The influence of a continuous rate infusion of dexmedetomidine on the nociceptive withdrawal reflex and temporal summation during isoflurane anaesthesia in dogs

Dexmedetomidine Sedation in Dogs: Impact on Electroencephalography, Behavior, Analgesia, and Antagonism with Atipamezole

This study aimed to assess the impact of dexmedetomidine constant rate infusion (CRI) on key parameters in dogs. Six dogs received a 60 µg/kg/h dexmedetomidine infusion over 10 min, followed by three 15 min decremental CRIs (3, 2, and 1 µg/kg/h). A subsequent reversal phase employed 600 µg/kg/h atipamezole over 5 min. Continuous electroencephalogram (EEG) assessment, and cardiorespiratory and analgesia monitoring (every 3 min) were conducted, including analgesia evaluation through responses to electric stimulation. Dexmedetomidine induced profound sedation, evidenced by lateral recumbency and immobility. Patient State Index (PSI) decreased from awake (90.4 ± 4.3) to Phase 1 (50.9 ± 30.7), maintaining sedation (29.0 ± 18.1 to 33.1 ± 19.1 in Phases 2-4). Bradycardia (37.8 ± 3.5 bpm, lowest at Phase 3) and hypertension (133.7 ± 17.0 mmHg, highest at Phase 1) were observed, with minimal analgesia. Atipamezole promptly reversed sedation, restoring cognitive function (tail wagging behavior), and normalizing cardiovascular parameters. During atipamezole CRI, the EEG exhibited a transition from delta waves to alpha and low beta waves. This transition was observed alongside gradual increases in PSI and electromyographic activities. Additionally, spindle activities disappeared during this process. This study's results suggest potential clinical utility for EEG-guided dexmedetomidine sedation with reversal using atipamezole, warranting further investigation.

Inhibition and facilitation of nociceptively evoked muscular activity by fentanyl or dexmedetomidine in isoflurane-anaesthetized pigs

OBJECTIVE

To investigate motor and cardiovascular responses to dexmedetomidine or fentanyl in isoflurane-anaesthetized pigs.

STUDY DESIGN

Experimental, balanced, block randomized, two-group design.

ANIMALS

A group of 16 crossbred pigs, 55 ± 8 days (mean ± standard deviation) old.

METHODS

Deltoid electromyography (EMG) was recorded during isoflurane anaesthesia. Electrical stimulation using 5, 10, 20 and 40 mA of the distal right thoracic limb elicited a nociceptive withdrawal reflex (NWR), quantified by the area under the curve (AUC) for the simulation intensity versus EMG amplitude response curve. Latency to movement evoked by clamping a claw for maximum 60 seconds was noted. Arterial blood pressure and pulse rate were recorded. Data were sampled at baseline and during dexmedetomidine 0.25, 0.5, 1.0, 2.0, 4.0 and 8.0 μg kg^-1 hour^-1 or fentanyl 5, 10, 20, 40, 80 and 160 μg kg^-1 hour^-1 infusions. The influence of infusion rate on NWR AUC and spontaneous EMG was analysed using a mixed model, with p < 5%.

RESULTS

NWR AUC increased at fentanyl 5 μg kg^-1 hour^-1 but decreased at fentanyl 40, 80 and 160 μg kg^-1 hour^-1 and dexmedetomidine 4.0 and 8.0 μg kg^-1 hour^-1. All pigs at fentanyl 80 μg kg^-1 hour^-1, and three pigs at dexmedetomidine 8.0 μg kg^-1 hour^-1 had mechanical latencies greater than 60 seconds. Spontaneous EMG activity increased accompanied by visually evident 'shivering' at fentanyl 5, 10 and 20 μg kg^-1 hour^-1 but decreased at dexmedetomidine 2, 4 and 8 μg kg^-1 hour^-1. Clinically relevant effects of increasing infusion rates on blood pressure or pulse rate were not observed.

CONCLUSION AND CLINICAL RELEVANCE

If anaesthetic plane or antinociception is evaluated in pigs, response to claw clamping and NWR will not necessarily give uniform results when comparing drugs. If only one method is used, results should be interpreted cautiously.

