Basics of Anaesthesia

INTRODUCTION
Many patients, and even some physicians, automatically assume that surgery requires general anaesthesia, and that the patient should be asleep during surgery. This is not true. Many procedures can be performed on awake patients, using local or regional anaesthesia. This not only avoids the risks and unpleasantness sometimes associated with general anaesthesia, but may also provide specific benefits such as reduced blood loss and better postoperative analgesia.
Patients are often concerned about having surgery under a local or regional anaesthetic. These concerns are not usually justified by the facts. The more patients understand the reasons for, and the benefits of, local or regional anaesthesia, the more likely they are to choose this type of anaesthetic. Unfortunately, in these days of cost-cutting and same day surgery, patients may never get the opportunity to discuss their anaesthetic options with an anaesthetist in detail prior to surgery. In the rush to get through a busy operating list the anaesthetist may, unfortunately, decide that it is quicker and simpler just to put the patient to sleep, rather than enter into the discussion and education necessary to allow the patient to make an informed choice about the most appropriate type of anaesthesia.
Patients are becoming more involved as consumers of health care. They are actively seeking out information about treatment choices, and some are turning to the Internet as a source of medical information. This site is dedicated to patients who want to learn more about local and regional anaesthesia. However, they must understand that this article provides background information only. The final decision about the best type of anaesthetic depends on the specific operation, patient, surgeon, and anaesthetist involved.
If you are faced with the possibility of needing surgery in the future, chances are you will need some type of anaesthesia to go along with it. There are many different types of anaesthesia. Which one you will need depends on a variety of factors such as the type of surgery you are having and your state of health. Some surgical procedures require only an injection of local anaesthesia into the incision area. Other procedures cannot be performed unless you are completely anesthetized -- unconscious and unaware of pain.

The Basics
Anaesthesia is divided into four basic categories:
· general anaesthesia
· regional anaesthesia
· local anaesthesia
· sedation
Each type of anaesthesia has an effect on a part of the nervous system, which results in a depression or numbing of nerve pathways. General anaesthesia affects the brain cells, which causes you to lose consciousness. Regional anaesthesia has an effect on a large bundle of nerves to a particular area of the body, which results in losing sensation to that area without affecting your level of consciousness. Local anaesthesia causes you to lose sensation in a very specific area.
Some of the drugs that produce general anaesthesia in large doses can be used to produce sedation, or "twilight sleep" in lower doses. Sedation can be given in many ways. A common example of an anesthetic gas that is used for sedation is nitrous oxide or laughing gas.
If you are scheduled to have surgery, you may be told not to eat anything for eight hours. It is very important that you follow whatever instructions you are given for not eating or drinking anything prior to surgery. Why? Because when you are given anaesthesia, you lose the ability to protect your lungs from inhaling something you're not supposed to inhale. When you are awake, you can usually swallow saliva and food without choking because part of the swallowing mechanism involves a reflex that results in covering the opening into the lungs. When you are anesthetized, you lose that reflex. So, if you have any solids or liquids in your stomach, they could come up into your mouth and be inhaled into your lungs. The result could be very serious lung damage.
Sleep is a state of reduced consciousness, depressed metabolism, and little activity of the skeletal muscles. Strong stimuli such as loud noise, bright light or shaking can arouse the sleeper. Consciousness is being clearly aware of yourself and your environment.
Unconsciousness is when you are completely or partially unaware of yourself and your environment, or you don't respond to sensory stimuli.
Conscious sedation is caused when an anesthesiologist administers depressant drugs and/or analgesics in addition to anaesthesia during surgery. Consciousness is depressed and you may fall asleep, but are not unconscious.

General Anaesthesia
General anesthetics produce an unconscious state. In this state a person is:
· unaware of what is happening
· pain-free
· immobile
· free from any memory of the period of time during which he or she is anesthetized
It is not completely clear exactly how general anesthetics work at a cellular level, but it is speculated that general anesthetics affect the spinal cord (resulting in immobility), the brain-stem reticular activating system (resulting in unconsciousness) and the cerebral cortex (seen as changes in electrical activity on an electroencephalogram).

