Pharmacokinetics and preliminary observations of behavioral changes following administration of midazolam to dogs

ACVIM Consensus Statement on the management of status epilepticus and cluster seizures in dogs and cats

BACKGROUND

Seizure emergencies (ie, status epilepticus [SE] and cluster seizures [CS]), are common challenging disorders with complex pathophysiology, rapidly progressive drug-resistant and self-sustaining character, and high morbidity and mortality. Current treatment approaches are characterized by considerable variations, but official guidelines are lacking.

OBJECTIVES

To establish evidence-based guidelines and an agreement among board-certified specialists for the appropriate management of SE and CS in dogs and cats.

ANIMALS

None.

MATERIALS AND METHODS

A panel of 5 specialists was formed to assess and summarize evidence in the peer-reviewed literature with the aim to establish consensus clinical recommendations. Evidence from veterinary pharmacokinetic studies, basic research, and human medicine also was used to support the panel's recommendations, especially for the interventions where veterinary clinical evidence was lacking.

RESULTS

The majority of the evidence was on the first-line management (ie, benzodiazepines and their various administration routes) in both species. Overall, there was less evidence available on the management of emergency seizure disorders in cats in contrast to dogs. Most recommendations made by the panel were supported by a combination of a moderate level of veterinary clinical evidence and pharmacokinetic data as well as studies in humans and basic research studies.

CONCLUSIONS AND CLINICAL RELEVANCE

Successful management of seizure emergencies should include an early, rapid, and stage-based treatment approach consisting of interventions with moderate to preferably high ACVIM recommendations; management of complications and underlying causes related to seizure emergencies should accompany antiseizure medications.

Heart rate, arterial pressure and propofol-sparing effects of guaifenesin in dogs

OBJECTIVE

To evaluate the heart rate (HR) and systemic arterial pressure (sAP) effects, and propofol induction dose requirements in healthy dogs administered propofol with or without guaifenesin for the induction of anesthesia.

STUDY DESIGN

Prospective blinded crossover experimental study.

ANIMALS

A total of 10 healthy adult female Beagle dogs.

METHODS

Dogs were premedicated with intravenous (IV) butorphanol (0.4 mg kg^-1) and administered guaifenesin 5% at 50 mg kg^-1 (treatment G50), 100 mg kg^-1 (treatment G100) or saline (treatment saline) IV prior to anesthetic induction with propofol. HR, invasive sAP and respiratory rate (f_R) were recorded after butorphanol administration, after guaifenesin administration and after propofol and endotracheal intubation. Propofol doses for intubation were recorded. Repeated measures analysis of variance (anova) was used to determine differences in propofol dose requirements among treatments, and differences in cardiopulmonary values over time and among treatments with p < 0.05 considered statistically significant.

RESULTS

Propofol doses (mean ± standard deviation) for treatments saline, G50 and G100 were 3.3 ± 1.0, 2.7 ± 0.7 and 2.1 ± 0.8 mg kg^-1, respectively. Propofol administered was significantly lower in treatment G100 than in treatment saline (p = 0.04). In treatments G50 and G100, HR increased following induction of anesthesia and intubation compared with baseline measurements. HR was higher in treatment G100 than in treatments G50 and saline following induction of anesthesia. In all treatments, sAP decreased following intubation compared with baseline values. There were no significant differences in sAP among treatments. f_R was lower following intubation than baseline and post co-induction values and did not differ significantly among treatments.

CONCLUSIONS AND CLINICAL RELEVANCE

When administered as a co-induction agent in dogs, guaifenesin reduced propofol requirements for tracheal intubation. HR increased and sAP and f_R decreased, but mean values remained clinically acceptable.

Effects of Cisatracurium in Sevoflurane and Propofol Requirements in Dog-Undergoing-Mastectomy Surgery

The purpose of the present study was to test whether the addition of cisatracurium in combination with propofol and sevoflurane would result in a change in doses of used anesthetic drugs. Ten dogs (Group A) undergoing elective unilateral mastectomy surgery were included in the study. To induce and maintain anesthesia, subjects received propofol and sevoflurane at varying doses; analgesia was performed with remifentanil. After three months, the same subjects (Group B) underwent contralateral mastectomy and received the same anesthetic protocol with the addition of cisatracurium at a dosage of 0.2 mg/kg−1. The following parameters were monitored during anesthesia: heart rate, systolic blood pressure, end-tidal CO2, oxygen saturation, halogenate requirement, and rectal temperature at baseline (T0), induction (T1), 5 (T5), 10 (T10), 15 (T15), 20 (T20), 25 (T25), 30 (T30), and 35 (T35) time points. In Group A, halogenate requirement was reduced at all the time points other than T1 (p < 0.001); in Group B, the percentage of halogenate requirement was already reduced at T1 and remained constant during the experimental period, showing no significant intragroup differences. The dose requirements of sevoflurane and propofol varied significantly between the two groups, with significantly lower dosages in the Group B (the cisatracurium-treated group). Moreover, patients treated with cisatracurium showed a stable anesthetic plan. The nondepolarizing-muscle-relaxant cisatracurium besylate could be considered a useful adjunct to anesthetic protocols.

Comparison of Certain Intrarectal versus Intramuscular Pharmacodynamic Effects of Ketamine, Dexmedetomidine and Midazolam in Cats

The aim of this clinical trial was to evaluate the impacts of administration via the intrarectal route (IR) in cats on their heart and respiratory rates, blood pressure, body temperature, and sedation quality compared to the intramuscular route (IM). The intramuscular group (IMG) received 0.003 mg kg−1 dexmedetomidine, 2 mg kg−1 ketamine, and 0.2 mg kg−1 midazolam while the intrarectal group (IRG) protocol was 0.003 mg kg−1 dexmedetomidine, 4 mg kg−1 ketamine, and 0.4 mg kg−1 midazolam. Cardiorespiratory values, temperature, and sedation score were measured 2 min after administration and then every 5 min up to the 40th minute. Cats belonging to IRG reacted less strongly to the drug, as opposed to those receiving intramuscular administration (2/10 in IRG vs. 8/10 in IMG). Average time between drug administration and standing position was 44.9 ± 5.79 in IRG and 57 ± 9.88 min in IMG. In IRG, maintenance of SpO₂ values is >95% at each time point. Median and range peak of sedation {7 (5)} in IMG occurs at 20th, 25th, and 30th minutes post drug administration while was lower in IRG. Cardiorespiratory values were slightly lower in IMG than in IRG, but always constant in both treatments. Temperature did not differ between groups. At this dosage, although sedation score was higher in IMG, intrarectal route could be efficacious for performing minimally invasive clinical and diagnostic procedures in cats.

