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Abstract

Objective—To compare the ability of 2 partial IV anesthesia (PIVA) techniques to maintain anesthesia, compared with isoflurane alone, in horses.

Animals—45 horses.

Procedures—Client-owned horses requiring general anesthesia for a variety of procedures of at least 1 hour's duration were randomly allocated to 3 groups (n = 15/group) that differed for the maintenance protocol. Anesthesia was maintained with isoflurane with a starting end-tidal isoflurane concentration of 1.3% (isoflurane group) or a concentration of 1% supplemented with an adjustable continuous infusion of guaifenesin-ketamine (IGK group) or romifidine-ketamine (IRK group). A predefined scoring system was used to assess anesthetic depth and to adjust anesthetic delivery. The need for rescue anesthetics and recovery quality were compared.

Results—A mean ± SD end-tidal isoflurane concentration of 1.36 ± 0.16% was necessary to maintain a surgical plane of anesthesia in the isoflurane group. Mean infusion rates of 5.0 ± 1.3 μL/kg/min and 5.1 ± 0.8 μL/kg/min were necessary to maintain a surgical plane of anesthesia in the IRK and IGK groups, respectively. A lower need for ketamine as a rescue anesthetic was observed in the IGK group, compared with the isoflurane group. Higher blood pressure and lower heart rates were found at selected time points for the IRK group, compared with the IGK and isoflurane groups.

Conclusions and Clinical Relevance—Both PIVA protocols were satisfactory to maintain smooth and stable surgical anesthesia in horses. The present study supports previous findings in which PIVA has isoflurane-sparing effects. Furthermore, PIVA did not impair recovery quality.

Full access
in American Journal of Veterinary Research

Abstract

Case Description—A 7-month-old male Siberian Husky affected by lower motor neuron tetraparesis was anesthetized for electrodiagnostic testing and collection of muscle and nerve biopsy specimens.

Clinical Findings—Preanesthetic physical examination revealed a high rectal temperature, and serum biochemical analysis revealed high muscle and liver enzyme activities. The dog was anesthetized twice. The dog was anesthetized with isoflurane and developed moderate hypercarbia and mild hyperthermia. Injectable anesthetic agents were used to anesthetize the dog the second time, during which the dog developed severe malignant hyperthermia. A genetic test performed after anesthesia did not reveal a mutation of the RYR1 gene, the gene that mediates calcium-release channels in skeletal muscle. On the basis of clinical features, and because other neuromuscular disorders were ruled out, a genetic channelopathy involving the skeletal muscle ion channels was suspected.

Treatment and Outcome—The dog was disconnected from the breathing system, and active cooling of the body was performed with ice packs applied to the body surface and alcohol applied to the foot pads. Cold crystalloid solutions were administered IV. Intermittent positive-pressure ventilation with 100% oxygen was performed to decrease end-tidal partial pressure of carbon dioxide. Because dantrolene was not available, acepromazine was administered to facilitate a decrease in body temperature. The dog recovered from malignant hyperthermia and was discharged to the owner after 13 days of hospitalization.

Clinical Relevance—Dogs affected by genetic muscle disorders should be considered at risk for perianesthetic malignant hyperthermia, even in the absence of an RYR1 gene mutation.

Restricted access
in Journal of the American Veterinary Medical Association

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)

Full access
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)

Full access
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

Abstract

Objective—To investigate the feasibility of evoking the nociceptive withdrawal reflex (NWR) from foreand hind limbs in conscious dogs, score stimulusassociated behavioral responses, and assess the canine NWR response to suprathreshold stimulations.

Animals—8 adult Beagles.

Procedure—Surface electromyograms evoked by transcutaneous electrical stimulation of ulnaris and digital plantar nerves were recorded from the deltoideus, cleidobrachialis, biceps femoris, and tibialis cranialis muscles. Train-of-five pulses (stimulustrain) were used; reflex threshold (It train) was determined, and recruitment curves were obtained at 1.2, 1.5, and 2 × It train. Additionally, a single pulse (stimulussingle) was given at 1, 1.2, 1.5, 2, and 3 × It train. Latency and amplitude of NWRs were analyzed. Severity of behavioral reactions was subjectively scored.

