GuidelinesfortheUseofAnalgesicsandTranquilizersinLaboratoryAnimal5

Anesthetic Induction and Maintenance

Injectable Anesthesia

Anesthetic induction using injectable anesthetics is fairly simple. It involves admininsistration of the drug and monitoring the depth of anesthesia. Supportive care may be needed. Maintenance of injectable anesthesia can be through repeated bolus doses of the drug or through a constant infusion. Infusion rates are calculated based on the clearance time of the drug. Bolus dosing is simpler. Typically, 1/2 of the original dose is given for repeat doses.

Injectable anesthetics can be administered by various routes depending upon the specific compound. The most frequently used routes of administration in laboratory animals are intraperitoneal, intramuscular and intravenous. Less frequently used routes, among others, are intrathoracic, oral and rectal. Techniques are described below. Contact RAR at 624-9100 for training materials on handling animals and administering injections.

• Intravenous(IV) 

  Method- An appropriate vein must be selected. For large animals, the saphenous, cephalic or jugular veins are best. For rodents, the tail veins are best. For rabbits and swine, ear veins may be used. The vein is held off proximal to the venipuncture site. The vessel may be stroked with a finger to stimulate blood flow into it. The needle is inserted at a 30-45° angle to the vessel. Then the needle is lowered to align with the longitudinal axis of the vessel and advanced slightly. Draw back. If blood appears in the hub of the needle, the drug may be injected. If not, try redirecting the needle (before you pull it out of the skin) and repeat. You may need to try several times while learning. Using a new, sharp needle for each stick, even if it is the same animal, will improve your chances for success. Once the needle is withdrawn, it is necessary to put pressure on the vessel to prevent bleeding.

  Advantages- rapid delivery of drug, ability to titrate dose, irritating substances may be given IV

  Disadvantages- small veins are hard to access (i.e. small animals), restraint is critical, developing skill in venipuncture takes experience

• Intramuscular (IM) 

  Method- Insert the needle into a large muscle mass. Draw back slightly. If blood is aspirated, you are in a blood vessel. Redirect the needle. When the needle is placed correctly, inject the drug. The best muscle masses to use are for small animals, the caudal thigh muscles. For larger animals, the lateral dorsal spinal muscles or the cranial or caudal thigh muscles may be used. When administering into thigh muscles, inject from the lateral aspect, or if from the caudal aspect, direct the needle slightly lateral. This will help avoid injecting into the sciatic nerve.

  Advantages-- Fairly rapid absorption, technique is simple

  Disadvantages- IM injections are painful, small volumes are necessary, the animal may try to bite or escape

• Intraperitoneal (IP) 

  Method- The animal is usually restrained in dorsal recumbency. The drug may be injected anywhere in the caudal 2/3 of the abdomen. However, it is best to try to avoid the left side in rodents and rabbits because of the presence of the cecum. After the needle is inserted, draw back. If anything is aspirated, you have likely hit the viscera. Withdraw and get a new needle before trying again. If the needle is placed correctly the drug may be injected.

  Advantages- relatively large volumes may be injected (0.5 ml in mice, 2 ml in rats, etc.)

  Disadvantages- technique is more difficult than IM injections, drug may be administered into the viscera resulting in no effect or in a complication.

• Subcutaneous (SQ) 

  Method- Pinch an area of loose skin. Inject into the center of the "tent" created by pinching.

  Advantages- Technique is the simplest of any, large volumes may be given (basically as much as the tent of skin will hold that doesn't cause discomfort to the animal)

  Disadvantages- Irritating substances cannot be given this way, absorption is slow

Inhalant Anesthesia

Induction of inhalation anesthesia can be difficult. Anesthetic gases are irritating to eyes and nasal passages. Animals may resist as they begin to lose consciousness or they may stop breathing temporarily. For this reason induction using a mask or nose cone held over the animal's nose can only be performed on smaller or non-fractious animals. In smaller animals gas can be delivered into an induction chamber large enough to contain the entire animal. Induction via a nose cone or chamber requires delivery of the anesthetic gas at 2-3x MAC. Frequently an injectable anesthetic is used to induce anesthesia and the inhalation agent is used for maintenance.

Maintenance of inhalation anesthesia is normally accomplished by delivering approximately 1.2 MAC to an animal via a mask or nose cone, or directly into the lungs via an endotracheal tube. Intubation is recommended whevever possible, particularly when a procedure will be prolonged. Endotracheal access is essential to provide ventilation support.

Gas Delivery Systems

The most complicated aspect of using inhalant anesthesia is the delivery system. A delivery system must provide the anesthetic gas to the animal at a known and constant rate. It must also ensure that animals receive adequate oxygen. There are several types of delivery systems typically used in laboratory animals. A more complete discussion of anesthetic delivery systems is available here.

Drop System

The drop system is the most basic type of anesthetic delivery system. It involves application of the anesthetic gas to an absorbent material that is then placed in the bottom of an anesthetic chamber or nose cone device. The gas mixes with the air in the chamber until it reaches a concentration equal to the vapor pressure of the gas. For this reason drop systems have been traditionally used with low vapor pressure anesthetics such as methoxyflurane or with slow acting drugs like ether. Some success has been achieved by mixing high vapor pressure drugs such as halothane or isoflurane at a concentration of 15% by volume with mineral oil and using this mixture in the drup system.

Problems with a drop system and how to deal with them

• The concentration of the gas being delivered to the animal is largely unknown 

• Place animals in the chamber. Remove them as soon as they lose consciousness (i.e. they lay down and don't respond to a gentle stimulus)

• There is a limited ability to adjust the concentration 

• For anesthetic maintenance (after animals lose consciousness) place absorbent material soaked with anesthetic in the bottom of a nose cone or empty syringe case.

• Move the cone closer to or further away from the animal's face to adjust the concentration

• If a closed chamber is used, there is a danger that the animal will not receive adequate oxygen 

• Don't leave animals in a closed chamber after they have lost consciousness

• Significant waste gas is produced. To minimize waste: 

• Perform anesthesia in a fume hood or other well-ventilated area

• Use a chamber with a tight-fitting cover

• Use a chamber with the smallest diameter mouth possible

• Keep the lid on except when the animal is being placed into or removed from the chamber 

• Add anesthetic to the absorbent material only in a fume hood.

Apparatus for Rodent Anesthesia

Left: a non-re-breathing nose cone that can be used with a large animal anesthetic machine; Middle: a typical drop system closed anesthetic chamber; Right: a gas scavenging system that can be used with a drop system.