General anaesthetics are drugs that produce reversible loss
of all sensation and consciousness. The main features of general anaesthesia
are:
(i)
Loss of all sensation
(ii)
Sleep (unconsciousness)
(iii) Muscle
relaxation and
(iv) Abolition
of reflexes
THEORIES OF ANAESTHESIA
The mechanism of action of the general anaesthetics is not
precisely known. A wide variety of drugs or chemicals produce general
anaesthesia. Therefore, general anaesthesia is probably not exerted through
specific receptors but by a more generalized membrane action. The different
theories of anaesthesia are:
1. Lipid/water
partition theory
If an anaesthetic agent is more lipid soluble then it will
produce more intense anaesthesia. It has been found that minimal alveolar
concentration (MAC) (i.e the lowest amount of anaesthetic in pulmonary alveoli
needed to produce immobility) decreases with inhalational anaesthetics having
higher o/w partition coefficient.
But this is not the perfect mechanism of action.
2. Surface tension
theory
General anaesthetics reduce surface tension at cell membrane
and thus affect its permeability, electrical and/or enzymatic properties.
However, this is not generally accepted.
3. Inhibition of
energy production/utilization
Higher concentrations of general anaesthetics in the brain
directly inhibit metabolic processes and oxygen consumption in neurons. But it
is an effect rather than the cause.
4. Clathrates
formation theory
Water is normally partially structured, i.e. has crystal
like molecular arrangement. General anaesthetics are believed to fill up the
spaces between the microcrystals (clathrates) and make water more structured;
thus they plug the pores and impede ionic fluxes. However, there is no evidence
of clathrate formation at body-temperature.
5. Membrane expansion
theory
The anaesthetics occupy space in the membrane and expand it
disproportionately (about 10 times their molecular volumes). This causes
increased surface pressure in the membrane, thereby closing ion channels. This
has been prompted by the fact that application of external pressure reverses
anaesthesia.
6. Membrane
fluidization / perturbation theory
Anaesthetic, by dissolving in the membrane lipids, increases
the degree of disorder in their structure, favoring a gel-liquid transition
(fluidization). Normally, fluidization occurs at high temperatures; anaesthetic
may make it possible to occur at lower temperature. This affects the state of
membrane bound proteins that regulate ionic fluxes.
STAGES OF ANAESTHESIA
When a general anaesthetic is administered first the higher
functions of brain are lost then progressively lower segments of the brain are
involved. In case of spinal cord lower segments are affected somewhat earlier
than the higher segments.
The vital centers located in the medulla are paralyzed the
last as the depth of anaesthesia increases.
When ether is
administered as inhalation the following four stages are seen. The IIIrd stage
is again divided into 4 planes.
1. Stage of Analgesia
Starts from beginning of anaesthetic inhalation and lasts
upto the loss of consciousness.
Pain is progressively abolished.
Patient remains conscious, can hear, and feels a dream like
state.
Reflexes remain normal.
·
Though some minor and even major operations can
be carried out during this state, it is rather difficult to maintain - Use
is limited to operations taking short time.
II. Stage of Delirium
From loss of consciousness to the beginning of regular
respiration.
Apparent excitement is seen, patient may shout, struggle and
hold his breath; muscle tone increases, jaws are tightly closed, breathing is
jerky; vomiting, involuntary micturation or defecation may occur.
Heart rate and blood pressure may rise.
Pupils dilate due to sympathetic stimulation.
No stimulus should be applied or operative procedure carried
out during this stage.
This phase is cut short by rapid induction, premedication, etc.
III. Surgical
anaesthesia
Extends from onset of regular respiration to cessation of
spontaneous breathing. This has been divided into 4 planes that may be distinguished
as:
Plane 1 Roving eye balls. T
This plane ends when eyes become fixed.
Plane 2 Loss of corneal and
laryngeal reflexes.
Plane 3 Pupil starts dilating
and light reflex is lost.
Plane 4 Intercostal paralysis,
shallow abdominal respiration, dilated pupil.
Most surgical operations are done in Plane 2 and Plane 3. Plane 4 is
never attempted.
IV. Medullary
paralysis
Cessation of breathing to failure of circulation and death.
Pupil is widely dilated, muscles are totally flabby, pulse is thready or imperceptible
and blood pressure is very low.
PROPERTIES OF AN IDEAL
ANAESTHESIA
A. For the patients:
It should be pleasant, non-irritating, should not cause
nausea or vomiting.
Induction and recovery should be fast with no after effects.
B. For the surgeon
It should provide adequate analgesia, immobility and muscle
relaxation.
It should not be inflammable and nonexplosive (so that hot
cautery may be used).
C. For the
anaesthetist
Its administration should be easy, controllable and
versatile.
Margin of safety should be wide - no fall in blood
pressure.
Heart, liver and other organs should not be affected.
It should be potent so that low concentrations are needed
and oxygenation of the patient does not suffer.
Rapid adjustments in depth of anaesthesia should be
possible.
It should be cheap, stable and easily stored.
It should not react wit rubber tubing or sodalime glass.
