Application of Dixon's Up-and-Down Design to Estimate the Minimum Alveolar Concentration of Sevoflurane in Rats with Refined Movement Classification

Summary

The protocol presented herein seeks to evaluate the depth of anesthesia utilizing Dixon's Up-and-Down Design in sedated rats subjected to a standardized pain stimulus. Furthermore, it delineates a classification of the various movement patterns observed.

Abstract

When using volatile anesthetic agents, the standard measure of potency is the minimum alveolar concentration (MAC), defined as the alveolar concentration at which 50% of subjects do not show a motor response to a painful stimulus. In clinical and experimental studies, the current consensus states that only "gross purposeful movements" count as a positive motor response and can, therefore, indicate too mild sedation. However, the appropriate level of anesthesia, in both humans and animals, varies individually based on various influencing factors, such as other medications administered. Two animal studies in rats were conducted to examine the effects of tetrahydrocannabinol and ethanol on sevoflurane, one of the most commonly used volatile anesthetics. To estimate the MAC, Dixon's Up-and-Down Design was employed. For the first animal, a predetermined dose of sevoflurane was chosen. After an equilibration period of 15 min, a standardized pain stimulus was applied to the distal third of the rat's tail using a surgical clamp. The animal's response to the stimulus was documented as positive or negative. If a positive motor reaction occurred, the dose for the next animal was increased. Conversely, if the rat did not exhibit a motor response, the dose for the following animal was decreased. Isotonic regression and the bootstrap method were used to estimate the MAC and 95% confidence intervals. This article outlines a detailed protocol for assessing anesthetic depth in rats using the up-and-down-design. It also provides a novel and detailed description and categorization of movement patterns in sedated rats when a standardized painful stimulus is applied to enhance reproducibility for future studies.

Introduction

Sevoflurane is one of the most frequently used volatile inhaled anesthetics¹. Its potency is assessed using the minimum alveolar concentration (MAC), which is defined as the alveolar concentration at which 50% of patients do not show a motor response to a painful stimulus^2,3,4. To establish the MAC, rats can be used as animal models due to their greater homogeneity compared to humans. The pain stimulus is typically applied using a surgical clamp on the distal third of the rat's tail². Using alternative methods for pain testi....

Protocol

All experimental animal studies have been approved by the Intramural Committee for Animal Experimentation of the Medical University of Vienna and the competent regulatory authority (Austrian Federal Ministry of Women, Science and Research). The experiments were conducted under constantly maintained anesthesia. The animals do not consciously perceive pain even if a movement should occur¹⁷. Adult Wistar rats at the ages of 81 to 95 days were used.

1. Study animals and housing

1. House the animals according to weight and sex. Separate males and females and house them for a minimum of 14 days be....

Results

In the THC study, the bootstrap estimate of the MAC was 2.8 vol% in the control group (95% CI: 2.7-2.9 vol%) and 2.1 vol% in the 10 mg/kg THC group (95% CI: 1.8-2.4 vol%)¹³.

In the ethanol study, the estimated MAC was 2.24 vol% in the control group (95% CI: 1.97-2.94 vol%). In the 1 g/kg ethanol group, it was 1.65 vol% (95% CI: 1.40-1.98 vol%), and in the 2 g/kg ethanol group, 1.08 vol% (95% CI: 0.73-1.42 vol%)¹⁴. The differing number of decimal .......

Discussion

This study refines the protocol for determining the MAC of sevoflurane in rats by detailing critical methodological steps and addressing common challenges to contribute to a more standardized and reproducible approach for assessing anesthetic depth.

Accurate MAC determination is essential for both experimental and clinical anesthetic practices. This work contributes to the existing literature by providing a clear and reproducible method for assessing sevoflurane potency in animal models. The r.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Airtight transparent chamber	N/A	N/A	Generic
Arterial clamp	KLS Martin	13-410-22-07	25 cm
Dräger Primus	Dräger Austria GmbH	Primus
Dräger Vapor 2000	Dräger Austria GmbH	Vapor 2000
Homeothermic Blanket Control Unit	Harvard Apparatus, Holliston, Massachusetts, USA	50-7137	230 V
Manual Mobile Infant Warmer	Fisher/Paykle, Auckland, New Zealand	IW9210/920