Ketamine and alpha-2 agonist combinations

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The text on this page is taken from an informal compilation of opinions of contributors to the online VOLE List. As such, they are not peer reviewed and may contain differences of opinion. Those wishing to contact the list may contact Adrian Smith.

We are wondering if any groups have experienced a change in adverse events using their established ketamine/a2 combinations, and if so what do you think were factors?

One of the groups I work with has recently started to experience increased dyspnoea/mortality using ketamine-xylazine combination intraperitoneally in BL6 based mice for a 45-90 minute long imaging protocol, with a ~30 minute top-up and atipamisol reversal, having used this for years with a reported ~1-5% mortality rate. The animals seemed to develop progressive respiratory failure, or if the alpha-2 agonist is reversed, start to recover regaining righting reflex but then decline and die.

We have made changes (reducing ketamine dosage, no longer giving a top-up, and swapping the alpha-2 agonist to medetomidine - now animals have a single induction dose of 70mg/kg ketamine and 0.5mg/kg medetomidine injected ip, plus reversal) which has improved things considerably, but we are now looking at investing in engineering solutions to permit isoflurane use instead as a long-term solution with multiple benefits.

We have engaged in brainstorming, but remain unsure what may have changed to explain the need for lower doses, except for housing in IVCs and the observation that 'it's been quieter' during Covid shutdown (possibly less baseline stress in the mice?) - although problems were noted back in December 2019 by this and a different group (using rats). We have reviewed many obvious possibilities e.g. strain, drugs in date, training of new staff, drug dilutions, dose calculations, health status has been consistent, subcutaneous fluids, pre-GA food availability, gender, incidence in control v. experimental animals, etc.

Just a thought if the mortality risk appeared linked to the bolus top up of anesthetic agents. For prolonged anesthesia where inhalation agents are contraindicated, we get a nice, uninterrupted plane of surgical anesthesia using an intraperitoneal continuous-rate infusion with ketamine/alpha-2 agonist in mice (to avoid a second bolus injection). You just need a syringe driver and an extension line/catheter, so it is technically simple and we do not see many adverse incidences (reverse as needed).

Here is the original research article:

Maybe you could also try reducing the alpha-2 dose if you are seeing significant respiratory depression (although the doses you quote look standard)?

I would very much appreciate your advice on long-term (up to 3.5 hours), repeat, recovery ketamine/xylazine general anaesthesia in rats.

A researcher wishes to perform thoracotomy cardiac infarction/reperfusion studies in Long Evans rats (200-250 g). Does not want to use halothane/isoflurane due to cardioprotective effects. Proposing to use ketamine (80 mg/kg) / xylazine (8 mg/kg) IP anaesthesia induction. Following induction, buprenorphine 0.05 mg/kg SC pre-emptive analgesia. Animals in supine position, on heating pad at 37C, intubated and ventilated with 100% O2 (2.5 ml/stroke) at 70 strokes/min. After ~20-30 mins of GA, on return of pedal withdrawal reflex (monitored every 5 mins), proposes 40 mg/kg IP ketamine boluses only (as required) to maintain anaesthesia. Total anaesthetic time (open chest) up to 2.5-3.5 hours. Following recovery, re-anaesthetize animals on same day after 2-12 hours with same anaesthetic regimen. Reperfusion of cardiac ischeamia then for 2 hours under anaesthesia. Buprenorphine 0.05 mg/kg BID SC for 5 days post- surgery. Re-anaesthetize after 28 days to harvest tissues.

I am concerned about the anaesthetic regimen, but cannot find solutions online or in standard texts. Are induction doses appropriate? Should only ketamine be given after initial 20-30 mins of anaesthesia, and should this be IM or IV rather than IP? Additional xylazine required during 3.5 hour surgery? Should alternative anaesthesia/analgesia agents be considered instead, that don't have significant cardioprotective effects?

This regimen should work, but a better means of "top up" would be to place an i/v catheter in the tail, and give diluted ketamine (10mg/ml), at about 5-10mg/kg, as a top up. This regimen is very cardiodepressant, and will produce a wide range of endocrine and sympathetic effects. It will also produce marked hyperglycaemia and an osmotic diuresis (increased by a direct ADH inhibition). The stability of the anaesthesia could probably be assisted by infiltration of bupivacaine as an intercostal block. Personally, I'd go for a quick knockdown with iso, put in a tail vein catheter and infuse proposal (either alone, or even better with an opioid) - once the rates for these rats has been established, it should be easy to manage, and the effects of a slight overdose would be resp depression, and they are ventilating the animals.