General anaesthesia can be administered as an inhaled gas or as an injected liquid. There are several drugs and gases that can be combined or used alone to produce general anaesthesia. The potency of a given anesthetic is measured as minimum alveolar concentration (MAC). This term describes the potency of anesthetic gases. (Aveolar is the area in the lung where gases enter and exit the bloodstream via the capillary system). Technically, MAC is the alveolar partial pressure of a gas at which 50 percent of humans will not move to a painful stimulus (e.g. skin incision). Injected liquid anesthetics have a "MAC equivalent" which is the blood concentration of the liquid anesthetic that provides the same effect. Using MAC as a guideline, the amount of anesthetic given to a patient depends on that particular patient's needs.
When anesthetics reach the bloodstream, the drugs that affect the brain pass through other blood vessels and organs so they are often affected too. Therefore, patients must be carefully monitored. The anesthesiologist continuously monitors the patient's heart rate, heart rhythm, blood pressure, respiratory rate, and oxygen saturation. Some patients may have even more extensive monitoring depending on their health and which type of procedure or surgery they are having.

Most adults are first anesthetized with liquid intravenous anesthetics followed by anesthetic gases after they are asleep. Children, however, may not like having an injection or intravenous catheter placed in them while they are awake. Therefore, they often breathe themselves to sleep with anesthetic gases given through a mask.
What is local or regional anaesthesia?
Anaesthesia means the absence of sensation. Regional anaesthesia means blocking the nerve supply to part of the body, such as an arm, so the patient cannot feel pain in that area. Local anaesthesia, strictly speaking, means putting local anaesthetic ("freezing") around the affected area to make it pain free. However, many people use the phrase loosely to include regional anaesthesia.
Local Anaesthesia
Local anaesthesia involves numbing a small area by injecting a local anesthetic under the skin just where an incision is to be made. When used alone, this type of anaesthesia has the least number of risks. Local anesthetics are thought to block nerve impulses by decreasing the permeability of nerve membranes to sodium ions. There are many different local anesthetics that differ in absorption, toxicity, and duration of action.
One of the most commonly used local anesthetics is lidocaine (Xylocaine). Lidocaine can be administered as an injection or placed topically on mucous membranes. Another topical anesthetic is cocaine. Cocaine is primarily used to anesthetize the nasal passages for surgical procedures. A topical anesthetic that is gaining popularity for anesthetizing the skin prior to painful procedures, such as injections, is known as eutectic mixture of local anesthetics (EMLA) cream which contains lidocaine and prilocaine. This white cream is placed on the skin and then covered with an occlusive dressing for approximately one hour to obtain a good numbing effect. In addition, EMLA can be used to numb the skin prior to giving injections or pulling superficial splinters.

Regional Anaesthesia
Regional anaesthesia is so named because a "region" of the body is anesthetized without making the person unconscious. One example of this is spinal anaesthesia, which is often used on women during childbirth. A local anesthetic is injected into the spinal fluid and causes a loss of sensation of the lower body. Spinal anaesthesia can be used for surgery on the legs or lower abdomen (below the bellybutton).
Epidural anaesthesia is similar to spinal anaesthesia in that a patient loses sensation in the legs and lower abdomen, but instead of injecting the local anesthetic into the spinal fluid, the anesthetic is injected into a space outside the spinal canal called the epidural space. A small tube or catheter can be placed into this space and a local anesthetic can be infused (fed) through the tube for hours, days, or even weeks. This type of anaesthesia can be used for surgery with larger doses of anesthetic, or for chronic pain relief with lower doses of anesthetic. Regional anaesthesia techniques can be used to block very specific areas such as one foot, one leg, one arm, or one side of the neck. In these cases, a smaller group of nerves is blocked by injection of the local anesthetic into a specific area. For spinals and epidurals, narcotic painkillers such as morphine and fentanyl can be used in addition to a local anesthetic.

Sedation
Some of the drugs that produce general anaesthesia in large doses can be used to produce sedation or "twilight sleep" in lower doses. Sedation can be given in many ways. A common example of an anesthetic gas that is used for sedation is nitrous oxide or laughing gas. Liquid sedating drugs are usually given by injection but some can also be given by mouth. Ketamine and Versed are examples of sedating drugs that can be given by injection or by mouth. The oral route is particularly useful for sedating children who do not like injections.
Children who refuse to drink medications may also receive sedation through the rectum via a small, lubricated tube or via the nasal route by spraying it into the nose. Regional and local anaesthesia can be combined with sedation to make patients more comfortable during a procedure in which general anaesthesia is not necessary, or when general anaesthesia may be too large a risk for the patient.