EVALUATION OF THE EFFECTS OF MIDAZOLAM AND FLUMAZENIL IN THE BALL PYTHON (PYTHON REGIUS)

The study objective was to evaluate the sedative, muscle relaxant, and cardiorespiratory effects of midazolam and flumazenil in the ball python (Python regius). Ten healthy adult female ball pythons were used in a randomized and blinded crossover trial evaluating the effects of two dosages (1 and 2 mg/kg intramuscular [i.m.] in the cranial third of the body). In a subsequent open trial, nine ball pythons received 1 mg/kg i.m. of midazolam followed by 0.08 mg/kg i.m. of flumazenil 60 min later. Heart rate, respiratory rate, temperature, and the level of sedation and muscle relaxation (using a semiobjective scoring system) were evaluated. There were no significant differences between midazolam dosages for any of the parameters evaluated. Sedation scores were significantly increased compared with baseline from 15 min (1 mg/kg) and 10 min (2 mg/kg) postinjection up until 56 hr (1 mg/kg) and 72 hr (2 mg/kg) postinjection. Peak effect was reached 60 min postinjection, with 60% of snakes (6/10) being unable to right themselves. One snake developed paradoxical excitation with the 2 mg/kg dosage. Heart rates were significantly lower than baseline from 30 min to 128 hr postinjection with both midazolam dosages. Respiratory rates were significantly lower than baseline at four time points, with the highest dosage only: 15, 45, 60 min, and 8 hr postinjection. Flumazenil resulted in reversal of sedation and muscle relaxation in all snakes within 10 min of administration. However, resedation was evident in all snakes 3 hr after reversal. Midazolam administered at 1 and 2 mg/kg i.m. provides a moderate to profound, although prolonged, sedation and muscle relaxation in ball pythons. Flumazenil reverses the effects of midazolam in ball pythons, but its duration of action at the evaluated dosage is much shorter than midazolam, leading to resedation.

Evaluation of alfaxalone and midazolam with or without flumazenil reversal in Egyptian fruit bats (Rousettus aegyptiacus)

OBJECTIVES

To evaluate alfaxalone-midazolam anesthesia in Egyptian fruit bats (Rousettus aegyptiacus) and the effect of flumazenil administration on recovery time and quality.

STUDY DESIGN

Randomized, blinded, crossover and controlled, experimental trial.

ANIMALS

A total of 10 male Egyptian fruit bats.

METHODS

Bats were anesthetized with alfaxalone (15 mg kg^-1) and midazolam (2 mg kg^-1) administered subcutaneously. During anesthesia, vital signs, muscle tone and reflexes were monitored every 10 minutes. Flumazenil (0.3 mg kg^-1) or saline at an equal volume was administered subcutaneously 60 minutes after anesthetic administration. Time to induction, time to first movement and recovery time (flying) were measured. Quality of induction, anesthesia and recovery were assessed on a 1-3 scale (1, poor; 2, good; 3, excellent).

RESULTS

Time to induction was 4.2 ± 1.9 minutes (mean ± standard deviation), with median quality score of 2 (range, 1-3). Anesthesia quality score was 3 (1-3). During anesthesia, heart rate and respiratory frequency decreased significantly and penis relaxation, indicating muscle tone, increased significantly. Administration of flumazenil significantly reduced mean recovery time compared with saline (10 ± 5 versus 45 ± 17 minutes, respectively), and significantly improved the quality of recovery [2.5 (2-3) versus 1 (1-2), respectively].

CONCLUSIONS AND CLINICAL RELEVANCE

Alfaxalone-midazolam anesthesia resulted in good induction, muscle relaxation and sufficient anesthesia to perform routine diagnostic and therapeutic procedures for approximately 40 minutes. Reversal of midazolam with flumazenil is recommended, resulting in quicker and better recovery.

Pharmacokinetics and efficacy of trazodone following rectal administration of a single dose to healthy dogs

OBJECTIVE

To determine the pharmacokinetics and efficacy of trazodone following rectal administration of a single dose to healthy dogs.

ANIMALS

6 healthy adult dogs.

PROCEDURES

Each dog received a single dose of trazodone (approx 8 mg/kg) per rectum. Trazodone tablets were crushed into a powder, mixed with 5 mL of tap water, and injected into the rectum via a red rubber catheter. Sedation scores were assigned, and blood samples were collected for determination of plasma trazodone concentration at predetermined times before and after drug administration. Pharmacokinetic parameters were estimated by noncompartmental analysis.

RESULTS

Plasma trazodone concentration remained below the detection limit for 1 dog even though it became moderately sedate. Median (interquartile [25th to 75th percentile] range [IQR]) maximum plasma trazodone concentration and volume of distribution and clearance corrected for bioavailability were 1.00 μg/mL (0.66 to 1.40 μg/mL), 10.3 L/kg (7.37 to 14.4 L/kg), and 639 mL/kg/h (594 to 719 mL/kg/h), respectively. Median time to maximum plasma trazodone concentration and elimination half-life were 15 minutes (range, 15 to 30 minutes) and 12 hours (IQR, 7.99 to 12.7 hours), respectively. All dogs became mildly or moderately sedate, and the extent of sedation was maximal at a median of 30 minutes (IQR, 30 to 60 minutes) after trazodone administration. No adverse effects were observed.

CONCLUSIONS AND CLINICAL RELEVANCE

Rectal administration of trazodone may be a viable option for sedation and treatment of anxiety in dogs for which administration of sedatives and anxiolytics by other routes is contraindicated. Further research is necessary to better elucidate the pharmacokinetics and efficacy of trazodone following rectal administration and determine optimal dosing.

First dose in neonates: pharmacokinetic bridging study from juvenile mice to neonates for drugs metabolized by CYP3A

First dose prediction is challenging in neonates. Our objective in this proof-of-concept study was to perform a pharmacokinetic (PK) bridging study from juvenile mice to neonates for drugs metabolized by CYP3A. We selected midazolam and clindamycin as model drugs. We developed juvenile mice population PK models using NONMEM. The PK parameters of these two drugs in juvenile mice were used to bridge PK parameters in neonates using different correction methods. The bridging results were evaluated by the fold-error of 0.5- to 1.5-fold. Simple allometry with and without a correction factor for maximum lifespan potential could be used for a bridging of clearance (CL) and volume of distribution (V_d), respectively, from juvenile mice to neonates. Simulation results demonstrated that for midazolam, 100% of clinical studies for which both the predictive CL and V_d were within 0.5- to 1.5-fold of the observed. For clindamycin, 75% and 100% of clinical studies for which the predictive CL and V_d were within 0.5- to 1.5-fold of the observed. A PK bridging of drugs metabolized by CYP3A is feasible from juvenile mice to neonates. It could be a complement to the ADE and PBPK models to support the first dose in neonates.

