Chris Thompson  

Anaesthesia - an introduction

What is anaesthesia?

Conscious Sedation - sedation and anxiolysis with retention of consciousness at all times.

Unconscious ('deep') Sedation - sedation sufficient to induce sleep from which arousal to consciousness is easy

Neurolept Analgesia - analgesia, disinterest and psychomotor retardation typically induced by a combination of a major tranquiliser (e.g. droperidol) and a narcotic analgesic. Patient may appear calm but be anxious.

General Anaesthesia - unrousable unconsciousness, reversible, and usually drug induced.

Local anaesthesia - rendering a part of the body numb.

Topical anaesthesia - anaesthesia of skin or mucous membranes by topical appliation of local anaesthetics.

Infiltration anaesthesia - anaesthesia of tissues by direct injection of local anaesthetic where it is needed - ie for excision of skin lesions.

Regional anaesthesia (`conduction blocks' or `blocks') - anasthesia of a part of the body by injecting local anaesthetic into the nerves that go there. Simple blocks include finger blocks, ankle blocks, etc; more complex blocks include plexus blocks, and `major regionals' mean epidural or spinal anaesthesia.

 

What do Anaesthetists do?

5-6 years medical school, 2 years in hospital (intern and resident), a further 6 years specialist training, focusing on physiology, pharmacology, intensive care, and technical aspects of anaesthetic practice.

The College of Anaesthetists supervises registrar training, exams, post-Fellowship training, ongiong education and policy matters; the Australian Society of Anaesthetists assists more with practice management and political issues. Fees to these bodies are about $2,400 in total annually. Medical Defence is about $7,000 for Staff Specialists and $15,000 for VMO's each year.

Patients are usually seen by an anaesthetist (or a trainee anaesthetist) preoperatively in a pre-admission clinic. This person may or may not actually do the case. The patient's health, pre-operative tests, and any specific concerns are reviewed in the context of the planned procedure. A discussion of the risks and benefits of the various approaches or techniques required should result in an agreed plan. Selecting the safest technique in the circumstances is important, and the details of that technique, including the management of post-operative pain, should be explained. Common risks (pain, nausea, sore throat, muscle aches, etc) and rare but serious risks (death, paraplegia, etc) should be discussed. The patient should be sufficiently well informed that they can make an informed decision as to whether or not they should proceed with the anaesthesia plan for the procedure.

Management of the anaesthetic is the responsibility of the anaesthetist, not the surgeon.

The anaesthetist stays with the patient from the time anaesthesia starts until care is passed on to recovery staff, however their responsibility legally covers to the entire period from premedication until full recovery from the anaesthetic.

Once the patient is asleep, the Anaesthetist continuously monitors the adequacy of breathing and the circulation; additional monitoring is used in special circumstances, ie brain monitoring in neurosurgery. Positioning is performed carefully so as to not cause nerve or skin damage.

Anaesthetists adjust the doses of the anaesthesia-inducing drugs indivually on a patient by patient basis - they are generally titrated to a measurable end-point of some kind. Subconscious hearing is preserved during most anaesthetics. Fortunately memory is strongly impaired with small doses of anaesthetics, so most people don't remember much from just before they go to sleep until some time after they actually wake up. During most anaesthetics there is no way to be certain that the patient is asleep. Brain monitoring can be helpful especially if patients are at high risk of awareness, for example if the anaesthetic requires paralysis of the muscles, in which case the patient can't move even if they want to, or if the patient had an episode of awareness in the past. Drug adminstration is part of the art of anaesthesia.

Anaesthesia practice is a combination of technical skill, experience, compassion and science; experience is far more important than knowledge alone.

At the end of the operation the anaesthetic drugs are turned off or reversed, and once the patient is awake they will be transported to recovery, or ICU. The anaesthetist will wait awhile to make sure the patient is OK. We arrange appropriate post-operative analgesia.

Anaesthetists run most acute pain services and often have substantial input to ICU and the ER.

