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

Sedation and Anesthesia in Exotic Animal Critical Care

Sedation and anesthesia of exotic animals in inherently challenging, but often facilitates the best care for patients. Critical illness or injury adds on another layer of complexity to their management for obtaining diagnostics and providing treatments. This article serves to review some of the more recent literature of sedation and anesthesia within exotics practice, bringing to light some nuances and considerations for when those patients are critically ill or injured.

Snake Sedation and Anesthesia

Snakes can be more challenging to anesthetize compared with other animals because of anatomic and physiologic differences, a wide range of patient sizes, and variable responses to anesthetic agents. Snakes have preferred optimal temperature zones, which, along with physiologic characteristics, such as the ability to shunt blood toward or away from the lungs, can have an impact on anesthesia. Injectable agents, including benzodiazepines, α₂-agonists, opioids, propofol, and alfaxalone, as well as inhalant anesthetics can be used to anesthetize snakes. Pain management must be incorporated to the anesthetic plan when performing procedures that are expected to produce nociception.

Evaluation of a dexmedetomidine-midazolam-ketamine combination administered intramuscularly in captive ornate box turtles (Terrapene ornata ornata)

OBJECTIVE

To characterize the effects of a combination protocol of dexmedetomidine-midazolam-ketamine (DMK) administered intramuscularly (IM) in ornate box turtles (Terrapene ornata ornata).

STUDY DESIGN

Prospective experimental trial.

ANIMALS

A total of 16 apparently clinically healthy adult ornate box turtles (eight male, eight female).

METHODS

Each turtle was treated with dexmedetomidine (0.1 mg kg^-1), midazolam (1 mg kg^-1) and ketamine (10 mg kg^-1) administered IM. Time to first response, time to maximal effect, the plateau phase and time to recovery from reversal administration were recorded. Physiologic variables, muscle tone, reflexes and the ability to perform endotracheal intubation were recorded at 5 minute intervals. Movement in response to an IM injection of 0.1 mL sterile 0.9% NaCl administered in the left pelvic limb, using a 25 gauge needle to a depth of just past the bevel of the needle, was assessed every 15 minutes. Atipamezole (0.5 mg kg^-1) IM and flumazenil (0.05 mg kg^-1) SC were administered 60 minutes after the initial DMK injections.

RESULTS

The mean time to first response, time to maximal effect, the plateau phase and time to recovery were 2.1, 14.9, 38.7 and 7.8 minutes, respectively. A respiratory rate was not observed in most turtles. The body temperature significantly increased over time. The palpebral reflex was persistent in 43% of turtles and the tail pinch reflex remained intact in 13% of turtles. All turtles recovered with no observed adverse effects.

CONCLUSIONS AND CLINICAL RELEVANCE

In this study, this DMK protocol administered to ornate box turtles resulted in a rapid-onset, light anesthesia lasting approximately 40 minutes and a smooth recovery with no adverse effects noted.

A Pharmacokinetic-Pharmacodynamic Study of Intravenous Midazolam and Flumazenil in Adult New Zealand White-Californian Rabbits (Oryctolagus cuniculus)

Flumazenil, a competitive GABAA receptor antagonist, is commonly used in rabbits to shorten sedation or postanesthetic recovery after benzodiazepine administration. However, no combined pharmacokinetic (PK) and pharmacodynamic (PD) data are available to guide its administration in this species. In a prospective, randomized, blinded, crossover study design, the efficacy of IV flumazenil (FLU; 0.05 mg/kg) or saline control (SAL; equal volume) to reverse the loss of righting reflex (LORR) induced by IV midazolam (1.2 mg/kg) was investigated in 15 New Zealand white rabbits (2.73 to 4.65 kg, 1 y old). Rabbits were instrumented with arterial (central auricular artery) and venous (marginal auricular vein) catheters. After baseline blood sampling, IV midazolam was injected (T0). Flumazenil or saline (FLU/SAL) was injected 30 s after LORR. Arterial blood samples were collected at 1 and 3 min after midazolam injection, and at 1, 3, 6, 10, 15, 21, 28, 36, 45 and 60 min after injection with flumazenil. Plasma samples for midazolam, 1-OH-midazolam and flumazenil were analyzed using high performance liquid chromatography-high-resolution mass spectrometry and the time to return of righting reflex (ReRR) was compared between groups (Wilcoxon test). FLU terminal half-life, plasma clearance and volume of distribution were 26.3 min [95%CI: 23.3 to 29.3], 18.74 mL/min/kg [16.47 to 21.00] and 0.63 L/kg [0.55 to 0.71], respectively. ReRR was 25 times faster in rabbits treated with FLU (23 [8 to 44] s) compared with SAL (576 [130 to 1141] s; 95%CI [425 to 914 s]). Return of sedation (lateral recumbency) occurred in both groups (7/13 in FLU; 12/13 in SAL) with return of LORR in a few animals (4/13 in FLU; 7/13 in SAL) at 1540 [858 to 2328] s. In the population and anesthesia protocol studied, flumazenil quickly and reliably reversed sedation induced by midazolam injection. However, the potential return of sedation after flumazenil administration warrants careful monitoring in the recovery period.

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.

Effects of midazolam and nitrous oxide on the minimum anesthetic concentration of isoflurane in the ball python (Python regius)

OBJECTIVE

To evaluate the effects of midazolam and nitrous oxide (N₂O) on the minimum anesthetic concentration of isoflurane (MAC_ISO) in ball pythons.

STUDY DESIGN

Prospective, crossover, randomized, semi-blinded study.

ANIMALS

A total of nine healthy adult female ball pythons (Python regius) weighing 2.76 ± 0.73 kg.

METHODS

In each snake, three protocols were evaluated with 2 week washouts: treatment MID-O₂, midazolam (1 mg kg^-1) administered intramuscularly (IM) and anesthesia induced with isoflurane-oxygen; treatment SAL-O₂, saline (0.2 mL kg^-1) IM and anesthesia with isoflurane-oxygen; and treatment SAL-N₂O, saline IM and anesthesia with isoflurane and 50% nitrous oxide (N₂O):50% oxygen. In each treatment, isoflurane was administered by face mask immediately after premedication. Snakes were endotracheally intubated and inspired and end-tidal isoflurane concentrations were monitored. The study design followed a standard bracketing technique, and the MAC_ISO was determined using logistic regression. Electrical stimulation using a Grass stimulator connected to the base of the tail (50 V, 50 Hz, 6.5 ms pulse^-1) was used as the supramaximal stimulus. Blood-gas analysis was performed on cardiac blood collected immediately following intubation and after the last stimulation. Blood-gas variables were compared over time and between treatments using linear mixed models.

RESULTS

MAC_ISO at a body temperature of 30.1 ± 0.4 °C was 1.11% (95% confidence interval, 0.94-1.28%) in SAL-O₂ and was significantly decreased to 0.48% (0.29-0.67%) in MID-O₂ (p < 0.001) and to 0.92% (0.74-1.09%) in SAL-N₂O (p = 0.016). PO₂ was significantly lower in MID-O₂ and SAL-N₂O than in SAL-O₂.

CONCLUSIONS AND CLINICAL RELEVANCE

Midazolam significantly decreased the MAC_ISO by 57% in ball pythons, whereas addition of N₂O resulted in a modest, although significant, decrease (17%). MAC_ISO in ball pythons was lower than those previously reported in reptiles.