Plasma levels of a methadone constant rate infusion and their corresponding effects on thermal and mechanical nociceptive thresholds in dogs

Peripheral warming for prevention of hypothermia in small dogs during soft tissue surgery: A randomized controlled trial

OBJECTIVE

To evaluate the effects of wrapping the extremities of small dogs with table leg covers to prevent hypothermia during anesthesia.

STUDY DESIGN

Randomized parallel-group study.

ANIMALS

A total of 60 adult dogs with a body mass <15 kg anesthetized for soft tissue surgery.

METHODS

Dogs were randomly assigned to one of two groups. The control group received routine intraoperative thermal support, while the limb-wrapping group had their peripheral limbs covered with table leg covers up to the mid-metacarpal/metatarsal region, in addition to routine thermal support. Rectal temperature during anesthesia was recorded and compared between the two groups. Data analyses were performed using Student's t-test for rectal temperature, Fisher's exact test for hypothermia incidence and analysis of covariance for the effect of limb-wrapping while taking other factors into account.

RESULTS

Thirty dogs were included per group. Rectal temperature did not differ between the groups at the time of intubation, but it was significantly higher in the limb-wrapping group (36.7 ± 1.0 °C) than in the control group (35.9 ± 0.8 °C) at the end of surgery (p = 0.003). The mean difference was 0.81 °C (95% confidence interval of mean difference 0.33-1.29 °C). The incidence of hypothermia (<37.0 °C) was significantly lower in the limb-wrapping group than in the control group (19/30 versus 28/30 dogs, respectively; p = 0.010).

CONCLUSIONS

For dogs with body masses <15 kg, limb-wrapping with table leg covers slowed the reduction in intraoperative rectal temperature. Limb-wrapping is inexpensive and easy to perform, making it a practical method for minimizing hypothermia during anesthesia in small dogs undergoing soft tissue surgery.

CLINICAL RELEVANCE

Peripheral warming with table leg covers has the potential to reduce hypothermia during soft tissue surgery in small dogs.

Pharmacokinetics of methadone after intravenous and subcutaneous administration in domestic ferrets (Mustela putorius furo)

OBJECTIVE

To determine the pharmacokinetic profile of methadone after intravenous (IV) and subcutaneous (SC) administration in domestic ferrets (Mustela putorius furo).

STUDY DESIGN

Crossover experimental study.

ANIMALS

A group of eight healthy adult ferrets weighing 1.01 ± 0.23 kg (mean ± standard deviation).

METHODS

Methadone hydrochloride (0.3 mg kg-1) was injected IV or SC to each ferret with a 3 week washout period. Blood samples were collected via a jugular catheter before and 5, 10, 15, 20, 30, 45, 60, 90, 120, 180, 240, 360 and 480 minutes after drug administration. Liquid chromatography-tandem mass spectrometry was used to determine plasma methadone concentrations. A nonlinear mixed effects model was used to analyze the data.

RESULTS

After IV injection, systemic clearance (Clss) and volume of distribution (Vdss) were 78.9 mL min-1 kg-1 and 9.8 L kg-1, respectively. Elimination half-life was 2.0 hours and SC bioavailability was fixed at 1. The maximum observed plasma concentration after SC injection was 92.1 ± 76.8 ng mL-1. Behavioral changes were observed after both routes.

CONCLUSIONS AND CLINICAL RELEVANCE

The pharmacokinetic profile of IV methadone was characterized by a high Clss and large Vdss, with high bioavailability and absorption rate after SC administration. Half-life was short and mean plasma methadone concentrations stayed above the minimum effective concentration (MEC) reported in humans only after SC administration for 5 minutes, but remained above that reported in dogs for 45 minutes following both routes. Further studies investigating the MEC and pharmacodynamics of methadone in ferrets are warranted.

Anesthesia update - Incorporating methadone into companion animal anesthesia and analgesic protocols: A narrative review

Opioid analgesics are routinely used during the perioperative period, to provide analgesia and reduce anesthetics doses required to maintain a surgical plane of anesthesia in companion animals. Acting on receptors in the brain, spinal cord, and peripheral nervous system, opioids provide reliable and consistent analgesia; however, they are not without adverse effects. Methadone, a mu agonist opioid analgesic, was recently licensed for veterinary use in Canada. In addition to its action on opioid receptors, methadone contributes to analgesia through other pathways, including inhibition of N-methyl-D-aspartate (NMDA) receptors. It has physiologic effects similar to other mu opioid agents, but fewer adverse gastrointestinal effects. This review discusses methadone's mechanism of action, pharmacologic characteristics, and clinical effects in dogs and cats. Current recommendations for using methadone in companion animals are also provided.

