The study of anesthesia and analgesia in reptiles has significantly expanded and there is readily available information in the literature to help make sound clinical decisions for these patients. While reptiles are often thought of as stoic animals that do not show many behavioral changes, this is a misconception and anyone working with them should become familiar with the signs of pain and discomfort. It is also important to remember that the analgesic effect provided by general anesthesia is short lived, up to the point of recovery, and analgesic protocols must be instituted in all case when painful stimuli is unavoidable. We must think of anesthesia and analgesia as being two different components of one modality, both being essential for its success.
|Lecture Objectives will include:|
|I. Introduction – Definitions
About the Presenter
Dr. Nevarez became a Diplomate of the American College of Zoological Medicine and the European College of Zoological Medicine (Herpetology) in 2011. He has been a faculty member of the Zoological Medicine service at the LSU School of Veterinary Medicine since 2003 where he also serves as director of the Wildlife Hospital of Louisiana. [MORE]
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Download Dr. Nevarez’s PowerPoint presentation: PDF (6 MB)
|General principles||Inhalant anesthesia||Injections|
|Signs of pain||Monitoring||Miscellaneous|
|Preoperative period||Catheters and fluids||References|
|Agents and/or routes||Venipuncture|
Q: Do external temperature fluctuations significantly affect drug metabolism?
A: YES. There’s definitely going to be an effect with temperature. The closer the reptile is to its preferred optimum temperature zone for the species, the more efficiently they will metabolize drugs and they will also get rid of the anesthetic drugs more efficiently so that will also help with recovery of the animals.
Q: What is the mortality rate associated with anesthesia in reptiles?
A: I am not aware of any studies in the literature that document what the mortality rate is per se. Of course it’s going to depend on the health status of the animal, the type of procedure, etc. I would say in general, compared to mammals or birds, the mortality rate is probably going to be a little bit less in reptiles. We don’t see as many mortalities, but you WILL see a lot more prolonged recoveries.
SIGNS OF PAIN
Q: Is there any correlation between pain and leg use in chelonians with midline carapacial injuries?
A: That’s a really good question. I don’t know about correlation between pain and leg use, but one thing we know about reptiles is they tend to have a lot of peripheral innervation that is independent of central innervation. Although more research is needed, the short answer to that question is: There are no studies that I’m aware of that show a particular correlation between pain and leg use.
We have seen chelonians that have mid-carapacial injuries or even complete, transverse, spinal injuries—with no connection whatsoever—and they’re still able to move their limbs. The reptile nervous system is very different. There’s a whole different pathway here, a lot more peripheral pathways. You certainly will see animals that might have decreased use of those limbs, but whether that’s from the nervous injury itself or pain, that’s hard to tell.
Q: So are you not as concerned about doing bloodwork in older reptile patients prior to giving an NSAID like we often do for mammals?
A: In general, exotics seem to be a lot more tolerant to NSAIDs and don’t get as many of the side effects as in mammals. That is evidenced also by the higher dosages that we use in exotic species. Of course having bloodwork is always ideal. If you can have that bloodwork, especially a biochemistry panel, that is always going to be good.
Hydration status is also a consideration. So we are always get concerned about patients that are very dehydrated while using NSAIDs. Give fluids concurrently with NSAID administration. It’s definitely something to monitor, but reptiles definitely aren’t as sensitive to the side effects of NSAIDs as dogs and cats.
Q: How long should you fast reptiles?
A: This will depend on the species. We typically do a few (2-3) hour fast. One big advantage of reptiles is that the clinician can have much better control of the glottis, so aspiration pneumonia is a lot less likely to happen compared to a bird. But again it depends on the species and what they’re eating. Obviously it’s based on the metabolism; that’s going to change quite a bit too.
Most reptiles you can really just fast them for a day or two before the procedure. There are only rare instances when you’re going to have reptiles that are constantly eating like a dog or cat or other mammals. So fasting is typically not that big of a concern.
Clinically, it’s not something we think about other than making sure that if we have a snake, for example, don’t feed them the day before. If we have some of the chelonians, they do tend to eat a little bit more constantly, we’ll typically pull the food 2-3 hours before the procedure.
Q: Is preoperative oxygen therapy contraindicated as well?
A: That’s a good question. I don’t know that preoxygenation is CONTRAINDICATED but I don’t think that anyone has done any studies to look at the benefit of preoxygenation in reptiles as in other species. I don’t think you’re necessarily going to see a significant detrimental effect for anesthesia, but you do have to keep in mind that if you preoxygenate, the reptile will probably become apneic a lot sooner but most reptiles under anesthesia will become apneic anyway and you’ll just have to begin intermittent positive pressure ventilation regardless.
