Anaesthesia and Its Purpose | Drugs

After reading this article you will learn about:- 1. Purpose of Anaesthesia 2. History of Anaesthesia 3. Characteristics of an ideal Anaesthetic 4. Clinical Stages of Anaesthesia 5. Preparation of Patient for Anaesthesia 6. General Consideration in Selection of Anaesthetics.

Purpose of Anaesthesia:

Anaesthesia is a reversible process by which loss of sensation in a body part or in the whole body, is induced-usually by the administration of a drug that depresses the activity of nervous tissue either locally or generally.

The purpose of anaesthesia is to provide a safe, convenient, and inexpensive means of restraint to enable clinical procedures to be performed with the minimum of pain, discomfort and toxicity to the patient and anaesthetist.

History of Anaesthesia:

Various techniques to induce unconsciousness have been described in ancient literature. Ancient Greeks were aware that pressure on the carotid artery would induce unconsciousness. The surgeon Pare adopted this method for inducing anaesthesia.

Drugs of various kinds have also been used for many centuries to reduce the distress of surgical operations. Arabian physicians used drugs like opium and henbane. Alcohol was also used to dull sensation-during emergency amputations performed in wartime, strong rum was often administered to the patients.

A brief account of history of anaesthesia is given below century wise:

16th Century:

Ether was discovered.

17th Century:

Christopher Wren and Robert Boyle induced narcosis in a dog through intravenous injection of opium in 1665.

18th Century:

(i) Nitrous oxide was discovered by Joseph Priestley in 1776.

(ii) Humphrey Davy inhaled nitrous oxide and found that it produced euphoria and mirth in 1799. Later discovered its anaesthetic effect.

19th Century:

(i) Morphine was isolated and pure drug was applied by scarification in painful conditions in 1806.

(ii) Benjamin Brodie demonstrated the anaesthetic property of ether in 1821 but did not report it till 1851.

(iii) William Sewell, a Professor at the London veterinary college treated two cases of equine tetanus in 1835 using curare.

(iv) Nitrous oxide contained in bladders was passed around and inhaled for its soporific effect at parties in Britain and united states in late 1830s. Travelling shows also used the mirth-provoking equality of the gas to attract audiences. The US dentist Horace Wells conceived the idea that nitrous oxide may be used for teeth extraction after watching a demonstration at one such show.

(v) Crawford Long, a US physician, operated on a tumour on the neck of a friend under ether anaesthesia in 1842. A ledger entry was used to establish his priority as the first man to use an anaesthetic for a surgical operation, though the credit for this is often given to the person who demonstrated it in public.

(vi) Wells demonstrated that nitrous oxide could be used an anaesthetic during teeth extractions in 1844 but kept it a secret.

(vii) Thomas Morton, on October 16, 1846 was the first person to demonstrate ether anaesthesia in public at an operation held at Massachusetts General Hospital. In the same year i.e. 1846, armed with the knowledge of the use of diethyl ether in the US, British Physician John Snow devised an apparatus for controlling its administration. He is also credited with classify the stage of anaesthesia into five stages.

(viii) The scottish obstetrician, J.Y. Simpson, was the first to administer an anaesthetic-diethyl ether-for obstetrics in 1847. Later, he used chloroform for the same purpose. There was stiff opposition from the British clergy and others to the use of anaesthetics during child birth, they regarded painful childbirth varyingly as God’s will or as advantageous to the patients or both.

The first report of veterinary anaesthesia was published in the veterinarian- British practitioner, Edward Mayhew described his experiments on ether anaesthesia in dogs and cats.

(ix) Snow’s administration of chloroform to Queen Victoria during the birth of her son in 1853 helped in overcoming religious and medical prejudice towards anesthesia.

(x) Though, ice had been in use as a regional anaesthetic for years, a true local anaesthetic- ether spray was invented by Benjamin Richardson in 1867.

(xi) In late 1860s

American dentist, J.H. Smith attended a show put on by one Gardener colton, which was the same show which had given wells his inspiration. Smith took on colton as a partner and together the two apparently extracted nearly 4000 teeth under nitrous oxide anaesthesia.

(xii) First surgical operation under nitrous oxide anaesthesia was performed in 1868.

(xiii) By 1875, chloral hydrate was being used as a narcotic and anaesthetic in the horse.

(xiv) In 1884, Cocaine was used for eye operations by Carl Koller.

