Anesthesia In Your Dog & Cat
Ron Hines DVM PhD
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When your pet is scheduled to undergo a surgical procedure your first thought may be of the risks of anesthesia. This is perfectly natural – the risks of general anesthesia are usually greater than that of the surgery itself.
All veterinarians know this and most of us have a great deal of empathy for your concerns. We veterinarians are fortunate in that older, less predictable, anethesthetics have been replaced by compounds that are very predictable and safe when properly used. Injectable anesthetics now each have their specific reversal agents and the gas anesthetics we use are quickly reversed by ventilation as your pet breaths them out of it's system. These agents have dramatically reduced the risk to your pet. My clients often as me which anesthetic is the safest. The answer is that which ever anesthetic your veterinarian is most familiar using is the safest.
of the drugs that we use for anesthesia at one dose are sedatives
at a lesser dose. So the difference between your pet being sedated
and your pet being anethetized it blurry. A rule all veterinarians
and physicians use it to never give more than just enough of an
anethetic to do the procedure skillfully.
Because it is common for pets to not sit still or become terrified in the animal hospital, We use general anesthetics more frequently than a human physician would. Procedures such as x-ray, joint examinations and laparoscopy that a human physician would do while you were awake can not be done safely animals without sedation or anethesia.
There are two components of safety in the use of anesthetics.
The first is the proficiency in the use. To attain this you must select a competent veterinarian with long experience in the use of a particular anethetic.
The second component is evaluating anesthetic risk of the individual pet. Many times, I have forgone anesthesia in a pet when I was dissatisfied with the results of my physical exam. Veterinarians that have worked in the Profession for as long as I have, usually develope the ability to sense the general health of the animal they are examining. When examination leaves me with uncertainties I schedule a group of biochemical tests to gauge the health of the major organs of the body.
Preanethetic Blood Work Up
Before any major surgery I suggest you have your veterinarian perform a BUN. and creatinine test to gauge kidney health and ALT, AP, bilirubin, and cholesterol tests to detect liver problems. Glucose analysis and total blood proteins detect diabetes or other debilitating diseases and an EKG may detect heart problems. Include a hematocrit, a differential and total white blood cell count. It is extremely rare for all these tests to be normal in a dog that is a high anesthetic risk. When we know of problems in advance we can make modifications in anesthesia that protect your pet's life.
I often give a pre-anesthetic sedation agent such as acepromazine or Valium to pets to calm them down before surgery. Excited pets often require more anesthetic and may even react differently or badly to anesthetic drugs. Once the sedative or tranquilizer has taken effect I shave the patient’s arm and place an intravenous catheter in its recurrent radial vein. This gives me easy access to the pet’s blood stream for fluids and other medications in the event of an emergency.
Selection Of The Proper Anesthetic Agent:
Various anesthetics are administered in one of four ways. They can be injected locally around nerves,injected into the muscle to work systemically, injected intravenously to work more rapidly or they can be inhaled as a gas. Injected systemic and inhaled anesthetics move through the blood stream to the brain where they render the pet unconscious or insensitive to pain. Anesthetics that are injected near nerves block the sensation of pain distant to the point of injection.
As I mentioned earlier, ideal anesthesia uses neither more or less of the anesthetic than the amount necessary to perform the surgical procedure. This must be monitored throughout the procedure. So a special non-distracted person(s) is generally assigned to watch the pet closely. I try to give just enough of the drugs to maintain the pet pain-free and relaxed during the procedure. This amount is different for each patient. More painful procedures, such as intra-abdominal surgery, or spaying and orthopedic surgery require more anesthetics than procedures such as teeth cleaning or superficial tumor removal. Often, a combination or two or more anethetics, given in different ways, works better than a single agent given alone.
Lidocaine, marcaine and carbocaine or agents similar to them are related compounds that I use to perform local or regional nerve block anesthesia on pets. I use more of these agent than most veterinarians. I do this because they are safer in general practice than general gas or injectable systemic anesthetics. When I use them to remove superficial tumors I combine these agents with epinephrine which decreases hemorrhage during surgery. When I use local anesthetics, I tranquilize the pets so they are not terrified during surgery. Although I have read that overdoses with these agents are possible, I have not seen a bad reaction to these agents in the many years that I have used them. I also use these agents when I perform a cesarean section.
Gas anesthetics cause a deep sleep-like state in animals, completely eliminating the sensation of pain. All these agents work directly on the brain. Some that are currently used by veterinarians include sevoflurane, isofluorane and halothane. This group of chemicals are all halogenated (non-flammable) ethers.
Barbiturates such as sodium pentobarbital were used many years ago as general anesthetics in the United States and are still used in part of the World. Although they are powerful anesthetics they are also dangerous because once given they could not be reversed or removed quickly from the body. They also caused sharp drops in blood pressure and heart rate. The only drugs of this class still in use are thiamylal and pentothal which are ultra-short acting anesthetics used to place an endotracheal tube in your pet prior to administering maintenance gas anesthetics.
