Anesthesia

I

Introduction

Anesthesia, absence of physical sensation in part or all of the body. The term more commonly refers to a reversible condition that is induced using anesthetic drugs. These drugs may be injected, inhaled, or applied directly to the surface of the body. Each year about 40 million patients in the United States receive anesthetic drugs for surgery, obstetrics, dentistry, or other medical procedures. Induced anesthesia may be local, involving only part of the body, or general, involving lack of sensation in the entire body as well as a loss of consciousness. Localized anesthesia can also be a result of natural causes, such as nerve injury, leprosy, or diabetes. The lack of sensation caused by these conditions is not easily reversible, and patients’ unawareness of pain and other sensations can put them at risk of serious harm. This article focuses on induced anesthesia used in medical procedures.

II

Pain Pathways and Anesthetic Action

Anesthesia is distinct from analgesia. An analgesic, or pain-relieving drug such as aspirin, may relieve a headache, but a person who takes an aspirin still feels other physical sensations, such as pressure, heat, cold, and vibration. In contrast, anesthetic drugs block all physical sensations, though for medical purposes their ability to block pain is among their most important effects. Pain is a crucial warning system that tells us when our bodies are in danger, but without anesthesia, pain would make surgery and various other medical procedures much more difficult—or even impossible.

The sensation of pain results from communication between nerve cells in the brain, spinal cord, and elsewhere in the body (see Nervous System). The process begins when certain nerve cell endings, known as pain receptors or nociceptors, are stimulated. Pain receptors are located in the skin, joints, muscles, the linings of the body cavities, and elsewhere in the body. Nerve impulses travel from pain receptors along nerve fibers to the spinal cord and then to the brain. Pain impulses are relayed through a brain structure known as the thalamus and then to the cerebral cortex, the area of the brain that interprets messages and generates the conscious sensation of pain. At several points along their journey from pain receptor to cerebral cortex, pain impulses can be modified. For example, chemicals known as endorphins, the body’s natural painkillers, interact with nerve cells in the brain and spinal cord to dampen the sensation of pain.

The various drugs used in anesthesia work by several different mechanisms to block the transmission or perception of pain and other sensations. Some sleep-producing drugs used as part of general anesthesia are injected into a patient’s veins. These drugs are taken up by organs, muscles, and brain tissue and interfere with the complex and poorly understood biochemical mechanisms of consciousness. Anesthetic drugs that are inhaled dissolve in the blood and circulate to the brain. There they interact with brain cells, especially cells in the cerebral cortex that are involved in sensory perceptions. Opiates, a family of opium-derived pain relievers that includes morphine, codeine, and fentanyl, act like the body’s own natural endorphins to dampen the sensation of pain. Drugs used as local anesthetics block pain impulses in a specific part of the body, preventing these nerve impulses from reaching the brain. These drugs interfere with the chemicals inside nerve fibers that are involved in transmission of nerve impulses.

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III

General Anesthesia

General anesthesia, which is usually used for major surgery, produces an absence of pain and sensation in the entire body, loss of consciousness, muscle relaxation, and amnesia. These effects make complicated surgical procedures easier to perform. For instance, muscle relaxation and loss of consciousness prevent movement by the patient, helping doctors perform more accurate surgery. Surgery under general anesthesia is less traumatic for the patient because he or she remembers nothing about the procedure.

Anesthetic drugs must be administered throughout the length of an operation in order to maintain the proper depth of general anesthesia. Most commonly, general anesthesia is produced with a combination of several different drugs, each used for a specific effect, such as producing sleep, pain control, or muscle relaxation. General anesthesia often begins with the injection of a sedative medication, such as midazolam. A patient may also receive medications to reduce the production of saliva, which could cause choking, and of stomach acid, which could damage the lungs if inhaled. Propofol and sodium pentothal are common sleep-inducing drugs that are administered intravenously. These drugs act rapidly but their effect does not last long, so they are often used for the early stages of anesthesia. Anesthesia is then maintained for the length of the surgical procedure with longer acting drugs. Isoforane and desflurane are sleep-inducing drugs that are inhaled. They are mixed with oxygen in an anesthesia machine, then inhaled by the patient through a facemask. Nitrous oxide, a gas that produces light anesthesia when inhaled alone, is often used in combination with other anesthetic drugs to increase their effect. Commonly used muscle relaxants include curare and vecuronium, which work by blocking impulses from nerves to muscles. These drugs paralyze muscles throughout the body, including those involved in breathing. For this reason, doctors place patients who are under general anesthesia on a breathing machine. A pain-blocking drug such as fentanyl is also injected as part of general anesthesia.