Effect of Midazolam on Vestibular Signs in Two Geriatric Dogs with Vestibular Disease

An abrupt balance impairment, including leaning, falling, and rolling, occurred after IV administration of 0.2 mg/kg midazolam as a preanesthetic medication in two geriatric dogs with a history of nystagmus and head tilt. In the second case, leaning, falling, and rolling recurred after recovery from general anesthesia but gradually ceased after IV administration of 0.01 mg/kg flumazenil. These two cases suggest that the IV administration of midazolam was responsible for the balance impairment in dogs who were suspected to have idiopathic peripheral vestibular disease.

Sedative and cardiorespiratory effects of intramuscular administration of alfaxalone and butorphanol combined with acepromazine, midazolam, or dexmedetomidine in dogs

OBJECTIVE

To evaluate the sedative and cardiorespiratory effects of IM administration of alfaxalone and butorphanol combined with acepromazine, midazolam, or dexmedetomidine in dogs.

ANIMALS

6 young healthy mixed-breed hounds.

PROCEDURES

Dogs received each of 3 treatments (alfaxalone [2 mg/kg] and butorphanol [0.4 mg/kg] combined with acepromazine [0.02 mg/kg; AB-ace], midazolam [0.2 mg/kg; AB-mid], or dexmedetomidine [0.005 mg/kg; AB-dex], IM) in a blinded, randomized crossover-design study with a 1-week washout period between treatments. Sedation scores and cardiorespiratory variables were recorded at predetermined time points. Data were analyzed by use of mixed-model ANOVA and linear generalized estimating equations with post hoc adjustments.

RESULTS

All treatments resulted in moderate to deep sedation (median score, ≥ 15/21) ≤ 5 minutes after injection. Sedation scores did not differ among treatments until the 40-minute time point, when the score was higher for AB-dex than for other treatments. Administration of AB-dex resulted in median scores reflecting deep sedation until 130 minutes, versus 80 and 60 minutes for AB-ace and AB-mid, respectively, after injection. Heart rate, cardiac output, and oxygen delivery decreased significantly after AB-dex, but not AB-ace or AB-mid administration. Respiratory variables remained within clinically acceptable ranges after all treatments. Undesirable recovery characteristics were observed in 4 dogs after AB-mid treatment. Four dogs required atipamezole administration 180 minutes after AB-dex injection.

CONCLUSIONS AND CLINICAL RELEVANCE

All protocols produced reliable sedation. The results indicated that in young, healthy dogs, AB-mid may produce undesirable recovery characteristics; AB-dex treatment caused cardiovascular depression and should be used with caution.

Effects of ketamine or midazolam continuous rate infusions on alfaxalone total intravenous anaesthesia requirements and recovery quality in healthy dogs: a randomized clinical trial

OBJECTIVE

To determine the alfaxalone dose reduction during total intravenous anaesthesia (TIVA) when combined with ketamine or midazolam constant rate infusions and to assess recovery quality in healthy dogs.

STUDY DESIGN

Prospective, blinded clinical study.

ANIMALS

A group of 33 healthy, client-owned dogs subjected to dental procedures.

METHODS

After premedication with intramuscular acepromazine 0.05 mg kg^-1 and methadone 0.3 mg kg^-1, anaesthetic induction started with intravenous alfaxalone 0.5 mg kg^-1 followed by either lactated Ringer's solution (0.04 mL kg^-1, group A), ketamine (2 mg kg^-1, group AK) or midazolam (0.2 mg kg^-1, group AM) and completed with alfaxalone until endotracheal intubation was achieved. Anaesthesia was maintained with alfaxalone (6 mg kg^-1 hour^-1), adjusted (±20%) every 5 minutes to maintain a suitable level of anaesthesia. Ketamine (0.6 mg kg^-1 hour^-1) or midazolam (0.4 mg kg^-1 hour^-1) were employed for anaesthetic maintenance in groups AK and AM, respectively. Physiological variables were monitored during anaesthesia. Times from alfaxalone discontinuation to extubation, sternal recumbency and standing position were calculated. Recovery quality and incidence of adverse events were recorded. Groups were compared using parametric analysis of variance and nonparametric (Kruskal-Wallis, Chi-square, Fisher's exact) tests as appropriate, p < 0.05.

RESULTS

Midazolam significantly reduced alfaxalone induction and maintenance doses (46%; p = 0.034 and 32%, p = 0.012, respectively), whereas ketamine only reduced the alfaxalone induction dose (30%; p = 0.010). Recovery quality was unacceptable in nine dogs in group A, three dogs in group AK and three dogs in group AM.

CONCLUSIONS AND CLINICAL RELEVANCE

Midazolam, but not ketamine, reduced the alfaxalone infusion rate, and both co-adjuvant drugs reduced the alfaxalone induction dose. Alfaxalone TIVA allowed anaesthetic maintenance for dental procedures in dogs, but the quality of anaesthetic recovery remained unacceptable irrespective of its combination with ketamine or midazolam.

Pharmacokinetics of midazolam in sevoflurane-anesthetized cats

OBJECTIVE

To estimate the pharmacokinetics of midazolam and 1-hydroxymidazolam after midazolam administration as an intravenous bolus in sevoflurane-anesthetized cats.

STUDY DESIGN

Prospective pharmacokinetic study.

ANIMALS

A group of six healthy adult, female domestic cats.

METHODS

Anesthesia was induced and maintained with sevoflurane. After 30 minutes of anesthetic equilibration, cats were administered midazolam (0.3 mg kg^-1) over 15 seconds. Venous blood was collected at 0, 1, 2, 4, 8, 15, 30, 45, 90, 180 and 360 minutes after administration. Plasma concentrations for midazolam and 1-hydroxymidazolam were measured using high-pressure liquid chromatography. The heart rate (HR), respiratory rate (f_R), rectal temperature, noninvasive mean arterial pressure (MAP) and end-tidal carbon dioxide (Pe'CO₂) were recorded at 5 minute intervals. Population compartment models were fitted to the time-plasma midazolam and 1-hydroxymidazolam concentrations using nonlinear mixed effect modeling.

