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

Pharmacokinetics and pharmacodynamics of dextroketamine alone or combined with midazolam in Caiman crocodilus

Pharmacokinetics studies of anesthetic agents are important for understanding of the pharmacology and metabolism of anesthetic agents in reptilians. This study was designed to examine the pharmacokinetic and pharmacodynamic properties of intravenous dextroketamine alone or combined with midazolam in Caiman crocodilus. Eight caimans were anesthetized with dextroketamine (10 mg/kg; group D) or dextroketamine and midazolam (10 and 0.5 mg/kg respectively; group DM) into the occipital venous sinus. The pharmacokinetic parameters were calculated by HPLC using a non-compartmental modeling. Serial blood samples were collected at baseline and within 15 and 30 min, and 11.5, 2, 4, 8, 12, 24 and 48 h of drug administration. Sedation status over time differed between groups. All animals in group D (8/8; 100%) showed signs of light sedation at t10. Half (4/8; 50%) of these caimans did not progress to deeper levels of sedation. In spite of light sedation at t10, animals in group DM were deeply sedated within 13.13 ± 7.04 min of anesthetic agent injection. The area under the plasma concentration-time curve (AUC0-48) and half-life of dextroketamine changed significantly after combination with midazolam. Even without significant changes in clearance, the almost two-fold increase in the half-life of dextroketamine suggests a slower rate of elimination.

Effect of atropine and propofol on the minimum anaesthetic concentration of isoflurane in the freshwater turtle Trachemys scripta (yellow-bellied slider)

OBJECTIVE

To determine if the administration of atropine would reduce the measured minimum anaesthetic concentration of isoflurane (MAC_isoflurane) in freshwater turtles - the yellow-bellied slider (Trachemys scripta scripta).

STUDY DESIGN

Paired, blinded, randomized, prospective studies of 1) the effect of atropine in isoflurane anaesthetized freshwater turtles (T. scripta scripta) and 2) the effect of atropine in yellow-bellied sliders in which anaesthesia was induced with propofol and maintained with isoflurane.

ANIMALS

T. scripta scripta (n = 8), female, adult.

METHODS

Atropine (2 mg kg^-1) or an isovolumetric control injection of saline was administered intraperitoneally 15 minutes prior to induction of anaesthesia with isoflurane. Individual MAC_isoflurane was then determined by end-tidal gas analysis in a bracketing design by an experimenter blinded to the administered drug, with a 2 week washout period. The experiment was repeated, with atropine (2 mg kg^-1) or saline administered intravascularly in combination with propofol for anaesthetic induction. Linear mixed modelling was used to determine the effects of atropine and propofol on the individual MAC_isoflurane. Data are presented as mean ± standard deviation.

RESULTS

Premedication with atropine significantly reduced MAC_isoflurane (p = 0.0039). In isoflurane-induced T. scripta scripta, MAC_isoflurane decreased from 4.2 ± 0.4% to 3.3 ± 0.8% when atropine had been administered. Propofol as an induction agent had a MAC-sparing effect (p < 0.001) such that MAC_isoflurane following propofol and a control injection of saline was 2.3 ± 1.0%, which decreased further to 1.5 ± 0.8% when propofol was combined with atropine.

CONCLUSIONS AND CLINICAL RELEVANCE

Atropine, presumably by inhibiting parasympathetically mediated pulmonary artery constriction, decreases right-to-left cardiac shunting and the MAC_isoflurane in yellow-bellied sliders, and thereby may facilitate control of inhalant anaesthesia. Propofol can be used for induction of anaesthesia and reduces the required concentration of inhaled anaesthesia assessed 1.5 hours following induction.

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.

Ectothermy and cardiac shunts profoundly slow the equilibration of inhaled anaesthetics in a multi-compartment model

The use of inhalational anaesthesia is ubiquitous in terrestrial vertebrates. Given the dependence of these agents on delivery by the cardiorespiratory system, we developed a new computational model predicting equilibration of inhaled anaesthetics in mammalian and ectotherm conditions including the ability of reptiles to maintain vascular shunts. A multi-compartment model was constructed from simultaneously-solved equations, verified by comparison to the literature for endo and ectotherm physiology. The time to 90% equilibration of anaesthetic in arterial blood (t₉₀) is predicted and used to compare anaesthetics and physiologies. The five to tenfold lower cardiac output and minute ventilation of ectothermic vertebrates is predicted to slow equilibration times by five to ten times leading to 90% equilibration in ectotherm arterial blood of over 200 min, compounded by reduction in body temperature, and the extent of right-to-left vascular shunts. The impact of these findings is also influenced by the solubility coefficient of the anaesthetic, such that at net right-to-left shunt fractions of over 0.8, sevoflurane loses the advantage of faster equilibration, in comparison with isoflurane. We explore clinical strategies to regulate anaesthetic uptake in ectotherms by managing convectional flow especially by supportive ventilation and reduction of the right-to-left shunt.