Anesthesia and analgesia
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Anesthesia and analgesia · Dec 1998
Nitrous oxide and carbon dioxide have no effect on the blood-gas solubilities of sevoflurane and isoflurane.
Nitrous oxide (N2O) has been shown to decrease the solubility (lambdaB:G) of volatile anesthetics in human blood and, consequently, affect their rate of uptake. If this is true, then carbon dioxide (CO2) may also have an effect, which is important because methods that measure the tension of volatile anesthetics in blood washout CO2 in the process. Blood samples were obtained from fasted, healthy volunteers and patients undergoing major surgery. Each sample was divided into two aliquots: one was equilibrated at 37 degrees C in a closed glass tonometer with a mixture of isoflurane 1% and sevoflurane 2% in a test gas mixture of either 50:50 N2O/O2 or 5:95 CO2/O2; the other aliquot was equilibrated with isoflurane and sevoflurane in O2 alone as a control. Using a two-stage headspace technique using gas chromatography, we measured the lambdaB:G of isoflurane and sevoflurane in the presence and absence of the test gas in each subject. There was no significant difference between the lambdaB:G of sevoflurane and isoflurane obtained from the N2O group and their controls or between the CO2 group and their controls. We conclude that neither N2O nor CO2 has an effect on the lambdaB:G of sevoflurane or isoflurane in the concentrations tested. ⋯ The blood solubilities of sevoflurane and isoflurane were measured with and without nitrous oxide and carbon dioxide. No differences were found. Nitrous oxide does not affect the kinetics of other anesthetics by altering their solubility. Carbon dioxide tensions need not be controlled when measuring anesthetic tensions in blood.
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Anesthesia and analgesia · Dec 1998
Characteristics of tolerance to somatic and visceral antinociception after continuous epidural infusion of morphine in rats.
A continuous epidural infusion of morphine may cause a complicated tolerance to develop, depending on the spinal and supraspinal sites. We designed this study to clarify the characteristics of the tolerance to somatic and visceral antinociception after epidural morphine infusion. Rats received epidural infusion of morphine at the rates of 50 or 100 microg kg(-1) h(-1), or isotonic sodium chloride solution for 7 days. The tail-flick (TF) test and colorectal distension (CD) test were used to measure the somatic and visceral antinociceptive effects, respectively. Nociceptive tests were performed on Days 1, 2, 3, 4, and 7. After 7 days, time-response curves after epidural morphine (10 microg) or intraperitoneal morphine (3 mg) challenge tests were conducted to assess the magnitude of tolerance. Epidurally infused morphine significantly increased percent maximal possible effects (%MPEs) (P < 0.05) in both the TF and CD tests, depending on the concentration of morphine. In the epidural morphine challenge test, increases in %MPEs were significantly attenuated (P < 0.05) in the morphine-infused group compared with the isotonic sodium chloride solution-infused group. The increases in %MPEs in the intraperitoneal challenge test were also attenuated in the morphine-infused group. We conclude that morphine tolerance to both somatic and visceral antinociception develops rapidly during epidural infusion and is based on the development of tolerance at the systemic, as well as the epidural, level. ⋯ A continuous epidural infusion of morphine rapidly induces tolerance to visceral and somatic antinociception in rats. This development is based on the development of tolerance at the systemic, as well as the epidural, level.
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Anesthesia and analgesia · Dec 1998
Hypoxemia decreases the shivering threshold in rabbits anesthetized with 0.2 minimum alveolar anesthetic concentration isoflurane.
Shivering has been proposed as an etiology of postoperative hypoxemia. The difficulty with this theory is that hypoxemia inhibits shivering in unanesthetized cats, rats, and humans. However, anesthesia inhibits many protective reflexes, including the ventilatory response to hypoxemia. We therefore tested the hypothesis that arterial hypoxemia fails to inhibit shivering in lightly anesthetized rabbits. Rabbits were intubated and instrumented during exposure to surgical concentrations of anesthesia, and anesthesia was then maintained with 0.2 minimum alveolar anesthetic concentration isoflurane. The core was cooled at a rate of 2-3 degrees C/h by perfusing water at 10 degrees C through a colonic thermode. Core temperatures were recorded from the distal esophagus. Sustained, vigorous shivering was considered physiologically significant. The core temperature that triggering significant shivering identified the thermoregulatory threshold for this response. Arterial blood was sampled for gas analysis at the shivering threshold in each rabbit. Hypoxemia linearly reduced the shivering threshold from 36.7 degrees C at 130 mm Hg to 35.4 degrees C at 50 mm Hg (threshold = PaO2.0.019 + 34.3; r2 = 0.49). We failed to confirm our hypothesis: instead, even mild hypoxemia reduced the shivering threshold >1 C. A 1 C decrease in the shivering threshold is likely to prevent or stop most postoperative shivering because it exceeds the reduction produced by many effective anti-shivering drugs. These data do not support the theory that shivering causes postoperative hypoxemia. ⋯ Shivering has been proposed as an etiology of postoperative hypoxemia. Our data, in contrast, show that mild hypoxemia inhibits shivering. Shivering is thus unlikely to be a cause of postoperative hypoxemia.
