Anesthesiology
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Fifteen patients undergoing thoracotomy were given 0.25 or 0.50 mg morphine intrathecally (L2-L3 or L3-L4) for an analgetic and pharmacokinetic study. Administration of morphine at the end of the operation resulted in a highly variable duration of analgesia ranging from 1-20.5 and 1-40 h for the 0.25 and 0.50 mg groups, respectively. Calculation of cumulative consumption pattern of additional analgesics given im indicated a dose-related analgesia lasting around 12 h. ⋯ For the 0.50 and 0.25 mg groups, the terminal elimination half-life in CSF was 175 +/- 9 min and 196 +/- 13 min, respectively: the volume of CSF distribution was 0.88 +/- 0.16 ml X kg-1 and 1.06 +/- 0.17 ml X kg-1, respectively: and the clearance from CSF was 2.81 +/- 0.41 microliter X kg-1 X min-1 and 3.41 +/- 0.55 microliter X kg-1 X min-1, respectively (means +/- SEM). The study indicates that the significant pharmacokinetic parameter related to the long duration of analgesia after intrathecal morphine administration probably is the high CSF concentrations found, since the rate of elimination from CSF is similar to what is reported for morphine in plasma. Furthermore, modulation of nociceptive input in the thoracic region also may be achieved by lumbar administration, but a slower onset should be anticipated.
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Comparative Study
Effects of halothane, enflurane, and isoflurane on hypoxic pulmonary vasoconstriction in rat lungs in vitro.
Rat lungs were ventilated and perfused at a constant rate in vitro. The maximal hypoxic pulmonary vasoconstrictor (HPV) response was recorded by measuring the pulmonary artery pressure change when the inspired oxygen concentration was changed from 21% to 3% (with 5.5% carbon dioxide) in the absence of anesthetic vapor. In different experimental groups, the effects of halothane, enflurane, and isoflurane on HPV were examined. ⋯ All three agents depressed HPV in a dose-related manner. The concentrations in MAC units at which 50% depression of HPV (ED50) occurred was 0.47, 0.60, and 0.56 for halothane, isoflurane, and enflurane, respectively, and neither the ED50 values nor the slopes of these dose response curves were significantly different. It was concluded that these halogenated general anesthetics inhibit HPV with essentially the same potency.
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The results of erroneous filling of agent-specific anesthetic vaporizers were studied. The fraction of gas flow through the vaporizer was calculated for three vaporizers set to deliver essentially equipotent final concentrations: halothane, 1% (1.25 MAC); enflurane, 2% (1.19 MAC); and isoflurane, 1.5% (1.30 MAC). These fractional flows, at 22 degrees C, were 0.0188 for 1% halothane, 0.0615 for 2% enflurane, and 0.0295 for 1.5% isoflurane. ⋯ Enflurane and isoflurane could not be separated satisfactorily by gas chromatography. Halothane, when mixed with enflurane or isoflurane, enhanced vaporization of each agent, as well as being somewhat more easily vaporized itself. Halothane, enflurane, and isoflurane do not form ideal solutions when mixed and the resultant vapor concentrations of each of two agents when mixed may be far from those predicted by an assumption of ideality.
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The exact role of calcium in nerve conduction in neurons that have been blocked by local anesthetics remains controversial. Recently, attention has been drawn to the importance of examining both frequency-dependent and nonfrequency-dependent conduction block, since it is felt that frequency-dependent block provides a model that more closely approximates the normal physiologic state. The present study was designed to examine the effects of calcium on both the nonfrequency-dependent and frequency-dependent components of lidocaine nerve block. ⋯ Compound action potentials were measured, and both frequency-dependent block and nonfrequency-dependent block were compared in each solution. Low calcium concentrations significantly enhanced both nonfrequency- and frequency-dependent lidocaine block. The effect of low concentrations of calcium was greater at higher frequencies of stimulation.