STATISTICAL ANALYSIS: Pharmacokinetic accuracy was determined by the percentage prediction error, bias and precision, as were wobble and divergence. The clearance of propofol from the central compartment was determined for each patient using the computerized record of the infusion profile delivered to each patient, together with relevant blood propofol concentration estimations. For each patient, the nonlinear mixed-effects modelling (NONMEM) objective function was employed to determine the goodness of fit. RESULTS: The population distribution of propofol clearance was subsequently found to have a Gaussian distribution only in the log domain (mean value equivalent to 26.1 mL/kg/min). The distribution in the normal domain was consequently asymmetric, with a slight predominance of patients with high values of clearance (5% and 95% confidence limits 17.7 and 42.1 mL/kg/min, respectively). Using regression analysis, gender and age covariates were derived that optimized the performance of the target controlled infusion system. The clearance (CL) of propofol in male patients changed little with age (CL [mL/kg/min]=26.88-0.029xAge; r2=0.006) whereas that in female patients had a higher initial value but decreased progressively with age (CL [mL/kg/min]=37.87-0.198xAge; r2=0.246). CONCLUSION: We achieved a relatively simple and practical covariate model in which the variability of pharmacokinetics within the study population could be ascribed principally to variability in clearance from the central compartment. Pharmacokinetic simulation predicted an improved performance of the TCI system when employing the derived covariates model, especially in elderly female patients.

Ultrasound guided fascia iliaca block: a comparison with the loss of resistance technique.

BACKGROUND AND OBJECTIVES: The aim of this study was to compare the efficacy of fascia iliaca block, performed by loss of resistance and ultrasound guidance techniques. METHODS: Eighty patients undergoing either unilateral hip or knee joint replacement surgery were randomly assigned to undergo fascia iliaca compartment block by either loss of resistance or ultrasound guidance. Sensation in the anterior (femoral nerve), lateral (lateral cutaneous nerve) and medial (femoral and variable contribution from obturator nerve) aspects of the thigh were assessed prior to block placement. Femoral motor block (knee extension) was also evaluated. Obturator motor block (hip adduction) was measured using a sphygmomanometer. Sensation and motor function were reassessed after block placement. RESULTS: Using ultrasound guidance, there was a statistically significant increase in the incidence of sensory loss in the medial aspect of the thigh from 60% to 95% (P = .001). Complete loss of sensation in the anterior, medial, and lateral aspects of the thigh increased from 47% to 82% of patients using ultrasound (P = .001). Ultrasound-guided fascia iliaca block resulted in a statistically significant increase in the incidence of femoral (P = .006) and obturator (P = .033) nerve motor block. CONCLUSIONS: Ultrasound-guided fascia iliaca block increased the frequency of sensory loss in the medial aspect of the thigh. Ultrasound guidance also increased the frequency of femoral and obturator motor block.

The contribution of remifentanil to middle latency auditory evoked potentials during induction of propofol anesthesia.

There is a debate regarding whether opioids, as a component of general anesthesia, are adequately reflected in the assessment of anesthesia based on derivatives of the electroencephalogram. To test the hypothesis of a possible quantitative contribution of remifentanil on middle latency auditory evoked potentials, we studied its interaction with propofol anesthesia in 45 unpremedicated male patients undergoing elective lower limb orthopedic surgery. They were allocated randomly to three groups. The first two groups received remifentanil either with a high (8 ng mL(-1)) or a low (3 ng mL(-1) target concentration using target-controlled infusion (TCI). The third group received spinal anesthesia instead of remifentanil. Anesthesia was induced by a stepwise increase in propofol concentration using TCI. The auditory evoked potential index (AEPex) and calculated propofol effect site concentrations were determined at loss of consciousness and the reaction to laryngeal mask airway insertion was noted. The propofol infusion was then converted to a closed-loop TCI using an AEPex value of 40 as the target. We found no significant contribution of remifentanil alone on the auditory evoked response, whereas increasing concentrations of remifentanil led to a significant decrease of the calculated propofol effect site concentrations (P = 0.023) necessary for unconsciousness. Prediction probability for AEPex was inversely related to the remifentanil concentration and was best for the control group, which received propofol alone. These results support previous findings of a quantitative interaction between remifentanil and propofol for loss of consciousness but question the specific contribution of remifentanil to auditory evoked potentials.

