Comparison of Anesthetic Potency and Cardiopulmonary Effects of Isoflurane and Sevoflurane in Colubrid Snakes
A comparison of the characteristics of isoflurane and sevoflurane in colubrid snakes was conducted to determine if potency and physiologic effects were similar to those in mammals. Isoflurane is widely used to anesthetize reptiles because of its characteristic rapid induction, ease of adjustment of anesthetic depth, and rapid recovery. The pungent odor of isoflurane is associated with breath-holding in human pediatric anesthesia. Because breath-holding in some reptiles is easily elicited, the use of isoflurane has raised concern among veterinarians caring for reptiles.
Sevoflurane is a potent inhalant anesthetic recently introduced into veterinary medicine. For humans, dogs, and cats, the advantages of sevoflurane are its low solubility, resulting in rapid anesthetic induction and recovery, and a mild odor that causes minimal breath-holding and smooth mask inductions. Whether these characteristics apply to colubrid snakes has not previously been studied.
The minimum infundibular concentration (MIC) of two inhalant anesthetics, isoflurane and sevoflurane, was calculated for colubrid snakes of the species Elaphe radiata, the radiated, or copperhead, rat snake. Seven snakes, three males and four females, were studied. All animals were captive bred, had no recorded history of disease, and were of known reproductive status. Since each animal was to be anesthetized twice, once each for isoflurane and sevoflurane, a random number generator was utilized to establish the order of the anesthetic agent and the animal. Anesthetic protocols were run no less than 6 d apart on each animal to ensure complete recovery and to negate any potential effects of residual anesthetic from the previous treatment.
Anesthesia was administered by mask, sized to minimize dead air space, connected to a Bain non-rebreathing circuit. The snakes were allowed to breathe spontaneously throughout the study. The mask was placed over the head and neck, and the animal was manually restrained and allowed to breathe O2 while preanesthetic data, and 0.25 ml blood from the tail vein, were collected. Temperature, % saturation of hemoglobin (SpO2), heart rate, inspired oxygen, inspired and end-tidal anesthetic, and end-tidal CO2 concentrations were measured.
The MIC for sevoflurane and isoflurane was determined by a bracketing method similar to that described for the determination of minimum alveolar concentration (MAC) for mammals and in birds. Initially, the inspired concentration was set at 2 x MAC for the dog. The snakes were allowed to equilibrate for a minimum of 10 min, or until the inspired and expired concentrations of anesthetic agent were within 0.1% of each other, as reported by the analyzer (Model 5250 RGM gas analyzer, Ohmeda Corporation, Louisville, Colorado). After the animal reached a stable anesthetic level, the degree of anesthesia was scored using three criteria. First, the righting reflex was assessed based on the snake ability and attempt to roll from dorsal to sternal recumbency. A complete lack of righting attempts was scored as 0, a minimal attempt as 1, moderate attempt 2, and success at righting was scored as 3. Secondly, a Satinsky (atraumatic vascular) forceps was placed across the last 1/3 of the tail and pressure applied for 15 sec. The forceps was closed to two full clicks of the locking mechanism. If there was no motion or reaction, either at the time of clamping or after the clamp had been removed, the score was recorded as 0; withdrawal or active motion at the time of clamping was scored as a 3. Delayed reaction after the clamp had been removed or motion while the clamp was applied would score a 1 or a 2, depending on the intensity of motion. Thirdly, body muscle tone was assessed. If the entire length of the body was flaccid or had mild, non-returning muscular tone upon flexure, tone was scored as 0. A score of 1 was assigned if mild to moderate muscular tone was present that did not resolve upon flexure and was limited to the caudal half of the snake. If tone was present along the entire length of the animal, with or without mild motion, it was scored as a 2. Overall tone with deliberate motion was scored as 3. Scoring was performed at each anesthetic concentration; the process took 1–3 min to complete.
Following assessment of anesthesia, the anesthetic concentration was changed, such that the new level was half the difference between the previous two settings (example: first setting 2%, second setting 1%, third setting 1.5%). Calculation of the MIC for each snake was the anesthetic concentration midway between the lowest concentration with a sum score equal to 0 and the highest concentration where the sum score was greater than 0. Once a MIC had been established, blood was drawn for analysis. At the end of each study, the vaporizer was turned off, the animal was recovered on 100% oxygen, and the time to recovery was recorded. Following anesthesia, each animal was housed alone for 12–18 h to assess condition before being returned to group housing.
MIC was found to be 1.68%±0.30% (SD) for isoflurane and 2.42%±0.57% (SD) for sevoflurane. Respiratory and cardiac depression was found to occur with both anesthetics, with isoflurane causing a 23% decrease in heart rate compared to a 20% decrease in heart rate by sevoflurane. Respiratory depression was found to have greater differences between the two anesthetics, with isoflurane decreasing mean respiratory rates by 63% compared to a mean sevoflurane-induced depression of 83%. Neither agent caused significant decreases in SpO2 despite depression of respiratory rate. Although respiratory rate decreased with both agents, sevoflurane decreased end-tidal CO2 less than isoflurane with drops of 39% and 55%, respectively. Subjectively during induction, less breath-holding was observed with isoflurane, with more time required for sevoflurane induction than isoflurane induction. Both isoflurane and sevoflurane provided safe and effective anesthesia.
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