GENERAL ANESTHESIA PDF Print E-mail

Sub-topics

Induction (“going off to sleep”)
Intravenous induction
Inhalation (gas) induction
Not just a recipe !
Breathing during anesthesia
Muscle relaxation
Airways and breathing tubes
Awake intubation
Induction of anesthesia in children
Your role as a parent during induction of anesthesia
Emergency induction of general anesthesia
Maintenance of anesthesia (“keeping you asleep”)
Emergence (“waking up”)

Doctor at work

induction (“going off to sleep”)

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our anesthesiologist may start your anesthesia or induce ‘sleep’ in one of three ways. Induction may be:

  • intravenous (into the vein): the most common method

  • inhalation (by breathing in) sometimes called “gas induction”: often used in children

  • intramuscular (into the muscle) injection: now used very rarely.

Intravenous induction

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Drip

 

Before having an intravenous induction, you may have had local anesthesia cream applied to the skin over the vein to be used for the initial injection. The location of the vein depends on the anesthesiologist’s preference, the site of the operation, and the appearance of your veins. Often the veins on the back of the hand or forearm are used. The choice of hand depends on whether you are left- or right-handed, because having a bruise on the back of your dominant hand may cause discomfort afterwards. Also, if your intravenous line must remain in place for some time, you will find it easier to be able to do things, such as combing your hair or brushing your teeth, if your intravenous is not in the hand with which you normally do these things.

Having wiped away the cream and applied some cleaning alcohol to the skin, your anesthesiologist inserts a cannula or fine plastic tube into the vein. This is accompanied by a sensation varying between slight pain and a feeling of light pressure. In the absence of local anesthesia cream, you feel a short sharp pain. The cannula is secured to the skin with tape and may be attached to an intravenous ‘line’ or long clear plastic tube connected to a bag of saline or similar fluid. This fluid may feel cold when it runs into the vein (usually in your arm).

Your anesthesiologist may then have you breathe oxygen from a mask. This process is known as preoxygenation. Your anesthesiologist may also give you one or more medications, before giving you the actual medication which makes you lose consciousness. For example, if you are scheduled to have your gallbladder removed, your anesthesiologist might start by giving you an injection of a medication to relax you, and then another medication to decrease the chance of postoperative vomiting. You might also be given an injection of a potent pain-reliever (opiate or narcotic), such as fentanyl. This medication also helps minimise any marked rises in heart rate and blood pressure that can occur at a slightly later stage of anesthesia and surgery. Sometimes anesthesiologists give these additional medications after you have lost consciousness.

The anesthesiologist then injects the induction medication through the cannula into your vein. This is the time when he or she may ask you to count (often backwards, from 100). Counting is a means of distracting you and also shows when the medication has achieved its effect. The induction medication works very quickly, especially in younger patients. It takes only the time for the blood carrying the medication to return from the arm to the heart and then be pumped through the lungs, back to the heart, and then to the brain. (Anesthesiologists call this the ‘arm-brain circulation time’.) In most people this time is about ten seconds, but it may be faster in children and slower in elderly or very ill patients.

Doctor at work 2

Inhalation (gas) induction

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This method is common in children but is also used in some adults. It involves having the anesthesiologist or the patient hold a mask over the patient’s nose and mouth. The patient then breathes in a mixture of gases through the face mask until loss of consciousness occurs. Induction by mask usually takes longer than the intravenous method, and achievement of the appropriate depth of anesthesia is often preceded by a period of restlessness. This is quite normal and the patient is already unconscious at this time.

Then the anesthesiologist has an assistant (nurse, technician or another anesthesiologist) hold the mask and ensures that the patient is continuing to breathe well. The anesthesiologist then inserts an intravenous cannula (as above), unless one has previously been started. This is more likely to have been done in adults. From this point, anesthesia is similar, whether an intravenous or inhalation technique has been used.

Not just a recipe !

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Notes


How does your anesthesiologist know how much to give you?

Individuals vary in their requirements for anesthesia medications. The dose of the induction medication is generally given slowly to patients who are to have an elective operation. Your anesthesiologist has calculated the expected dose you should need, from your weight, your age, your sex, and your state of health. However, as the medications are injected, the dose of each is adjusted as necessary, according to the effects produced. This is known as titrating the medications according to their effect. In an emergency it is sometimes necessary to give the medications quickly, and a predetermined dose is calculated.

Will you have the same anesthesia as the patient in the bed next to you?

Every anesthesia given is a very individual thing and each anesthesia depends on the patient to whom it is given. The doses of medications that you are given are calculated according to your weight, age and state of health; the operation or examination for which it is given; and even the anesthesiologist who gives them. There is no fixed recipe.

What happens once you are asleep?