Anaesthesia alone in healthy patients has a mortality (due to unexpected drug reactions, haste, device malfunction, etc) of 1:500,000. This contrasts with the 1:30 chance of being killed or maimed for drivers from the age of 17 to 25, and is far less than the risk of surgery. Overall the risk of death due to anaesthesia in all patients undergoing surgery is about 1:30,000. This is about the same risk of death and/or disablility as driving 10km to and from work in Sydney traffic for a few months.

History

Opium, alcohol etc were used in antiquity.

Bromide first used 1853, and an account of Bromism in Evelyn Waughs "The Ordeal of Gilbert Pinfold". Before 1900, Chloral Hydrate, Paraldehyde, Urethan, and Sulfonal were in use. Barbiturates were first synthesised in 1864 on St. Barbaras Day (which celebrates the day long ago when the father of a 4th century virgin and martyr was struck dead by lightning after unjustifiably beheading same. St. Barbaras is also the Patron Saint of Gunners and Arsenals). Phenobarbital first used in 1912 Thiopentone introduced in World War II - killed many in Pearl Harbour because too much was unknowingly given to patients in hypovolaemic shock. Benzodiazepines first synthesised 1933, Chlordiazepoxide first clinically used in 1960. Propofol is relatively recent - 1993.

Charpentier first synthesised and used Promethazine, the first major tranquiliser, in the 1930's and Chlorpromazine in the 1950's.

Nitrous Oxide was first `abused' by travelling entertainers; then in 1890's successfully used for dental extractions and anaesthesia.

Ether and Chloroform were the first drugs to make inhalational anaesthesia possible. Ether was used first, but Chloroform was easier to administer. Commenced in 1847, Simpson, for obstetrics; use ended in 1900's due to cardiac irritability and hepatic damage (obligate hepatotoxin via phosgene). Ether then regained popularity. They were usually given by open drop onto wire frames, but machines not unlike those in use now were made by Boyle in the 1920's. Halothane, the first potent volatile agent was first use in 1956; Isoflurane in the 1980's.

Spinal anaesthesia became popular as an alternative to ether, but suffered in the late 1930's when several patients became paraplegic. Now spinal and epidural anaesthesia are much more common.

Early anaesthetics were given by assistants to the surgeon, and in the USA it was more common for a non-medical person to do so under the supervision of the surgeon (these are now the CRNA's of today). In England, doctors with a particular interest in anaesthesia became anaesthetists and the specialist practice of anaesthetics is the result.

The likelihood of death or injury attributable to anaesthesia have declined dramatically over the last 50 years not only because the drugs and equipment have improved greatly, but also because of a much more accurate and detailed understanding of how how the body works and a far greater level of training before a doctor can call themselves an Anaesthetist.

Drugs Used

Intravenously administered anaesthetic agents

Benzodiazepines

Used mostly as oral premeds (Temazepam 10-30 p.o. an hour pre-op) or parenteral sedatives, anxiolytics and induction agents. Diazepam is slow in offset and irritant to veins - cannot be given imi. Midazolam is relatively new agent that is not too irritant to veins and can be given orally, imi, or ivi; it is often used for sedation for blocks.

All benzodiazepines are protein bound moderately lipid soluble drugs; their duration of effect depends primarily on the rate of metabolism of the main active metabolite, except with IV use where relatively brief responses (after boluses) are due to redistribution. Metabolised mostly in the liver; long half-life in the elderly. Nor-Diazepam (N-desmethyl-diazepam) is active with a half-life of 50 to 100 hours.

They act by polysynaptic pathway inhibition via interaction with specific receptors (GABA, BZ, Barbiturate modified Chloride channel) which enhance the inhibitory effect of GABA in the Reticular Activating System and Amygdala. Benzodiazepines reduce the dose of other intravenous anaesthetic agents.

Main effects are:-

1) CNS Depression - decreased anxiety, tranquility, sedation, anterograde amnesia, ultimately unconsciousness and respiratory depression. Anxiety reduced more than apparrent drowsiness. Sleep changes include delayed and reduced REM sleep and reduced stage 3 and 4 sleep, but increased overall sleep times.

2) Anticonvulsant effects - mostly with Diazepam.

3) Muscle relaxation - due to relaxation and reduced tone - do not enhance paralysing drugs.