RESULTS

The pharmacokinetic model was fitted to the data from five cats, as 1-hydroxymidazolam was not detected in one cat. A five-compartment model best fitted the data. Typical values (% interindividual variability where estimated) for the volumes of distribution for midazolam (three compartments) and hydroxymidazolam (two compartments) were 117 (14), 286 (10), 705 (14), 53 (36) and 334 mL kg^-1, respectively. Midazolam clearance to 1-hydroxymidazolam, midazolam fast and slow intercompartmental clearances, 1-hydroxymidazolam clearance and 1-hydroxymidazolam intercompartment clearance were 18.3, 63.5 (15), 22.1 (8), 1.7 (67) and 3.8 mL minute^-1 kg^-1, respectively. No significant changes in HR, MAP, f_R or Pe'CO₂ were observed following midazolam administration.

CONCLUSION AND CLINICAL RELEVANCE

In sevoflurane-anesthetized cats, a five-compartment model best fitted the midazolam pharamacokinetic profile. There was a high interindividual variability in the plasma 1-hydroxymidazolam concentrations, and this metabolite had a low clearance and persisted in the plasma for longer than the parent drug. Midazolam administration did not result in clinically significant changes in physiologic variables.

Pharmacokinetics of midazolam and its major metabolite 1-hydroxymidazolam in the ball python (Python regius) after intracardiac and intramuscular administrations

Midazolam is a benzodiazepine with sedative, muscle relaxant, anxiolytic, and anticonvulsant effects. Twelve ball pythons (Python regius) were used in a parallel study evaluating the pharmacokinetics of 1 mg/kg midazolam following a single intracardiac (IC) or intramuscular (IM) administration. Blood was collected from a central venous catheter placed 7 days prior, or by cardiocentesis, at 15 time points starting just prior to and up to 72 hr after drug administration. Plasma concentrations of midazolam and 1-hydroxymidazolam were determined by the use of high-performance liquid chromatography tandem-mass spectrometry and pharmacokinetic parameters were estimated using noncompartmental analysis. The mean ± SD terminal half-lives of IC and IM midazolam were 12.04 ± 3.25 hr and 16.54 ± 7.10 hr, respectively. The area under the concentration-time curve extrapolated to infinity, clearance, and apparent volume of distribution in steady-state of IC midazolam were 19,112.3 ± 3,095.9 ng*hr/ml, 0.053 ± 0.008 L hr^-1  kg^-1 , and 0.865 ± 0.289 L/kg, respectively. The bioavailability of IM midazolam was estimated at 89%. Maximum plasma concentrations following an IM administration were reached 2.33 ± 0.98 hr and 24.00 ± 14.12 hr postinjection for midazolam and 1-hydroxymidazolam, respectively, and 22.33 ± 20.26 hr postinjection for 1-hydroxymidazolam following IC administration.

Prolonged Anesthetic Recovery after Continuous Infusion of Midazolam in 2 Domestic Cats (Felis catus)

Two healthy research cats involved in a randomized, blinded prospective pharmacodynamics study evaluating midazolam continuous-rate infusion as a means to decrease sevoflurane concentrations experienced unexpectedly prolonged recoveries. Midazolam loading doses, infusion rates, and the targeted plasma midazolam concentrations at steady-state were determined by pharmacokinetic modeling based on the results of a preliminary pharmacokinetic study using a single dose of midazolam. In the pharmacodynamics study, cats remained oversedated after recovery from anesthesia, and plasma concentrations of midazolam and its primary metabolite (1-hydroxymidazolam) remained elevated. The use of flumazenil was unsuccessful in timely treatment of oversedation. Administration of intravenous lipid emulsion was used in one of the cats to facilitate recovery and appeared to be effective in both reducing the depth of midazolam-induced oversedation and significantly reducing the plasma concentration of 1-hydroxymidazolam. The effects after the administration of both treatment modalities on clinical signs and plasma drug concentrations in cats are discussed. The observations suggest that cats may eliminate 1-hydroxymidazolam more slowly than expected; consequently dose adjustments may be required when continuous infusion of midazolam is intended. In addition, intravenous lipid emulsion may facilitate recovery from midazolam oversedation, particularly in cases unresponsive to traditional treatment modalities. However, further investigations are warranted to delineate the efficacy of this modality in the treatment of midazolam oversedation.

Effect of midazolam on the quality and duration of anaesthetic recovery in healthy dogs undergoing elective ovariohysterectomy or castration

OBJECTIVE

To determine whether the use of a single dose of midazolam affects quality and duration of the recovery period in healthy dogs undergoing elective castration or ovariohysterectomy.

STUDY DESIGN

Prospective, randomized, placebo-controlled, masked clinical trial.

ANIMAL POPULATION

Seventy-four client-owned dogs undergoing neutering.

METHODS

Following cage demeanour scoring using a simple descriptive scale (SDS), dogs were premedicated with acepromazine (0.03 mg kg^-1) and pethidine (3 mg kg^-1) intramuscularly (quadriceps muscle). Twenty minutes later sedation was scored with an SDS. Male dogs were induced with midazolam (0.25 mg kg^-1) (group M) or an equivalent amount of Hartmann's solution (group P) and propofol intravenously (IV). Female dogs were induced with propofol alone and were administered midazolam (group M) or Hartmann's solution (group P) 5 minutes before intraoperative manipulation of the first ovary. Anaesthesia was maintained with isoflurane in oxygen. Intraoperative analgesia was provided with morphine (0.3 mg kg^-1 IV) prior to the start of surgery. Male dogs were administered intratesticular lidocaine (1 mg kg^-1). All dogs were administered meloxicam (0.2 mg kg^-1 IV) at the end of the procedure, and recovery was scored with an SDS following extubation and 30 minutes later. Time to extubation, head lift, sternal position and standing and complications during recovery were recorded. Data are presented as median (range).

RESULTS

Time to standing was significantly longer in animals in group M [56 (13-179) minutes] than in group P [44 (4-137) minutes], and the early recovery score in group M [3 (2-6)] was overall worse than in group P [3 (1-5)]. Significantly more dogs in group M (n = 30) than in group P (n = 22) displayed hypotension.

CONCLUSIONS AND CLINICAL RELEVANCE

The administration of midazolam prolonged time to standing and had a mild negative effect on the quality of recovery in a pooled population of healthy male and female dogs undergoing neutering.

Use of midazolam in combination with medetomidine for premedication in healthy dogs

OBJECTIVE

To assess the sedative effects, propofol sparing properties and impact on quality of induction and intubation of intravenous (IV) medetomidine and midazolam administered consecutively at different doses compared to medetomidine alone in healthy dogs for premedication.

STUDY DESIGN

Prospective, randomized, blinded, clinical study.

ANIMALS

A total of 40 adult healthy client owned dogs, weighing 18 ± 7 kg (mean ± standard deviation).

