Anesthetics and Anesthesia for Dogs

Anesthetics are drugs used to block the sensation of pain. They are divided into two categories: local and general.

Local anesthetics are used to numb the surface of the body. They are injected into tissue and around regional nerves. They may also be applied topically to mucous membranes. Local anesthetics, such as xylocaine, have fewer risks and side effects than general anesthetics, but they are not suitable for major surgery.

General anesthetics render the dog unconscious. They can be given by injection or inhalation. Light anesthesia sedates or relaxes the dog and may be suitable for short procedures, such as removing porcupine quills. For prolonged surgeries or extremely painful procedures such as surgery on the eye or orthopedic procedures, the dog may need to be kept at a deeper level of anesthesia. This is normally done using inhaled anesthetics. Inhaled gases, such as isoflurane, are administered through a tube placed in the dog's trachea. By adjusting the flow of the gas, the dog can be kept at a lighter or deeper level of anesthesia.

The guideline dose of an injectable anesthetic is computed according to the weight of the dog. For gas anesthesia, the mixture of oxygen and anesthetic is balanced and the dose adjusted according to the breathing of the dog. Many factors require that the exact dosage be customized to the individual dog.

Certain breeds have an increased sensitivity to barbiturates and other anesthetics, and that must also be taken into account. Toy breeds and breeds with a low percentage of body fat, particularly Greyhounds and Border Collies, require less anesthetic per pound of body weight. This is one reason why anesthetics should be given by someone who is trained to determine the degree of sedation each drug produces.

Combinations of anesthetics are often given to lessen the potential toxicity of each.

Anesthetics are removed from the bloodstream by the lungs, liver, and kidneys. Impaired function of these organs can cause dose-related complications. If your dog has a history of lung, liver, kidney, or heart disease, the risk from anesthesia and surgery is increased. Presurgery bloodwork will help your veterinarian to determine the safest drug and dose for your dog.

A major risk of general anesthesia is the dog vomiting when going to sleep or waking up. The vomitus refluxes into the trachea and produces asphyxiation. This can be avoided by keeping the stomach empty for 12 hours before scheduled surgery. If you know your dog is going to have an operation the next day, do not give him anything to eat or drink after 6 p.m. the night before. This means picking up the water dish and keeping the dog away from the toilet bowl and other sources of water. The endotracheal tube used to administer anesthesia has a small inflatable balloon that helps block off the trachea and inhaling vomitus.

This article is excerpted from “Dog Owner’s Home Veterinary Handbook” with permission from Wiley Publishing, Inc.