(xv) August Bier used cocaine as a spinal anaesthetic in 1898.

20th Century:

(i) Procaine was discovered in 1904.

(ii) Barun used procaine as a local anaesthetic in 1905.

(iii) Retzgen demonstrated epidural anaesthesia in horses in 1925 ; Benesch demonstrated it in cattle.

(iv) In 1940, James Farquharson introduced the paravertebral nerve block.

After World War II:

Many pain clinics were founded in countries like Australia, Canada, Denmark and USA. Gerbershagen et al (1975) described one such clinic in Malmo, Sweden, where a patient with a pain is referred after a diagnosis is made, for nerve block therapy.

The history of anaesthesia, according to Smith Corns (1971), “offers mute testimony to the regrettable fact that medical men, sometimes, have been loathe to need the lessons of history”. This is apparent from the time lag between the demonstration of the anaesthetic properties of ether and nitrous oxide and their eventual adoption as anaesthetics.

The history of anaesthesia also has its share of high drama. The Morton-Wells and Morton-Jackson imbroglios are a poignant reminder of the underlying pressures faced by researchers all over the world, regardless of country or century. An account of the incidents during the time is provided in Smithcors (1971).

The use of nitrous oxide and ether during tooth extractions were experimented upon individually by several dentists and doctors in America. Crawford Long, an US surgeon, had used ether in his practice since 1842 but did not make his findings public till 1849. In 1844 itself, the American dentist Horace wells had extracted teeth using nitrous oxide.

However, during a demonstration at Harvard medical school where Wells was to publicize his discovery, “apparently as a practical joke, the students subject howled as if in pain and Wells was laughed out of the arena”.

Thomas Morton, a dentist in the same country, is generally regarded as the person who convinced the medical world that general anaesthesia was a practical proposition. Morton has been a partner of Wells and was apparently aware of his success with nitrous oxide. He approached a chemist, Charles Jackson, for a substitute for nitrous oxide. Jackson suggested the use of ether.

On October 16, 1846, Morton administrated ether to a patient during a demonstration held at Massachusetts general hospital and was given the credit of being the first person to demonstrate the use of ether anaesthesia in public.

Unsuccessful in his attempts to establish priority of discovery, Wells committed suicide. Morton and Jackson also became involved in a controversy to establish claim to the discovery of ether anaesthesia.

Morton was apparently deranged as a result of this and died after plunging into a lake while in a frenzy. Jackson, anxious to establish his claim, issued reports on work apparently done by him. However, apart from being acknowledged as the person who gave the suggestion to Morton, he was discredited and later died in an asylum.

Important Terms:

A. Sedative:

CNS depressant drugs decrease the normal activity of CNS. A drug which produces mild depression of CNS is termed as sedative.

B. Hypnotic:

The drugs that cause depression of the CNS equal to normal sleep are called hypnotics.

CNS depression equal to deep or profound sleep which can be aroused with difficulty is narcosis and drugs employed for narcosis are narcotics e.g., opium.

Depression of the CNS when the individual is completely unconscious to all the sensations and external environment is anaesthesia.

Depression of CNS to cause death is called euthanesia and drugs that produce this state are euthanesics. Any degree of depression may be produced with a particular drug depending upon the dose used. Therefore, the same drug may act as a sedative or hypnotic, narcotic or euthanesic. Hence, dose of these drugs is more important.

C. Balanced Anaesthesia:

An ideal anaesthetic should possess potent anaesthetic, adequate analgesic and sufficient muscle relaxant activities. Unfortunately there is no single anaesthetic available which possess all these qualities in adequate amount. Therefore, a combination of different drug substances, such as anaesthetic, analgestic and muscle relaxant is prepared which gives the above mentioned activities.

Such combination is known as balanced anaesthetic and the condition produced after its administration is called the balanced anesthesia. For example, combination of barbiturate (for CNS depression, pethedine for analegesia) and D-tubocurarine (for muscle relaxation).

D. Pre anaesthetic:

Pre anaesthetic (not necessarily a CNS depressant) is a drug which is administered before the administration of general anaesthetic to make the anaesthesia easy and safe. The administration of pre-anesthetics suppress bronchial, salivary and lacrimal secretions e.g., atriopine, hyoscine, chlorpromazine hydrochloride.

In veterinary, the administration of pre-anesthetics make the animals cooperative and docile. Because of the sedative effect of pre-anesthetic the dose of basal anaesthetic is reduced and thus reduces the chances of toxicity. Pre-anesthetics block the effect of vagus on heart and reduce apprehension and anxiety.

