My last post on rodent anesthesia looked at the use of chemical anesthesia, in particular, the use of ketamine-xylazine to attain surgical unconsciousness. I identified several issues with the use of this combination, most importantly the lack of dosage control and lack of predictability in response.
So what are the alternatives?
If ketamine-xylazine is not used, most often researchers will turn to an inhalent anesthetic. Inhalents use a vaporizer to deliver a gaseous form of a chemical directly to the rodent via a hose or face mask (or through intubation for larger animals) in combination with oxygen. The most common inhalant used in rodents is isofluorane gas.
Isoflurane is a halogen-based gas which causes muscle relaxation and unconsciousness. It used to be used in human medicine, but is now principally used in veterinary medicine and research.
Gaseous anesthetics require more equipment than injectible anesthetics. In addition to the vaporizer and assorted tubes and hoses, a scavenging system must be used. Scavengers gather up leftover gases to prevent worker exposure. Charcoal canisters which absorb residual gases are a common form of passive scavenging, while various types of ventilated hoods can be used for active scavenging. Never use anesthetics without a proper scavenging system! Chronic isoflurane exposure has been linked to cognitive decline.
[Aside: I had some bad exposure to isoflurane in the past and it is like being in a fog – you just can’t think. Little bit terrifying. Thank God for new careers! Computers can only ruin my posture and eye sight! … ]
But using all this complicated equipment has it’s advantages! The vaporizer allows the technician to control precisely how much gas is being delivered to the animal. If you need to increase or decrease it, you know exactly how much you are raising or decreasing the dosage. It is much harder to accidentally cause an overdose. As well, since the gas is being continually delivered, there is no risk that the animal might being to wake up in the middle of a procedure.
For the animals, recovery from isoflurane is much, much quicker than recovery from ketamine-zylazine. Isoflurane is excreted entirely via the lungs – no residual chemical is left in the system. So once an animal is removed from the gas flow, it beings to recover immediately. Full recovery occurs in only a few minutes and aside from regular post-surgical care, rodents do not generally require additional monitoring.
Unfortunately, isoflurane does not provide any analgesic properties aside from unconsciousness during the procedure. So, analgesic must still be given before and after surgical procedures.