Anesthesia and analgesia
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To examine the physicochemical stability of combinations of propofol-lidocaine mixtures frequently used in clinical practice, we added lidocaine 5, 10, 20, or 40 mg to commercially available 1% propofol 20 mL. To assess chemical stability, propofol concentrations were determined by gas chromatography assay for 24 h after preparation of the mixture. In addition, scanning electron microscopy was used to determine the maximum detectable droplet size in randomly selected fields. Macroscopically, separate, colorless layers were first seen at 3 and 24 h after the addition of 40 and 20 mg of lidocaine to propofol, respectively, whereas the mixture with 5 or 10 mg of lidocaine was macroscopically stable. Propofol concentrations in the mixture with 40 mg of lidocaine decreased linearly and significantly from 4 to 24 h after preparation, whereas those combined with other lidocaine doses were unchanged compared with baseline concentrations. Scanning electron microscopy showed that droplets with diameters >or=5 microm first appeared 30 min after the addition of 40 mg of lidocaine to propofol, and the emulsion droplets were enlarged in a time- and dose-dependent fashion. Our results indicate that the addition of lidocaine to propofol results in a coalescence of oil droplets, which finally proceeds to a visible separate layer. Depending on the dose of lidocaine and the duration between its preparation and administration, this combination may pose the risk of pulmonary embolism. ⋯ The addition of lidocaine to propofol results in time- and dose-dependent increases in oil droplet diameters in emulsion. This mixture is physicochemically unstable over time and may cause pulmonary embolism, depending on the dose of lidocaine.
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Anesthesia and analgesia · Dec 2003
Propofol suppresses the cortical somatosensory evoked potential in rats.
The dose-response curve for the effect of volatile anesthetics on the somatosensory evoked potential (SEP) is well described, but for propofol, the large dose segment of the curve is undefined. We describe the effect of increasing plasma concentrations of propofol on cortical SEPs in 18 rats. After surgical preparation under ketamine anesthesia, a remifentanil infusion was begun at 2.5, 5, or 10 microg x kg(-1) x min(-1). After 20 min, the propofol infusion was initiated at 20 mg x kg(-1) x h(-1) and was increased to 40, 60, and 80 mg x kg(-1) x h(-1) at 20-min intervals. SEP was recorded before remifentanil infusion, before propofol infusion rate changes, and 30 min after discontinuing propofol infusion. In six additional rats, the plasma concentrations of propofol after each 20-min infusion were measured using gas chromatography. Remifentanil did not have a significant effect, but propofol significantly depressed the SEP amplitude and prolonged the latency at infusion rates of 40 mg x kg(-1) x h(-1) and more. Propofol's effect was dose-dependent, but even at 80 mg x kg(-1) x h(-1) with an estimated plasma concentration of 31.6 +/- 3.4 microg/mL (10.8 50% effective concentration), a measurable response was present in 44.5% of rats. These results suggest that even at large doses, propofol and remifentanil provide adequate conditions for SEP monitoring. ⋯ Rats demonstrate dose-dependent somatosensory evoked potential (SEP) suppression with propofol but not with remifentanil. However, SEP suppression by 50% occurred only at large (1.5 EC(50)) concentrations of propofol, and a measurable SEP was present in 8 of 18 rats, even at 10.8 EC(50).