Anesthesia for Dogs

By Malcolm Weir, DVM, MSc, MPH; Tammy Hunter, DVM; Ernest Ward, DVM

What is anesthesia?

The word anesthesia comes from the Greek word meaning "lack of sensation." Anesthesia is accomplished by administering drugs that depress nerve function. With general anesthesia, the patient is made unconscious for a short period. During this unconscious state, there is muscular relaxation and a complete loss of pain sensation.

Other types of anesthesia include local anesthesia, which involves numbing a localized area of skin or a tooth, and spinal anesthesia, such as an epidural block commonly used in humans during childbirth.

What are the risks of anesthesia?

There is always the risk of a harmful reaction when we use any anesthetic agent, whether it is for short-term sedation or for general anesthesia lasting several hours.

It is estimated that approximately 1 in 100,000 animals will have some sort of harmful reaction to an anesthetic agent. These reactions may range from mild swelling at the site of injection or a mild decrease in cardiac output, to a full-blown episode of anaphylactic shock or death. However, many experts estimate the risk of anesthetic death is lower than the risk of driving to and from the hospital to have the anesthetic procedure. A recent study put the risk of death in dogs at 0.17% and in cats at 0.24%.

"Only when the benefits outweigh the risks will they perform anesthesia on your dog."

Another potential danger may arise if the dog is not properly fasted prior to anesthesia. Anesthetized patients lose the normal reflex ability to swallow. If there is food in the stomach, the dog could vomit while under anesthesia or in the early post-anesthetic period. If vomiting occurs in the absence of the swallowing reflex, vomited material can be aspirated (enter into the lungs), causing aspiration pneumonia, a potentially life-threatening condition.

Other rare complications of anesthesia include organ system failure (such as kidney, liver, or heart failure), visual impairment, clotting disorders, and seizures. Your veterinarian will take every precaution to minimize these risks during your dog's anesthesia. Only when the benefits outweigh the risks will they perform anesthesia on your dog.

What can be done to minimize the risks?

Pre-surgical physical examination, preoperative blood and urine tests, and radiographic examination may detect clinical and sub-clinical medical conditions that could increase the risk associated with anesthesia. These conditions include heart disease, liver disease, kidney disease, diabetes mellitus, anemia, dehydration, and certain infectious diseases such as heartworm disease.

Blood tests increase the chance of detecting a hidden problem that could be life-threatening. In older animals, chest X-rays (radiographs) and an electrocardiogram (ECG) are often recommended to ensure there is no pre-existing disease in the heart or lungs that might increase the risk of an adverse reaction.

Immediate intravenous access for emergency drug administration is one of the most important factors in the successful treatment of heart or breathing problems. By placing an intravenous (IV) catheter and line before anesthesia, your veterinarian can ensure that this lifeline is already in place, if the need arises. Anesthetics, fluids, and emergency drugs can be administered through the IV line.

Intravenous fluids help maintain blood pressure in the anesthetized patient and replace lost fluids (due to evaporation from body cavity surfaces, through bleeding, and in any tissues that are being removed). Upon completion of the procedure, intravenous fluid therapy speeds the recovery process by diluting anesthetic agents circulating in the bloodstream and enhancing their metabolism and elimination through the liver and kidneys. Patients that receive IV fluid therapy generally wake up faster than those that do not.

"You should ensure your dog's complete medical history is available to your veterinarian, especially if your dog has been seen at another veterinary clinic."

In addition, studies have shown that 0.9%–2.0% of all patients that receive general anesthesia will develop kidney dysfunction 7–14 days after anesthesia. This risk is significantly reduced in patients that receive intravenous fluid therapy with their surgery. Although 98% of all pets will have no problem, your veterinarian's goal is to eliminate that unknown 2%. For these reasons, all surgery patients should receive intravenous catheterization and fluid therapy.

You should ensure your dog's complete medical history is available to your veterinarian, especially if your dog has been seen at another veterinary clinic. Before anesthetizing your dog, your veterinarian needs to know about any medications or supplements that your dog has received in the past few weeks, any pre-existing medical conditions, any known drug reactions, the results of previous diagnostic tests, and whether the dog has undergone any anesthetic or surgical procedures in the past. Other useful information includes the dog's vaccine status and reproductive status (i.e., when its last estrus was, or heat cycle, if they are not spayed).

