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Pet Anesthetic

Posted Dec 20 2010 7:11am

Before the anesthetic procedure
A thorough history of current and past medical problems can provide valuable information about your pet’s physical condition. A history of poor ability to exercise may indicate abnormal heart or lung function, an important consideration when planning an anesthetic procedure. Physical examination may reveal abnormalities of the heart or lungs that may require further evaluation such as an electrocardiogram (ECG), chest x-rays, or heart ultrasound prior to performing general anesthesia. Many anesthetic drugs affect blood flow to major organs and are inactivated by the liver. Your veterinarian may evaluate a blood sample for anemia and for kidney and liver function prior to deciding which anesthetic technique to use.

There are many types of anesthetic regimes including the use of drugs that are injected into a vein or muscle or inhalant drugs that are breathed in and out of the body. Your veterinarian will select the anesthetic regime based on the health of your pet and on the type of surgical procedure to be performed.
Tranquilizers  are often given before a general anesthetic.           Pre-Anesthetic Evaluation
Many anesthetic techniques involve the administration of a sedative or tranquilizer before the anesthetic agent is given. Tranquilizers and sedatives can allow for less of the general anesthetic to be used. pre-anesthetic sedation may calm a pet excited by unfamiliar surroundings. Excited animals may require higher doses of general anesthetics and excitement may predispose to development of an irregular heart rhythm (cardiac arrhythmia).  pre-anesthetic sedation may also make recovery from anesthesia smoother.

An endotracheal tube and intravenous catheter are often placed prior to general anesthesia.           After pre-anesthetic sedation, an intravenous (IV) catheter is placed into a vein in either a front or hind limb, or occasionally in the neck. An IV catheter is the patient’s lifeline while (s)he is under the effects of general anesthesia. Through the IV catheter your veterinarian will have ready access to your pet’s blood stream to administer fluids and other drugs during surgery. To prevent life threatening bacterial infections, it is important that the skin overlying the catheter site be clipped of fur and scrubbed with a surgical disinfectant.

General anesthesia is often begun by giving a short-acting anesthetic agent IV. As soon as the pet loses consciousness, a soft plastic tube (endotracheal tube or ET tube) is inserted into the windpipe and is connected to an anesthesia machine. The anesthesia machine is used to deliver an inhalant anesthetic in oxygen and other gases. Many anesthetic drugs can depress breathing; having an ET tube in place allows the veterinarian to assist or control breathing if it becomes necessary. The loss of consciousness that occurs during anesthesia is often accompanied by loss of the ability to cough and gag. In awake animals, coughing and gagging are protective reflexes which prevent inhaling stomach contents or other foreign materials into the lungs. Insertion of a proper size ET tube prevents inhalation of stomach contents into the airways and lungs during anesthesia.

At the completion of the surgical procedure, the concentration of the anesthetic that the animal is breathing is reduced and the animal slowly regains consciousness. When the pet regains its swallowing reflexes, the ET tube is removed and the patient is monitored until it is fully conscious.
Close monitoring of your pet’s vital signs during anesthesia will allow early detection and correction of problems.           Monitoring General Anesthesia
There are several methods by which the pet’s response to anesthesia and surgery can be monitored during the procedure. Close monitoring during anesthesia allows for early recognition and correction  of problems.

Vital signs such as heart rate, respiratory rate, and blood pressure can be monitored using specialized devices.

Rapid-acting, anesthetic drugs are easier to control if they are continuously administered. The anesthetic is “titratedâ€Â? to achieve the desired level of anesthesia and analgesia. In other words, most general anesthetics are “dosed to effect”.

The animal’s depth of anesthesia is determined by evaluating reflexes, muscle tone, and response of vital signs to surgical stimulation. If an animal is judged to be too light for the surgical procedure being performed, an increased amount of anesthetic is administered. Conversely, if the patient is judged to be in an excessively deep plane of anesthesia, the amount of anesthetic administered is decreased.
All anesthetic drugs are potent and can cause serious problems although new drugs are much safer.           Anesthetic Drugs
Modern day inhalant anesthetics are a tremendous improvement over older anesthetics. Inhalant agents are those that are breathed in and out rather than being injected into a blood vessel or muscle. While the newer anesthetics are “kinder” to an animal’s organ systems, even the “safestâ€Â? anesthetics are extremely potent and potentially lethal.

Newer anesthetics such as Isoflurane® are less likely to aggravate a pre-existing abnormal heart rhythm. Additionally, inhalation agents such as Isoflurane® enter and exit the brain rapidly, allowing for a rapid onset of and recovery from anesthesia.

Anesthetic Considerations
Some think of general anesthesia as a relaxing sleep. It is probably much more accurate to compare general anesthesia to a period of very strenuous exercise. Just as young, healthy animals are more able to exercise vigorously, they are more able to tolerate the depression in heart function caused by general anesthesia because they have such great cardiovascular reserve. Older or debilitated animals have less cardiovascular reserve and may have less tolerance for general anesthesia. Older or debilitated animals often recover from general anesthesia and surgery more slowly than a young patient, in part because of their decreased reserve.
After general anesthesia,  the anesthetic drugs may have an effect for several days. This may include temporary behavioral changes.           Post-Anesthetic Considerations
After general anesthesia, animals are likely to be affected by the anesthetic drugs for several days.

