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Stages of Anesthesia in Animals
Veterinary & Aquatic Services Department, Drs. Foster & Smith
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General anesthesia is attained in stages. The success of the anesthesia depends on all the stages being reached correctly.

Stage 1: Preparation (premedication)

Several types of premedications are given prior to administering the general anesthetic. The animal is given a tranquilizer or a sedative to help relax him for the procedure. A sedative decreases excitement and causes relaxation and reduced mental activity. Some sedatives such as Xylazine, also provide pain relief. A tranquilizer is a drug that calms an anxious human or animal. Tranquilizers do not provide any analgesia (pain relief). Tranquilizers include diazepam, midazolam, and acepromazine. Tranquilizers and sedatives may be used singly or in various combinations. Combining medications can minimize side effects because less of each medication is used. For more information on some of these medications see 'Anesthetics, Sedatives, and Tranquilizers.'

A medication such as atropine or glycopyrrolate may also be used as a preanesthetic to help maintain the heart rate during anesthesia and to decrease the amount of saliva produced. Pain medication such as morphine or butorphanol may also be given at this stage to help make for a smoother, less painful recovery.

An intravenous catheter is placed in the vein through which injectable anesthetics and fluids may be given. The catheter is also available in case emergency drugs need to be administered.

Stage 2: Induction of anesthesia

Usually an injectable anesthetic is given to place the animal in an unconscious state. Injectable anesthetics include thiopental, propofol, and etomidate. Once the animal is unconscious, an endotracheal tube is placed in the pet's trachea (windpipe) to allow for oxygen and gas anesthetic to be given. In animals that do not receive injectable anesthetic agents, the inhalant (gas) anesthesia may be given via a mask over the face to bring them to an unconscious state. At that point, an endotracheal tube would be placed.

Stage 3: Maintenance of anesthesia

This is the stage where surgery is performed. The animal is kept unconscious and pain-free. Inhalant anesthesia offers more precise control of the level of anesthesia than do injectable forms. Examples of inhalant anesthetics are isoflurane and halothane. The anesthetist monitors the animal's vital signs including heart rate and rhythm, breathing rate, oxygenation of the tissue, and body temperature.

Stage 4: Recovery

A good recovery is one that is uneventful. During this time, the animal's vital signs should still be monitored until the pet is fully awake and able to stand. Recovery is easiest on the pet if it occurs in a warm, quiet environment. The animal should be able to awaken slowly and quietly. During this stage, vomiting or urinating may occur. Shivering is normal in the early stage of recovery as the body's temperature regulation returns to normal.

Rate of recovery: Individuals have different rates at which they recover from anesthesia depending on health, length of anesthesia, and type of anesthetic used. For short procedures such as a routine dental cleaning, animals are typically awake and ready to go home within 4-8 hours after the procedure. Other longer surgeries, such as a complicated fracture repair that require several hours of surgery, will require a longer time-frame for recovery.

Post operative pain management: Managing post operative pain has become an important aspect to be considered during the recovery period and beyond. In general, it is best to give pain relief medication before or during surgery so it is already working as the animal is waking up. Depending upon the surgical procedure, pain medication may be continued for several days post surgery.

Giving food and water: Once the pet is able to stand and walk, very small amounts of food and water can usually be offered every several hours. Some pets are nauseous after anesthesia, especially if they were anesthetized for a long period of time, and may not want to eat or drink for several hours or overnight. If fluids have been given during the surgery, the fast should not harm the pet. If the pet has been released from the hospital and will not eat the day after the procedure, the veterinarian should be notified in case follow-up care is required.


Anesthesia is not entirely without risk, but it is an invaluable tool that is used in veterinary and human medicine. The risks can be minimized if proper advanced planning is done when possible and each stage of anesthesia is reached correctly. Today's anesthetics can be individualized for each animal's needs and veterinarians are highly trained in their use.

References and Further Reading

Tufts University School of Veterinary Medicine. Catnip. January 1998; Vol. 5. No. 10.

Cornick-Seahorn, J. Formulating an anesthetic plan for your patients. Veterinary Forum 2000; 46-53.

McKelvey, D; Hollingshead, W.K. Small Animal Anesthesia Canine and Feline Practice. Mosby. St. Louis, MO; 1994.  

Parts & Function of a Gas Anesthesia Machine for Pets 
Types of Anesthesia, Sedatives & Tranquilizers in Animals 
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