E. Basal Anaesthesia:

Basal anaesthesia is lighter level of anaesthesia produced generally by pre-anesthetic medication. The patient becomes unconscious but is not sufficiently depressed for undergoing surgery, e.g., pentothal sodium (5% solution) and paraldehyde (10-30% diluted in normal sline solution).

F. Local Anaesthesia:

Involves loss of sensation of limited area.

G. Regional Anaesthesia:

Loss of sensation involves in a larger although in a limited area in the body.

Characteristics of an Ideal Anaesthetic:

(i) The anaesthetics should be safest.

(ii) Anaesthesia should be smooth easy and there should be quick recovery.

(iii) It should be balanced.

(iv) It should be least depressant to respiratory and cardio vascular system.

(v) It should be non irritant and non explosive.

(vi) Preferably, it should not diffuse into the foetus/placenta.

(vii) It should not promote capillary bleeding and should be compatible with preanesthetic and other ancillary therapeutic measures (that is drugs used after anaesthesia).

(viii) The anaesthetic should be neutralized with non-toxic antidote so that the duration of anesthesia can be shortened at will.

(ix) It should be non explosive and stable during storage.

(x) Anaesthetic should not produce any pronounced physiological change (e.g., marked depression in B.P., Heart rate and respiration).

(xi) It should be cheap.

Unfortunately, we do not have all the characteristics in one anaesthetic agent so, in order to achieve the approximation to ideal drugs of complementary.

Clinical Stages of Anaesthesia:

The depth of anaesthesia may be classified into four stages using the neuromuscular reflexes:

1. Stage of Voluntary Excitement or Stage of Analgesia (Stage I):

This stage begins from the beginning of the inhalation to loss of sensation. Excitement and struggling is the most characteristic features. Heart beat becomes faster and stronger and the respiration is rapid and deep.

Pupil is dilated, there is excitement, and excessive salivation (due to irritation of vapour). There is no change in muscular tone. There is analgesia without loss of consciousness (sensory depression). In horse, struggling is more violent and voluntary breath holding may occur.

2. Stage of Involuntary Excitement or Stage of Delirium (Stage II):

This stage begins with the depression of cortical centres with loss of consciousness and voluntary centres. There is exaggerated reflex, struggling or purposeless movement. The horse may neigh or dog may whine and whimper (emit small bark). Generally impulse is rapid and strong. Blood pressure elevated (commonly in the early part).

The respiration is uneven in depth and rate and breath holding may occur. Mucous membrane is flushed. The eyelids remain widely open and pupil dilated. Reactivity of the iris to light (photomoter reflex) is retained.

In horse, nystagmus (occilation of eyeball in the orbit) is characteristic of this stage but not in dog and other species. In dog there is vomition (if not fasted) and muscular tone is enhanced. This stage is very short staying. Stage I and stage II are known as induction stage.

Stage II has been subdivided into stage II A, IIB and IIC. Stage IIA and IIB are characterized by abnormal posture and behaviour and hallucination in man. Stage IIC is characterized by cataleptoid. Cataleptoid stage is a waxy type of rigidity of the muscle but animal remains in position of placement. This may extend to muscle twitching, CNS excitation and convulsion.

Ether and chloroform induce all stages (I, II, III and IV) and therefore are totipotent anaesthetic (i.e. 100% anaesthesia producing). Methoxy flurane, halothane and barbiturate do not produce stage II (i.e. no excitement). Thus, they may be considered good agents for inducing anaesthesia.

Nitrous oxide (N₂O) and trichloroethylene do not produce stage III or stage IV and therefore, are called incomplete anaesthetics (50% anaesthesia). Alfa-chlorolase, iso-flurane and enflurane do not produce stage II and stage III. Ketamine produces stage I and II producing seizure and catalepsy.

3. Surgical Anaesthesia (Stage III):

During this stage, depressant action of anesthetics is extended from the cortex and the mid brain to spinal cord. Pain sensation, consciousness and spinal reflexes are abolished. Muscular relaxation occurs and coordinated movement disappears.

Nearly all surgical procedures on animals are performed in this stage. Stage III is further divided in plane I, plane II, plane III and plane IV. Plane I and II are known as light surgical anaesthesia and plane III and IV is considered as deep surgical anaesthesia.

