BioSpace Collaborative - Combining Nitrous Oxide with Carbon Dioxide Decreases the Time to Loss of Consciousness during Euthanasia in Mice

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Anesthesiology and Pain Management - Physiology - Science Policy

Combining Nitrous Oxide with Carbon Dioxide Decreases the Time to Loss of Consciousness during Euthanasia in Mice — Refinement of Animal Welfare?
Published: Thursday, March 15, 2012
Author: Aurelie A. Thomas et al.

by Aurelie A. Thomas, Paul A. Flecknell, Huw D. R. Golledge

Carbon dioxide (CO₂) is the most commonly used euthanasia agent for rodents despite potentially causing pain and distress. Nitrous oxide is used in man to speed induction of anaesthesia with volatile anaesthetics, via a mechanism referred to as the “second gas” effect. We therefore evaluated the addition of Nitrous Oxide (N₂O) to a rising CO₂ concentration could be used as a welfare refinement of the euthanasia process in mice, by shortening the duration of conscious exposure to CO2. Firstly, to assess the effect of N₂O on the induction of anaesthesia in mice, 12 female C57Bl/6 mice were anaesthetized in a crossover protocol with the following combinations: Isoflurane (5%)+O₂ (95%); Isoflurane (5%)+N₂O (75%)+O₂ (25%) and N₂O (75%)+O₂ (25%) with a total flow rate of 3l/min (into a 7l induction chamber). The addition of N₂O to isoflurane reduced the time to loss of the righting reflex by 17.6%. Secondly, 18 C57Bl/6 and 18 CD1 mice were individually euthanized by gradually filling the induction chamber with either: CO₂ (20% of the chamber volume.min-1); CO₂+N₂O (20 and 60% of the chamber volume.min^-1 respectively); or CO₂+Nitrogen (N₂) (20 and 60% of the chamber volume.min-1). Arterial partial pressure (P_a) of O₂ and CO₂ were measured as well as blood pH and lactate. When compared to the gradually rising CO₂ euthanasia, addition of a high concentration of N₂O to CO₂ lowered the time to loss of righting reflex by 10.3% (P<0.001), lead to a lower P_aO₂ (12.55±3.67 mmHg, P<0.001), a higher lactataemia (4.64±1.04 mmol.l^-1, P?=?0.026), without any behaviour indicative of distress. Nitrous oxide reduces the time of conscious exposure to gradually rising CO₂ during euthanasia and hence may reduce the duration of any stress or distress to which mice are exposed during euthanasia. More...