So I don’t think it’s necessarily a bad idea, but there’s just not enough information about how helpful it is. It’s something clinicians can try. I think preoxygenation would be more of a consideration if the reptile has respiratory disease and the ability to move oxygen may be altered in any way, shape, or form. Or preoxygenation might be a consideration for anemic animals. In animals that don’t suffer from these conditions, it might be hard to tell how beneficial preoxygenation might be.
SPECIFIC AGENTS AND/OR ROUTE OF ADMINISTRATION
Q: Do you inject alfaxalone all at the same site? Because volume is usually pretty high for large animals.
A: It depends on the volume and the size of the animal. In larger animals, alfaxalone does come to a large volume, so sometimes you need to split it between multiple sites if you’re going IM but if you’re going intravenously you can give it intravenously in the full volume.
Q: Do you use a new bottle of alfaxalone every time, how long do you store it. We have very variable effectiveness.
A: At least in the United States, alfaxalone is labeled for single use, so you do have to discard the bottle after you use it. So we [LSU-SVM] don’t store it. The bottle is opened up, and it has to be discarded that same day.
So if someone is looking at effectiveness after storage, then they need to look at the formulation depending on what country they’re in. I’m not sure if any formulations from other countries can be kept longer or not.
Q: What is the CRI dose of alfaxalone?
A: No real research has been done with CRI on alfaxalone; there are no good doses available in the literature at this time. Alfaxalone was mentioned because it is certainly a drug that CAN be used because alfaxalone is not going to accumulate in the system; it will be metabolized and excreted a lot faster.
If you look at dog and cat medicine, for example, and we extrapolate from their dosing system: Induction for a dog will be about 1-3 mg/kg and then for a CRI they will do 4-7 mg/kg, so basically they’re doubling the dose. In the reptile, there’s really a wide range of doses for alfaxalone, anywhere from 5-20, or even 30 mg/kg, depending on the species. So if someone wanted to attempt an alfaxalone CRI, I would probably start at 5-10 mg/kg, see what you get, then bump up from there.
Q: Any info on butorphanol?
A: For a number of years now, butorphanol has not been selected for analgesia in reptiles and that’s because butorphanol mostly affects kappa receptors and reptiles are now found to have more mu receptors. So that’s why the trend has been over the years to go away from butorphanol and start using drugs like morphine and hydromorphone.
In the presentation, I did not mention the drugs that are not used anymore, that don’t work in reptiles. I strictly described the drugs that are currently recommended and that are being used. So butorphanol is certainly not something that we recommend any more for analgesia whatsoever. I realize in some parts of the world, like Latin America, there is easy access to butorphanol but not so much to the other opioids. So you could still potentially use butorphanol as a sedative or as part of your anesthetic protocol, but it’s not going to necessarily provide analgesia. So in those instances, you’re going to have to rely more on meloxicam, or tramadol potentially.
Q: Are you using butorphanol or methadone in reptiles for sedation or premedication?
A: We don’t really use butorphanol in reptiles anymore because the main reason for the use of butorphanol in the past was as an analgesic, and now we know that reptiles are more receptive to mu-opioid agonists, not the kappa. You can certainly use butorphanol as an anesthetic or sedative, however the effectiveness of butorphanol as a sedative compared to hydromorphone is difficult to evaluation in reptiles, so we really don’t use butorphanol anymore in reptiles.
With regards to methadone, there is no good data on methadone use in reptiles yet.
Q: Any research into the use of desflurane?
A: I have no information on desflurane. I think isoflurane and sevoflurane are prevalent in veterinary medicine, and inexpensive enough. I think desflurane gets significantly more expensive, however desflurane would work. I don’t see any reason why it would not.
Q: What was the ketamine and dexmedetomidine dose?
A: I always encourage people to refer back to the literature. There are plenty of formularies out there. There’s the Exotic Animal Formulary, there’s a number of different textbooks that formularies in them. So it’s always best to go back.
Ketamine-dexmedetomidine doses are also going to vary with the species, the health status of the patient, the procedure. So make those educated determinations based on what you find in the formularies and based on the assessment of your patient.
Editor’s note: The doses mentioned in Dr. Nevarez’s presentation were ketamine (2-5 mg/kg) dexmedetomidine (0.025-0.05 mg/kg)
I am also interested in the use of the fentanyl patch, but I don’t understand if the dose was 2.5 – 12.5 μg/reptile/h or μg/kg/h.
