Objective—To investigate effects of isoflurane at approximately the minimum alveolar concentration (MAC) on the nociceptive withdrawal reflex (NWR) of the forelimb of ponies as a method for quantifying anesthetic potency.
Animals—7 healthy adult Shetland ponies.
Procedure—Individual MAC (iMAC) for isoflurane was determined for each pony. Then, effects of isoflurane administered at 0.85, 0.95, and 1.05 iMAC on the NWR were assessed. At each concentration, the NWR threshold was defined electromyographically for the common digital extensor and deltoid muscles by stimulating the digital nerve; additional electrical stimulations (3, 5, 10, 20, 30, and 40 mA) were delivered, and the evoked activity was recorded and analyzed. After the end of anesthesia, the NWR threshold was assessed in standing ponies.
Results—Mean ± SD MAC of isoflurane was 1.0 ± 0.2%. The NWR thresholds for both muscles increased significantly in a concentration-dependent manner during anesthesia, whereas they decreased in awake ponies. Significantly higher thresholds were found for the deltoid muscle, compared with thresholds for the common digital extensor muscle, in anesthetized ponies. At each iMAC tested, amplitudes of the reflex responses from both muscles increased as stimulus intensities increased from 3 to 40 mA. A concentration-dependent depression of evoked reflexes with reduction in slopes of the stimulus-response functions was detected.
Conclusions and Clinical Relevance—Anesthetic-induced changes in sensory-motor processing in ponies anesthetized with isoflurane at concentrations of approximately 1.0 MAC can be detected by assessment of NWR. This method will permit comparison of effects of inhaled anesthetics or anesthetic combinations on spinal processing in equids.
Objectives—To evoke and measure the nociceptive
withdrawal reflex (NWR) by use of electromyographic
recordings and to score the behavioral nociceptive
responses to electrical pulses in standing nonsedated
Animals—10 adult horses.
Procedure—The lateral palmar digital nerve of the forelimb
was transcutaneously stimulated, and surface
electromyographic responses were recorded from the
ulnaris lateralis, extensor carpi radialis, and common
digital extensor muscles. Stimuli consisted of a 25-millisecond
train of 5 constant-current pulses delivered by
a computer-controlled stimulator. The 80- to 250-milliseconds
poststimulation interval was analyzed to
detect the NWR. The current intensity was increased in
steps of 0.5 mA until the NWR threshold intensity (It)
was reached. The stimulus at It was repeated twice.
Latency and amplitude of the NWR, together with the
behavioral reaction of horses, were analyzed. The latter
was scored according to a scale from 0 (no reaction) to
5 (vigorous reaction). Finally, 3 suprathreshold stimuli at
1.2 × It were analyzed.
Results—The median It to elicit NWR was 2.5 mA.
Median onset latency of the NWR was 96.0 milliseconds
at It and 89.6 milliseconds for suprathreshold
stimuli. The amplitude of the reflexes was higher for
suprathreshold stimulations, and behavioral reactions
were slightly stronger when stimulus intensity
Conclusions and Clinical Relevance—Results of our
study indicate that it is possible to record NWR in conscious
standing horses, to define a reflex threshold,
and to measure reflexes in response to increasing
stimulus intensity. (Am J Vet Res 2002;63:1551–1556)