Dose:-
Most inhalational anaesthetics have a steep concentration -
response curve. Increasing the concentration only by 1/3 over MAC makes almost
all individuals immobile (at MAC only 50% of the patients are immobilized), and
2-4 MAC is often lethal.
Complications of
general anaesthesia:
A. During anaesthesia
(i)
Respiratory depression.
(ii)
Salivation, respiratory secretions - less
now as non-irritant anaesthetics, are mostly used.
(iii) Cardiac
arrhythmias.
(iv) Fall
of blood pressure.
(v)
Aspiration of gastric contents.
(vi) Laryngospasm
and asphyxia.
(vii)Delirium, convulsions.
(viii)Fire and explosion.
B. After anaesthesia
(i)
Nausea and vomiting.
(ii)
Persisting sedation: impairs psychomotor function.
(iii) Pneumonia
(iv) Organ
toxicities: liver and kidney damage.
CLASSIFICATION OF GENERAL ANAESTHETICS
1. Inhalation anaeshetics:
(a) Gases: Nitrous oxide (N2O)
Cyclopropane
(b)
Volatile liquids Ether
Chloroform
Halothane
Enflurane
Trichloroethylene
Isoflurane
2. Intravenous anaesthetics:
(i) Inducing agents: Thiopentane
sodium
Methohexitone
sodium
(ii) Slower acting drugs:
(a)
Benzodiazepines e.g. Diazepam
Lorazepam
Midazolam
(b)
Dissociative anaesthesia e.g. Ketamine
(c)
Neutoleptic analgesia i.e. Fentanyl + droperidol.
GASEOUS ANAESTHETICS
Nitrous oxide (N2O)
It is also called laughing gas. It is an inorganic gas,
colorless and sweet in taste.
It is heavier than air. It is insoluble in blood and it does
not combine with haemoglobin. It is carried in blood in the form of physical
solution.
It does not decompose in the body. So the oxygen of nitrous
oxide is not available for tissue respiration. It is excreted unchanged through
the lungs. A small quantity may be eliminated through skin.
Advantages:
1.
Induction and recovery are quick.
2.
No irritation to mucous membranes.
3.
It can produce analgesia which can be used for minor
surgery or tooth extraction.
4.
It is non-toxic to liver and kidney.
Disadvantages:
1.
It is not a potent anaesthetic.
2.
Muscle relaxation is not adequate.
3.
Special apparatus is required for its use.
4.
It produces euphoria and excitation during induction.
Cyclopropane
It is the simplest cyclic hydrocarbon. it is colorless gas
with a sweet odour and taste.
It is heavier than air.
It is rapidly absorbed and completely eliminated through lung.
A small amount is metabolized in the body and eliminated as
CO2 and water.
Advantages:
1.
Induction is quick and pleasant and recovery is smooth
and rapid.
2.
Useful for major surgery like thoracic and abdominal
surgery.
3.
Less-irritant to mucous membranes. So coughing and
respiratory secretions are less.
4.
Does not affect blood pressure and cardiac rate.
5.
It does not produce any visceral damage.
Disadvantages:
1.
Explosive when combined with oxygen.
2.
It causes increased capillary bleeding.
3.
Produces cardiac arrhythmia and reflux bradycardia.
4.
Nausea and vomiting are produced during bradycardia.
5.
It requires complicated instruments for use.
VOLATILE LIQUIDS
Ether (C2H5
- O - C2H5) [Diethyl ether]
It is a colorless and volatile liquid with a pungent odour.
It is highly inflammable and explosive. It is quickly absorbed and eliminated through
lungs.
Advantages;
1.
It produces satisfactory muscle relaxation.
2.
It does not produce liver or kidney damage.
3.
It can be given by open drop (through congestion of
eye, soreness of trachea and ether burns on face can occur) without the need
for any equipment, and is relatively safe in inexperienced hands.
4.
It is cheap hence still popular in developing
countries.
Disadvantages:
1.
It is highly soluble in blood - induction is prolonged.
2.
Unpleasant with struggling, breath holding, salivation
and marked respiratory secretions (atropine must be given as premedication to
prevent the patient from drowning in his own secretions).
3.
Recovery is slow.
4.
Produces nausea and vomiting during recovery.
5.
Inflammable and explosive.
Chloroform:
It is a clear liquid with a sweet odour.
Advantages:
1.
Non-inflammable and non-explosive.
2.
Induction is rapid and pleasant.
3.
It is less irritant to mucous membranes.
4.
Does not produce capillary bleeding.
Disadvantages:
1.
It decomposes into phosgene, a poisonous gas and this
can be prevented by the addition of 1% alcohol.
2.
Produces cardiac arrhythmias due to sensitization of
myocardium to adrenaline.
3.
Fall in blood pressure and decrease in cardiac output
are produced.
4.
It is toxic liver.
5.
Produces nausea and vomiting during recovery.
Halothane:
It is volatile liquid wit sweet odour, non-irritant and
non-inflammable.
Advantages:
1.
Non-inflammable and non-explosive.
2.
It has intermediate solubility in blood -
induction is reasonably quick and pleasant.
3.
It is a potent anaesthetic.
4.