A: In slide 12 of Dr. Nevarez’s presentation, the dose is written as 2.5-12.5 μg/h q24-72h (Gutwillig et al 2012, Darrow et al 2010, Gamble 2008)
Q: Is there any benefit to giving hydromorphone PO for analgesia?
A: The problem with oral administration, is we don’t necessarily have pharmacokinetic (PK) information, so I recommend before using any drug: Go online, try to find papers in the literature, see if there are any PK studies out there and then try to extrapolate. Based on that, making the best educated decision that we can. This is something we do on a daily basis when deciding on the use of some drugs. Another thing to add about oral administration in reptiles is you have to make sure the patient is healthy enough that the gut is working properly. And snakes are a very big challenge because adults usually eat once a week or every other week, so giving a lot of oral medication on a daily basis is probably not going to have significant absorption. So whenever possible, stay away from oral administration. Before giving any drugs orally to a reptile, you have to make sure that they are at the proper temperature, that metabolism is working properly to ensure they’re actually maximizing absorption of the drug.
Q: Can meloxicam be administered to reptiles orally?
A: I think a lot of people have to give meloxicam orally for outpatient care, but whenever possible, give an injectable. This will be a lot better than the oral route.
Q: What about medetomidine?
A: Medetomidine is certainly something we used to use. We don’t have medetomidine easily available in the States anymore, that’s why everyone has switched over to dexmedetomidine, but yes, if you have medetomidine available, it is something that was used very successfully in reptiles for a long time. So that’s certainly an option as part of a balanced anesthetic protocol.
Q: Do you combine midazolam with butorphanol?
A: We use midazolam a lot, [but] we don’t really use butorphanol in reptiles anymore because the main reason for the use of butorphanol in the past was as an analgesic, and now we know that reptiles are more receptive to mu-opioid agonists, not the kappa. You can certainly use butorphanol as an anesthetic or sedative., The effectiveness of butorphanol as a sedative compared to hydromorphone is hard to tell, so we really don’t use butorphanol anymore in reptiles. Midazolam typically goes with hydromorphone or morphine.
Q: Can you do constant rate infusion (CRI) of propofol?
A: Yes, you can certainly do a CRI of propofol. I think the one thing to really be careful of is propofol can stay around in the system a lot longer than [something like] alfaxalone.
Editor’s note: A maintenance range of 0.3–0.5 mg/kg/min IV has been described in the literature.
Q: Tiletamine use?
A: Telazol can certainly be used and there are still some species out there in which people use it. I think with the combinations of ketamine-dexmedetomidine-midazolam in which you can provide more balanced anesthesia, single use of Telazol has fallen out of favor. There are doses for tiletamine out there, you have to go back to the formularies, but again tiletamine by itself is not going to be adequate for true, balanced anesthesia, so I would obviously add some other compounds to that.
Q: Do you use tramadol orally?
A: We routinely do not use tramadol, but…if you talk to a small animal anesthesiologist, they’re not necessarily big fans of tramadol. They don’t really feel it has that great of an effect.
Q: If you have surgical patients that are very painful, post op meds? Meloxicam and re-dose hydromorphone?
A: Yes, for post-op meds yeah we’ll use meloxicam, then re-dose hydromorphone. Probably no more than q12h with the hydromorphone, so every 12-24 hours. It is very important to monitor the patient, then assess for pain and discomfort, then re-dose as the veterinarian deems appropriate. One thing to be careful of with more frequent dosing of hydromorphone is to look for evidence of sedation, which can also happen.
Q: What do you recommend for long-term pain relief for reptiles?
A: That’s an area where more research needs to be done, but based on the current research mu-opioid agonists are going to be more effective so they’re certainly an option.
We also use a lot of meloxicam, although there’s a lot of concern about its true effectiveness, but that’s why I typically use a dose of 0.5-1.0 mg/kg. Most of the studies are using the lower dosages in the 0.2 mg/kg range. Sometimes I’ll put reptiles on 1 mg/kg for a few days, then bring them down to 0.5 mg/kg. Clinically, I think it does seem to make a difference but again research is needed to look at the actual effectiveness of these higher dosages.
So again we sometimes use a combination of both non-steroidals and opioids for long-term pain relief.
Q: What doses of these drugs for intrathecal administration?
A: That is very well described in the paper cited in the presentation. Those interested in more information can go to the article. It explains the procedure and provides the dosages there.
Lidocaine (4 mg/kg, 2%), bupivacaine (1 mg/kg, 0.5%) and morphine (0.1-0.2 mg/kg) were used to provide spinal analgesia in male red-eared sliders over 0.5 kg BW (Mans 2014)
Q: How to give oral analgesic to snakes for long periods?