Light surgical anaesthesia (plane I and II):

(i) Major operations in large animals are performed under light surgical anesthesia.

(ii) Respiration slow and regular and consists of diaphragmatic and inter costal movements.

(iii) Pulse and B.P. are normal

(iv) Nystagmus may occur but at slower rate.

(v) Pupil becomes constricted. The corneal and palpabral reflexes are still present but slow to respond to stimuli.

(vi) The pedal reflex (withdrawal of limb on painful stimulation) disappears almost at the onset of stage III.

(vii) Cough reflexes disappear in plane II.

(viii) The swallowing and skin reflexes also gone.

(ix) Palpebral, corneal and pedal reflexes are frequently present in all planes when barbiturate anaesthesia is used.

(x) Muscle tone which was normal at the beginning of this stage decreases due to depression of ordinary postural reflexes.

(xi) In horse, tail becomes flaccid and limb and penis often protrude from the sheath.

(xii) Cutting skin and muscle does not produce reflex (muscular contraction).

Deep surgical anaesthesia (plane III and IV):

(i) Plane III is used for performing operation in human being and small animals like dogs and cats.

(ii) Body temperature falls rapidly (Hypothermia).

(iii) All reflexes disappear and the respiration is chiefly abdominal. The thoracic respiration is depressed and disappears in plane IV.

(iv) The pulse becomes weak, the B.P. drops in plane IV.

(v) The eyeball becomes fixed.

(vi) Muscle tone is too much decreased and completely relaxed. The patient becomes flaccid.

(vii) Faeces and urine may pass involuntarily.

(viii) Closure of eyelid in horse indicates the approximate safe maximum of anaesthesia.

4. Medullary Paralysis and Death (Stage IV):

(i) Vital medullary centres are paralyzed.

(ii) There is respiratory arrest with fall in B.P. to the shock level.

(iii) Heart usually beats quickly for sometimes after respiration cease. Hypoxia worsens and cardiac arrest occurs within five minutes of respiratory arrest.

(iv) Large animals can not be resuscitated.

(v) Small animals may be resuscitated by artificial respiration.

Preparation of Patient for Anaesthesia:

Majority of the orations are planned however, emergency operations are non planed where little preparation is possible. The following preparatory measures before performing operations may be taken.

(i) Withhold Food:

The food is withheld for 12-24 hours before operation is to be started. Because, the presence of ingesta may interfere with free movement of diaphragm and can dispose to respiratory difficulty. Moreover, gaseous distension occurring from fermentation processes may result in interference of venous return to heart in ruminants and non ruminants. Vomition may produce hazard of mechanical pneumonia and can contaminate the instruments and operative field.

(ii) Glucose Infusion:

In toxaemic patients, liver is reinforced by glucose before operation.

(iii) Urine test and haematological examination.

General Consideration in Selection of Anaesthetics:

1. The anatomical and physiological peculiarities of the species and breed involved as well as the temperament and size of the animal influence the selection of anaesthetic agents.

(a) Dog and Cat:

(i) Ether with basal anaesthetics

(ii) Barbiturates (i/v) preferred.

Care should be taken in case of brachiocephalic dog when general anaesthetic used and especially when endotracheal tube is not employed e.g., Pekingese bull dog.

(b) Ruminants:

Large four stomachs with great amount of ingesta invariably present makes use of inhalation anesthetic impossible. Therefore, local or regional anesthetics with hypnotics are preferred in ruminants.

(c) Horse:

(i) Chloroform inhalent

(ii) Chlorhydrus, Magnesium sulfate, Pentobarb.

(iii) For operation on foot and leg, perineural injection (local anaesthetic) is preferred in standing condition.

(d) Swine:

(i) Pentobarbital sodium combined with local and regional (Epidural) anaesthetics are preferred.

(ii) Because of obese characteristic, general anaesthetics are dangerous.

2. The nature of operation performed, its size, duration and location are necessary consideration, e.g.,

(i) Caesarian section-In dog, barbiturate is not used because it will kill foetus in uterus.

(ii) Caped elbow in Horse: There is sub-dermal fibrosis which is deep seated and thus local anaesthetic agent can not be effectively infiltrated. Therefore, general anaesthetic is used.

(iii) Deeply seated abcess in the area of pharynx nearby structures produce movement of operative site and thus, general anaesthesia is preferred.

3. Species Susceptibilities:

(i) Opium and its derivatives are good in dog but it should be avoided in cat and unsatisfactory in other species.

