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  • Author or Editor: Luciano Spadavecchia x
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Abstract

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.

Full access
in American Journal of Veterinary Research

Abstract

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)

Full access
in American Journal of Veterinary Research

Abstract

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 horses.

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 increased.

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)

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in American Journal of Veterinary Research

Abstract

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)

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in American Journal of Veterinary Research

Abstract

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 frequencies.

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)

Full access
in American Journal of Veterinary Research