Effect of Medetomidine, Dexmedetomidine, and Their Reversal with Atipamezole on the Nociceptive Withdrawal Reflex in Beagles

The objectives were: (1) to compare the antinociceptive activity of dexmedetomidine and medetomidine, and (2) to investigate its modulation by atipamezole. This prospective, randomized, blinded experimental trial was carried out on eight beagles. During the first session, dogs received either medetomidine (MED) (0.02 mg kg-1 intravenously (IV)] or dexmedetomidine (DEX) [0.01 mg kg-1 IV), followed by either atipamezole (ATI) (0.1 mg kg-1) or an equivalent volume of saline (SAL) administered intramuscularly 45 min later. The opposite treatments were administered in a second session 10-14 days later. The nociceptive withdrawal reflex (NWR) threshold was determined using a continuous tracking approach. Sedation was scored (0 to 21) every 10 min. Both drugs (MED and DEX) increased the NWR thresholds significantly up to 5.0 (3.7-5.9) and 4.4 (3.9-4.8) times the baseline (p = 0.547), at seven (3-11) and six (4-9) minutes (p = 0.938), respectively. Sedation scores were not different between MED and DEX during the first 45 min (15 (12-17), p = 0.67). Atipamezole antagonized sedation within 25 (15-25) minutes (p = 0.008) and antinociception within five (3-6) minutes (p = 0.008). Following atipamezole, additional analgesics may be needed to maintain pain relief.

Alternatives to Opioid Analgesia in Small Animal Anesthesia: Alpha-2 Agonists

Alpha-2 agonists have potent analgesic effects, in addition to their sedative actions. Alpha-2 agonists provide analgesia through any of several routes of administration, including parenteral, oral, epidural or intrathecal and intraarticular, because of spinal and supraspinal actions. Systemic doses are short acting, whereas local administration at the site of action result in longer analgesic effects. The potent cardiovascular and respiratory effects of alpha-2 agonists should be considered when used as analgesics.

Polymelia in a chimeric Simmental calf: nociceptive withdrawal reflex, anaesthetic and analgesic management, anatomic and genetic analysis

BACKGROUND

Polymelia is a congenital defect characterized by one or more supernumerary legs. The genetics and aetiology of this condition in cattle have not yet been thoroughly investigated even though several case reports do exist. The model of the nociceptive withdrawal reflex (NWR) has been characterized in several species to study spinal nociceptive processing. It is a polysynaptic spinal reflex that can be elicited by noxious electrical stimulation and recorded by electromyography. Thorough nociceptive examination and preventive analgesic management has not yet been an aspect in the perioperative management of polymelia cases.

CASE PRESENTATION

A 4-month-old female Simmental calf was presented with notomelia. The animal was in good health and showed no neurologic deficiencies. Preoperatively, computed tomography was performed to gain more detailed anatomical information. To evaluate the sensitivity of the accessory limb, NWR testing was performed and revealed a connection of the afferent reflex pathway of the accessory limb to the efferent of the normal limb. The accessory limb was surgically removed under general anaesthesia. Intensive care included multimodal pain therapy adapted to the pain intensity scored during regular pain assessment. A gross anatomical dissection as well as a genetic analysis of the accessory limb were performed postoperatively. The calf was identified as a chimera.

CONCLUSION

This calf was successfully relieved of its accessory limb. Chimerism has not been described in the congenital defect polymelia. As the accessory limb was pain sensitive and a common nociceptive reflex pathway was identified, thorough perioperative pain management was performed with the intention to prevent chronic neuropathic pain development.

Electrophysiological characterisation of central sensitisation in canine spontaneous osteoarthritis