(ii) Barbiturates in horse are not preferred.

(iii) CH.Cl₃ is not preferred in small animals because it produces respiratory and CVS failure but it is good in large animals, particularly in horse.

4. Condition of the Patient:

(i) Prolonged fasting depletes glycogen reserve in liver thus, reducing the ability of that organ to detoxify poisonous materials.

(ii) Phenoparb is avoided in renal pathology.

(iii) Inhalation anaesthetics are not employed in respiratory diseases.

(iv) Chloroform is not used in patients suffering from heart disorder.

5. Legal Responsibility:

Exists in some countries such as Great Britain, where the animal anaesthetics act of 1919 requires that certain operation be performed only under a general anaesthetic.

Pre Anaesthetic Medication:

Preanesthetic is a general term that refers to the administration of some agents shortly before anaesthesia is administered. Such agents are morphine, meperidine, methadone, atropine, or a barbiturate for small animals and chloral hydrate for large animals.

The objective of preanesthetics are:

(i) To quieten the patient so that it is more easily restrained with minimum of danger to animal and veterinarian.

(ii) To decrease the amount (dose) of the anaesthetic agent required to produce the surgical anaesthesia.

(iii) To decrease the flow of saliva and respiratory secretions which might hamper the breathing.

(iv) To protect the animal patient from vagal reflexes and cardiac arrest induced by anaesthetics.

Examples of Pre-Anaesthetics:

(a) Barbiturates:

Thiopental i.v. and then maintain on ether. Thus, induction stages pass and off without struggling.

Disadvantages:

(i) Excitement in large animals (horse) and occasionally in human due to greater of pain

(ii) Tolerance to many dependent drugs.

(b) Morphine:

Used in man and dog.

(i) It is a vary good analgesic.

(ii) Reduces anxiety.

(iii) Cuts down struggling during induction stage and thus induction more smooth.

(iv) Compatible with hyoscine.

(v) Depress cough reflex.

Disadvantages:

(i) Constipation and urinary retention effect.

(ii) Pin point pupil.

(iii) Depression of respiration.

(iv) May also induce salivation and bronchial secretions.

(v) Emesis.

(vi) Its stimulant effect on smooth muscles may cause spasm of bile product or of the uterus, colicy pain.

(c) Pethidine or Meperidine hydrochloride:

(i) It is non constipatory.

(ii) Depresses secretion (atropine like effect).

(iii) Do not produce sedation of that extent as do morphine.

(iv) Does not produce emesis.

(d) Atropine Sulfate:

(i) It produces anti-secretory effect.

(ii) Incidence of bronchial and laryngeal spasm blockade due to vagal stimulation.

(iii) Protects the heart from vagal stimulation.

(iv) Mild stimulation effect on respiration.

(v) Cuts down post operative vomition and pulmonary complication.

Disadvantages:

(i) Alters pupillary signs (Mydriasis and cycloplegia).

(e) Scopolamine or Hyoscine:

(i) Similar to atropine but may produce CNS depression.

(ii) Excitement lessened, produces amnesia.

(iii) Counteracts the respiratory depression of morphine.

(iv) Superior to atropine as antisialagogue

Disadvantages:

(i) Icterus.

(ii) Very rarely produce delirium.

(f) Tribromoethane (Avertin):

Used mainly as basal anaesthetic in man for brain surgery and it lowers the C.S.F. pressure. It is also used in cat (135 mg/kg 3% solution) per rectum but replaced by thiopental. The main advantages is that it is easy to administer.

Disadvantages:

As given per rectum, one does not know how much drug is going to be absorbed due to erratic absorption and thus, it may be dangerous.

(g) Tranquilizers:

Phenothiazine derivatives are used. It potentiates the administration of barbiturates and other anaesthetics, thus reduces the dose. It also reduces the apprehension, anxiety, is antihistaminic, antiemetic.

(h) Butyrophane:

Droperidol and pentanyl (50:1) is used as neurolept analgesic.

Post Anaesthetic Medication:

Post anaesthetic medication with morphine or meperidine may be indicated to avoid post operative pain with disturbance of operation area by patient. On the other hand sleep should not be too long, too deep because of a greater opportunity for hypostatic pneumonia to develop. Post anaesthetic fluid medication frequently is indicated following surgery to compensate for fluid loss from the body (due to haemorrhage, vomition, diarrhoea).