General anesthesia is the induction of a balanced state of unconsciousness, accompanied by the absence of pain sensation and the paralysis of skeletal muscle over the entire body. It is induced through the administration of anesthetic drugs and is used during major surgery and other invasive surgical procedures.
General anesthesia is intended to bring about five distinct states during surgery:
- analgesia, or pain relief
- amnesia, or loss of memory of the procedure
- loss of consciousness
- weakening of autonomic responses
A complete medical history, including a history of allergies in family members, is an important precaution. Patients may have a potentially fatal allergic response to anesthesia known as malignant hyperthermia, even if there is no previous personal history of reaction.
General anesthetics should be administered only by board-certified medical professionals. Anesthesia providers consider many factors, including a patient's age, weight, allergies to medications, medical history, and general health when deciding which anesthetic or combination of anesthetics to use. The American Society of Anesthesiologists has compiled guidelines for classifying patients according to risk levels as follows:
- I: healthy patient
- II: patient with mild systemic disease without functional limitations
- III: patient with severe systemic disease with definite functional limitations
- IV: patient with severe systemic disease that is life-threatening
- V: dying patient not expected to survive for 24 hours without an operation
Equipment for general anesthesia should be thoroughly checked before the operation; all items that might be needed, such as extra tubes or laryngoscope blades, should be available. Staff members should be knowledgeable about the problems that might arise with the specific anesthetic being used, and be able to recognize them and respond appropriately. General anesthetics cause a lowering of the blood pressure (hypotension), a response that requires close monitoring and special drugs to reverse it in emergency situations.
General anesthetics may be gases or volatile liquids that evaporate as they are inhaled through a mask along with oxygen. Other general anesthetics are given intravenously. The amount of anesthesia produced by inhaling a general anesthetic can be adjusted rapidly, if necessary, by adjusting the anesthetic-to-oxygen ratio that is inhaled by the patient. The degree of anesthesia produced by an intravenously injected anesthetic cannot be changed as rapidly and must be reversed by administration of another drug.
The precise mechanism of general anesthesia is not yet fully understood. There are, however, several hypotheses that have been advanced to explain why general anesthesia occurs. The first, the so-called Meyer-Overton theory, suggests that anesthesia occurs when a sufficient number of molecules of an inhalation anesthetic dissolve in the lipid cell membrane. The second theory maintains that protein receptors in the central nervous system are involved, in that inhalation anesthetics inhibit the enzyme activity of proteins. A third hypothesis, proposed by Linus Pauling in 1961, suggests that anesthetic molecules interact with water molecules to form clathrates (hydrated microcrystals), which in turn inhibit receptor function.
Stages of anesthesia
There are four stages of general anesthesia that help providers to better predict the course of events, from anesthesia induction to emergence.
- Stage I begins with the induction of anesthesia and ends with the patient's loss of consciousness. The patient still feels pain in Stage I.
- Stage II, or REM stage, includes uninhibited and sometimes dangerous responses to stimuli, including vomiting and uncontrolled movement. This stage is typically shortened by administering a barbiturate, such as sodium pentothal, before the anesthetic agent.
- Stage III, or surgical anesthesia, is the stage in which the patient's pupillary gaze is central and the pupils are constricted. This is the target depth of surgical anesthesia. During this stage, the skeletal muscles relax, the patient's breathing becomes regular, and eye movements stop.
- Stage IV, or overdose, is marked by hypotension or circulatory failure. Death may result if the patient cannot be revived quickly.
Types of anesthetic agents
There are two major types of anesthetics used for general anesthesia, inhalation and intravenous anesthetics. Inhalation anesthetics, which are sometimes called volatile anesthetics, are compounds that enter the body through the lungs and are carried by the blood to body tissues. Inhalation anesthetics are less often used alone in recent clinical practice; they are usually used together with intravenous anesthetics. A combination of inhalation and intravenous anesthetics, often with opioids added for pain relief and neuromuscular blockers for muscle paralysis, is called balanced anesthesia.
INHALATION ANESTHETICS. The following are the most commonly used inhalation anesthetics:
- Halothane causes unconsciousness but provides little pain relief; often administered with analgesics . It may be toxic to the liver in adults. Halothane, however, has a pleasant smell and is therefore often the anesthetic of choice when mask induction is used with children.
- Enflurane is less potent, but produces a rapid onset of anesthesia and possibly a faster recovery. Enflurane is not used in patients with kidney failure.
- Isoflurane is not toxic to the liver but can induce irregular heart rhythms.
- Nitrous oxide (laughing gas) is used with other such drugs as thiopental to produce surgical anesthesia. It has the fastest induction and recovery time. It is regarded as the safest inhalation anesthetic because it does not slow respiration or blood flow to the brain. However, because nitrous oxide is a relatively weak anesthetic, it is not suited for use in major surgery. Although it may be used alone for dental anesthesia, it should not be used as a primary agent in more extensive procedures.
- Sevoflurane works quickly and can be administered through a mask since it does not irritate the airway. On the other hand, one of the breakdown products of sevoflurane can cause renal damage.
- Desflurane, a second-generation version of isoflurane, is irritating to the airway and therefore cannot be used for mask (inhalation) inductions, especially not in children. Desflurane causes an increase in heart rate, and so should be avoided for patients with heart problems. Its advantage is that it provides a rapid awakening with few adverse effects.
