Surgical Anesthesia & Analgesia

Surgical Anesthesia and Analgesia

Anesthesia

Rodent surgery is performed with the animal 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. Injectable anesthetics are typically administered to rodents via subcutaneous or intraperitoneal injection. The anesthetic drugs must be sterile and administered using a sterile needle and syringe. The amount of drug given to each animal must be calculated based on the animal's body weight.

Inhalant anesthetics require anesthetic equipment to deliver a controlled drug dose with the animal remaining anesthetized only as long as it is breathing the anesthetic gas. Animals tend to recover rapidly from the inhalant anesthetic (isoflurane) that is used most commonly at Penn State. The delivery of inhalant anesthetics requires the use of an anesthetic vaporizer (see photo above right), oxygen source and waste gas scavenging system. The duration and depth of anesthesia is easily controlled and recovery is rapid. Chronic exposure to inhalant anesthetics is a human health hazard and must be avoided. Pregnant women should not work around inhalant anesthetics. Inhalant anesthetics may only be used in locations where adequate waste gas scavenging equipment, such as a biosafety cabinet or downdraft table, is available to direct anesthetic fumes away from the user.

Information on specific anesthetics for rodents and rabbits may be found on the ARP website.

Monitoring the Depth of Anesthesia

The ideal dose of general anesthetic drug acts on the central nervous system to eliminate the sensation of pain as well as consciousness. Under-anesthetized ('light') animals can still feel pain even though they may not be able to move. Animals given too much anesthetic ('too deep') can die due to drug-induced heart and respiratory depression.

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 for painful procedures.
  • Make sure the animal is not too deeply anesthetized by monitoring respiration, heart rate and mucous membrane color. The breathing and heart rates will slow as the animal becomes more deeply anesthetized. Although respiratory and heart rates are not commonly counted during anesthesia in rodents, with practice, the speed and general character of these parameters can be recognized. Changes (e.g., shallow, infregquent breathing, pale or blue mucous membranes, decreasing heart rate) can be indicators that the animal is too deeply anesthetized. Note: anesthetized animals that appear to be gasping with an open mouth may be showing 'agonal breathing' and close to death.
  • Instruments such as a pulse oximeter may be used to monitor the heart rate and blood oxygen levels.
  • Capnographs can be used to measure the amount of carbon dioxide expelled during each breath and are another way to monitor respiratory function during anesthesia.

 

Frequency of monitoring:

  • Perform a toe or tail pinch about every 5 minutes
  • Respiration and heart function should be monitored frequently. At the minimum, you should check respiration and mucous membrane color about every 5 minutes when you check toe/tail 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.

 

Analgesia

Post-operative analgesic drugs 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 drugs immediately before, or in some cases, just after the surgical procedure. This allows the drugs to take effect before the animal recovers from anesthesia and is called preemptive analgesia. The administration of analgesia before surgery and recovery from anesthesia can reduce the severity and duration of post-operative pain and decrease the amount of analgesic drug needed to provide effective pain relief. Preemptive analgesia ensures 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 will be administered by injection. Analgesic drugs can sometimes be placed in drinking water or food, but this method is not generally recommended. Animals in pain reduce their food and water intake and so are not likely to consume enough drug to provide pain relief using this method. Further information on analgesia may be found on the ARP website. 

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