Constructing an Isoflurane Box - Venomous Snake Anesthesia
If your local reptile veterinarian is not initially comfortable with the idea of working with venomous snakes, you might be able to make him or her feel more comfortable by stating that you will bring the animal in a sealed container that will not be opened until the snake is fully anesthetized. All the veterinarian needs to do in order to induce anesthesia is to inject isoflurane into the securely sealed box that already contains your snake.
This is a standard 7 cup, 4.0 liter Rubbermaid container. It has a known volume which is printed on the container itself for easy reference, and a tight fitting lid. Any transparent container of known volume that can be sealed tightly will work well as an induction chamber for venomous snake anesthesia. This is a modification of the "open drop technique" commonly used on laboratory rodents.
The larger and deeper the box, the easier and safer it is for the handler to place a venomous snake inside and close the lid, and the more isoflourane will be needed to produce a 5% concentration of volatile gases inside the chamber. This 4 liter chamber needs 1cc of liquid isoflurane. The formula is .05 ml of isoflurane per 1% concentration per liter. A 1 liter container would need .25cc (one fourth of one cc) to reach a 5% concentration. An eight liter container needs 2cc's.
Prepare the chamber by drilling a tiny hole and taping a cotton ball securely to the inside of the box directly in front of the hole. For a larger chamber you may want to tape a small wire-mesh kitchen strainer to the side of the box to hold several cotton balls. A larger chamber may also have a small fan affixed to one side to better distribute the gas throughout the container. An inexpensive microprocessor fan is adequate for the task and can be purchased at any computer supply store. The chamber should be clean and free of sharp or rough edges that a snake could injure itself on, but it does not need to be sterile. Make sure the chamber is still airtight (or can be made airtight) when you have finished making your modifications. You may find it helpful to seal any holes you have made for wires, switches, etc, with a caulking gun. Be sure to let the compound dry completely before using the box to induce a snake.
Using a hook to manipulate the snake and long hemostats to slide the lid, place your subject in the chamber. If you are a zookeeper needing to secure the animal on your own premises for safe transport to the vet clinic, you should modify the lid so that it has a few small (1") mesh ventilation holes that can be securely duct taped over once your vet is ready to administer the isoflourane. A snake should never be transported in a sealed, non ventilated container. Some clinics will permit you to bring a venomous snake in a properly ventilated container and transfer it on their premises, but other veterinarians will be more comfortable if you bring the animal in a sealed container that will not be opened until the snake is fully anesthetized. Once the snake is inside your mesh ventilated induction chamber, further secure it for transport by duct taping the lid closed. Do not tape over the ventilation holes while the animal is being transported. The chamber can then be placed inside a sturdy lockable container such as a plastic tool box that is also ventilated.
Tape up the ventilation holes securely just before introducing the isoflurane. If tape alone is not sufficient to make a temporarily airtight seal over the mesh covered ventilation holes you have installed in the lid, consider the use of nontoxic, nonreactive artist's putty or soft modelling clay to help make your seal. If the chamber is not airtight when you introduce the isoflurane, you will be able to smell the leakage immediately if you put your face close to the container.
When you are ready to induce anesthesia, inject the appropriate amount of isoflurane through the drilled hole into the cotton ball and cover the hole with tape. Isoflurane is an irritant, so be careful not to drip any into the chamber or on the animal's skin. Tape over the hole with clear packing tape and observe the snake's behavior. Do not leave the induction chamber unattended as you will be removing the animal as soon as it has lost its righting reflex. The time it will take for a snake to get to this stage can vary widely. Different species and individuals of the same species may react very differently to the same concentration of drug depending on their size, age, health, percentage of body fat, etc. Some animals will breath-hold and remain conscious for fifteen or twenty minutes or more. Others will induce within minutes. Watch for breathing motions, or the lack of them.
Keep a careful eye on your subject and check for the loss of righting reflex as soon as you see the animal stop moving, especially if its head is drooping or resting at an abnormal angle and it appears to have lost muscle tone. If you turn the box slowly and gently upside down, and the snake no longer moves or attempts to right itself, the animal has lost righting reflex and can be removed from the box for intubation or for brief and minimally invasive procedures.
Typically a snake will go through an excitatory phase where its motion increases. This normally signifies the end of breath holding and the onset of anesthesia within a short time. Once the snake has lost righting reflex, cut the duct tape and remove it from the box immediately to avoid an anesthetic overdose. You have from five to fifteen minutes to do simple procedures, or you may administer additional drugs or intubate the animal for more extended procedures. Local analgesic agents such as Lidocaine will give you a lot more leeway for your procedures as pain is a very effective stimulant in reviving a snake that would otherwise remain safe to handle at a deeper plane of anesthesia. Additional injectable drugs can also be administered at this time to extend the depth and duration of anesthesia.
Recovery from anesthesia in snakes is caudal to cranial; induction is cranial to caudal. That means that they lose muscle control and reflexes starting with the head, and regain responsiveness starting with the tail. This is a very convenient physiological mechanism as it allows a good safety margin while the veterinarian is performing surgical procedures. A handler should be constantly monitoring muscle tone and withdrawal reflex in the tail. When responsiveness is detected, the patient is no longer at a surgical plane of anesthesia, but there is still plenty of time before the head will regain consciousness.
This is a 33 cup, 7.8 liter Rubbermaid container which is about to be used to induce a blacktail rattlesnake (Crotalus molossus). Once the animal has lost its righting reflex and can no longer right itself when turned over, it will be removed from the box and maintained on a ventilator for surgery on an infected fang sheath.
Snakes at a surgical plane of anesthesia cannot breathe voluntarily and must be ventilated either manually or with a standard small-animal ventilator. While snakes can tolerate long periods of hypoxia (lack of oxygen) they can suffer some ill effects, especially if the patient is already sick or injured. It is important to keep your patient properly ventilated during surgical or scientific procedures done under anesthesia. It is also important to consider the patient's comfort. Prolonged pain and stress can be immunosuppressive and all invasive or painful procedures should be done with good analgesics already on board to prevent post-operative discomfort, such as Buprenorphine and/or Butorphanol.
A snake undergoing anesthesia cannot thermoregulate and must be kept exactly within the preferred optimum temperature zone for its species. Overheating can be rapidly fatal; overcooling can contribute to immunusuppression and subsequent illness and infection. Careful monitoring of the unconscious patient is critical throughout the procedure and the recovery period.
Remember not to anesthetize yourself while inducing the patient. When a larger chamber is opened, there may be some noticeable isoflurane fumes. It is a good idea to ventilate the room as well as the patient when using this type of anesthetic induction.