Barbiturates:

These are general anaesthetic agents (like alcohol, only stronger) with no specific receptor sites (except in low doses via weak interactions at the GABA receptor. When used as sleeping pills they cannot be safely used in conjunction with alcohol and have a very low therapeutic index, hence are frequently used orally for successful suicides. Anxiolysis is not clearly seen, and euphoria and antalgesia may occur.

Thiopentone is the most commonly used induction agent in the world. It is a yellow powder that comes in 20ml ampoules (500mg) and has to be dissolved in water before it can be given. The dose required varies a lot - from 50 to 750mg. Patients wake up fairly quickly after ordinary doses (ie, 5-15 minutes), but may have a depressed, tired feeling afterwards. It can't be given by infusion without accumulating so much in the body that the patient remains drowsy for ages.

Overdoses cause profound respiratory and cardiac depression and death from extreme hypotension; it is easy to overdoes little old ladies and people in shock.

Propofol

This is a recently introduced modified phenol (di-isopropyl phenol to be exact) dissolved in a soya emulsion like intralipid. Kind of disinfectant in soy milk. 20ml in a glass ampoule or 50ml in a larger bottle. About twice as expensive as thiopentone. More rapid recovery, less hangover, clear sensorium make it appealing; tendency for euphoria on recovery and also tends to cause a lot of hypotension. Cautious use in the elderly is necessary. Laryngeal reflexes are suppressed more than thiopentone and much more than the volatile agents, so it is often used if a laryngeal mask is to be used.

Propofol can be used quite effectively as the sole anaesthetic drug by infusion and even after a long time patients wake up fairly well.

Ketamine

This agent maintains BP and muscle tone and can be used intramuscularly as the sole anaesthetic agent, especially in trauma. Unfortunately most patients wake up feeling very odd (dissociated, hallucinatory) so it is not used much for elective surgery.

Gaseous anaesthetics, eg N2O, Cyclopropane.

Mode of action not clearly understood; probably cell membrane expansion (`pressure reversal' theory). Rapid onset and offset.

Cyclopropane

Virtually unused today - highly explosive when mixed with oxygen. Very rapid induction of anaesthesia is possible - just inhale one deep breath and away you go!

Nitrous oxide - N2O or laughing gas

Used in almost every anaesthetic these days as the main `background' anaesthetic, typically at about 66% of the inspired gas, with the remainder oxygen. Comes in blue cylinders.

Once breathed in, the patient rapidly equilibrates (5-10 minutes) with the inspired concentration. Similarly rapid offset.

You need 105% N2O at sea level for full anaesthesia so some other drugs have to be given as well to make sure the patient will stay still and remain unconscious; usually volatile inhaled agents and narcotics are added.

Volatile anaesthetics (ie, Sevoflurane, Isoflurane, Desfurane)

These drugs are thought to act like the gaseous agents. They are pungent liquids at room temperature (smell like metho or cleaning fluid) and are breathed in from a calibrated device called a vapouriser, which is usually a shiny metal thing on the back bar of an anaesthetic machine. For isoflurane, 1% concentration in 66% N2O is enough, usually, to maintain anaesthesia.

As increasing doses are inhaled, patients experience increasing disinhibition, numbness, drowsiness, excitement, disorientation, agitation, unconsciousness, reflex suppression, cardiovascular depression, respiratory depression, and ultimately death.

Ether is a very irritable, highly flammable volatile anaesthetic. Patients in whom ether is used to induce anaesthesia never forget it! They vividly remember being held down and gradually drowning and falling into some kind of terrible nightmare. It can be used (and often is in developing countries) for the maintenance of anaesthesia after an inhalational induction (6-7%).

Halothane was the first modern potent inhalational anaesthetic. It is not flammable and is not as pungent as ether, so it can be used for the inhalational induction and maintenance of anaesthesia. It has rare complications such as malignant hyperthermia and hepatic failure, and potentiates cardiac dysrhthmias, so it is used much less these days, except in children.

Isoflurane is now the most popular inhalational agent; patients wake up more quickly than with Halothane and it has fewer side-effects.

