Anesthesiology
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Comparative Study
Anesthetic potencies of isoflurane, halothane, and diethyl ether for various end points of anesthesia.
In experiments with rats, the authors compared potency ratios and slopes of dose-effect curves of isoflurane, halothane, and diethyl ether for three end points of anesthesia: loss of righting reflex (RR), abolition of purposeful movement (PM) response to painful stimuli, and abolition of heart rate (HR) response to painful stimuli. Determinations of potency were based on the direct measurement of brain concentrations of anesthetics with the use of gas chromatography. It was found that the ratio of the PM ED50 to RR ED50 was 2.41 for isoflurane, 1.74 for halothane, and 1.25 for diethyl ester. ⋯ The ratios of HR ED50 to PM ED50 were not significantly different for the studied agents and there were no differences found between the slopes of the dose-effect curves for PM and HR. The results suggest that heart rate response to a noxious stimuli in contrast to the righting reflex is depressed by inhalation anesthetics through a mechanism similar to that underlying the depression of purposeful movement response to a noxious stimuli. Heart rate response to a noxious stimuli might be used as an alternative index for the measurement of anesthetic potency.
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The characteristics of the neuromuscular blockade produced by prolonged succinylcholine infusion were compared in 40 patients anesthetized with either nitrous-oxide-isoflurane (0.75-1.50% inspired) or nitrous-oxide-fentanyl. Neuromuscular transmission was monitored using train-of-four stimulation and the infusion rate was adjusted to keep the first twitch at 10-15% of its control value. Initially, all patients exhibited a depolarizing-type block, and the infusion rates were similar in the isoflurane (61 micrograms . kg-1 . min-1) and fentanyl (57 micrograms . kg-1 . min-1) groups. ⋯ The block in all 13 patients (eight with isoflurane, five with fentanyl) who did not recover spontaneously was antagonized successfully with atropine and neostigmine. It was concluded that with succinylcholine infusion of 90 min or less, isoflurane accelerates the onset of tachyphylaxis and phase II neuromuscular block without affecting succinylcholine requirements. These results, with isoflurane, were similar to those reported previously with enflurane or halothane.
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Biography Historical Article
The Gill-Merrill expedition: penultimate chapter in the curare story.