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Surgical Anesthesia & Analgesia


Most rodent surgery will be conducted under general anesthesia, a condition in which the animal is unconscious and completely insensible to pain. There are two types of general anesthetic agents.

Injectable anesthetics are administered via needle and syringe. The duration of injectable anesthesia varies depending on the drug used but is typically around 20-30 minutes in rodents. Recovery tends to be prolonged, usually significantly longer than the duration of anesthesia.

  • These anesthetics are usually administered to rodents via subcutaneous or intraperitoneal injection.
  • Some injectable anesthetics will provide residual analgesia until the drug is metabolized by the body, however, this is not generally considered adequate for post-operative pain relief.
  • Injectable anesthetic drugs must be sterile and administered using a sterile needle and syringe.
  • The amount delivered must be calculated based on the animal's body weight.

Inhalant anesthetics require an anesthetic vaporizer, oxygen source and pressure regulator to deliver a controlled anesthetic dose. The animal remains anesthetized only as long as it is breathing the anesthetic gas. Animals recover rapidly from most inhalant anesthetics. Isoflurane is the most commonly used inhalant anesthetic at Penn State.

  • The duration and depth of anesthesia is easily controlled and recovery is rapid.
  • Most inhalant anesthetics are excreted primarily via the lungs and do not require metabolism by the liver or kidneys.
  • Inhalant anesthetics do not provide any residual analgesia. Once the animal has awakened after anesthesia other drugs must be used to supply analgesia.
  • Chronic exposure to inhalant anesthetics such as isoflurane represents a potential human health hazard and should be avoided. Pregnant women should not administer inhalant anesthetics.
  • Isoflurane may only be used in locations where equipment is available to direct anesthetic fumes away from the user.
  • You must be trained to use this equipment.

Specific information on injectable and inhalant anesthetics available for various species may be found on the ARP website.

Monitoring the Depth of Anesthesia

The ideal dose of general anesthesia acts on the central nervous system to eliminate the sensation of pain as well as consciousness. However, anesthetics may also affect breathing and heart function such that high doses can kill the animal. The goals of monitoring are to:

  • Verify that the animal is not able to feel pain. Firmly pinch the paw or tail. If the animal moves it is too lightly anesthetized.
  • Eyelid and corneal reflexes are not accurate indicators of anesthetic depth in rodents. The eyelids of an anesthetized rodent remain open. If anesthesia is to last more than a few minutes, sterile ophthalmic ointment must be applied to the eyes help prevent corneal injury.
  • Make sure that the animal remains at an adequate plane of anesthesia during the entire procedure. Animals that are too lightly anesthetized will show increased heart and respiratory rates, have withdrawal responses to toe or tail pinch and may begin to move.
  • As an animal becomes more deeply anesthetized the breathing rate will slow and breathing will become shallow. If the animal becomes too deeply anesthetized it's breathing rate will become irregular and may stop. Gasping-type respiration attempts may be seen prior to death.
  • Instruments such as a pulse oximeter and/or capnograph may be used to monitor cardiac and respiratory function.

Anesthetic depth should be monitored by performing a toe or tail pinch and observing respiration at least every 5 minutes. Clear surgical drapes can make it easier to monitor the animal during surgery. Feet and/or legs must not be restrained such that withdrawal reflexes are not easily seen during a toe pinch.

Important: It is easy to forget about monitoring the animal when you are concentrating on the surgical procedure. Consider using an alarm set to beep at 5 minute intervals to remind you to check.


Post-operative analgesic medication must be provided for research animals subjected to procedures which may be expected to induce more than momentary or slight discomfort. Pain adversely impacts the welfare of animals and if not controlled, is a variable that can confound the interpretation of experimental results. Investigators are responsible for the assessment and management of pain in their research animals and must include a detailed pain management plan in their IACUC protocol.

It is often advisable to administer analgesic medication immediately before, or in some cases, just after the surgical procedure. The intention is to establish effective analgesia before the animal recovers from anesthesia. This is termed preemptive analgesia. The administration of analgesia before surgery and recovery from anesthesia may reduce the severity and duration of post-operative pain and decrease the amount of analgesic medication needed to provide effective pain relief.

Specific information on analgesics available for various species may be found on the ARP website.

Preemptive analgesia insures that all animals receive analgesic medication prior to the onset of post-operative pain. However, this requires the investigator to make an accurate prediction of how much post-operative pain the surgical procedure will cause the animal. The investigator must also continue to observe and evaluate each animal post-operatively for pain and discomfort and treat accordingly.

In most cases analgesics should be administered by injection. Analgesic drugs can be placed in drinking water or food for oral delivery, but this method is not generally recommended. Animals in pain reduce their food and water intake and so will likely not consume sufficient drug to provide pain relief using self-administration methods.

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