Neuromusclar Blockers (muscle relaxants)

These drugs cause nerve stimuli that would usually cause movement to not get conducted to the muscles - resulting in paralysis.

Suxamethonium is a very rapid acting `depolarising' muscle relaxant and is used where it's rapid onset makes anaesthesia safer - for example when it is necessary to intubate a patient urgently or when the stopmach is full - and for brief procedures that need muscle relaxation. Sux cannot be reversed; you just have to wait for it to go away.

Pancuronium, Vecuronium and Atracurium are all slower onset longer duration drugs which can (an usually need to) be reversed with an anatagonist at the end of the operation.

Common anaesthetic techniques

Premedication

Mostly we tell the patient to fast from midnight but to continue all the their usual medications, and give a premed to relieve anxiety (a benzodiazepine, usually orally). Sometimes something to relieve pain (pethidine or morphine imi) and dry secretions (atropine) may be given, but this is less common now. The most important thing is to be sure that the patient's general medical condition is as good as it could be and to make sure that appropriate tests have been done, checked, and, if necessary, acted on.

General Anaesthesia

In adults anaesthesia usually starts with the application of basic monitoring, an iv cannula, then an intravenous anaesthetic agent (thiopentone, propofol, or midazolam) in the right dose. Often multiple small doses titrated to effect. Small children are induced with N2O and a volatile agent by mask.

Once the patient is asleep N2O and some volatile anaesthetic is given by mask. The patient's airway may be managed by intubation, the laryngeal mask airway, or just a facemask with or without a guedel airway; intubation usually goes hand in hand with the need for muscle relaxants.

Towards the end, the inhalational agents are turned off and they gradually get breathed out; if you get it right the patient wakes up at the end rather than before or half an hour later! Often narcotics are given as well to reduce the amount of other agents that are needed and to help the patient be pain free at the end.

Continuous infusions of propofol are used sometimes in place of a volatile agent.

Spinal Anaesthesia

Here a small (25g by 3 inch) needle goes in the back and ends up in the CSF; where about 3 ml of 0.5% plain bupivacaine (Marcaine) is injected the patient finds that within 10-15 minutes they can't feel or move their legs - or in fact anything from the waist down.

Recovery usually occurs 4 to 8 hours later.

Spinals are good for operations on the pelvis and lower limbs, particularly in the elderly. Unfortunately CSF leaks out of the hole in the dura and in young people and day surgery it is common to get quite a bad `post-spinal' headache. Spinals are really much more common in the elderly - for fractured hips, TUR's etc.

Basic monitoring only is used and the patient needs a drip.

Epidural Anaesthesia

This is similar to a spinal but the needle is bigger (14G) and the local (about 10-20ml) is injected into the epidural space (just outside the dura, and NOT into the CSF). Onset of the block is slower and a little less predictable.

Usually a fine plastic catheter is left in place and `top-ups' of local can be injected to maintain the block for many hours - this can't be done with spinals. Dilute local anaesthetics can give good analgesia for labour while retaining a lot of movement and some sensation, though getting it right is an art.

Neurolept Analgesia

The "Neuroleptic" effects of the major tranquilisers was first noted by Charpentier. The patient loses interest in their surroundings, emotions are dulled and spontaneous and complex movements are reduced. There is no gross impairment of intelligence or the ability to perform tasks and unconditioned or reflex activity remains unimpaired (in contrast to the effects of benzodiazepines.

Neurolept Analgesia refers to the use of major tranquilisers, ie droperidol, in conjunction with narcotics such as morphine or fentanyl. Unfortunately one often gets a `locked-in' state of `outer calm inner panic' that patients recall vividly and unpleasnantly, so this technique is not used much these days. It is not so bad if the patient has received a good dose of bezodiazepines - when it is quite good in small doses for restless fidgety old people who won't stay still while having operations under local.

Conscious Sedation

Small doses of midazolam or infusions of propofol are commonly used to sedate patients who are undergoing unpleasant (ie, gastroscopy) or boring (ie, eye) procedures. Midazolam is associated with more sleepiness afterwards but more amnesia for what exactly happened. The trick is not to give so much that the person goes unconscious!

 


Last updated Tuesday, April 13, 2010
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