A: Again oral administration is not the most efficient route because the gut is not working when the reptile is not digesting prey items. So absorption is not going to be very efficient. We certainly have seen snakes that we’ve had on oral antibiotics long term and they certainly do seem to improve on that, but one trick when you’re doing that is, in addition to giving the oral analgesic like meloxicam, give a little bolus of fluid in an effort to stimulate gut motility, but again there’s no real data out there on that.
Q: When using your combinations are you mixing all into one syringe or giving the different drugs at intervals?
A: It depends on which drugs you’re using, the volume, the purpose. When we’re doing combos of ketamine, dexmedetomidine, hydromorphone, and midazolam in a large reptile, like a crocodilian or large sulcate for example, often I will put all that in one syringe and give that all IV. If you have a smaller patient and you have to go intramuscular, sometimes you do have to split that a little bit more. And in that case, I would probably give the hydromorphone and midazolam as a premedication IM first in one site. And then come back with the ketamine-dexmedetomidine in a different site.
Q: Do you have to mechanically ventilate all reptiles under general anesthesia?
A: Yes, a lot of reptiles will become apneic under general anesthesia and much of that probably has to do with the 100% oxygen flow, so you either have to manually ventilate them or put them on a mechanical ventilator, depending on patient size. For most reptiles, you only have to ventilate them 2-4 times per minute, depending on the species, size, etc.
Q: How long do you have from discontinuation of oxygen to movement/recovery of the patient?
A: That is going to depend on your anesthetic protocol, and assuming the patient is warm, well hydrated, and all the other things that we talked about.
If you’re using something like alfaxalone-midazolam-hydromorphone, for example, those animals typically recover fairly quickly. If you’re using a combination of ketamine-dexmedetomidine-hydromorphone-midazolam, they’ll be a little bit more of a lag but it depends on the duration of the procedure. With any of those combinations you’re typically get a good 30-45 minutes just on injectables, that’s not adding gas anesthetic. So in my experience, if you discontinue oxygen as soon as you start closing the incision when doing a coelomic surgery, by the time you’re done with that incision—I’d say within 20-30 minutes of discontinuing oxygen and gas—you should start seeing some movement from the patient and early signs of recovery. And for sure within an hour, you should start seeing that animal recover back. So typically within 30 minutes to an hour, the animal should be either fully recovered or significantly recovered.
Q: How to do corneal reflex in patient with spectacles?
A: The corneal reflex is not an option in species with spectacles, like snakes, for monitoring anesthesia.
Q: What brand of temperature monitor do you use for esophageal monitoring?
A: As far as brand of temperature monitor, it really doesn’t matter. You can use whatever brand you have available to you. We [LSU-SVM] typically use temperature probes that are attached to our anesthetic monitoring equipment. There are also temperature probes that can be attached to pulse oximeters, so there are a wide, wide range of monitors [available].
We’re [LSU-SVM] now working on a study comparing esophageal versus cloacal temperatures. One important thing we found is that some of the flexible probes will actually break down and go bad over time. So you have to make sure you are using probes that are working properly. We found that some probes were reading even a 2-3-degree difference. So upkeep of those probes is going to be important.
CATHETER PLACEMENT & FLUID THERAPY
Q: Where would you recommend placing an intravenous (IV) cannula if required during surgical procedures?
A: Visit Administration of Medication in Reptiles for a list reptile IV catheter sites that do not require a cutdown. Additional advice on catheter placement can be found in the brief article: Catheters in Reptiles.
Q: With intraosseous (IO) how much slower does the infusion need to be compared to IV? (e.g. for constant rate infusions and intravenous fluid therapy)
A: The infusion rate will vary quite a bit with the size of the animal, the size of their bones, the location of the needle. If you have an intraosseous catheter needle that’s too close to the middle of the bone, where you have more of the fat tissue within the medullary cavity, the absorption is going to be slower.
What I typically do is, I will go for a maintenance rate of about 10-30 ml/kg/day. Typically, I’ll start reptiles at about 10 ml/kg/day. So when I start an IO catheter, is I start them at half that rate, monitor the patient, make sure we’re not getting any leakage, and then slowly bump up the rate from there if you’re not getting any leakage. If your rate is too high, often what will happen, even if the catheter is seated correctly, you’ll start getting either leakage at the insertion point of the catheter or sometime through some of the nutrient foramina of the bone, and you’ll start getting some edema around the bone, so that will be an indication to slow down the rate.
Q: Is there a need for special follow-up care if using IO catheters? Do you use those only in surgery or in intensive care patients also?
