Volatile anesthetics depress spinal motor neurons.
BACKGROUND
Depression of spinal alpha-motor neurons apparently plays a role in the surgical immobility induced by isoflurane. Using the noninvasive technique of F-wave analysis, the authors tested the hypothesis that depressed motor neuron excitability is an effect common to other clinically relevant inhaled anesthetics.


METHODS
The authors measured F-wave amplitude in rats anesthetized with desflurane, enflurane, halothane, or sevoflurane. Each animal received one anesthetic at five equipotent anesthetic concentrations (0.6, 0.8, 1.2, and 1.6 minimum alveolar concentration [MAC] and 0.8 MAC with 65% N2O). F waves were detected as late potentials in electromyographic responses evoked in the intrinsic muscles of the hind paw after monopolar stimulation of the ipsilateral posterior tibial nerve.


RESULTS
All tested inhaled anesthetics depressed F-wave amplitude but not M-wave (orthodromic, early muscle activation) amplitude, and increased M-F latency in a dose-dependent manner. At 1.0 MAC, the estimated F/M ratio was 70 +/- 13% SD of that at baseline (0.6 MAC). Nitrous oxide added to 0.8 MAC of the potent vapors depressed F/M ratio by 63 +/- 17%.


CONCLUSIONS
All anesthetics tested appeared to depress the excitability of spinal motor neurons. This effect may contribute to surgical immobility, and its magnitude is comparable at equipotent concentrations of agents. The authors hypothesize that this effect is due to hyperpolarization, although, currently, there is insufficient information to discriminate between pre- and postsynaptic mechanisms.
Rampil
I J
IJ
Department of Anesthesia, University of California, San Francisco 94143-0648, USA. ira rampil@vaxine ucsf.edu or http://ira-mac.ucsf.edu
King
B S
BS
D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D009046:Motor Neurons / Q000187:drug effects* / Q000502:physiology; D009609:Nitrous Oxide / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D013116:Spinal Cord / Q000187:drug effects* / Q000502:physiology
D018685:Anesthetics, Inhalation; D009609:Nitrous Oxide