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.
Objective—To compare nociceptive withdrawal
reflexes (NWRs) evoked from the distal aspect of the
left forelimb and hind limb in conscious standing horses
and to investigate NWR recruitment for graded
electrical stimulation intensities.
Animals—20 adult horses.
Procedure—Surface electromyographic (EMG) activity
evoked by transcutaneous electrical stimulation of
the digital palmar (or plantar) nerve was recorded from
the common digital extensor and cranial tibial muscles.
Stimuli consisted of 25-millisecond train-of-5 constant
current pulses. Current intensity was gradually
increased until NWR threshold intensity was reached.
The EMG signal was analyzed for quantification of the
NWR. Behavioral responses accompanying the reflex
were scored (scale, 0 to 5). The NWR recruitment
curves were determined at 0.9, 1.1, 1.2, and 1.3 times
the NWR threshold intensity.
Results—The NWR threshold was significantly higher
for the hind limb (median value, 6.6 mA; range, 3 to
10 mA) than the forelimb (median, 3 mA; range, 1.7 to
5.5 mA). The NWR of the hind limb had a significantly
longer latency (median, 122.8 milliseconds; range,
106 to 172 milliseconds), compared with the forelimb
(median, 98 milliseconds; range, 86 to 137 milliseconds),
and it was associated with significantly
stronger behavioral reactions. Gradual increase of
NWR amplitude was evident at increasing stimulation
intensities and supported by the behavioral observations.
Conclusions and Clinical Relevance—We documented
NWRs evoked from the forelimb and hind
limb and their recruitment with stimuli of increasing
intensity in horses. These results provide a basis for
use of NWRs in studies on nociceptive modulation in
horses. (Am J Vet Res 2003;64:700–707)
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)
Objective—To investigate the action of a single IV
administration of romifidine on the thresholds of the
nociceptive withdrawal reflex (NWR) and temporal
summation in conscious horses.
Animals—10 adult horses.
Procedure—Single electrical stimulations were
applied on the digital nerves to evoke NWR from the
left forelimb and hind limb. Repeated electrical stimulations
(10 stimuli, 5 Hz) were given to obtain temporal
summation. Surface electromyographic reflex
activity was recorded from the common digital extensor
and cranial tibial muscles. After baseline assessment
of NWR and temporal summation thresholds,
romifidine (80 µg·kg−1, IV) was administered.
Successive determinations of NWR and temporal
summation thresholds were performed 5, 25, and 55
minutes after administration.
Results—Romifidine significantly increased the current
intensities necessary to evoke NWR and temporal
summation in forelimbs and hind limbs of horses.
Values were significantly higher than baseline values
55 minutes after romifidine administration. After
administration of romifidine, a facilitation of reflex
components of tactile origin was observed when
repeated stimulations were applied.
Conclusions and Clinical Relevance—Results confirm
antinociceptive activity of romifidine and may
represent an objective demonstration of the wellknown
hypersensitivity to tactile stimuli observed in
horses receiving α2-adrenoreceptor agonists in clinical
practice. Romifidine can be included in analgesic
and anesthetic protocols to provide additional analgesia
in horses. (Am J Vet Res 2005;66:1992–1998)
Objective—To investigate whether facilitation of the
nociceptive withdrawal reflex (NWR) can be evoked
and quantified as a measure of temporal summation
from the distal aspect of the left forelimb and hind
limb in standing nonsedated horses via repeated
stimulations of various subthreshold intensities and
Animals—10 adult horses.
Procedure—Surface electromyographic activity
evoked by stimulation of the digital palmar and plantar
nerves was recorded from the common digital
extensor and cranial tibial muscles. For each horse,
the NWR threshold intensity to a single stimulus was
determined for the forelimb and hind limb. Repeated
stimulations were performed at subthreshold intensities
and at frequencies of 2, 5, and 10 Hz. The reflex
amplitude was quantified, and the behavioral responses
accompanying the stimulations were scored.
Results—Repeated stimulations at subthreshold
intensities were able to summate and facilitate the
NWR in conscious horses. The reflex facilitation was
significantly related to the intensity of the repeated
stimuli, whereas no effect of stimulation frequency
was found. Reaction scores increased significantly for
increasing stimulation intensities.
Conclusions and Clinical Relevance—Temporal summation
obtained by repeated stimulations of subthreshold
intensity appears to represent a new tool for
investigating nociceptive pathophysiologic processes
in horses; this experimental model may be useful to
examine the mode of action and efficacy of analgesic
and anesthetic interventions and possibly to assess
sensory dysfunction in clinical settings. (Am J Vet Res 2004;65:901–908)