After the induction medication has caused you to lose consciousness, your anesthesiologist gives you one or more other medications (a mixture of pain-relievers, sedatives, and anesthesia gases) to ensure that you remain unconscious. If these other anesthesia agents were not given, you would regain consciousness in a few minutes, after the induction medication had worn off.

Breathing during anesthesia

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Operation

 

Once you are unconscious, your anesthesiologist will take over the management of your breathing, while attending to any changes in your pulse, blood pressure and the amount of oxygen in the blood. This management might consist of holding the mask over your mouth and nose, ensuring that you are breathing clearly and without snoring; or holding the mask and breathing for you by squeezing a bag attached to the breathing circuit; or inserting a breathing tube into your mouth.

Throughout the operation you are given oxygen, first with the mask, and then usually through a plastic airway. There are several types of airway, each of which is a different size, depending on your age and size. The presence of an airway helps to ensure that your breathing is adequate and, in the case of an endotracheal (breathing) tube, that acid from your stomach does not pass into your lungs.

Muscle relaxation

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To help manage your breathing, your anesthesiologist might inject a muscle relaxant, to relax or weaken your throat and abdominal muscles. Muscle relaxants have two major useful effects.

  • They make it easier for your anesthesiologist to insert a breathing (endotracheal) tube through your mouth or, on occasion, through your nose, into your trachea or windpipe. (This process is known as tracheal intubation.) Without muscle relaxants, your anesthesiologist would have to give higher doses of other medications so as to weaken the muscles of your mouth and throat, to make insertion of the tube (intubation) easier.

  • They actually make it possible for the surgeon to perform many operations, without causing any damage to muscle fibres. Indeed, it is difficult for a surgeon to operate inside your abdomen if the muscles are not relaxed. The same applies to other operations, such as those on the hip or in the chest, but not for those on the skin or the body surface.

If you have been given a muscle relaxant, all of your muscles will be relaxed or weakened, including the muscles that help with breathing. In that case, your anesthesiologist ‘breathes for you’. This is usually done with a ventilator, which pushes gas around the anesthesia circuit and into your lungs. Ventilation may also be done by hand, with your anesthesiologist squeezing a bag attached to the anesthesia circu

Airways and breathing tubes

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Airways

 

 

Breathing Tubes

 

The smallest airway is the oral airway. An average adult airway is about four inches (ten centimetres) in length and one-half inch (one centimetre) in diameter and is curved to fit over the back of the tongue. An oral airway is most often used for minor operations, such as those on a limb, particularly if the duration of the procedure is to be short. The laryngeal mask airway is longer and fits over the top of the larynx. Many anesthesiologists now use the laryngeal mask for cases for that would previously have had an oral airway and for cases that may have required an endotracheal tube.

The endotracheal tube is long enough to reach from just outside your mouth or nose and down to just below your vocal cords. The decision to use an endotracheal tube is determined by your condition, the operation to be performed, and the position in which you are placed during the operation. Usually, an endotracheal tube is used if the surgeon is to operate on the brain, the head and neck region, the chest, the back, the abdomen, or the pelvis. Although anesthesia is started while you are lying on your back, your surgeon may need you to be in a different position for the operation. For example, if you are to have an operation on your back, the Operating Room team will turn you over onto your stomach after you are unconscious and an endotracheal tube has been inserted.

An airway is placed in your mouth after you become unconscious, although rarely an endotracheal tube must be inserted before any medications are given and you are still conscious. This is known as ‘awake intubation’ and is only likely if you have a tumour or severe obstruction in your throat.

 

Awake intubation

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Before placement of an endotracheal tube while still conscious, you would be given a solution of local anesthesia to gargle, which numbs your mouth and throat, and decreases any gagging or coughing as the tube is inserted. Your anesthesiologist would explain the process beforehand.

If your anesthesiologist has chosen to use a laryngeal mask or endotracheal tube, it is connected to the circuit after it has been inserted. Your anesthesiologist controls and monitors the flow and concentration of gases that enter and leave the circuit and your body, so that you receive the appropriate amount of anesthesia and breathe adequately.

How does your anesthesiologist know that the tube is where it should be?

If the anesthesiologist has inserted an endotracheal tube into your trachea (windpipe), you breathe carbon dioxide out through the tube. (Carbon dioxide is the gas produced by the body as it uses oxygen to generate energy. Carbon dioxide is then excreted from the body through the lungs.) Carbon dioxide can be measured with a specific monitor, normally attached to the endotracheal tube. The presence of carbon dioxide in the endotracheal tube suggests that the tube is in your trachea.

There are other methods to help confirm the correct position of the tube, but they are less accurate than the carbon dioxide monitor. Your anesthesiologist might also use a stethoscope to listen for the sounds of air moving in and out of your lungs on both sides of your chest and carefully observe how your chest moves up and down with each breath, noting whether or not this movement is symmetrical, which usually occurs when the tube is in the trachea.

