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Isoflurane minimum alveolar concentration decreases during anesthesia and surgery
S Petersen-Felix1, A M Zbinden, M Fischer
1Institute for Anesthesiology and Intensive Care, University Hospital, Bern, Switzerland.
Background:
It generally is assumed that the potency of inhalational anesthetics remains unchanged during the course of the administration of an anesthetic. Only one study has indicated a decrease of minimum alveolar concentration with time. In this study, an effect of the duration of anesthesia administration and surgery on the potency of isoflurane was investigated by determining MACtetanus (the minimum alveolar concentration that prevents movement in response to electrical tetanic stimulation in 50% of patients) before and after surgery.
Methods:
Ten patients who underwent removal of a herniated intervertebral disc were anesthetized with isoflurane only. Reaction to a standardized electrical stimulation applied to the forearm was observed and was graded as movement or no-movement. The isoflurane concentration was increased in steps of 0.10 vol% if the patient moved and decreased in steps of 0.10 vol% if no reaction was seen, until a "movement/no-movement/movement" or "no-movement/movement/no-movement" pattern, respectively, was achieved.
Results:
MACtetanus decreased in all patients from 1.28 +/- 0.22 vol% (mean +/- SD) before surgery to 1.04 +/- 0.22 vol% after surgery (P < 0.01). When the prestimulation arterial blood pressure or the maximal increase in blood pressure caused by stimulation at the individual MACtetanus before surgery were compared to the corresponding values at the individual MACtetanus after surgery, no significant difference could be found. The prestimulation heart rate and the maximal increase in heart rate were significantly lower after surgery, even though the end-tidal isoflurane concentration was 0.24 vol% lower at the individual MACtetanus after surgery.
Conclusion:
The authors conclude that MACtetanus decreases during the administration of anesthesia and the performance of surgery.
Insights
The potency of isoflurane anesthesia, measured as MACtetanus, decreased significantly during surgery. This indicates that anesthetic requirements change over time, contrary to common assumptions.
Area of Science:
- Anesthesiology
- Pharmacology
Background:
- The potency of inhalational anesthetics is generally assumed to be constant during administration.
- Limited research suggests a potential decrease in minimum alveolar concentration (MAC) over time.
Purpose of the Study:
- To investigate the effect of anesthesia duration and surgical procedures on isoflurane potency.
- To determine MACtetanus before and after surgery.
Main Methods:
- Ten patients undergoing spinal surgery were anesthetized solely with isoflurane.
- MACtetanus was determined by observing patient response to electrical stimulation, adjusting isoflurane concentration incrementally.
- Stimulation response was categorized as movement or no-movement.
Main Results:
- MACtetanus decreased significantly from 1.28 +/- 0.22 vol% before surgery to 1.04 +/- 0.22 vol% after surgery (P < 0.01).
- No significant differences were observed in arterial blood pressure responses before and after surgery.
- Prestimulation and maximal heart rates were significantly lower post-surgery, despite lower end-tidal isoflurane concentrations.
Conclusions:
- MACtetanus diminishes during the course of anesthesia administration and surgical intervention.
- Anesthetic potency is not constant and can decrease over time during procedures.