Kinetics and potency of halothane, isoflurane, and desflurane in the Northern Leopard frog Rana pipiens.

Equilibration between delivered and effect site anesthetic partial pressure is slow in frogs. The use of less soluble agents or overpressure delivery may speed equilibration. Ten Northern leopard frogs were exposed to 3-4 constant concentrations of halothane, isoflurane or desflurane and their motor response to noxious electrical stimulation of the forelimb evaluated every 30 minutes until a stable proportion of frogs were immobile. Each frog received each anesthetic and concentration in random order and allowed at least 14 hours to recover between anesthetic exposures. An overpressure technique based upon the kinetics in the first study was then tested with 4 concentrations of desflurane. For halothane, isoflurane and desflurane respectively; the proportion of frogs immobile in response to stimulus became stable after 510, 480 and 180 minutes, and ED50 values were 1.36, 1.67 and 6.78 % atm. Desflurane ED50 delivered by overpressure was not significantly different at 6.85 % atm. Halothane, isoflurane and desflurane are effective general anesthetics in frogs with potencies similar to those reported in mammals. The time required for anesthetic equilibration is fastest with desflurane and can be hastened further by initial delivery of higher partial pressures.

Isoflurane potency in the northern leopard frog Rana pipiens is similar to that in mammalian species and is unaffected by decerebration.

Amphibians are commonly used in biomedical research, including studies of mechanisms of anaesthetic action. There is, however, little published work describing the kinetics of inhaled anaesthetic agents or the potency of isoflurane in amphibians. Ten Northern leopard frogs were exposed to a constant isoflurane concentration of 1.0%, 1.2% or 1.5% atm for 4 h, and their response to a noxious stimulus was tested every 20 min. Each frog was anaesthetized with each concentration in random order and allowed at least 16 h to recover between anaesthetic exposures. Frogs were then pithed and the protocol was repeated. Frogs first displayed immobility during stimulus application at 80 min, and the proportion of animals becoming immobile steadily increased to reach a stable level at 4 h. The 50% effective dose for isoflurane in intact and pithed frogs did not differ, and was 1.15 and 1.25% atm, respectively. The potency of isoflurane in leopard frogs was similar to that reported in mammalian species. Cutaneous uptake of anaesthetic is effective given sufficient time, approximately 4 h in this study. Forebrain structures appear to be unimportant for the immobilizing action of isoflurane in the frog.

Intrathecal picrotoxin minimally alters electro-encephalographic responses to noxious stimulation during halothane and isoflurane anesthesia.

BACKGROUND: Isoflurane and halothane act in the spinal cord to blunt ascending transmission of impulses to the brain resulting from noxious stimulation. Because intrathecal picrotoxin (an antagonist at the gamma-aminobutyric acid-A receptor) partially reverses the immobilizing effect of isoflurane and halothane, we hypothesized that the electroencephalographic response to noxious stimulation would likewise be partially reversed by intrathecal picrotoxin. METHODS: Rats were anesthetized with isoflurane (n = 8) or halothane (n = 8) and a laminectomy performed. Following determination of minimum alveolar concentration (MAC), the electroencephalogram (EEG) was recorded during separate applications of a hindpaw clamp, tail clamp and electrical current to the tail at 0.8 and 1.2 MAC. Picrotoxin was then applied to the exposed spinal cord and the EEG response to noxious stimulation again determined. RESULTS: The EEG was more active during halothane anesthesia than isoflurane (spectral edge frequency for 95% power: 25.6 +/- 2.1 Hz vs. 23.1 +/- 1.6 Hz, P < 0.05). Noxious stimulation usually caused the EEG to shift to higher frequencies (e.g. for 0.8 MAC halothane, median edge frequency for 50% power: from 7.6 +/- 3.1 Hz to 10.7 +/- 2.6 Hz, P < 0.05). Picrotoxin minimally affected this response. CONCLUSIONS: Noxious stimulation evokes an EEG response that is minimally altered by intrathecal picrotoxin. This suggests that isoflurane and halothane do not have GABAergic actions in the spinal cord that indirectly suppress the EEG response.