Your anesthesiologist might also listen to your chest to ensure that the tracheal tube has not been placed too far down into one lung. This is known as an endobronchial intubation and is sometimes done on purpose. If the surgeon wants to operate on the left lung, then the tube is intentionally placed into the right lung.

Equipment

Induction of anaesthesia in children

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Children vary greatly in the way they react to induction of anesthesia. All children exhibit fear in some way, because of the strange environment, separation from their parents, and the uncertainty about what is to happen to them.

Less than six months

Less than six months

Infants of less than six months do not react strongly to being separated from their parents and usually respond appropriately to a parent substitute. The anesthesiologist should be accustomed to caring for small children and, together with other staff, be empathetic with both child and parents.

It is uncommon for parents to accompany infants of less than six months during induction of anesthesia. This is for two reasons: a child of this age does not suffer major separation anxiety; and everything occurs much more quickly in a baby. This includes the action of medications and the need to act to correct problems such as breath holding. The anesthesiologist must devote his or her whole attention to the child without also having to be concerned about parents.

Six months to four years

Children in this age group do not tolerate separation from their parents well and are not able to comprehend explanation. They react to the unknown with fear, withdrawal and struggling. Induction of anesthesia is best performed either with a parent present, or premedication, or both. With a parent present, the child tends to cling. Induction of anesthesia can be difficult in this age group. Adequately sedated, there is little problem and usually no recollection of events. However, the sedative medications may prolong the recovery phase and delay discharge from hospitals after minor or day stay operations.

With a parent present, either an intravenous or inhalation (gas) induction may be used. For intravenous induction, the parent is asked to hold the child firmly, with the parent either sitting on a chair or leaning over the child who is in a cot or on a bed. The parent is then asked to interact with the child by talking, singing or playing with a toy. At the same time, an assistant secures an arm or a leg where local anesthesia cream has been applied, while the anesthesiologist inserts a cannula.

Inhalation induction is preferred by some anesthesiologists. However, usually a mask cannot be placed over a child’s face without a struggle. Sometimes this struggle may be minimised by the anesthesiologist applying a few drops of a common food flavouring, such as strawberry, orange or bubblegum, to the mask. These scents help to disguise the smell of the anesthesia gases. Alternatively, some anesthesiologists use their hand as a mask. Induction by mask takes longer than intravenous induction.

Four to six years

Children in this age group are still anxious about separation but are more accepting of explanations and reassurance. As with younger children, they benefit from having a parent present during induction, although less physical restraint is required.

Six to ten years

Children aged six to ten years have less of a problem with separation from parents and are much more amenable to reassurance. They do, however, fear anesthesia and surgery, and particularly pain. They may have fantasies of mutilation and require reassurance about the exact nature of the operation. They will be irritable and impatient.

Intravenous induction is usually well tolerated, although the fear of needles may be so strong that even application of local anesthesia cream is not enough to overcome the fear. Cooperation can usually be obtained for an inhalation induction with a mask. Sometimes a child indicates a preference, especially if he or she has had previous anesthesias.

The presence of a parent or guardian can be of great assistance to the child and the anesthesiologist.

Adolescents

This group of patients may fear loss of control and death. It is important to reassure them of the safety of modern anesthesia and that they can be in control of their pain management after the operation.

Intravenous induction is commonly used in adolescents. However, some patients request an inhalation induction, particularly if they have undergone several (or multiple) operations.

Your role as a parent during the induction of anesthesia

You can be an enormous help during induction of your child’s anesthesia. Your presence, in most cases, means a calmer, more cooperative patient, with less likelihood of bad memories of the hospitalisation.

There are several points to consider. Just as your child needs to be prepared for the event, so you need to learn as much as you can about what will happen.

Part of your preparation includes recognising that you, too, may be distressed by the experience. The final decision rests with the anesthesiologist as to your presence. Although many anesthesiologists are now used to having parents present at induction, some find their presence stressful. For the child’s safety, an anesthesiologist may prefer not to have this added distraction.

Your presence may not be encouraged in every situation. This applies particularly if your child needs an emergency operation. Should something happen, such as your child vomiting, then the anesthesiologist needs to focus attention on the child.

You should not feel pressured to be involved. Not everyone is comfortable with the idea of staying during induction and you are free to decline the invitation. Your child’s care will be no less professional.

You should be prepared for your child’s appearance after induction. Your child will become anesthetized within seconds and may suddenly look lifeless, but often with the eyes still open. This is normal. At the same time the anesthesiologist will be concentrating on the next step in the process of caring for your child. He or she usually cannot talk with you or to answer questions at that time.