METHODS

Dogs were assigned to four groups: medetomidine 15 μg kg^-1 (positive control group), medetomidine 10 μg kg^-1 and midazolam 0.2 mg kg^-1, medetomidine 5 μg kg^-1 and midazolam 0.3 mg kg^-1, and medetomidine 5 μg kg^-1 and midazolam 0.2 μg kg^-1. The same clinician assessed sedation after administration at T2.5 minutes and T5 minutes using a composite simple descriptive sedation scale ranging between 0 and 15 (0 = no sedation; 15 = profound sedation). The dose of propofol for induction, quality of induction, ease of intubation and any adverse events were recorded.

RESULTS

There was no significant difference in sedation scores between treatment groups at T2.5 minutes or T5 minutes (p = 0.82 and p = 0.63, respectively). Administration of midazolam in combination with medetomidine resulted in 71% of dogs displaying paradoxical behaviours (p < 0.0001) such as agitation, excitation, restlessness, aggression and vocalization, which was different from pre-sedation. Propofol requirement was not different between groups. Induction and tracheal intubation quality was smooth in all groups.

CONCLUSION

In healthy dogs, at the doses studied, the combination of medetomidine-midazolam administered IV for premedication provided moderate sedation but was associated with a high incidence of paradoxical behaviours. This drug combination IV is not recommended for premedication in healthy dogs.

Randomised clinical trial comparing clinically relevant sedation outcome measures in dogs after intramuscular administration of medetomidine in combination with midazolam or butorphanol for routine diagnostic imaging procedures

The aim of this study was to investigate the sedative effects of medetomidine in combination with midazolam or butorphanol for routine imaging procedures in dogs. Eighty client owned dogs were recruited in a prospective, randomised, blinded clinical study and randomly assigned to receive one of four treatments intramuscularly (IM): (1) 30μg/kg medetomidine (Med30); (2) 20μg/kg medetomidine combined with 0.3mg/kg butorphanol (Med20But0.3); (3) 20μg/kg medetomidine combined with 0.3mg/kg midazolam (Med20Mid0.3); and (4) 10μg/kg medetomidine combined with 0.3mg/kg midazolam (Med10Mid0.3). The level of sedation was evaluated using a composite sedation scale assessed by one investigator (0=no sedation, 15=profound sedation). The number of dogs deemed to be adequately clinically sedated and the dose of propofol administered as rescue sedation were recorded. Mean±standard deviation sedation scores at 30min after the commencement of treatment in the groups that received Med20But0.3 (9.8±4) and Med20Mid0.3 (8.9±4.4) were not statistically significantly different from each other, but were significantly different from the group receiving Med10Mid0.3 (5.6±3.6). Only Med20But0.3 was significantly associated with adequate clinical sedation, while Med10Mid0.3 was associated with 85% sedation failure. The rescue sedation dose of propofol (1.5±1mg/kg) for the Med10Mid0.3 group was significantly higher than for other treatments. A sedation score≥10 out of 15 was a satisfactory cut-off to predict adequate clinical sedation. In healthy dogs, the combination of medetomidine with midazolam did not provide comparable sedation to the same dose of medetomidine in combination with butorphanol in a clinical setting.

Pharmacokinetic evaluation of novel midazolam gel formulations following buccal administration to healthy dogs

OBJECTIVE To determine the physiochemical properties and pharmacokinetics of 3 midazolam gel formulations following buccal administration to dogs. ANIMALS 5 healthy adult hounds. PROCEDURES In phase 1 of a 2-phase study, 2 gel formulations were developed that contained 1% midazolam in a poloxamer 407 (P1) or hydroxypropyl methylcellulose (H1) base and underwent rheological and in vitro release analyses. Each formulation was buccally administered to 5 dogs such that 0.3 mg of midazolam/kg was delivered. Each dog also received midazolam hydrochloride (0.3 mg/kg, IV). There was a 3-day interval between treatments. Blood samples were collected immediately before and at predetermined times for 8 hours after drug administration for determination of plasma midazolam concentration and pharmacokinetic analysis. During phase 2, a gel containing 2% midazolam in a hydroxypropyl methylcellulose base (H2) was developed on the basis of phase 1 results. That gel was buccally administered such that midazolam doses of 0.3 and 0.6 mg/kg were delivered. Each dog also received midazolam (0.3 mg/kg, IV). All posttreatment procedures were the same as those for phase 1. RESULTS The H1 and H2 formulations had lower viscosity, greater bioavailability, and peak plasma midazolam concentrations that were approximately 2-fold as high, compared with those for the P1 formulation. The mean peak plasma midazolam concentration for the H2 formulation was 187.0 and 106.3 ng/mL when the midazolam dose administered was 0.6 and 0.3 mg/kg, respectively. CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that buccal administration of gel formulations might be a viable alternative for midazolam administration to dogs.

Comparison of midazolam and diazepam as co-induction agents with ketamine for anaesthesia in sedated ponies undergoing field castration

OBJECTIVE

To compare intravenous (IV) midazolam and diazepam administered with ketamine for induction of anaesthesia in ponies, already sedated with detomidine, undergoing field castration.

STUDY DESIGN

Prospective, randomised, 'blinded', clinical study.

ANIMALS

Twenty Welsh pony yearlings.

METHODS

After IV injection of detomidine (20 μg kg(-1) ) and phenylbutazone (4.4 mg kg(-1) ) ponies were allocated to receive either IV midazolam (group M) or diazepam (group D) (both 0.06 mg kg(-1) ) with ketamine (2.2 mg kg(-1) ) for induction of anaesthesia. Using simple descriptive scales, quality of sedation, induction, endotracheal intubation, surgical conditions and recovery were scored by observers blinded to treatment. Time from sedation to induction of anaesthesia, IV injection to lateral recumbency, induction to start of surgery, induction to first head lift and to standing, and total surgical time were measured. Cardiorespiratory function was assessed every 5 minutes. Time, number and total quantity of additional IV ketamine as well as any adverse effects were documented. Data were tested for normality and analysed using two-way anova with Bonferroni post hoc tests, unpaired t-tests and Mann-Whitney U tests as appropriate. Significance was set at p < 0.05.

RESULTS

There were no significant group differences in any of the measured variables except bodyweight (mean ± SD: group M 163 ± 12 kg; group D 150 ± 7 kg; p = 0.01). One pony in group M required ketamine 15 minutes after induction of anaesthesia. Surgical conditions were good in all cases; time from induction to standing was 50 ± 11 minutes in group M and 48 ± 12 minutes in group D. There were no adverse effects. Recoveries were uneventful with minimal ataxia.