Results—Fore- and hind limb It train values (median; 25% to 75% interquartile range) were 2.5 mA (2.0 to 3.6 mA) and 2.1 mA (1.7 to 2.9 mA), respectively. At It train, NWR latencies in the deltoideus, cleidobrachialis, biceps femoris, and cranial tibialis muscles were not significantly different (19.6 milliseconds [17.1 to 20.5 milliseconds], 19.5 milliseconds [18.1 to 20.7 milliseconds], 20.5 milliseconds [14.7 to 26.4 milliseconds], and 24.4 milliseconds [17.1 to 40.5 milliseconds], respectively). Latencies obtained with stimulustrain and stimulussingle were similar. With increasing stimulation intensities, NWR amplitude increased and correlated positively with behavioral scores.

Conclusions and Clinical Relevance—In dogs, the NWR can be evoked from limbs and correlates with behavioral reactions. Results suggest that NWR evaluation may enable quantification of nociceptive system excitability and efficacy of analgesics in individual dogs.

Full access
in American Journal of Veterinary Research

Abstract

Objective—To investigate the facilitation of the nociceptive withdrawal reflex (NWR) by repeated electrical stimuli and the associated behavioral response scores in conscious, nonmedicated dogs as a measure of temporal summation and analyze the influence of stimulus intensity and frequency on temporal summation responses.

Animals—8 adult Beagles.

Procedures—Surface electromyographic responses evoked by transcutaneous constant-current electrical stimulation of ulnaris and digital plantar nerves were recorded from the deltoideus, cleidobrachialis, biceps femoris, and cranial tibial muscles. A repeated stimulus was given at 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, and 1.1 × It (the individual NWR threshold intensity) at 2, 5, and 20 Hz. Threshold intensity and relative amplitude and latency of the reflex were analyzed for each stimulus configuration. Behavioral reactions were subjectively scored.

Results—Repeated sub-It stimuli summated and facilitated the NWR. To elicit temporal summation, significantly lower intensities were needed for the hind limb, compared with the forelimb. Stimulus frequency did not influence temporal summation, whereas increasing intensity resulted in significantly stronger electromyographic responses and nociception (determined via behavioral response scoring) among the dogs.

Conclusions and Clinical Relevance—In dogs, it is possible to elicit nociceptive temporal summation that correlates with behavioral reactions. These data suggest that this experimental technique can be used to evaluate nociceptive system excitability and efficacy of analgesics in canids.

Full access
in American Journal of Veterinary Research

Abstract

Objective—To evaluate pharmacokinetics of ketamine and norketamine enantiomers after constant rate infusion (CRI) of a subanesthetic dose of racemic ketamine or S-ketamine in ponies.

Animals—Five 6-year-old Shetland pony geldings that weighed between 101 and 152 kg.

Procedures—In a crossover study, each pony received a CRI of racemic ketamine (loading dose, 0.6 mg/kg; CRI, 0.02 mg/kg/min) and S-ketamine (loading dose, 0.3 mg/kg; CRI, 0.01 mg/kg/min), with a 1-month interval between treatments. Arterial blood samples were collected before and at 5, 15, 30, 45, and 60 minutes during drug administration and at 5, 10, 30, and 60 minutes after discontinuing the CRI. Plasma ketamine and norketamine enantiomers were quantified by use of capillary electrophoresis. Individual R-ketamine and S-ketamine concentration-versus-time curves were analyzed by use of a monocompartmental model. Plasma disposition curves for R-norketamine and S-norketamine were described by estimating the area under the concentration-versus-time curve (AUC), maximum concentration (Cmax), and time until Cmax.

Results—Plasma concentrations of S-ketamine decreased and biodegradation products increased more rapidly after S-ketamine CRI, compared with results after racemic ketamine CRI. The R-norketamine was eliminated faster than was the S-norketamine. Significant differences between treatments were found for the AUC of S-ketamine and within the racemic ketamine CRI for the AUC and Cmax of norketamine isomers.

Conclusions and Clinical Relevance—CRI of S-ketamine may be preferable over CRI of racemic ketamine in standing equids because the S-enantiomer was eliminated faster when infused alone instead of as part of a racemic mixture.

Full access
in American Journal of Veterinary Research