Anesthesia and analgesia
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Anesthesia and analgesia · Dec 1998
Comparative StudyA comparison of pharyngeal mucosal pressure and airway sealing pressure with the laryngeal mask airway in anesthetized adult patients.
We measured pharyngeal mucosal pressures at six different locations on the laryngeal mask airway (LMA) and tested the hypothesis that the efficacy of the seal is not related to pharyngeal mucosal pressure. Twenty anesthetized, paralyzed adult patients were studied. Microchip sensors were attached to the size 5 LMA at locations corresponding to the lateral and posterior pharynx, the hypopharynx, the pyriform fossa, the base of tongue, and the oropharynx. Mucosal pressures and airway sealing pressures were recorded during inflation of the cuff from 0 to 40 mL in 10-mL increments. The highest mean mucosal pressure was in the oropharynx (26 cm H2O), and the lowest was in the posterior pharynx (2 cm H2O). Mucosal pressures increased with increasing intracuff pressure and cuff volume, but the rate of increase varied among locations. Airway sealing pressure increased with increasing intracuff volume from 0 to 10 mL (P < 0.0001) and 10 to 20 mL (P = 0.0001), was unchanged from 20 to 30 mL, and decreased from 30 to 40 mL (P = 0.005). The airway sealing pressure was higher than pharyngeal mucosal pressure until the intracuff volume was > or =30 mL. There was no correlation between mucosal pressures and airway sealing pressure at any location. We conclude that the efficacy of the seal is not related to pharyngeal mucosal pressure. Pharyngeal mucosal pressures are generally lower than those considered safe for the tracheal mucosa during prolonged intubation. ⋯ We measured pharyngeal mucosal pressures at six different locations on the laryngeal mask airway and showed that the efficacy of the seal is not related to pharyngeal mucosal pressure. Pharyngeal mucosal pressures are generally lower than those considered safe for the tracheal mucosa during prolonged intubation.
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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
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.