CONCLUSIONS AND CLINICAL RELEVANCE

Midazolam and diazepam at 0.06 mg kg(-1) can be used interchangeably in combination with ketamine for IV induction of short term anaesthesia in ponies.

The pharmacokinetics of midazolam after intravenous, intramuscular, and rectal administration in healthy dogs

Intravenous benzodiazepines are utilized as first-line drugs to treat prolonged epileptic seizures in dogs and alternative routes of administration are required when venous access is limited. This study compared the pharmacokinetics of midazolam after intravenous (IV), intramuscular (IM), and rectal (PR) administration. Six healthy dogs were administered 0.2 mg/kg midazolam IV, IM, or PR in a randomized, 3-way crossover design with a 3-day washout between study periods. Blood samples were collected at baseline and at predetermined intervals until 480 min after administration. Plasma midazolam concentrations were measured by high-pressure liquid chromatography with UV detection. Rectal administration resulted in erratic systemic availability with undetectable to low plasma concentrations. Arithmetic mean values ± SD for midazolam peak plasma concentrations were 0.86 ± 0.36 μg/mL (C0) and 0.20 ± 0.06 μg/mL (Cmax), following IV and IM administration, respectively. Time to peak concentration (Tmax ) after IM administration was 7.8 ± 2.4 min with a bioavailability of 50 ± 16%. Findings suggest that IM midazolam might be useful in treating seizures in dogs when venous access is unavailable, but higher doses may be needed to account for intermediate bioavailability. Rectal administration is likely of limited efficacy for treating seizures in dogs.

Bioavailability of a novel midazolam gel after intranasal administration in dogs

OBJECTIVE

To compare the pharmacokinetics of a novel bioadhesive gel formulation of midazolam after intranasal (IN) administration with that of midazolam solution after IN, IV, and rectal administration to dogs.

ANIMALS

10 (5 males and 5 females) healthy adult Beagles.

PROCEDURES

Dogs were assigned to 4 treatment groups for a crossover study design. Initially, midazolam solution (5 mg/mL) was administered (0.2 mg/kg) IV to group 1, rectally to group 2, and IN to group 3; a 0.4% hydroxypropyl methylcellulose midazolam gel formulation (50 mg/mL) was administered (0.2 mg/kg, IN) to group 4. Each dog received all 4 treatments; there was a 7-day washout period between subsequent treatments. Blood samples were collected before and after midazolam administration. Plasma concentration of midazolam was determined by use of high-performance liquid chromatography.

RESULTS

The peak plasma concentration after IN administration of the gel formulation was significantly higher than that after IN and rectal administration of the solution. Mean ± SD time to peak concentration was 11.70 ± 2.63 minutes (gel IN), 17.50 ± 2.64 minutes (solution IN), and 39 ± 14.49 minutes (solution rectally). Mean bioavailability of midazolam was 70.4% (gel IN), 52.0% (solution IN), and 49.0% (solution rectally). Bioavailability after IN administration of the gel formulation was significantly higher than that after IN and rectal administration of the solution.

CONCLUSIONS AND CLINICAL RELEVANCE

IN administration of midazolam gel was superior to both IN and rectal administration of midazolam solution with respect to peak plasma concentration and bioavailability.

The effect of midazolam on the end-tidal concentration of isoflurane necessary to prevent movement in dogs

OBJECTIVE

To determine the possible additive effect of midazolam, a GABA(A) agonist, on the end-tidal concentration of isoflurane that prevents movement (MAC(NM) ) in response to noxious stimulation.

STUDY DESIGN

Randomized cross-over experimental study.

ANIMALS

Six healthy, adult intact male, mixed-breed dogs.

METHODS

After baseline isoflurane MAC(NM) (MAC(NM-B) ) determination, midazolam was administered as a low (LDS), medium (MDS) or high (HDS) dose series of midazolam. Each series consisted of two dose levels, low and high. The LDS was a loading dose (Ld) of 0.2 mg kg(-1) and constant rate infusion (CRI) (2.5 μg kg(-1) minute(-1)) (LDL), followed by an Ld (0.4 mg kg(-1)) and CRI (5 μg kg(-1) minute(-1)) (LDH). The MDS was an Ld (0.8 mg kg(-1)) and CRI (10 μg kg(-1) minute(-1)) (MDL) followed by an Ld (1.6 mg kg(-1)) and CRI (20 μg kg(-1) minute(-1)) (MDH). The HDS was an Ld (3.2 mg kg(-1)) and CRI (40 μg kg(-1) minute(-1)) (HDL) followed by an Ld (6.4 mg kg(-1)) and CRI (80 μg kg(-1) minute(-1)) (HDH). MAC(NM) was re-determined after each dose in each series (MAC(NM-T)).

RESULTS

The median MAC(NM-B) was 1.42. MAC(NM-B) did not differ among groups (p > 0.05). Percentage reduction in MAC(NM) was significantly less in the LDS (11 ± 5%) compared with MDS (30 ± 5%) and HDS (32 ± 5%). There was a weak correlation between the plasma midazolam concentration and percentage MAC(NM) reduction (r = 0.36).

CONCLUSION AND CLINICAL RELEVANCE

Midazolam doses in the range of 10-80 μg kg(-1) minute(-1) significantly reduced the isoflurane MAC(NM) . However, doses greater than 10 μg kg(-1) minute(-1) did not further decrease MAC(NM) indicating a ceiling effect.

Abnormal motor activity during anaesthesia in a dog: a case report

Seizures or convulsions that occur during anaesthesia in veterinary patients are infrequently reported in the literature. Consequently, the incidence of such events is unknown. Several drugs commonly used in clinical veterinary anaesthesia have been shown to induce epileptiform activity in both human clinical patients and experimental candidates. The present case report describes convulsions in a four-year old male Bernese mountain dog during maintenance of anaesthesia with isoflurane after premedication with acepromazine and methadone followed by co-induction with propofol and ketamine. The dog had no history of previous convulsions. The use of several sedative and anaesthetic drugs makes it difficult to find one single causative pharmaceutical.