In man, central sensitisation (CS) contributes to the pain of osteoarthritis (OA). Dogs with spontaneous OA may also exhibit CS. Electrophysiological reflex measurements are more objective than behavioural assessments and can be used to evaluate CS in preclinical and clinical studies. It was hypothesised that dogs suffering from OA would exhibit electrophysiological characteristics indicative of CS, associated with reduced diffuse noxious inhibitory controls (DNICs). One hundred and seventeen client-owned dogs were recruited to the study. Hind limb nociceptive withdrawal reflex thresholds, stimulus response, and temporal summation characteristics were recorded, during alfaxalone anaesthesia, from 46 OA dogs, 29 OA dogs receiving nonsteroidal anti-inflammatory drugs (OANSAIDs), and 27 breed- and weight-matched control dogs. Efficacy of DNIC was evaluated in 12 control and 11 of the OA dogs, by application of a mechanical conditioning stimulus to the contralateral forelimb. Nociceptive withdrawal reflex thresholds were higher in OA compared with control dogs (P = 0.02). Stimulus response characteristics demonstrated an augmented response in OANSAID dogs compared with OA (P < 0.001) and control (P < 0.001) dogs. Temporal summation demonstrated exaggerated C-fibre-mediated responses in both OA (P < 0.001) and OANSAID (P = 0.005) groups, compared with control animals. Conditioning stimulus application resulted in inhibition of test reflex responses in both OA and control animals (P < 0.001); control animals demonstrated greater inhibition compared with OA (P = 0.0499). These data provide evidence of neurophysiological changes consistent with CS in dogs with spontaneous OA and demonstrate that canine OA is associated with reduced DNIC.

Dexmedetomidine combined with etomidate or emulsified isoflurane for induction reduced cardiopulmonary response in dogs

To investigate the effects of etomidate, emulsified isoflurane, and their combination with dexmedetomidine on physiological parameters, electrocardiogram (ECG) results, and the quality of induction and recovery during isoflurane maintenance anaesthesia. 5 mixed-breed dogs received each of four treatments: etomidate (E group); emulsified isoflurane (EI group); both dexmedetomidine and etomidate (DE group); or both dexmedetomidine and emulsified isoflurane (DEI group). All drugs were IV injection administered for induction, followed by 1.5 MAC (minimal alveolar concentration) of isoflurane to maintain anaesthesia. Rectal temperature (RT), respiratory rate (RR), heart rate (HR), mean arterial pressure (MAP), and ECG were measured at baseline, 0, 5, 10, 20, 40, and 60 minutes after intubation. The quality of induction and recovery was evaluated for all dogs. All the anaesthetic procedures provided good conditions for induction of anaesthesia. The quality of induction and recovery in the E group was worse than other groups. The decrease of RR in the E and DE groups was stronger than that in the EI and DEI groups. The dogs in the E group had the most significant prolongation of the Q-T interval and changes in the S-T segment. Deviation and extension of the S-T segment were noted in the El group. The dogs in the DE and DEI groups had fewer changes in the ECG results than those in the E and EI groups. The addition of dexmedetomidine caused less effect on cardiopulmonary parameters and the ECG results than either etomidate or emulsified isoflurane alone. Thus, etomidate or emulsified isoflurane in combination with dexmedetomidine may be useful clinically for the induction of anaesthesia.

Effect of dexmedetomidine on the minimum infusion rate of propofol preventing movement in dogs

OBJECTIVE

To determine the effect of dexmedetomidine on induction dose and minimum infusion rate of propofol preventing movement (MIR_NM).

STUDY DESIGN

Randomized crossover, unmasked, experimental design.

ANIMALS

Three male and three female healthy Beagle dogs weighing 10.2 ± 2.8 kg.

METHODS

Dogs were studied on three occasions at weekly intervals. Premedications were 0.9% saline (treatment P) or dexmedetomidine (1 μg kg^-1, treatment PLD; 2 μg kg^-1, treatment PHD) intravenously. Anesthesia was induced with propofol (2 mg kg^-1 and then 1 mg kg^-1 every 15 seconds) until intubation. Anesthesia was maintained for 90 minutes in P with propofol (0.5 mg kg^-1 minute^-1) and saline, in PLD with propofol (0.35 mg kg^-1 minute^-1) and dexmedetomidine (1 μg kg^-1 hour^-1), and in PHD with propofol (0.3 mg kg^-1 minute^-1) and dexmedetomidine (2 μg kg^-1 hour^-1). The stimulus (50 V, 50 Hz, 10 ms) was applied to the antebrachium, and propofol infusion was increased or decreased by 0.025 mg kg^-1 minute^-1 based on a positive or negative response, respectively. Data were analyzed using a mixed-model anova and presented as mean ± standard error.

