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MAC-awake of isoflurane, enflurane and halothane evaluated by slow and fast alveolar washout
D M Gaumann1, J P Mustaki, E Tassonyi
1Department of Anaesthesiology, University Hospital of Geneva, Switzerland.
Abstract:
End-tidal anaesthetic concentrations at first eye opening in response to a verbal command during recovery from anaesthesia (MAC-awake), were measured for isoflurane (n = 16), enflurane (n = 16) and halothane (n = 14). MAC-awake was measured during either slow or fast alveolar washout. Slow washout was obtained by decreasing anaesthetic concentrations in predetermined steps of 15 min, assuming equilibration between brain and alveolar partial pressures. Fast alveolar washout was obtained by discontinuation of the inhalation anaesthetic, which had been maintained at 1 MAC for at least 15 min. Mean MAC-awake obtained with slow alveolar washout was similar for isoflurane (0.25 (SD 0.03) MAC), and enflurane (0.27 (0.04) MAC) and significantly greater than values obtained by fast alveolar washout (isoflurane: 0.19 (0.03) MAC; enflurane: 0.20 (0.03) MAC). The MAC-awake of isoflurane and enflurane was significantly less than that of halothane, which was 0.59 (0.10) MAC as evaluated by the slow and 0.50 (0.05) MAC as evaluated by the fast alveolar washout method. Recovery time from anaesthesia with fast alveolar washout was 8.8 (4.0) min for halothane, which was not different from isoflurane (15 (2.5) min), but significantly shorter than for enflurane (22 (10) min), reflecting differences in the anaesthetic concentration gradient between MAC and MAC-awake values. These data do not support the hypothesis of a uniform ratio between MAC and MAC-awake values.
Insights
The minimum alveolar concentration for awakening (MAC-awake) varies between isoflurane, enflurane, and halothane, depending on the alveolar washout method used during anesthetic recovery.
Area of Science:
- Anesthesiology
- Pharmacology
Background:
- The minimum alveolar concentration (MAC) is a standard measure for anesthetic potency.
- Understanding MAC-awake is crucial for predicting and managing anesthetic recovery times.
- Previous research has explored the relationship between MAC and MAC-awake, but findings on their ratio remain inconsistent.
Purpose of the Study:
- To compare the MAC-awake values for isoflurane, enflurane, and halothane.
- To investigate the influence of slow versus fast alveolar washout methods on MAC-awake measurements.
- To determine if a uniform ratio exists between MAC and MAC-awake for these agents.
Main Methods:
- MAC-awake was measured in patients recovering from anesthesia with isoflurane, enflurane, or halothane.
- Two alveolar washout methods were employed: slow (gradual decrease) and fast (discontinuation of anesthetic).
- End-tidal anesthetic concentrations at eye opening to verbal command were recorded.
Main Results:
- Mean MAC-awake was significantly higher with slow alveolar washout compared to fast washout for isoflurane and enflurane.
- Halothane exhibited a significantly higher MAC-awake than isoflurane and enflurane, irrespective of the washout method.
- Recovery times varied, with halothane showing shorter recovery than enflurane, correlating with anesthetic concentration gradients.
Conclusions:
- The ratio between MAC and MAC-awake is not uniform across isoflurane, enflurane, and halothane.
- Alveolar washout methodology significantly impacts MAC-awake values.
- These findings have implications for predicting anesthetic emergence and recovery durations.