Using this regimen, at what frequency would you expect to give the diluted ketamine "top up" in the tail vein? Would you be guided mainly by pedal withdrawal reflex?

Given the diuresis, what intra-/ post-operative fluid supplementation (s/c boluses?) would you suggest?

I don't think propofol would be considered, due to cardioprotective effects.

If not propofol, how about alfaxan (there should be some data on cardio-protection as alphaxalone/alphadolone ("althesin") was a human product). To start with, I'd expect to top up every 30-40 mins, but once a few animals have been done, then about 2/3 the total "anticipated" top up could be given as a continuous infusion, starting about 20-30 mins after induction. This makes the whole process easier to manage - pumps are very cheap now - and if you need to buy one, the latest "Genie" pump from Kent Scientific is great value and very versatile. As you say, being guided by the withdrawal response will let them adjust the top ups - one thought is to add a pulse ox (needs to be a good one as the peripheral vasoconstriction can stop some devices functioning reliably) and then you will have heart rate responses as additional monitoring of depth (sudden increases of 10-20% may indicate its time for a top up), also, as they are ventilating, any asynchronous diaphragm movements (easiest to spot if you are monitoring ET co2) also can indicate lighter anaesthesia - the surgeon will probably spot this as it will annoy them!

Regarding the ketamine top ups – after the first few rats, would you administer the 2/3 of the total anticipated top-up by CRI over the full operative time (at 10 mg/ml?); with smaller IV ketamine boluses as required?

Yes, but as the operation times should become fairly standard, stop the infusion 10-15 minutes before completion, to speed recovery.

The text below is also taken from an informal compilation of opinions of contributors to the online VOLE List. As such, they are not peer reviewed and may contain differences of opinion. Those wishing to contact the list may contact Adrian Smith.

Use of atipamezole in mice and rats:

Are there any thoughts about the use of a reversing agent when using medetomidine and ketaine In rats and mice? I have been asked  a question at our current training course about the unpleasant recovery experienced after ketamine in larger animals. Is this the case in rodents - should we discourage the sue of reversal agents for this reason?

We are indeed encouraging our researchers to reverse any alpha 2 agonist as much as possible, in order to significantly shorten the recovery period (up to 2 hours without, to only approx 5 min with atipamezole).

Ketamine's duration of action is much shorter than medetomidine, so by the time your surgery is over, ketamine has gone (no need to reverse it) and the alpha 2 is the only one on board.  So unless atipamezole is administered very shortly after the initial alpha2/ketamine cocktail (i.e. less than 10-15 min) we are very unlikely to see some frantic ketamine recovery. I should add that even if the animal was to show some (mild, in any case) paradoxical excitement, it would be potentially much less damaging to the animal than a prolonged period of (sometimes severe) hypothermia, for instance. I appreciate where the apprehension comes from, and we would definitely avoid to inject ketamine to any large animal (horse of cattle) just before recovery, because of the risk of self injury or fracture during recovery. Having said that, even in horses, we use ketamine infusion during anaesthesia and surgery as part of a multimodal analgesic plan. Ketamine was actually shown not to have any deleterious effect on horses' recovery unless it is infused at high doses for more than 2 hours (accumulation of norketamine).

As long as the atipamezole isn't administered IV, everything should be alright.

Sometimes after short surgeries we have seen that rats can become a bit hyperactive when the atipamezole is given (not ideal when they've just had surgery), so if this happens then the subsequent rats only get half the reversal dose. They are recovered in incubator cabinets so a slightly slower recovery is not too much of an issue.

When considering the possible excitation from residual ketamine -  how long do you want the rats to be a risk of hypothermia, have marked cardiovascular system depression, an osmotic diuresis, hyperglycaemia, respiratory depression and a lot of endocrine disruptions  - all of which will be reversed by the atipamezole - for a bit more detail see "Laboratory Animal Anaesthesia".

Addiitonal resources: