Anesthesiology
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During combined epidural/isoflurane anesthesia, the core temperature triggering finger-tip vasoconstriction is approximately 1 degree C less than that triggering redilation. This hysteresis suggests that thermoregulatory responses are not dependent entirely on current thermal status (state-dependence), but may be influenced also by the system's recent thermal history (direction-dependence). Once triggered, the gain and maximum response intensity of many thermoregulatory responses is nearly normal during isoflurane anesthesia. However, it remains unknown whether preserved gain and maximum response intensities are a characteristic paradigm describing thermoregulatory responses to general anesthetics. Also unknown is whether the sweating and pre-capillary vasodilation thresholds are comparably impaired by different volatile anesthetics. Accordingly, the authors tested the hypotheses that, during one minimum alveolar concentration of enflurane anesthesia: (1) there is a direction-dependent hysteresis for sweating; (2) the sweating and active vasodilation thresholds increase approximately 1.2 degrees C, as they do during one minimum alveolar concentration of isoflurane; and (3) the gain and maximum intensity of sweating are well preserved. ⋯ One minimum alveolar concentration of enflurane increased the sweating threshold only slightly more than previously reported for isoflurane. As in previous studies of sweating and vasoconstriction during isoflurane anesthesia, gain and maximum response intensity were well preserved during enflurane anesthesia. An increase in the interthreshold range (temperatures not triggering thermoregulatory responses), with little change in gain and maximum response intensities, appears to be the typical effect of volatile anesthetics. Sweating during enflurane anesthesia appears to be state-dependent and little influenced by the direction of core temperature perturbations.
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Comparative Study
Delayed onset of malignant hyperthermia induced by isoflurane and desflurane compared with halothane in susceptible swine.
Desflurane (difluoromethyl 1-fluoro 2,2,2-trifluoroethyl ether) is a new inhalational anesthetic currently under investigation for use in humans. Recently, the authors showed that desflurane is a trigger of malignant hyperthermia (MH) in susceptible swine. To date, there has been no in vivo comparison of the relative ability of inhalational anesthetics to trigger MH. The effects of desflurane, isoflurane, and halothane on six MH-susceptible purebred and six MH-susceptible mixed-bred Pietrain swine were examined. ⋯ Although all three volatile anesthetics triggered MH, exposure to halothane resulted in significantly shorter times to MH triggering when compared with desflurane and isoflurane.
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Hypothermia develops rapidly during the 1st h of anesthesia and results in part from evaporative heat loss during surgical skin preparation. The authors tested the hypothesis that evaporation of skin preparation solution contributes significantly to hypothermia. ⋯ Heat loss was significantly less with water-based than with alcohol-based solutions. Though heating the solutions and radiant warming decreased heat loss, such loss under each tested condition, even per square meter of washed surface, was small compared to other causes of perioperative hypothermia. Consequently, the authors recommend that efforts to maintain intraoperative normothermia be directed elsewhere.
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Although lung volume may be changed by certain procedures during anesthesia and mechanical ventilation, dependence of the dynamic mechanical properties of the lungs on lung volume are not clear. Based on studies in dogs, the authors hypothesized that changes in lung mechanics caused by anesthesia in healthy humans could be accounted for by immediate changes in lung volume and that lung resistance will not be decreased by positive end-expiratory airway pressure if tidal volume and respiratory frequency are in the normal ranges. ⋯ Increases in dynamic lung elastance and lung resistance with anesthesia can be explained by the accompanying, acute decreases in lung volume, although other factors may be involved. Increasing lung volume by increasing mean airway pressure with positive end-expiratory pressure will decrease lung resistance only if the original lung volume is low compared to awake, seated functional residual capacity.
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Although propofol and alfentanil are given in combination in clinical practice, the pharmacodynamic interaction between these drugs has not been described. ⋯ The alfentanil requirements in ASA physical status 1 female patients undergoing lower abdominal surgery are less when given as a supplement to propofol (4 micrograms/ml) compared to 66% N2O.