RESULTS

Propofol induction doses were 8.68 ± 0.57 (P), 6.13 ± 0.67 (PLD) and 4.78 ± 0.39 (PHD) mg kg^-1 and differed among treatments (p < 0.05). Propofol MIR_NM values were 0.68 ± 0.13, 0.49 ± 0.16 and 0.26 ± 0.05 mg kg^-1 minute^-1 for P, PLD and PHD, respectively. Propofol MIR_NM decreased 59% in PHD (p < 0.05). Plasma propofol concentrations were 14.04 ± 2.30 (P), 11.30 ± 4.30 (PLD) and 7.96 ± 0.72 (PHD) μg mL^-1 and dexmedetomidine concentrations were 0.68 ± 0.12 (PLD) and 0.89 ± 0.08 (PHD) ng mL^-1 at MIR_NM determination.

CONCLUSIONS AND CLINICAL RELEVANCE

Dexmedetomidine (1 and 2 μg kg^-1) decreased propofol induction dose. Dexmedetomidine (2 μg kg^-1 hour^-1) resulted in a significant decrease in propofol MIR_NM.

Alfaxalone Anaesthesia Facilitates Electrophysiological Recordings of Nociceptive Withdrawal Reflexes in Dogs (Canis familiaris)

Naturally occurring canine osteoarthritis represents a welfare issue for affected dogs (Canis familiaris), but is also considered very similar to human osteoarthritis and has therefore been proposed as a model of disease in humans. Central sensitisation is recognized in human osteoarthritis sufferers but identification in dogs is challenging. Electromyographic measurement of responses to nociceptive stimulation represents a potential means of investigating alterations in central nociceptive processing, and has been evaluated in conscious experimental dogs, but is likely to be aversive. Development of a suitable anaesthetic protocol in experimental dogs, which facilitated electrophysiological nociceptive withdrawal reflex assessment, may increase the acceptability of using the technique in owned dogs with naturally occurring osteoarthritis. Seven purpose bred male hound dogs underwent electromyographic recording sessions in each of three states: acepromazine sedation, alfaxalone sedation, and alfaxalone anaesthesia. Electromyographic responses to escalating mechanical and electrical, and repeated electrical, stimuli were recorded. Subsequently the integral of both early and late rectified responses was calculated. Natural logarithms of the integral values were analysed within and between the three states using multi level modeling. Alfaxalone increased nociceptive thresholds and decreased the magnitude of recorded responses, but characteristics of increasing responses with increasing stimulus magnitude were preserved. Behavioural signs of anxiety were noted in two out of seven dogs during recordings in the acepromazine sedated state. There were few significant differences in response magnitude or nociceptive threshold between the two alfaxalone states. Following acepromazine premedication, induction of anaesthesia with 1–2 mg kg^-1 alfaxalone, followed by a continuous rate infusion in the range 0.075–0.1 mg kg^-1 min^-1 produced suitable conditions to enable assessment of spinal nociceptive processing in dogs, without subjecting them to potentially aversive experiences. This methodology may be appropriate for obtaining electrophysiological nociceptive withdrawal reflex data in client-owned dogs with naturally occurring osteoarthritis.

Postoperative analgesic effects of either a constant rate infusion of fentanyl, lidocaine, ketamine, dexmedetomidine, or the combination lidocaine-ketamine-dexmedetomidine after ovariohysterectomy in dogs

OBJECTIVE

To evaluate the postoperative analgesic effects of a constant rate infusion (CRI) of either fentanyl (FENT), lidocaine (LIDO), ketamine (KET), dexmedetomidine (DEX), or the combination lidocaine-ketamine-dexmedetomidine (LKD) in dogs.

STUDY DESIGN

Randomized, prospective, blinded, clinical study.

ANIMALS

Fifty-four dogs.