Non-irritant to mucous membranes.
5.
It produces relaxation of bronchial smooth muscles and
hence is helpful in asthmatic patients.
Disadvantages:
1.
Halothane causes direct
depression of myocardial contractility by reducing inter-cellular Ca++
concentration. Cardiac output is reduced with falling of blood pressure.
Myocardium is sensitized to adrenaline
and may produce cardiac arrhythmia.
2.
Muscle relaxation is not adequate and analgesia is
poor. (This poor analgesia and muscle relaxation and neuromuscular blockers.)
3.
It is toxic to liver.
4.
It is expensive.
Trichloroethylene
It is clear, colorless liquid with a characteristic odour.
Advantages:
1.
Induction is rapid.
2.
It does not irritate mucous membranes.
3.
It is useful for short operations and in dentistry.
Disadvantages:
1.
Muscle relaxation is not adequate.
2.
Produces cardiac arrhythmias due to sensitization of
myocardium to adrenaline.
3.
Produces nausea and vomiting during recovery.
4.
It is not useful for long operations.
INTRAVENOUS ANAESTHETICS
INDUCING AGENTS
These are drugs which on intra-venous injection produce loss
of consciousness in one arm-brain circulation time (approximately 11 seconds);
are generally used for induction because of rapidity of onset of action.
Anaesthesia is then usually maintained by an inhalation agent. They also serve
to reduce the amount of maintenance anaesthetic. Supplemented with analgesics
and muscle relaxants, they can also be used as the sole anaesthetic.
1. Ultrashort acting
barbiturates:
E.g. Thiopentone sodium, methohexitone sodium.
These ultra short-acting barbiturates produce hypnosis, deep
sleep and unconsciousness in that order. These drugs have short duration of action
due to high lipid solubility rapid destruction in the liver.
Advantages:
1.
Injection produces unconsciousness in 15 to 20 secs.
Its undissociated form has high lipid solubility -enters brain almost
instantaneously. Other less vascular tissues (like muscles, fats, etc.);
gradually takes up the drug - blood concentration falls and drug comes out from the
brain. Consciousness is regained in 10 to 20 mins.
2.
Respiration is quiet and salivation is absent.
3.
Less post-anaesthetic complications.
Disadvantages:
1.
Extravenous injection or inadvertent intra-arterial
injection produces intense pain - necrosis and gangrene may occur. Poor analgesic.
Painful procedures should not be carried out under its influence unless an opioid
or N2O has been given, otherwise the patient may struggle, shout and
show reflex changes in blood pressure and respiration.
2.
It is weak muscle relaxant.
3.
Respiratory depression with large doses is severe.
2. SLOWER ACTING DRUGS
1. Benzodiazepines
e.g. Diazepam, Lorazepam, Midazolam
Advantages:
(i)
Benzodiazepine in large dose injected i.v. produce
sedation, amnesia and then unconsciousness in 5-10 minutes.
(ii)
Does not markedly depress respiration, cardiac
contractility or blood pressure.
Disadvantages:
(i)
Benzodiazepines decrease muscle tone by central action
but require neuromuscular blocking drugs for muscle relaxation of surgical
grade.
(ii)
Relatively large dose is required.
2. Ketamine
It is a non-barbiturate general anaesthetic. It produces
“dissociative anaesthesia” characterized by superficial sleep and complete analgesia.
[Light sleep and feeling of dissociation from ones own body and the surroundings].
Advantages:
(i)
It is useful only for short procedures like dressing of
burns and bronchoscopy.
(ii)
It does not affect respiration.
Disadvantages:
(i)
Delirium and hallucinations are produced during
induction and recovery.
(ii)
Blood pressure and heart rate are increased.
(iii) Muscle
relaxation is poor.
(iv) It
produces laryngospasm and salivation.
3.
Fentanyl-Droperidol combination
Fentanyl is a short acting (30 to 50 minutes), potent opioid
analgesic related to pethidine, while droperidol is a rapidly acting potent
neuroleptic related to haloperidol. When a combination of these i injected i.v
a state of “neurolept analgesia” is produced, i.e., intense analgesia without unconsciousness.
This state lasts for 30-40 mins.
Patients remain drowsy but conscious and his cooperation can
be commanded.
Neurolept analgesia is quite suitable for endoscopies,
angiographies, burn dressing, etc. and has been used for a variety of minor
surgical procedures in severely ill or otherwise poor risk patients.
PREANAESTHETIC MEDICATION
The term ‘preanaesthetic medication’ refers to the use of
drugs prior to the administration of anaesthetics, so as to make anaesthesia
safe and agreeable to the patient. Pre-medication aims at providing the
following effects:
1.
Sedation
: To reduce anxiety of the patient before surgery. This can be achieved
by sedative, hypnotics and tranquilizers. Usually morphine and its derivatives
and barbiturates are given.
2.
Analgesia: achieved by morphine and pethidine.
They reduce pain-sensation and also reduce the amount of general anaesthesia
required.
3.
Inhibition of parasympathetic activity: so as to
decrease bronchial and salivary secretion induced by drugs like ether.
No comments:
Post a Comment