Anesthetic potency of sevoflurane with and without nitrous oxide in mechanically ventilated Dumeril monitors

Sedation and Anesthesia of Lizards

The field of lizard sedation, anesthesia, and locoregional anesthesia is advancing with new drug protocols being evaluated, and new locoregional techniques being developed and evaluated. Inducing and maintaining effective and safe chemical restraint in lizards can be challenging, particularly in systemically diseased individuals. Understanding the anatomic and physiologic adaptations of lizards, using reversible or partially reversible injectable protocols, and using locoregional anesthesia may increase the quality of chemical restraint, facilitate faster recoveries, and limit anesthesia-related morbidity and mortality.

Comparison of isoflurane, sevoflurane, and desflurane as inhalant anesthetics in prairie rattlesnakes (Crotalus viridis)

OBJECTIVE

To characterize induction and recovery characteristics of 3 commonly used inhalant anesthetics in prairie rattlesnakes (Crotalus viridis): isoflurane, sevoflurane, and desflurane.

ANIMALS

12 healthy adult prairie rattlesnakes.

PROCEDURES

In a randomized crossover design, snakes underwent anesthetic induction with 5% isoflurane, 8% sevoflurane, or 18% desflurane, with a washout period of ≥ 7 days between anesthetic events. Anesthetic depth parameters were recorded throughout induction and recovery, including time to loss and return of righting reflex, muscle tone, ability to intubate, response to pressure, and time to return to spontaneous respiration. Every 5 minutes throughout the anesthetic procedures, heart rate, respiratory rate, and percentage expired anesthetic gas were recorded.

RESULTS

No snakes died during the study. Sevoflurane anesthesia resulted in anesthetic gas avoidance behavior in snakes during induction and had the significantly longest recovery time to extubation and time to return of pressure response, compared with the other inhalant anesthetics. Anesthesia with isoflurane resulted in a significantly longer time to return of righting reflex, compared with sevoflurane or desflurane. No significant difference was noted in time to loss of pressure response among the 3 anesthetic gases. Desflurane anesthesia resulted in the significantly quickest loss of righting reflex among the anesthetic protocols; despite this, 4 of 12 desflurane anesthetized snakes did not achieve an anesthetic plane deep enough for intubation.

CONCLUSIONS AND CLINICAL RELEVANCE

Isoflurane and sevoflurane, but not desflurane, inhalation anesthesia resulted in consistent and predictable loss of righting reflex and induction of anesthesia deep enough to allow intubation in snakes.

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.

The efficacy of intracoelomic fospropofol in red-eared sliders (Trachemys scripta elegans)

Intravenous anesthetic delivery in reptiles can be challenging. Current injectable techniques have varied induction/recovery times and anesthetic quality. This study hypothesized that intracoelomic administration of a new anesthetic, fospropofol, in turtles would result in dose-dependent anesthesia and respiratory depression. A two-part prospective trial using adult red-eared slider turtles (Trachemys scripta elegans) weighing 764 +/- 17 g was conducted to determine an effective anesthetic dose and to evaluate the anesthetic quality, duration, and respiratory effects of an efficacious dose. In part 1, six turtles were randomly administered 25-mg/ kg (low-dose [LD]) and 50-mg/kg (high-dose [HD]) fospropofol in a crossover design. Respiratory rate, immobility, and muscle relaxation scores were evaluated for 180 min. In part 2, eight turtles were administered HD fospropofol. Immobility and muscle relaxation (front and hind limb) scores and time to endotracheal intubation/extubation were evaluated until scores returned to baseline. In part 1, the LD group had significantly lower immobility and muscle relaxation scores versus the HD group over time (both P < 0.05); scores were significantly elevated from baseline for 20-120 min and 15-180 min, respectively (all P < 0.05). Although not significantly different between groups (P > 0.05), respiratory rate was significantly decreased from baseline from 10 to 120 min (all P < 0.05). In part 2, HD fospropofol decreased respiratory rate from 21.5 +/- 2.9 breaths/min to 0.1 +/- 0.1 breaths/min, similar to the results in part 1. Maximal reductions in mobility and front and hind limb motor tone occurred at 39.0 +/- 4.1, 30.8 +/- 3.6, and 24.0 +/- 3.6 min, respectively. Intubation in 7/8 turtles occurred at 45.7 +/- 5.4 min and extubation at 147.0 +/- 23.2 min. However, 2/8 turtles showed prolonged anesthetic effects, requiring resuscitative efforts for recovery. Due to the unpredictable quality and duration of anesthesia with intracoelomic fospropofol, it should be used with caution for general anesthesia in red-eared sliders at the doses and administration route investigated.

Determination of the minimum anesthetic concentration of sevoflurane in thick-billed parrots (Rhynchopsitta pachyrhyncha)

OBJECTIVE

To determine the minimum anesthetic concentration (MAC) of sevoflurane in thick-billed parrots (Rhynchopsitta pachyrhyncha) and compare MAC obtained via mechanical and electrical stimulation.

ANIMALS

15 healthy thick-billed parrots.

PROCEDURES

Anesthesia was induced in each parrot by administration of sevoflurane in oxygen. An end-tidal sevoflurane concentration of 2.5% was established in the first bird. Fifteen minutes was allowed for equilibration. Then, 2 types of noxious stimulation (mechanical and electrical) were applied; stimuli were separated by 15 minutes. Responses to stimuli were graded as positive or negative. For a positive or negative response to a stimulus, the target end-tidal sevoflurane concentration of the subsequent bird was increased or decreased by 10%, respectively. The MAC was calculated as the mean end-tidal sevoflurane concentration during crossover events, defined as instances in which independent pairs of birds evaluated in succession had opposite responses. A quantal method was used to determine sevoflurane MAC. Physiologic variables and arterial blood gas values were also measured.

RESULTS

Via quantal analysis, mean sevoflurane MAC in thick-billed parrots determined with mechanical stimulation was 2.35% (90% fiducial interval, 1.32% to 2.66%), which differed significantly from the mean sevoflurane MAC determined with electrical stimulation, which was 4.24% (90% fiducial interval, 3.61% to 8.71%).

CONCLUSIONS AND CLINICAL RELEVANCE

Sevoflurane MAC in thick-billed parrots determined by mechanical stimulation was similar to values determined in chickens and mammals. Sevoflurane MAC determined by electrical stimulation was significantly higher, which suggested that the 2 types of stimulation did not induce similar results in thick-billed parrots.