How is it used?
Local or regional anaesthesia can often be used to prevent pain during surgery. Sometimes it is used by itself, with no other medications, so that the patient remains wide awake during surgery. It can also be combined with sedative drugs to make the patient relaxed or sleepy during surgery.
Sometimes local or regional anaesthesia is used in addition to a general anaesthetic (i.e., in patients who are asleep during surgery). This is done to reduce the stress associated with surgery, to allow a lighter level of anaesthetic during surgery, and to provide pain relief after surgery.
Inhaled Anesthetics
Many adults may remember having ether for their anesthetic when they were young. Ether is an inflammable anesthetic that is no longer used in the United States. Today, the commonly used inhaled anesthetics are nitrous oxide (also known as laughing gas), sevoflurane, desflurane, isoflurane and halothane.
Why do we have so many different kinds of gases? Because each gas has its own special properties. For example, sevoflurane and halothane are easy to inhale while desflurane is very irritating to inhale and has a shorter duration of action. If you need to breathe yourself to sleep, halothane or sevoflurane would be easiest to inhale. If a very short-acting anesthetic is needed, the anesthesiologist can switch to desflurane after you fall asleep. Nitrous oxide is easy to inhale, but when used alone is not potent enough to be a complete general anesthetic. However, it can be used alone for sedation, or combined with one of the other inhaled anesthetics or injected liquid anesthetics for general anaesthesia.
These gases have different effects on other organs as well. For example, halothane may cause the heart rate to slow down and the blood pressure to decrease while desflurane may cause the heart rate to speed up and the blood pressure to increase. How do these inhaled anesthetics reach the brain? When an anesthetic gas is inhaled into the lungs, the blood that travels through the lungs carries the anesthetic gas to central nervous system cells. The rate at which the bloodstream takes up the anesthetic is dependent on many factors including the concentration of the inspired gas, the rate of flow of the gas from the anaesthesia machine, the solubility of the gas in blood, the rate and depth of breathing, and the amount of blood the heart pumps each minute in the person breathing the gas.
An important property of anesthetics is reversibility. When the surgery is over, the anesthesiologist wants to shut off the anesthetic and have the patient wake up from the anesthetic-induced sleep. Once the anesthetic gas is turned off, the blood stream brings the gas back to the lungs where it is eliminated. The more soluble the gas is in blood, the longer it takes to eliminate. Nitrous oxide and desflurane are the shortest-acting anesthetic gases because they are the least soluble in blood.

Injected Anesthetics
A liquid anesthetic drug is delivered to the brain by injecting it directly into the bloodstream, usually through an intravenous catheter. Examples of injected drugs are barbiturates, propofol, ketamine, and etomidate, as well as larger doses of narcotics (such as morphine) and benzodiazepines (Valium-like drugs). These drugs quickly reach the brain and their effect is dependent on several factors including the volume in which the drug is distributed in the body, the fat-solubility of the drug, and how quickly the body eliminates the drug.
A commonly used injected barbiturate anesthetic is sodium thiopental, also known as Pentothal. This drug is fat-soluble and acts very quickly. If you receive sodium thiopental and then you are asked to count backward from 100 after the drug is injected, you probably won't remember counting past 95. Some injected anesthetics are used in low doses for sedation. A small dose of a narcotic or a benzodiazepine like Valium or Versed can significantly decrease anxiety. These drugs are used in these doses either as a premedication prior to general anaesthesia or as "twilight sleep" or sedation when used in conjunction with local or regional anaesthesia.

DISCLAIMER: This information is solely for informational purposes. IT IS NOT INTENDED TO PROVIDE MEDICAL ADVICE. Neither the Editors of Health Mirror, the author nor publisher take responsibility for any possible consequences from any treatment, procedure, exercise, dietary modification, action or application of medication which results from reading or following the information contained in this information. The publication of this information does not constitute the practice of medicine, and this information does not replace the advice of your physician or other health care provider. Before undertaking any course of treatment, the reader must seek the advice of their physician or other health care provider.
In case of any queries please feel free to contact Dr Anil K Dhull