An animal may exhibit behavioral changes for several days after general anesthesia. They may act as if they do not recognize familiar surroundings, people or other animals. Behavioral changes after general anesthesia are extremely common; fortunately they usually resolve within a few days. Do not leave young children unattended with an animal that has just recovered from general anesthesia no matter how trustworthy that animal normally is. Remember, your pet has been through a lot and probably won’t fully recover and be himself/herself for several days. There are reports of normally well-behaved dogs returning home after surgery and anesthesia and biting young children for no apparent reason.

The pet’s ability to control its body temperature may be affected during the recovery period. Many anesthetics alter the temperature set point in the brain and cause blood vessels in the skin to dilate promoting heat loss. Conversely, an animal’s natural cooling mechanisms may be unable to adequately respond to increases in environmental temperature. For the first few days after general anesthesia, it is recommended to keep your pet in a warm, though not overly hot room. Cold weather breeds such as Malamutes and Huskies tend to retain heat easily and a cooler environment may be more appropriate for these breeds.

Obese animals often have delayed recoveries. Most general anesthetics are very fat soluble so the greater the amount of body fat and the longer the animal is anesthetized, the greater amount of anesthetic agent that will be absorbed into body fat. Anesthetic taken up by body fat will leach back into an animal’s blood for days or even weeks after anesthesia. This low residual amount of anesthetic may continue to affect an animal’s behavior for several days.
Some breeds are more sensitive to certain anesthetics than other breeds.           Breed Sensitivities to General Anesthesia
Purebred dogs have been selectively bred to perform specific tasks such as retrieving or racing or have been bred to develop specific physical characteristics. Often, less desirable traits emerge as a result of selective breeding.

A common question posed to veterinarians is about the “sensitivity” of a particular breed of dog or cat to anesthetic drugs. This is a difficult question to answer, as there are few scientific studies that have evaluated the sensitivities of different breeds of animals to anesthetics. Many of the reported breed sensitivities are based on the clinical experience of veterinarians. Many giant breed dogs seem to require less of a pre-anesthetic sedative (a smaller dose per unit of body weight) than miniature or toy breeds. The reason for this apparent difference is unclear.

It has been documented that sight hound breeds of dogs are more sensitive to some of the ultra short-acting thiobarbiturate induction drugs. The administration of thiobarbiturates for induction of anesthesia in sight hound breeds has been associated with a slower recovery from anesthesia. The reason for this breed sensitivity is unclear at this time but may be related to a difference in liver metabolism of the drug and/or differences in body fat. When sight hound breeds are to be anesthetized, it has been recommended that non-thiobarbiturate induction drugs be used to prevent a prolonged recovery from anesthesia.

Within the same breed, individual animals respond to the same anesthetic in differing degrees. Vigilant monitoring of vital signs during general anesthesia will enable the veterinarian to recognize and respond to life-threatening changes in heart and lung function.

A patient who displays an abnormal response to any drug (anesthetic or not) should be monitored carefully if the situation dictates that the same drug be re-administered at a later date.
There is no “perfectly safe” anesthetic agent.           The Perfect Anesthetic Drug
The perfect anesthetic is a drug that produces no heart or lung depression, provides adequate analgesia, provides excellent muscle relaxation, is not metabolized by the patient, is not toxic, and is readily reversible. Although the general anesthetics in use in veterinary anesthesia today are great improvements compared to the anesthetics of yesteryear, they still fall far short of the perfect agent.

Probably, the most desirable general anesthetic for a young, healthy dog is the one your veterinarian is most familiar with. There are a great many anesthetic drugs available to today’s practicing veterinarian, however most practitioners use a few carefully chosen anesthetics with which they have the most experience and the most confidence. Your veterinarian’s experience in the use of a certain anesthetic drug often will more than offset one or two undesirable properties of a general anesthetic agent.
Preparing your pet for general anesthesia…           Preparing your Pet for General Anesthesia
There are several steps that a pet owner may take to help your veterinarian plan for a successful general anesthesia and surgery.

1. Communicate concerns about your pet’s health to your veterinarian prior to scheduling surgery. Any signs of exercise intolerance, weight loss, a recent change in urination or defecation, and mental alertness are particularly informative and may require further diagnostic workup.
2. Withhold all food from pets scheduled for elective surgery for at least 12 hours prior to arriving at your veterinarian’s clinic. The presence of food in a pet’s stomach will greatly increase the likelihood of aspiration of food into the lungs, should your pet vomit during induction of general anesthesia. Usually water is NOT with held but check with your veterinarian.
3. Balance your desire to keep your pet in show condition with the knowledge that the risk of infections increases greatly if the skin overlying the IV catheter site is not properly disinfected prior to IV catheter placement. Proper disinfect ion requires clipping of the fur overlying a suitable vein.
4. Follow your veterinarian’s discharge instructions. If you have small children at home, be careful not to leave your dog alone with the small child even for just a few minutes. Remember, all animals recovering from all general anesthetics may react unpredictably for several days.

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