Cardiorespiratory Effects and Desflurane Requirement in Dogs Undergoing Ovariectomy after Administration Maropitant or Methadone

General anesthesia for ovariectomy in dogs is based on a balanced anesthesia protocol such as using analgesics along with an inhalant agent. While opioids such as fentanyl and methadone are commonly used for their analgesic potency, other drugs can also have analgesic effects. Maropitant, an antiemetic for dogs and cats, has also been shown to exert analgesic effects, especially on visceral pain. The aim of this study was to compare the cardiorespiratory effects and analgesic properties of maropitant and methadone combined with desflurane in dogs undergoing ovariectomy. Two groups of 20 healthy mixed-breeds bitches undergoing elective ovariectomy received intravenous either maropitant at antiemetic dose of 1 mg kg^-1 or methadone at the dose of 0.3 mg kg^-1. Cardiorespiratory variables were collected before premedication, 10 min after sedation and during surgery. Recovery quality and postoperative pain were evaluated 15, 30, 60, 120, 240 and 360 min postoperatively. Results showed that maropitant produced analgesia and reduced the requirement of desflurane in amounts similar to those determined by methadone (5.39 ± 0.20% and 4.91 ± 0.26%, respectively) without significant difference, while maintaining heart rate, arterial blood pressure, respiratory rate and carbon dioxide end-tidal partial pressure even at a more satisfactory level. Therefore, maropitant may be recommended as an analgesic drug for abdominal surgery not only in healthy dogs but also in those with reduced cardiorespiratory compensatory capacities or at risk of hypotension, especially when combined with a sedative such as dexmedetomidine.

Extradural anaesthesia-analgesia in dogs undergoing cholecystectomy: A single centre retrospective study

Objectives

To assess the effects of extradural anaesthesia-analgesia (EAA) in dogs undergoing cholecystectomy.

Materials and methods

Medical records of dogs undergoing cholecystectomy between 2011 and 2019 were retrieved and allocated to two groups depending if analgesia was provided systemically (group SA) or extradurally (EAA). Preoperative data, intraoperative antinociceptive medications, postoperative analgesia, perioperative complications, and food intake were compared.

Results

Overall 41 medical records were included in the study: 19 and 22 dogs were allocated to groups SA and EAA, respectively. In group EAA, an extradural catheter was placed preoperatively in 8 dogs; in the remaining, it was placed postoperatively but an extradural injection was performed preoperatively. The extradural catheter tip was between the 4th lumbar and the 10th thoracic vertebrae. Intraoperatively, nociception was more likely to occur in group SA [OR 55.42 (2.97–1,035.06)]. During the first 24 and 48 h postoperatively, more dogs in group SA required methadone [OR 24 (2.81–268.4) and OR 11.56 (2.37–45.06), respectively] and additional analgesic drugs [OR 25 (3.47–281.9) and OR 35.29 (1.86–668.2), respectively] compared to group EAA. Voluntary postoperative food intake was also significantly higher in group EAA.

Clinical significance

Compared to systemic analgesia, the use of extradural anaesthesia-analgesia reduced perioperative analgesic requirement and promoted postoperative food intake in dogs undergoing cholecystectomy.

Stability of dilutions of methadone alone, and in combination with lidocaine and ketamine

OBJECTIVES

To assess stability and degradation over time, of methadone alone, and mixed with lidocaine and ketamine, using various diluents and storage conditions.

MATERIALS AND METHODS

Solutions of methadone diluted in 0.9% NaCl, and methadone-lidocaine-ketamine diluted in 0.9% NaCl or Hartmann's solution, and stored at room temperature with exposure to light, or refrigerated at 4°C and protected from light, were maintained over 10 days. Chemical stability was determined using liquid chromatography immediately after preparation and following 4, 24, 48, 96 and 240 hours of storage. Physical stability of the solutions was evaluated by visual examination and absorbance of ultraviolet/visible light. A linear model assessed the impact of different diluent solutions and storage conditions on drug degradation over time.

RESULTS

There was no evidence of physicochemical incompatibility for any solution. Methadone concentration, when diluted alone or in methadone-lidocaine-ketamine with Hartmann's solution at 4°C, did not decline over time. Ketamine and lidocaine decreased to a similar extent over time, regardless of the diluent used or storage method, while methadone in methadone-lidocaine-ketamine diluted with 0.9% NaCl or with Hartmann's solution at room temperature exposed to light, also declined over time; however, all three methadone-lidocaine-ketamine components retained acceptable stability (<10% degradation) for at least 48 hours following preparation, irrespective of diluent or storage conditions.

CLINICAL SIGNIFICANCE

Regardless of the diluent or storage method, methadone-lidocaine-ketamine solutions degrade over time, but this only becomes clinically significant after 48 hours. Solutions of 1 mg/ml methadone in 0.9% NaCl are stable for at least 10 days under storage conditions likely to be encountered in general practice.