INTRAVENOUS ANESTHETICS. Commonly administered intravenous general anesthetics include ketamine, thiopental (a barbiturate), methohexital (Brevital), etomidate, and propofol (Diprivan). Ketamine produces a different set of reactions from other intravenous anesthetics. It resembles phencyclidine, which is a street drug that may cause hallucinations. Because patients who have been anesthetized with ketamine often have sensory illusions and vivid dreams during post-operative recovery, ketamine is not often given to adult patients. It is, however, useful in anesthetizing children, patients in shock, and trauma casualties in war zones where anesthesia equipment may be difficult to obtain.
General anesthesia in dental procedures
The use of general anesthesia in dental and oral surgery patients differs from its use in major surgery because the patient's level of fear is usually a more important factor than the nature of the procedure. In 1985, an NIH Consensus Statement reported that high levels of preoperative anxiety, lengthy and complex procedures, and the need for a pain-free operative period may be indications for general anesthesia in healthy adults and very young children. The NIH statement specified that at least three professionals are required when general anesthesia is used during dental procedures: one is the operating dentist; the second is a professional responsible for observing and monitoring the patient; the third person assists the operating dentist.
Although the United States allows general anesthesia for dental procedures to be administered outside hospitals (provided that the facility has the appropriate equipment and emergency drugs), Scotland banned the use of general anesthesia outside hospitals in 2000, after a ten-year-old boy died during a procedure to have a tooth removed.
Preparation for general anesthesia includes the taking of a complete medical history and the evaluation of all factors—especially a family history of allergic responses to anesthetics—that might influence the patient's response to specific anesthetic agents.
Patients should not eat or drink before general anesthesia because of the risk of regurgitating food and liquid or aspirating vomitus into the lungs.
Patients should be informed of the risks associated with general anesthesia as part of their informed consent . These risks include possible dental injuries from intubation as well as such serious complications as stroke, liver damage, or massive hemorrhage. If local anesthesia is an option for some procedures, the patient should be informed of this alternative. In all cases, patients should be given the opportunity to ask questions about the risks and benefits of the procedure requiring anesthesia as well as questions about the anesthesia itself.
Depending on the patient's level of anxiety and the procedure to be performed, the patient may be premedicated. Most medications given before general anesthesia are either anxiolytics, usually benzodiazepines; or analgesics. Patients in severe pain prior to surgery may be given morphine or fentanyl. Anticholinergics (drugs that block impulses from the parasympathetic nervous system) may be given to patients with a known history of bronchospasm or heavy airway secretions.
The anesthetist and medical personnel provide supplemental oxygen and monitor patients for vital signs and monitor their airways. Vital signs include an EKG (unless the patient is hooked up to a monitor), blood pressure, pulse rate, oxygen saturation, respiratory rate, and temperature. The staff also monitors the patient's level of consciousness as well as signs of excess bleeding from the incision.
Although the risk of serious complications from general anesthesia are low, they can include heart attack, stroke, brain damage, and death. The risk of complications depends in part on the patient's age, sex, weight, allergies, general health, and history of smoking, alcohol or drug use.
The overall risk of mortality from general anesthesia is difficult to evaluate, because so many different factors are involved, ranging from the patient's overall health and the circumstances preceding surgery to the type of procedure and the skill of the physicians involved. The risk appears to be somewhere between 1:1,000 and 1:100,000, with infants younger than age one and patients older than 70 being at greater risk.
Awareness during surgery
One possible complication is the patient's "waking up" during the operation. It is estimated that about 30,000 patients per year in the United States "come to" during surgery. This development is in part the result of the widespread use of short-acting general anesthetics combined with blanket use of neuromuscular blockade. The patients are paralyzed with regard to motion, but otherwise "awake and aware." At present, special devices that measure brain wave activity are used to monitor the patient's state of consciousness. The bispectral index monitor was approved by the FDA in 1996 and the patient state analyzer in 1999.
Nausea and vomiting
Post-operative nausea and vomiting is a common problem during recovery from general anesthesia. In addition, patients may feel drowsy, weak, or tired for several days after the operation, a combination of symptoms sometimes called the hangover effect. Fuzzy thinking, blurred vision, and coordination problems are also possible. For these reasons, anyone who has had general anesthesia should not drive, operate machinery, or perform other activities that could endanger themselves or others for at least 24 hours, or longer if necessary.
Inhalation anesthetics are sometimes toxic to the liver, the kidney, or to blood cells. Halothane may cause hepatic necrosis or hepatitis. Sevoflurane may react with the carbon dioxide absorbents in anesthesia machines to form compound A, a haloalkene that is toxic to the kidneys. The danger to red blood cells comes from carbon monoxide formed by the breakdown products of inhalation anesthetics in the circuits of anesthesia machines.
Malignant hyperthermia is a rare condition caused by an allergic response to a general anesthetic. The signs of malignant hyperthermia include rapid, irregular heartbeat; breathing problems; very high fever; and muscle tightness or spasms. These symptoms can occur following the administration of general anesthetics, especially halothane.
General anesthesia is much safer today than it was in the past, thanks to faster-acting anesthetics; improved safety standards in the equipment used to deliver the drugs; and better devices to monitor breathing, heart rate, blood pressure, and brain activity during surgery. Unpleasant side effects are also less common, in part because of recent developments in equipment that reduces the problems of anesthetizing patients who are difficult to intubate. These developments include the laryngeal mask airway and the McCoy laryngoscope, which has a hinged tip on its blade that allows a better view of the patient's larynx.
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Sam Uretsky, PharmD