METHODS

Anesthesia was induced with propofol and maintained with isoflurane. Treatments were intravenous (IV) administration of a bolus at start of anesthesia, followed by an IV CRI until the end of anesthesia, then a CRI at a decreased dose for a further 4 hours: CONTROL/BUT (butorphanol 0.4 mg kg(-1), infusion rate of saline 0.9% 2 mLkg(-1) hour(-1)); FENT (5 μg kg(-1), 10 μg kg(-1) hour(-1), then 2.5 μg kg(-1) hour(-1)); KET (1 mgkg(-1) , 40 μg kg(-1) minute(-1), then 10 μg kg(-1) minute(-1) ; LIDO (2 mg kg(-1), 100 μg kg(-1) minute(-1), then 25 μg kg(-1) minute(-1)); DEX (1 μgkg(-1), 3 μg kg(-1) hour(-1), then 1 μg kg(-1) hour(-1)); or a combination of LKD at the aforementioned doses. Postoperative analgesia was evaluated using the Glasgow composite pain scale, University of Melbourne pain scale, and numerical rating scale. Rescue analgesia was morphine and carprofen. Data were analyzed using Friedman or Kruskal-Wallis test with appropriate post-hoc testing (p < 0.05).

RESULTS

Animals requiring rescue analgesia included CONTROL/BUT (n = 8), KET (n = 3), DEX (n = 2), and LIDO (n = 2); significantly higher in CONTROL/BUT than other groups. No dogs in LKD and FENT groups received rescue analgesia. CONTROL/BUT pain scores were significantly higher at 1 hour than FENT, DEX and LKD, but not than KET or LIDO. Fentanyl and LKD sedation scores were higher than CONTROL/BUT at 1 hour.

CONCLUSIONS AND CLINICAL RELEVANCE

LKD and FENT resulted in adequate postoperative analgesia. LIDO, CONTROL/BUT, KET and DEX may not be effective for treatment of postoperative pain in dogs undergoing ovariohysterectomy.

Cardiorespiratory and anaesthetic effects of two continuous rate infusions of dexmedetomidine in alfaxalone anaesthetized dogs

Six Beagles were used in this prospective randomised crossover experimental study. Dexmedetomidine was administered at 0, 1 or 2 μg/kg IV for group C, LDA and HDA, respectively. Animals were induced and maintained with alfaxalone at 0.07 mg/kg/min with a CRI dexmedetomidine dose of 0, 0.5 or 1 μg/kg/h for group C, LDA and HDA, respectively. Cardiorespiratory variables, arterial blood gases and depth of anaesthesia were recorded. The recovery times and quality of recovery were scored. Group HDA produced a greater increase in the depth of anaesthesia than LDA. However, with both protocols, CI was halved compared to normal values in dogs. The use of oxygen before and during the anaesthetic maintenance is advisable, mainly if dexmedetomidine is going to be use as a pre-medicant and maintenance agent. The quality of recovery was better in groups receiving dexmedetomidine, without causing an increase in recovery time.

Assessment of the wind-up phenomenon in the equine nociceptive trigeminal system

Repeated sub-threshold nociceptive electrical stimulation resulting in temporal summation of the limb nociceptive withdrawal reflex is a well-established non-invasive model to investigate the wind-up phenomenon in horses. Due to structural similarities of the trigeminal sensory nucleus to the dorsal horn of the spinal cord, temporal summation should be evoked by repeated transcutaneous electrical stimulation of trigeminal afferents. To evaluate this hypothesis repeated transcutaneous electrical stimulation was applied to the supraorbital and infraorbital nerves of 10 horses. Stimulation intensities varied between 0.5 and 1.3 times the trigemino-cervical reflex threshold defined for single stimulation. Evoked electromyographic activity of the orbicularis oculi, splenius and cleidomastoideus muscles was recorded and the signals analysed in the previously established epochs typical to the early and late component of the blink reflex and to the trigemino-cervical reflex. Behavioural reactions were evaluated with the aid of numerical rating scale. The nociceptive late component and the trigemino-cervical reflex were not elicited by sub-threshold intensity repeated transcutaneous electrical stimulation. Furthermore, the median reflex amplitude for the 10 horses showed a tendency to decline over the stimulation train so temporal summation of afferent trigeminal inputs could not be observed. Therefore, the modulation of trigeminal nociceptive processing attributable to repeated Aδ fibre stimulations seems to differ from spinal processing of similar inputs as it seems to have an inhibitory rather than facilitatory effect. Further evaluation is necessary to highlight the underlying mechanism.