Anesthesia

There are three broad categories of anesthesia: local anesthesia, regional anesthesia, and general anesthesia. Local and regional anesthesia involve the injection of a drug, such as lidocaine or bupivacaine, that soaks into the nerves and blocks the electrical signals from traveling down the nerves. With local anesthesia the drug is injected under the skin in the area of the surgery where the nerves are diffusely spread about in the tissue, whereas in regional anesthesia the drug is injected next to large, discrete nerves traveling to the surgical area. For example, when injected at the right location in the armpit, the arm can be made completely numb, allowing surgery to proceed without the patient feeling any pain. A spinal anesthetic involves placing the needle between the vertebrae into the spinal sac. The drug then reaches the nerves that go to the lower half of the body, making the patient numb from approximately the upper abdomen down. An epidural anesthetic is similar to a spinal, only the needle is placed outside the spinal sac, and, typically, a catheter is inserted (and the needle removed). An advantage of the catheter is it is easier to give subsequent injections.

A general anesthetic renders the patient unconscious during surgery. Most often, unconsciousness is rapidly achieved by injecting a large dose of a sedative, such as pentothal or propofol. Since the drug wears off quickly, it is immediately followed by a gas anesthetic to keep the patient asleep. During surgery, narcotic painkillers may be used to reduce the amount of gas being used, and to get a head start on the pain control that may be required after surgery. Sometimes drugs that paralyze the muscles must also be used to facilitate the operation.

During the use of any anesthetic, the patient's vital signs are watched carefully and continuously. The electrical activity of the heart (electrocardiogram) is displayed on a monitor (see Figure 1); blood pressure is measured every few minutes with an automated machine; and the oxygen level in the arterial blood is measured via a device that clips to a finger. During a general anesthetic a machine will measure the concentration of the gas anesthetic, as well as the level of carbon dioxide coming from the lungs. Careful monitoring is important because all anesthetics can lower blood pressure, depress breathing, and impair many of the body's defense mechanisms. The amount of anesthetic given the patient must therefore be continuously adjusted to match the conditions present during surgery.