Carbon dioxide as a short-term restraint anaesthetic in rats with subclinical respiratory disease

The use of carbon dioxide (CO2) with, and without, oxygen (O2) as a short-term restraint anaesthetic for Wistar rats in which subclinical respiratory disease was endemic, was assessed in 3 separate experiments. In the first, rats were placed in a CO2 atmosphere generated from solid CO2 chips in a 70 l plastic bin, and removed at time intervals ranging from 0 to 120 s after disappearance of the pedal reflex. Eight of 25 rats died, including 2 which were removed immediately the pedal reflex disappeared; it was concluded that CO2 without O2 was not a suitable short-term anaesthetic for rats. In a second study, rats were anaesthetized in atmospheres of 50:50 and 80:20 (CO2:O2) provided from commercially available cylinders, in 2 different environments--a 3.41 glass jar and a 17 l plastic bin. Rats became excited in the plastic bin but not the glass jar. Rats in the glass jar displayed visible depression and cessation of whiskers movement significantly more quickly in the 80:20 (CO2:O2) than in the 50:50 mixture (4.2 +/- 0.98 s, n = 6, and 66.0 +/- 4.9 s, n = 6 vs 13.8 +/- 2.77 s, n = 5 and 152.0 +/- 20.8 s, n = 5, respectively). Rats in the 17 l plastic bin lost their pedal reflexes in a mean 41.5 +/- 4.55 s (n = 11) in the 50:50 mixture and in a mean 30.9 +/- 6.38 s (n = 11) in the 80:20 (CO2:O2) group. Those left in the 50:50 mixture for 60 s and 180 s after disappearance of their pedal reflexes, recovered these reflexes in 20.2 +/- 0.44 s and 21.5 +/- 7.23 s respectively after removal from the gas. Respiration and heart beat ceased in one rat remaining in the 50:50 mixture after 13 min 10 s. No untoward effects occurred in rats left in the 50:50 mixture for 180 s after disappearance of the pedal reflex, but 2 died when left for an equivalent period in the 80:20 mixture. In the third study, examples of the practical use of a 50:50 mixture as a short term restraint anaesthetic are described. It was concluded that this mixture was a cheap, safe, and effective means of short-term restraint for rats with subclinical respiratory disease, when the minimal time of exposure to the gases was employed.