The most common dissociative anesthetics used in veterinary medicine are Ketamine which is approved for use in cats and Telazol which is a combination of tiletamine and the tranquilizer, zolazepam. These chemicals are not anethetics in the strict sense of the word because they block pain by "de-linking" certain parts of the brain from each other. So an animal receiving one of these agents may not appear to be asleep but rather in a trance.
Xylazine is an alpha-2-adrenergic agonist sedative with profound pain control properties. Its sedative properties relate to central nervous system depression. Its muscle relaxant properties relate to inhibition of the transmission of impulses between nerve cells of the brain. A drug, yohimbine (Yobine) reverses the effects of xylazine almost instantaneously when the procedure is finished. I often use xylazine combined with Telazol in a “cocktail” that improves its safety. For safety, I tend to underdose with this drug, using it at less then the manufacturers recommended dose. When necessary, I fine tune respiration and stabilize respiration with another drug, doxapram hydrochloride injection (Dopram).
Medetomidine hydrochloride (Domitor) is an injectable, synthetic a2-adrenoreceptor agonist with strong sedative and pain relief properties. I do not use this drug but those that do say it is an excellent and safe anesthetic. This drug facilitates handling dogs and aid in conducting diagnostic and therapeutic procedures. It is excellent for minor surgical procedures with or without local anesthesia such as dental care when intubation is not required.
Preparing your Pet for General Anesthesia:
There are several things that you as a pet owner can do to help your veterinarian plan for successful general anesthesia and surgery.
First, if you have fears about your pet’s ability to tolerate anesthesia tell them to your veterinarian a week or so before the scheduled surgery. Most veterinarians will be happy to explain the benefits and risks of anesthesia and other treatment options that may be available. Let the veterinarian know if you have noticed any lethargy, exercise intolerance, weight loss, tendency to bleeding (read about why here) or changes in body functions that may need to be explored. Ask that complete pre-anesthetic blood work be performed on your pet prior to surgery. Be sure to let the veterinarian know if your pet has reacted badly to anesthetics or medications in the past! Dogs and cats are extremely perceptive as to your fears and it is not unusual for them to show psychosomatic illness in response to your worry and concern.
Withhold all food from your pet for at least twelve hours prior to surgery. This is to prevent the pet from vomiting and inhaling stomach contents during surgery. It will also lessen nausea in the pet as he regains consciousness. It is amazing how many pets that I have operated on have had food in their stomachs despite my advice not to feed them. All members of the household need to withhold food from your pet and removing kitchen garbage containers from their reach is also a good idea. I allow pets to drink water up to 4 hours before surgery but veterinarians differ in their advice.
If the pet is not ill, groom and comb it prior to surgery and allow it to relieve itself before entering the hospital.
Many veterinary hospitals do not have twenty-four hour staff to observe your pet after surgery. This might mean that your pet would spend the night after surgery alone in the hospital. If you are not squeamish, I suggest you pick the pet up just prior to hospital closing and watch over it yourself that evening. Keep the veterinarian’s emergency phone number or the number of a 24-hour emergency service at hand. Dogs and cats are homebodies who find the familiar odors, sites and sounds of home a powerful incentive to getting well.
You should leave the hospital with a set of discharge instructions. If you have small children at home be careful that they do not bother the pet during its recovery. It is not unusual for dogs and cats to be snappy for a few days subsequent to surgery.
Stages Of Surgical Anesthesia:
Induction is the first stage of anesthesia. Once the animal is induced with a pre-anesthetic or anesthetic compound, I place an endotracheal tube in its windpipe to facilitate breathing and to be sure its airways remain unobstructed. This is especially important in flat-faced breeds such as Persian cats, pekingeses, pugs and bulldogs.
During anesthesia I regulate the depth of the pet by increasing the amount of gas administered through the endotracheal tube or administering more injectable agent. Sick and older pets always require less of the anesthetic agent.
During surgery I constantly monitor the depth of anesthesia by checking the animal’s eye reflexes, pulse strength, heart rate, mucous membrane color and jaw muscle tone. I also place a pulse oxymeter clip on the patient’s tongue to monitor oxygen saturation of the blood. With the recent availability of pet blood pressure monitors, checking blood pressure during surgery is becoming common.
I monitor the same signs during recovery to be certain recovery remains uneventful. If I become worried or just want to wake the patient up quickly, I administer a specific antagonist to the anesthetic drug that neutralizes it. Pets are often chilled and shivering as they recover from anesthesia. They really appreciate being wrapping a heated towel, fresh from the dryer, or being placing on a hot water bottle.
Post Anesthetic Pain Control:
Pets, like people, are in considerable pain after major surgery. One must balance the desire for pain control with the side effects of all pain control medications. Most drugs that block pain also depress the heart, circulatory system and respiration.
Aspirin can cause vomiting and diarrhea and slow the ability of your pets blood to clot. Never give Tylenol (acetometaphen). It does not work well in dogs and it is very toxic to cats.
A better drug for pain control in both cats and dogs is torbutrol (butorphanol tartrate). Other medications that work well to block post-surgical pain are tramadol, amantadine, non-steroidal anti-inflamatory drugs (NSAIDs) and gabapentin. You can read about the many ways to control pain in pets here.