Animal dependence of inhaled anaesthetic requirements in cats.

BACKGROUND: The minimum alveolar concentration (MAC) of an inhaled anaesthetic describes its potency as a general anaesthetic. Individuals vary in their sensitivity to anaesthetics and we sought to determine whether an individual animal's sensitivity to inhaled anaesthetics would be maintained across different agents. METHODS: Six female mongrel cats, age 2 yr (range 1.8-2.3) and mean weight 3.5 (SD 0.3) kg, were studied on three separate occasions over a 12-month period to determine the MAC of isoflurane, sevoflurane and desflurane. Induction of anaesthesia in a chamber was followed by orotracheal intubation and maintenance of anaesthesia with the inhaled agent in oxygen delivered via a non-rebreathing circuit. MAC was determined in triplicate using standard tail-clamp technique. RESULTS: Mean MAC values for isoflurane, sevoflurane and desflurane were 1.90 (SD 0.18), 3.41 (0.65) and 10.27 (1.06)%, respectively. Body temperature, systolic pressure and Sp(O(2)) recorded at the time of MAC determinations for isoflurane, sevoflurane and desflurane were 38.3 (0.3), 38.6 (0.1) and 38.3 (0.3) degrees C; 71.2 (8.3), 74.6 (15.9) and 88.0 (12.0) mmHg; 99.2 (1.1), 99.1 (1.3) and 99.4 (0.8)%, respectively. Both the anaesthetic agent and the individual cat had significant effects on MAC. Correlation coefficients for comparisons between desflurane and isoflurane, desflurane and sevoflurane, and sevoflurane and isoflurane were 0.90, 0.89 and 0.97, respectively. CONCLUSIONS: These findings show that an individual has a consistent degree of sensitivity to a variety of inhaled anaesthetics, suggesting a genetic basis for sensitivity to inhaled anaesthetic effects.

Comparison of methods to assess dog owners' therapeutic compliance.

OBJECTIVE: To compare different methods for assessing the compliance of veterinary clients administering medication to their dogs. PROCEDURE: Thirty-one owners whose dogs were prescribed amoxycillin-clavulanate, twice and thrice daily, for a duration of five to seven days were recruited from three Sydney veterinary hospitals. Compliance was assessed by electronic monitoring devices, return medication counts, client self-reports and veterinarians' estimation of likely compliance. RESULTS: Electronic monitoring showed owners administered on average 84% (range 7 to 104%) of prescribed medication to their dogs. Both return medication counts and client self-reports tended to overestimate therapeutic compliance when compared with electronic monitoring. When questioned, the majority of owners (71%) claimed perfect compliance with the prescribed regimen. No correlation was found between veterinarians' estimates of owner compliance and that assessed electronically. CONCLUSION: Electronic monitoring provided valuable information on dose timing and variation, but proved costly and difficult to employ in veterinary practice. Simplicity and low cost of return medication counts makes this method attractive for use in veterinary compliance studies. Client self-reports reliably detected some noncompliers and permitted identification of individual problems or errors. For practical purposes a combination of return medication counts and client self-reports may have merit in future veterinary compliance studies.

Owner compliance with short term antimicrobial medication in dogs.

The degree to which dog owners complied with instructions to administer a 5 to 10 day course of antimicrobial medication to their pets was assessed using microprocessor based monitoring devices. Twenty two clients gave an average of 84% of prescribed doses of amoxycillin-clavulanate. No difference was found between twice and thrice daily dosing regimens in the overall percentage of prescribed doses given. However, timing of doses was far from ideal in many cases and only 34% of doses were given within the designated optimum time period. Adherence to desired dosing intervals tended to be better with twice daily than with thrice daily dosing, although the difference was statistically insignificant.