You should go when asked to leave.

Emergency induction of general anaesthesia

 

Your anesthesiologist might modify the induction of anesthesia by using a technique known as a rapid sequence induction’. This is a crucial technique in patients who must undergo an emergency operation and who have a full stomach, either because they have just eaten or because their stomachs take longer than normal to empty (as a result of pain, medications, or other conditions).

In a rapid sequence induction, you are given 100 per cent oxygen to breathe from a mask placed firmly over your mouth and nose for three to four minutes. This process is known as preoxygenation and replaces the nitrogen in your lungs (the most common gas in the air) with oxygen. As a result, the store of oxygen in your body is markedly increased and there is less chance of lack of oxygen ( hypoxia).

In the next step your anesthesiologist calculates the dose of two medications – the induction medication (usually propofol or pentothal) and a rapid-acting muscle relaxant. The dose of each medication is calculated on the basis of your weight and your general condition.

Your anesthesiologist then injects the two medications rapidly through the intravenous cannula and you quickly lose consciousness. This minimises any risk of your going through a stage during the loss of consciousness when you struggle or vomit.

As you lose consciousness, your anesthesiologist instructs an assistant to apply firm pressure to the front of your neck. The assistant normally stands on your right and uses the first three fingers of the right hand to apply the pressure. (You might feel the assistant’s fingers lightly touching your neck as you lose consciousness.) The specific part where the pressure is applied, called your cricoid cartilage, is a ring of cartilage that forms part of your trachea. Pressure on the cricoid cartilage ( cricoid pressure) seals off the esophagus and reduces the possibility of stomach contents flowing from the esophagus into the back of the throat and then down into the lungs. 

Maintenance of anaesthesia (“keeping you asleep”)

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Monitoring

 

During the maintenance phase of anesthesia, your anesthesiologist keeps you in a state of unconsciousness, using a mixture of inhaled (inhalational) and intravenous (injected) medications. The inhalational agents are administered through the breathing circuit. They include nitrous oxide and the ‘volatile’ anesthesia agents (because they pass easily from being a liquid to a gas). The volatile anesthesia agents are commonly used in proportions between 0.5 and 4 per cent, although this varies according to the agent and the desired effect. They are powerful medications and are used to keep you unconscious, as well as helping to control pain and to relax muscles. These medications can also have side effects, such as low blood pressure, changes in heart rhythm, and difficulties with breathing.

Nitrous oxide (N2O) or (‘laughing gas’) is often used in general anesthesia, in a mixture with oxygen of around 70 per cent nitrous oxide and 30 per cent oxygen. At that concentration the nitrous oxide may make you sleepy and able to tolerate mildly painful procedures, but that is all. Nitrous oxide does, however, provide a means of giving other stronger anesthesia gases through the breathing system.

Air, enriched with extra oxygen, is sometimes used when nitrous oxide is less desirable, such as during anesthesia in the elderly, for some brain surgery, some major heart and lung surgery, and in some tiny premature infants.

Usually during anesthesia, oxygen is added so that the usual proportion given to the patient is about 30 per cent. This extra oxygen provides some safety margin over the normal 21 per cent in room air. The critical aspect of anesthesia care is to ensure that you continue to receive adequate oxygen, which is necessary for preservation of life and the functioning of organs.

Your anesthesiologist may choose to give you other medications through the intravenous line. Depending on the medication, your anesthesiologist may do this to increase the depth of the anesthesia (how unconscious you are). Medications are also given to provide pain relief after the operation. If the surgeon needs your muscles to be relaxed (in order to perform the procedure), your anesthesiologist may give you further doses of the muscle relaxant medication given at the time of induction, or a different medication. Intravenously administered medications may be given in separate or discrete doses (sometimes known as 'bolus' doses) or by constant injection or 'infusion' regulated by a pump.

Sometimes your anesthesiologist will not use any inhalation anesthesia at all. When all anesthesia medications are given intravenously, it is referred to as Total Intravenous Anesthesia, or TIVA. These medications are usually given by carefully controlled infusion.

emergence (“waking up”)

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The third phase of the general anesthesia is emergence or regaining consciousness. During this phase your anesthesiologist stops giving you all inhalational anesthesia agents (except the oxygen) and also stops any intravenous anesthesia medications. You gradually regain consciousness. Your anesthesiologist usually needs to reverse the effects of the muscle relaxants, with the injection of two more medications. As consciousness returns, your anesthesiologist makes sure that you can breathe without help. Once you are regaining consciousness and able to breathe without any help from the anesthesiologist, the breathing tube is removed. By carefully calculating the right amounts of each medication, your anesthesiologist can ensure that you are completely unconscious during the operation, but awake and pain-free at the end of the procedure.