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Anesthesia and analgesia · Dec 1998
End-tidal sevoflurane concentration for tracheal extubation and skin incision in children.
We sought to determine minimum alveolar anesthetic concentrations for skin incision (MAC) and for tracheal extubation (MAC(Ex)) for sevoflurane and its associated awakening time and respiratory complications during emergence from sevoflurane anesthesia in children. We studied 40 (20 in each group) unpremedicated pediatric patients ranging in age from 2 to 8 yr. For MAC(Ex) determination, anesthetic induction, tracheal intubation, and maintenance of anesthesia were performed with sevoflurane and N2O in oxygen. However, N2O administration was discontinued at the end of surgery. The ratio of the predetermined end-tidal to inspiratory concentration was maintained at 0.95-1.00 for at least 15 min. The trachea was gently extubated, and smooth tracheal extubation was defined by the absence of gross purposeful muscular movements. In addition, the respiratory complications and awakening time were investigated during emergence from anesthesia for each patient. For MAC determination, anesthesia induction and tracheal intubation were performed with 5% sevoflurane in oxygen. After the predetermined end-tidal sevoflurane concentration had been established and maintained for at least 15 min, skin incision was attempted. Patients' responses to skin incision were described as "no movement" or "movements." The MAC or MAC(Ex) for sevoflurane was 2.22% +/- 0.13% (mean+/-SD) or 1.70%+/-0.12%, and the 95% effective dose (ED95) for smooth extubation was 1.87% (95% confidence limits 1.75%-2.62%), respectively, in children. During emergence from anesthesia, none of patients held their breath or experienced laryngospasm in the current study. One patient in a subgroup at 1.5% coughed before tracheal extubation. All 10 patients with smooth tracheal extubation had hemoglobin oxygen saturation levels of > or =98% in this study. Awakening time was 9.7+/-3.7 min in the subgroup that received 1.75% sevoflurane. In conclusion, the MAC(Ex) and ED95 values of sevoflurane were 1.64% and 1.87%, respectively, in children. The MAC(Ex) to MAC ratio for sevoflurane was 0.8 in children within the same age range and mean age. ⋯ Because tracheal extubation of anesthetized patients may be advantageous in certain clinical situations, we performed this study. The ratio minimum alveolar anesthetic concentrations for skin incision and for tracheal extubation for sevoflurane was 0.8 in children within the same age range and mean age. No patient in the current study had laryngospasm.
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Anesthesia and analgesia · Dec 1998
In vitro investigation of cerebrospinal fluid leakage after dural puncture with various spinal needles.
Postspinal headache is one of the most common complications of spinal anesthesia and has repeatedly led to controversy concerning needle size and configuration. In an in vitro investigation, we measured cerebrospinal fluid (CSF) leakage with Sprotte, Whitacre, Quincke, and Atraucan needles under physiological conditions in human dura. The puncture characteristics were examined under an electron microscope. The pencil-point needles show 2-3 times less leakage of CSF compared with the cutting Quincke needles of corresponding size. Between the Sprotte and the Whitacre needles, there were no significant differences. The least loss of CSF occurred with the 26-gauge Atraucan needle. Under the electron microscope, a sharply delineated, persistent perforation channel was shown with the Quincke needles, which may explain the high CSF loss. With pencil-point needles, which push the tissue apart bluntly, a large opening on the inside is found, with some tearing of the dura. However, in contrast to the cutting needles, a persistent perforation channel is not manifested. The 26-gauge Atraucan needle, which both cuts and pushes apart conically, shows a relatively discrete opening on the inside, with slight tears in the dura and arachnoidea but without a visible perforation channel. The results of our study show that larger needles (26-gauge Atraucan) that are easier to handle can lead to good and, in some cases even better, puncture results if they have characteristics of both the cutting and the pencil-point needles. ⋯ We compared several brands of pencil-point and standard cutting spinal needles of varying sizes. All pencil-point needles had less cerebrospinal fluid leakage, the least loss occurring with 26-gauge Atraucan needles. Electron microscopic examination of the dura after puncture showed characteristic findings with each needle type. We conclude that the combined cutting and pencil-point characteristics seen in the Atraucan needle may have clinical advantages.