A: A simple wrap using elastic bandage material and cotton or gauze is sufficient. Also, yes, an intraosseous catheter can definitely be used in an intensive care setting.
Q: What is a fluid rate for reptiles per hour?
A: I typically don’t think of fluid therapy on a per hour basis; I’m always treating my patients for their maintenance needs over a 24-hour basis. So you can just figure out the math using 10-30 ml/kg/day (or 0.42-1.25 ml/kg/hr) depending on species, hydration status, then adding replacement deficits as needed, but the standard is 10-30 ml/kg/day. If the patient is severely dehydrated, shock, etc., this rate can be adjusted to give a 5 ml/kg bolus. Clinical judgement is still needed to determine the best approach for a particular patient.
Q: Do you have preferred fluid replacement guidelines/amounts for chelonians?
A: Maintenance requirements for reptiles are estimated as 10-30 ml/kg/day, however chelonians do tend to get overhydrated a little bit faster, and a little bit easier than other reptiles so I tend to stay at the lower end of 10-20 ml/kg/day for chelonians. I don’t typically use 30 ml/kg.
Sometimes when people are using intracoelomic (IC) fluids, chelonians don’t absorb these fluids as well and will retain some of these fluids a bit longer. Some people will elect to make a dilution of fluids using a 1:1 mixture of lactated Ringer’s (or NormosolTM or PlasmaLyte®) with saline to try to reduce the osmolality of the fluid, making it more hypotonic. So the fluids can be absorbed into the body a bit faster.
Osmolality plays an important role. There are a number of studies looking at osmolality, especially in chelonians in which plasma osmolality changes significantly across species, health status, etc. Currently, we don’t have very good formulas to calculate the osmolality of every chelonian species. If the osmolality was known, then you could match the fluids to the osmolality and route. If you’re giving fluids intravenously, intraosseously or subcutaneously, you just want to go with isotonic fluids. But if you’re going intracoelomic, there’s still potentially an argument that you want to go with hypotonic fluids to try to make sure those fluids get pulled into the vascular system and they don’t just hang around in the coelomic cavity.
Q: How much blood can we be comfortable in removing in relationship to body size?
- Blood volume is estimated to be approximately 10% of body weight in grams. The actual amount will vary by species (and individual) of course.
- In a health specimen, approximately 10% of blood volume or 1% of body weight (BW) can be collected. One person wrote 0.8% and that is actually a good rule of thumb because you want to have a “buffer zone” for inadvertent patient bleeding or bruising after venipuncture. So for instance, in an 80-gram gecko, a maximum of 0.8 ml could be collected however ~0.64 would be a more prudent volume.
- In a debilitated and/or anemic patient, collection of a maximum of 0.05% of BW is recommended.
Q: Anybody have pictures of brachial venipuncture?
A: Unfortunately LafeberVet does not have photos available at this time. We can refer you to an excellent article by Dr. Christoph Mans, which includes brachial venipuncture photos: Venipuncture Techniques in Chelonian Species.
Q: What is the preferred intramuscular injection site in chelonians?
A: The forelimbs are the preferred site for intramuscular injections in turtles and tortoises. Go to Administration of Medication in Reptiles for a brief video that illustrates recommended injection techniques.
Q: What is your email address Dr. Nevarez?
Q: Would love the rest of your drug dosages! (anesthetic protocols)
A: I did put some doses in my presentation, but all of the doses we use come straight from the formularies or textbooks. Doses will also vary with the species and the procedure, so it’s very difficult to give one drug dose across the board because it’s going to vary quite a bit. We use the Exotic Animal Formulary a lot, as well as Current Therapy in Reptile Medicine and Surgery and Reptile Medicine and Surgery. Most of the big reptile books are going to have a formulary in the back.
REFERENCES MENTIONED IN Q+A
Darrow BG, Myers GE, Kukanich B. Fentanyl transdermal therapeutic system pharmacokinetic in ball pythons (Python regius). Proc Am Assoc Zoo Vet 2010:238-239.
Gamble KC. Plasma fentanyl concentrations achieved after transdermal fentanyl patch application in prehensile-tailed skinks, Corucia zebra. J Herpetol Med Surg 18(3/4):81-85, 2008.
Gutwillig A, Abbott A, Johnson SM, et al. Opioid-dependent analgesia in ball pythons (Python regius) and corn snakes (Elaphe guttata). Proc Assoc Reptile Amphibian Vets 2012; 66.
Mans, C. Clinical technique: intrathecal drug administration in turtles and tortoises. J Exotic Pet Med 23 (1):67-70, 2014.
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