Why do I have to sign an anesthetic consent form?

It is important that you fully understand what will happen to your dog, and that you acknowledge that you understand the risks associated with anesthesia. Usually, the form will also include consent to perform surgery or other specified diagnostic testing and will provide an estimate of the expected costs of the procedures. In many areas, the veterinarian is required by their regulatory organization to obtain written consent from the client before they perform anesthetic procedures.

Can you describe a typical anesthesia?

All anesthesia patients are weighed on admission and are given a thorough pre-anesthetic examination, which includes an examination of the chest, palpation of the abdomen, and assessment of the gums (checking for hydration status and evidence of good circulatory status). Before any anesthetic drugs are administered, the veterinary team will review the dog’s medical history and may perform additional diagnostics such as blood or urine testing, blood pressure, electrocardiogram (ECG) or X-rays.

"All anesthesia patients are weighed on admission and are given a thorough pre-anesthetic examination."

In the great majority of cases, a technique called “balanced anesthesia” is used. With balanced anesthesia, the patient receives a combination of sedatives and anesthetic agents that are based on body weight and best suited to the dog’s individual needs. The most common combination is a pre-anesthetic sedative and analgesic combination that is administered by injection, followed by an injectable induction agent that induces anesthesia.

A breathing tube, called an endotracheal tube, is then inserted into the windpipe or trachea. The endotracheal tube is used to deliver anesthetic gas, mixed with oxygen, to keep the dog anesthetized. Additionally, the endotracheal tube seals off the airway so that the patient does not accidentally aspirate fluids or other foreign material while unconscious and unable to swallow.

How do you monitor an anesthetized dog?

Anesthetic monitoring in a veterinary hospital is like that found in any human hospital. Below is a list of common methods of monitoring anesthesia:

The surgery assistant is the most important monitor during an anesthetic procedure. This staff member is trained to observe and monitor the patient throughout the entire procedure, from induction until recovery. The assistant adjusts the anesthetic levels according to the patient's vital signs and ensures that the patient remains stable throughout the procedure.

The electrocardiogram, abbreviated as ECG or EKG, shows the rate and pattern of the heartbeat. It detects and shows abnormal heartbeats called arrhythmias. If an arrhythmia is detected, the anesthetist will make appropriate changes in anesthesia and/or administer emergency medications.

The heart rate monitor measures the number of heart beats per minute. Heart rate must be maintained within a certain range. The depth of anesthesia and surgical stimulation can both affect heart rate. By monitoring heart rate, increases or decreases can be detected early, and anesthetic adjustments can be made quickly, resulting in smoother anesthesia for the patient.

The blood pressure monitor measures the systolic blood pressure (when the heart contracts or pumps) and occasionally the diastolic blood pressure (when the heart relaxes or refills). Coupled with other monitoring equipment, this gives detailed information on the cardiovascular status of the patient.

The respirometer measures the number of breaths per minute.

The core body temperature is monitored, especially during a prolonged surgery, by inserting a temperature probe into the esophagus or rectum. Low or high body temperature can cause dangerous complications. Maintenance of normal body temperature is especially important in small or pediatric patients, and aids in speedy recovery from anesthesia.

A pulse oximeter may be used to monitor the amount of oxygen in the patient's blood (Sp02) and the pulse rate. This instrument should always be used in conjunction with other pieces of monitoring equipment.

The end-tidal C02 monitor is often used in conjunction with a pulse oximeter. This device measures the amount of expired C02 and helps determine if the patient is receiving adequate oxygen during anesthesia.

How long will it take my dog to recover from anesthesia?

With today's anesthetics, many of which are reversible, your dog should be almost completely normal by the time of discharge. Many dogs are sleepy or tired for 12 to 24 hours after anesthesia. If your dog appears to be unusually sluggish or if you cannot arouse them easily, contact the hospital immediately for specific advice.

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