Antagonistic effects of atipamezole, flumazenil and 4-aminopyridine against anaesthesia with medetomidine, midazolam and ketamine combination in cats

Antagonistic effects of atipamezole (ATI), flumazenil (FLU) and 4-aminopyridine (4AP) alone and in various combinations after administration of medetomidine-midazolam-ketamine (MED-MID-KET) were evaluated in cats. Animals were anaesthetised with MED (50 microg/kg), MID (0.5 mg/kg) and KET (10 mg/kg) given intramuscularly. Twenty minutes later, physiological saline, ATI (200 microg/kg), FLU (0.1 mg/kg), 4AP (0.5 mg/kg), ATI-FLU, FLU-4AP, ATI-4AP or ATI-FLU-4AP was administered intravenously. FLU, 4AP alone, or FLU-4AP did not effectively antagonise the anaesthesia, hypothermia, bradycardia, and bradypnoea induced by MED-MID-KET. ATI alone was effective. ATI-FLU, ATI-4AP and ATI-FLU-4AP combinations produced an immediate and effective recovery from anaesthesia. The combination of ATI-FLU-4AP was the most effective in antagonising the anaesthetic effects, but was associated with tachycardia, tachypnoea, excitement, and muscle tremors. Combinations with ATI are more effective for antagonising anaesthesia, but ATI-FLU-4AP is not suitable.

Anticonvulsant Therapy in Dogs and Cats

This article reviews anticonvulsant therapies in current use for dogs and cats and briefly describes new modes of anticonvulsant therapy that are being investigated or pending publication. Most of the information contained within the article is based on published information. Some of the information, however, is based on the author's clinical experience and is identified as such.

Extrapolation of preclinical pharmacokinetics and molecular feature analysis of "discovery-like" molecules to predict human pharmacokinetics

The prediction of human pharmacokinetics from preclinical species is an integral component of drug discovery. Recent studies with a 103-compound dataset suggested that scaling from monkey pharmacokinetic data tended to be the most accurate method for predicting human clearance. Additionally, interrogation of the two-dimensional molecular properties of these molecules produced a set of associations which predict the likely extrapolative outcome (success or failure) of preclinical data to project human pharmacokinetics. However, a limitation of the previous analyses was the relative paucity of data for typical "discovery-like" molecules (molecular weight >300 and/or clogP >3). The objective of this investigation was to generate preclinical data required for extension of this dataset for additional discovery-like molecules and determine whether the aforementioned findings continue to apply for these molecules. In vivo nonrodent intravenous pharmacokinetic data were generated for 13 molecules, and data for 8 additional molecules were obtained from the literature. Additionally, the various scaling methodologies and molecular features analysis were applied to this new dataset to predict human pharmacokinetics. Whereas the predictive accuracies demonstrated across all of the various methodologies were lower for this higher clearance compound dataset, scaling from monkey liver blood flow continued to be an accurate methodology, and human volume of distribution was similarly well predicted regardless of scaling methodology. Lastly, application of the molecular feature associations, particularly data-dependent associations, afforded an improved predictivity compared with the liver blood flow scaling approaches, and provides insight into the extrapolation of high clearance compounds in the preclinical species to human.

A sensitive HPLC-MS-MS assay for quantitative determination of midazolam in dog plasma

The clinical pharmacokinetics of midazolam have been extensively studied, due to its high clearance by CYP3A4 and sensitivity to drug-drug interactions. In order to investigate the potential to model drug-drug interactions with midazolam in the dog, a selective and sensitive high performance liquid chromatography-tandem mass spectroscopy (HPLC-MS-MS) method has been developed, with sufficient sensitivity to allow analysis of dog plasma samples generated following administration of a clinically relevant dose. The method involves extraction of midazolam and internal standard (flunitrazepam) from dog plasma, using 96-well Oasis MCX solid phase extraction plates. The assay has been validated over a concentration range of 0.1-10 ng/ml and its specificity, accuracy and precision demonstrated. The relative bias of the assay was within +/-15% for all standards with intra- and inter-assay precision (coefficient of variation-%CV) of less than 15%. The assay was applied to the analysis of plasma samples (0.2 ml), generated following intravenous or oral administration of midazolam to male beagle dogs, at a dose level of 0.05 mg/kg, and pharmacokinetic parameters were derived from the resulting data.

Quantitative electroencephalography of medetomidine, medetomidine-midazolam and medetomidine-midazolam-butorphanol in dogs

The purpose of this study was to evaluate the effects of the administration of an alpha2-adrenoceptor agonist alone and in combination with other derivatives on brain wave activity. In addition, the diagnostic values of the electroencephalogram (EEG) for judging the depth of the balanced anaesthesia with an alpha2-adrenoceptor agonist was evaluated. The treatments comprised 20 microg/kg medetomidine (Me-20), 80 microg/kg medetomidine (Me-80), 20 microg/kg medetomidine and 0.5 mg/kg midazolam (Me-Mi) administered intramuscularly, and 20 microg/kg medetomidine with 0.5 mg/kg midazolam and 0.1 mg/kg butorphanol (Me-Mi-Bu). The EEG was recorded continuously at pre-administration, and at 7, 10, 20, 30, 45 and 60 min after administration. The recorded data were analysed by separating the power spectrum into 1-3, 4-7, 8-13 and 14-30 Hz bands. Spectral-edge analysis was used to calculate the spectral edge frequency 90 (SEF90) and the median edge frequency (MEF). Time-related changes in power spectrum analysis showed a significant increase in the Me-80 group in the 1-3 Hz band. The power for 1-3 Hz in the Me-80 group was significantly higher than in all the other groups. In the 14-30 Hz band, there was a significant reduction of power in all groups following administration of the agents. The SEF90 frequencies were significantly reduced in all groups except for the Me-20 group after administration of the agents. The SEF90 frequencies in the Me-20, Me-Mi and Me-Mi-Bu were all significantly higher than those in the Me-80 group. However, there was no significant difference between the Me-20, Me-Mi and Me-Mi-Bu groups in any analyses. Our results demonstrated that the changes in quantitative EEG made by the Me-Mi-Bu and Me-Mi groups were similar to those made by Me-20 groups. Present results suggest that the EEG should be interpreted with caution in assessing the anaesthetic level in balanced anaesthesia in dogs.

Comparison of sevoflurane with isoflurane for rapid mask induction in midazolam and butorphanol-sedated dogs

Rapid mask induction can be a useful induction technique for veterinary patients, although it is often accompanied by exaggerated excitement responses in unpremedicated animals (Mutoh et al.: Jpn. J. Vet. Anesth. Surg. 26, 109-116; J. Vet. Med. Sci. 57, 1007-1013; J. Vet. Med. Sci. 57, 1121-1124; 1995). The aim of this study was to compare sevoflurane with isoflurane for rapid mask induction in six dogs sedated by a combination of midazolam (0.1 mg/kg) and butorphanol (0.2 mg/kg). Induction with sevoflurane (5%, 2.4 minimum alveolar concentration [MAC]) in O2 resulted in shorter time to loss of the palpebral reflex, negative tail clamp response, and successful intubation than with isoflurane (3%, 2.4 MAC) in O2. There were no changes in heart rate or mean arterial blood pressure during induction with sevoflurane, whereas an increase in heart rate was observed in dogs induced with isoflurane. A decrease in respiratory rate compared with the pre-induction rate was observed during induction, and associated mild respiratory acidosis, characterized by an increase in arterial PCO2, was measured at the end of the induction period in both induction groups. None of the animals had episodes of induction-related complications. These results suggest that both sevoflurane and isoflurane produce a smooth onset of induction in midazolam and butorphanol-sedated dogs. Sevoflurane is a more suitable for rapid mask induction than isoflurane since it provides faster induction associated with a lower blood/gas partition coefficient.