Prediction of responses to various stimuli during sedation: a comparison of three EEG variables.

OBJECTIVE: To compare the ability of the aepEX, a derivative of auditory evoked potentials, the bispectral index, and SEF95% to predict responses to various stimuli. DESIGN: Prospective clinical study. SETTING: General Intensive Care Unit in a university hospital. PATIENTS AND PARTICIPANTS: Forty postsurgical, mechanically ventilated patients. INTERVENTIONS: Target concentrations of blood propofol were randomly set at 0.5 microg/ml, 1.0 microg/ml, 1.5 microg/ml, and 2.0 microg/ml, with a fixed fentanyl infusion rate between 0.5 microg.kg(-1).h(-1) and 1.5 microg.kg(-1).h(-1). MEASUREMENTS: Depth of sedation was subjectively assessed with the Ramsay Sedation Score. The aepEX was recorded using an auditory evoked potentials system. The bispectral index and SEF95% were measured using an Aspect A-1000 monitor. RESULTS: The aepEX, bispectral index, and SEF95% correlated with the Ramsay Sedation Score, the Pk value being greatest for the aepEX, followed by the bispectral index. All three variables could predict opening of the eyes in response to verbal commands or a glabellar tap, the aepEX being a better predictor than the bispectral index or SEF95%. All three EEG variables had Pk values >0.5 in predicting coughing or movement in response to tracheal suction, but they were unable to predict increases in heart rate or systolic blood pressure. CONCLUSIONS: The aepEX was the best predictor, followed by bispectral index. Although in most intensive care patients subjective sedation scales are sufficient to assess levels of sedation, the aepEX and bispectral index were potential alternatives to subjective scales when they do not work well in the setting of neuromuscular blockade or may not be sufficiently sensitive to evaluate very deep sedation.

Bayesian probabilistic network modeling of remifentanil and propofol interaction on wakeup time after closed-loop controlled anesthesia.

OBJECTIVE: Until now, the knowledge of combining anesthetics to obtain an adequate level of anesthesia and to economize wakeup time has been empirical and difficult to represent in quantitative models. Since there is no reason to expect that the effect of non-opioid and opioid anesthetics can be modeled in a simple linear manner, the use of a new computational approach with Bayesian belief network software is demonstrated. METHODS: A data set from a pharmacodynamic study was used where remifentanil was randomly given in three fixed target concentrations (2, 4, and 8 ng/ml) to 62 subjects. Target concentrations of propofol were controlled according to the closed-loop system feedback of the auditory evoked potential index to render modeling unbiased by the level of anesthesia. Time to open eyes was measured to represent wakeup time after surgery. The NETICA version 1.37 software was used on a personal computer for network building, validation, and prediction. RESULTS: After the learning phase, the network was used to generate a series of random cases whose probability distribution matches that of the compiled network. The sampling algorithms used are precise, so that the frequencies of the simulated cases will exactly approach the probabilities of the network and that of the data learned. The graphical display of the predicted wakeup time shows less variability but a more complex interaction pattern than with the unadjusted original data. CONCLUSIONS: Model building and evaluation with Bayesian networks does not depend on underlying linear relationships. Bayesian relationships represent true features of the represented data sample. Data may be sparse, uncertain, stochastic, or imprecise. Multiple platform software that is easy to use is increasingly available. Bayesian networks promise to be versatile tools for building valid, nonlinear, predictive instruments to further gain insight into the complex interaction of anesthetics.