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I have been at two universities now and the first permitted the precharging of a cage with co2 before a mouse is put in, but the second does not. I was wondering what the reasoning is behind the ban on precharging a cage with Co2.

When I think about this, it seems that it would be less stressful for the mice to fall unconscious very quickly rather than a slow and gradual suffocation. Is there some shock associated with the precharged cage that makes this more stressful in some way?

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  • $\begingroup$ Upon some research, I have found more guidelines from universities attributing to NOT PRECHARGING the chamber with CO2. Precharging actually results in more distress for the rodents. $\endgroup$ – Imtiaz Raqib Dec 13 '16 at 22:09
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Quoting from guidelines to CARBON DIOXIDE EUTHANASIA OF RODENTS from University of Pennsylvania >>

  1. Do not “pre‐fill” the euthanasia chamber with CO2.  Start with room air then slowly fill the chamber with CO2 over several minutes (OLAW).  CO2 is heavier than room air, thus the chamber may need to be “purged” between groups of cages.

    “Because inspiration of high concentrations of CO2 is both aversive and painful, a recommended procedure is to place animals into a chamber that contains room air and then to gradually introduce CO2.” (ACLAM)

  2. CO2 first renders the animal anesthetized and then, with adequate exposure time, will result in death by CO2 narcosis.  Animals should be left for additional time within the euthanasia chamber, after spontaneous movements have ceased, with CO2 continuing to flow.  

    “Animals should be left in the container until clinical death has been ensured.” (NIH)

Also, according to an article in The Scientist by Kerry Grens >>

It's been realized that at high concentrations, CO2 causes creatures torment. After reaching mucosal surfaces, it transforms into carbonic acid, which stings. Also, in people, even low fixations can be upsetting, bringing about shortness of breath.Therefore, explaining why high levels of CO2 is actually more painful for the rodents.

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    $\begingroup$ I know I'm not supposed to use comments for this, but +1 for a very well presented answer. $\endgroup$ – anongoodnurse Dec 14 '16 at 4:15
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    $\begingroup$ I have always understood that in all methods of "humanely" euthanising animals, the idea is not to kill them fast, but to first softly get them unconscious and then kill them in their "sleep", as whatever is killing them will usually cause distress in a lethal dose. But this is just from having to watch pets be put down as a child. $\endgroup$ – skymningen Dec 14 '16 at 12:07
  • $\begingroup$ @anongoodnurse Thank you. It is nice to get into this community. $\endgroup$ – Imtiaz Raqib Dec 14 '16 at 14:49
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    $\begingroup$ @skymningen in my experience, cervical dislocation (CD) is the most rapid, humane, and cost-efficient method of euthanasia, of course when performed by well-trained researchers or staff. The way I learned it, you keep the mouse in its cage up to the very end, so it doesn't become stressed. You then remove the animal, and (very important) out of sight of any other animals, you place the mouse perpendicular to the wires on the top of an old-fashioned regular cage and grip its tail near the base until it reflexively grabs onto a wire. [...] $\endgroup$ – MattDMo Dec 14 '16 at 18:26
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    $\begingroup$ @ImtiazRaqib knocking out the mice with $CO_2$ kind of defeats part of the purpose of CD, although in times when I've had to euthanize via $CO_2$ I've also done CD on the bodies just to make sure they're dead. Even if you do asphyxiation slowly (in my experience, at least), the mice still get panicked as they realize there's not enough air, but before they're knocked out. Maybe I was just adding the gas too quickly, I'm not sure. The reason I used CD in grad school was because I was in an immunology lab, and stress can activate the immune system, altering your data. Proper CD doesn't do that. $\endgroup$ – MattDMo Dec 14 '16 at 19:32

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