Anesthetic drugs can have various side effects, including nausea and vomiting or changes in blood pressure, breathing, and heart rate. In addition, many surgical patients already have other illnesses, such as problems with the heart, blood pressure, lungs, kidney, liver, or nervous system. These problems can make anesthetic drugs more dangerous. To detect and correct any complications that may develop, an anesthetized patient’s blood pressure, heart rate and rhythm, blood oxygen concentration, breathing rate, exhaled carbon dioxide, and temperature are monitored throughout surgery. The patient is also observed for signs such as tearing, sweating, and wrinkling of facial muscles that can indicate anesthesia depth may be lightening.

Using a combination of anesthetic drugs enables doctors to use lower doses of each drug and maintain the proper depth of anesthesia while minimizing the risk of side effects. This strategy, along with better monitoring of anesthetized patients and the development of improved anesthetic drugs, has greatly improved the safety of anesthesia in recent years. In the last decade alone, the number of deaths attributed to anesthetic drugs has dropped 25-fold, to about 1 in every 250,000 general anesthesias.

Anesthesia is reversed simply by halting the administration of anesthetic drugs. Any remaining anesthetic gases in the body are gradually exhaled, and as their concentration in the body falls, consciousness returns. The muscle relaxant circulating through the body is removed by the liver and kidneys, and its effects can also be reversed with other drugs. Medications to control pain continue to be given after the operation.

IV

Local Anesthesia

Local anesthesia is commonly used for a variety of relatively minor procedures, such as dentistry, sewing up wounds in the emergency room, and operations on the toes and fingers and other superficial areas of the body. Local anesthesia is less complicated and often safer than general anesthesia because it involves fewer drugs and has less effect on blood pressure, heart rate, and breathing because there is no loss of consciousness. In dental procedures, local anesthesia is advantageous because protective reflexes such as the gag reflex, which helps prevent choking, remain intact.

Local anesthetics are most commonly administered by injecting them into the part of the body that needs to be anesthetized. They can be injected directly into the tissue being operated on, a technique known as local infiltration. Or, they can be injected near the nerves that carry pain impulses from a particular part of the body, a technique known as nerve block. Topical anesthesia is the application of a local anesthetic directly onto the surface of a mucous membrane, such as the eye or the lining of the throat or nose.

Regional anesthesia is produced by injecting a local anesthetic around the spinal area, thus anesthetizing a larger area of the body than in local anesthesia. Spinal anesthesia is a type of regional anesthesia in which the anesthetic is injected into the fluid around the spinal cord. Epidural anesthesia is a similar technique, but the anesthetic is injected into the spinal canal between the membranes that surround the spinal cord. Spinal and epidural anesthesia block sensation everywhere in the body that is lower than the site of injection. These techniques are frequently used in childbirth and in surgery on the lower half of the body, such as prostate gland removal and surgery on the hips or knees.

The earliest known local anesthetic was cocaine, but it is rarely used today because it can cause convulsions and nervousness, and it is addictive. The most commonly used local anesthetic today is lidocaine. Procaine, more commonly known by its brand name Novocain, is sometimes used as a local anesthetic, particularly in dental procedures. However, it does not last as long as lidocaine, and it is more toxic. Bupivacaine is a very long acting local anesthetic that is useful for long operations and provides a greater duration of pain relief after surgery. To help allay patients’ anxiety, a sedative is often given as a part of local anesthesia.

All local anesthetics can be toxic if injected in too high a dose. Particularly when used as part of spinal or epidural anesthesia, local anesthetics may cause a dangerous fall in blood pressure. Many people experience nervousness and discomfort when local anesthetics are injected with epinephrine, a drug that constricts blood vessels to help keep the anesthetic in the area where it is needed. However, true allergic reactions to local anesthetics are very rare.