Anaesthetic and cardiopulmonary effects of balanced anaesthesia with medetomidine-midazolam and butorphanol in dogs

The anaesthetic and cardiopulmonary effects of combinations of medetomidine (Me), midazolam (Mi) and butorphanol (Bu) were evaluated in dogs. The characterization of anaesthetic effects was assessed using a scoring system. The combinations tested were 20 or 40 micrograms/kg Me and 0.5 mg/kg Mi (20Me-Mi or 40Me-Mi) followed by either an intravenous injection of physiological saline solution (PSS) or Bu (0.1 or 0.3 mg/kg). The mixture of Me and Mi was injected intramuscularly, followed 15 min later by an intravenous injection of Bu or PSS in all six groups. The combined Me-Mi induced deep sedation but not profound anaesthesia. The effect of the subsequent Bu administration was observed in the scores related to its analgesic effect. There were no significant differences between the two doses of Bu, following either 20Me-Mi or 40Me-Mi in the duration of anaesthesia, heart and respiratory rates, rectal temperature, and anaesthetic and analgesic scores except for palpebral reflex, and interdigital web clamping scores. Therefore, we concluded that the addition of 0.1 mg/kg Bu to Me-Mi elicits adequate anaesthesia with adequate analgesic effect, and side-effects such as bradycardia, hypertension, and slight respiratory acidosis in some dogs.

Comparison of routes of flumazenil administration to reverse midazolam-induced respiratory depression in a canine model

OBJECTIVE

To determine whether flumazenil, a drug used to reverse benzodiazepine-induced respiratory depression and approved only for i.v. use, is effective by alternative routes.

METHODS

A randomized, controlled, nonblinded, crossover canine trial was performed to evaluate reversal of midazolam-induced respiratory depression by flumazenil when administered by alternative routes. Mongrel dogs were sedated with thiopental 19 mg/kg i.v., then tracheally intubated. With the dogs spontaneously breathing, tidal volume, end-tidal CO2, and O2 saturation were observed until a stable baseline was achieved. Incremental doses of midazolam were administered until respiratory depression (30% decline in tidal volume, 10% decrease in O2 saturation, and 15% increase in end-tidal CO2) occurred. Flumazenil was administered by a randomly selected route [0.2 mg followed 1 minute later by 0.3 mg i.v., sublingual (s.l.) or intramuscular (i.m.); or 1 mg followed 1 minute later by 1.5 mg per rectum (PR)]. Time to return to baseline respiratory functions was recorded ("time to reversal"). Each of 10 dogs was studied using all 4 routes of flumazenil administration with a washout period of at least 7 days. An additional dog served as a control (no flumazenil).

RESULTS

The control time to reversal was 1,620 seconds. The i.v. route was significantly faster (mean 120 +/- 24.5 sec) than the other 3 routes (p < 0.005). The SL route was the second fastest (mean 262 +/- 94.5 sec), the IM route was the third fastest (mean 310 +/- 133.7 sec) and the PR route was the s;owest (mean 342 +/- 84.4 sec). The SL, IM, and PR routes did not differ significantly from one another.

CONCLUSIONS

Flumazenil administered by all 4 routes reversed midazolam-induced respiratory depression in a dog model. For the selected dosages used, the i.v. route was significantly faster than all 3 other routes, and SL was the second fastest.

The behaviour of healthy awake cats following intravenous and intramuscular administration of midazolam

The onset of action and behavioural effects following intravenous (i.v.) and intramuscular (i.m.) administration of 0.05, 0.5, 1.0, 2.0 and 5.0 mg/kg of midazolam were studied for 2 h in 20 awake, healthy cats. All cats, except one that received 0.05 mg/kg i.m., showed effects of the drug, whereas no effects were observed in cats administered only the vehicle in which midazolam was dissolved. The onset of action was rapid following both i.v. and i.m. administration, some cats became ataxic, while others assumed positions of sternal or lateral recumbency. Even after administration of the highest dose (5.0 mg/kg), anaesthesia was not induced, with swallowing reflexes and conscious perception of a clamp placed on the tail still present in all cats. An abnormal arousal state was observed in many cats after administration of midazolam. During the first hour, restlessness was more commonly observed, while from 1 to 2 h, sedation was more prominent in cats that received the highest dose. Ataxia occurred in all but one cat, was short-lived in cats that received the lower doses, but still present at 2 h in all cats that received 2.0 and 5.0 mg/kg. Midazolam caused some of the cats to behave differently when approached and restrained compared with behavioural patterns identified prior to administration of the drug. The cats were more likely to behave abnormally following i.v. administration rather than i.m. administration and, for the most part, abnormal behaviour was equally distributed between the two extremes; cats being easier to approach and restrain and cats being more difficult to approach and restrain. Food consumption increased significantly, during the 2 h period, following all i.m. doses and all but the highest (5.0 mg/kg) i.v. dose, with most of the food being consumed in the first hour after administration.

Intravenous midazolam suppresses noxiously evoked activity of spinal wide dynamic range neurons in cats

The effects of intravenously (i.v.) administered midazolam on noxiously evoked activity of spinal wide dynamic range (WDR) neurons were investigated in decerebrate, spinal-cord-transected cats. Extracellular, single-unit recordings were measured during stimulation by pinching the receptive field on the hind paw and the effect of midazolam at doses of 0.25, 0.5, 1, 2, and 4 mg/kg were measured. Two series of experiments were performed to characterize the analgesic effects of midazolam. In the first, dose-response experiments (n = 59) demonstrated a dose-dependent suppression of the noxiously evoked activity of spinal WDR neurons after midazolam administration. This effect of midazolam was maximal at a dose of 1 mg/kg i.v.. The second series of experiments (n = 14) demonstrated that a benzodiazepine antagonist, flumazenil (n = 8), promptly reversed the effect of midazolam, while an opioid antagonist, naloxone (n = 6), had no effect on the effect of midazolam. The present study demonstrates that i.v. administered midazolam suppresses noxiously evoked activity of spinal WDR neurons that is reversible by a benzodiazepine antagonist. This is consistent with an analgesic action of midazolam.