Download Introduction to anesthesiology

General Anaesthesia
By
Dr. Nimer Khraim
Mode of action of anaesthetic drugs

The mechanism by which the anaesthetic drugs
produces their action is not very well known.

But simply they may act by switching off excitatory
systems & turning on inhibitory ones

so that impulses from periphery to the brain are
blocked mainly at the thalamic level,
Mode of action of anaesthetic drugs

while a similar action on the fusiomotor
system may explain the loss of motor control
associated with general anacsthesia.

As cerebral cortex being responsible for the
maintenance of consciousness, it is the first
part of the brain to be affected during
induction of anacsthesia
Mode of action of anaesthetic drugs

when its function is abolished
unconsciousness is lost and the lower parts
of the brain are released and become more
active

As anaesthesia deepens various hypothetical
centers are depressed down until only the
neurons in the medulla and spinal cord
continue to maintain life.
Stage of General Anaesthesia and their
Signs

Four stages of anaesthesia are describing
the transition from conciseness to complete
anaesthesia.
Stage of General Anaesthesia and
Their Signs
Stage I: Introduction stage of voluntary excitement:

The animal is conscious and may make forcible
efforts to avoid being anaesthesia.

It characterized by analgesia and disorientation
without loss of consciousness and lasts from
beginning of anaesthesia to the loss of conscious.
Stage of General Anaesthesia and
Their Signs
Clinically characterized by:
a. Heart Rate
b. Respiratory rate are deep.
c. Salivation.
d. Pupils = normal or slightly dilated.
e. Struggling.

Stage of General Anaesthesia and
Their Signs

Stage II. Stage of involuntary excitement:

This stage started from the loss of conscious
to the onset of regular breathing pattern.
Stage of General Anaesthesia and
Their Signs

Characterized clinically by:

Violent limb movement and exaggerated
response to stimuli.

Pupils dilated and brisk nystgmus (eyeball)
moving from side to side.
Stage of General Anaesthesia and
Their Signs

Swallowing and vomiting (pharyngeal reflex)
are present, but become progressively
depressed and disappear on entering stage
III.

Respiration is very irregular and breathholding is an invariable accompaniment of
struggling.
Stage of General Anaesthesia and
Their Signs

Stage III. Stage of surgical anacsthesia:

Characterized by Unconsciousness loss of
sensation to pain and movement.
Stage of General Anaesthesia and
Their Signs

This stage divided into 3 plans:

First plane (light) anaesthesia: Indicated by:
Onset of regular automatic breathing.
Cessation of all limp movements.


Stage of General Anaesthesia and
Their Signs

Nystagmus i.e eyeballs moving from side to
side particularly in horses.

The palpebral, conjuctival and corneal
reflexes disappear as anaesthesia deepens
and are no longer present when the animal
enters the second plane.
Stage of General Anaesthesia and
Their Signs

Many diagnostic investigations and minor
operations can be performed in this plane of
anaesthesia.
Stage of General Anaesthesia and
Their Signs

Second plane (medium) anaesthesia:

eyeballs is fixed centrally in horses, cats,
sheep and pigs but in dogs the eyeball
rotates downwards.

Respiration rate increases but its amplitude
decreases.
Stage of General Anaesthesia and
Their Signs

The laryngeal reflex persist until the middle
of the second plane is reached.

Muscle relaxation becomes progressively
more pronounced.

Pedal reflex in the dog and cat becomes
absent
Stage of General Anaesthesia and
Their Signs

This plane of anaesthesia is adequate for all
except intra-abdominal surgery.
Stage of General Anaesthesia and
Their Signs

Third plane (deep) anaesthesia:

Automatic breathing is still present, but
respiratory rate increases while its depth
decreases and a noticeable pause appear
between inspiration and expiration.
Stage of General Anaesthesia and
Their Signs

All body reflexes abolished.

In the dog and cat the eyeballs once again
become central as the tone of the eye
muscles is lost.
Stage of General Anaesthesia and
Their Signs
Stage IV. Stage of Over dosage:

In this stage Paralysis of the thoracic
muscles is complete and only diaphragmatic
activity remains.
Stage of General Anaesthesia and
Their Signs

The stage characterized by:

irregular respiration & respiratory arrest.
The pulse become rapid and the pupils dilate.

Stage of General Anaesthesia and
Their Signs

Cyanosis appears & soon replaced by the
ashen-grey color, which indicate heart failure
& cardiac arrest follows closely.

At this point immediate Vigorous corrective
therapy is required to successfully
resuscitable patient.
Anesthetic Agents Used for Induction
of General Anaesthesia:

Two main types often used to induce. G.A. in
veterinary practice:

Intravenous anaesthesia drugs.

Inhalational anaesthesia drugs.
Anesthetic Agents Used for Induction
of General Anaesthesia:

intravenous anaesthesia drugs:

Generally divided into 2 main types
–
–
The Barbiturate I.V anaesthesia agents.
The Non- barbiturate anaesthesia drugs.
Anesthetic Agents Used for Induction
of General Anaesthesia:

Advantages of intravenous anaesthesia agents over
inhalation anaesthesia agents:

Minimal equipment required.
Economical i.e. simple non-expensive equipments.
Quickest method for induction of anaesthesia.
Good for short surgical or diagnostic procedures.



Anesthetic Agents Used for Induction
of General Anaesthesia:

Disadvantages of I.V anaesthesia agents:

The plan (level) of anaesthesia can’t be decreased
quickly i.e. if over dosage occurred, it cannot be
reversed rapidly.

Recovery from anaesthesia depends on the animals
ability to detoxify or excrete the drug i.e the general
condition of animal is important.
Anesthetic Agents Used for Induction
of General Anaesthesia:

Recovery period may be long & Stormy.

Long surgical procedures which needs
administration of I.V anaesthesia agents for longer
periods or in larger doses can cause sever
physiological and damages in the patient

Oxygen and other respiratory support equipments
(e.g., respiratory ventilator) may not be available.
Anaesthesia with Inject able agents

Equipment:

A suitable hypodermic needle and a
syringe are all that essential for parental
administration of anaesthesia drugs.
Intravenous (I.V) injectable anaesthesia
agents
The barbiturate drugs: These are divided
into 4 groups of compounds
a. Oxybarbiturates
b. Methylated oxybarbiturates.
c. Thiobarbiturates
d. Methylated thiobarbiturates.

Intravenous (I.V) injectable anaesthesia
agents

Compounds from group (a) & (c) are mainly
used in veterinary anaesthesia, because they
produce unconsciousness in few site-brain
circulation times.
Intravenous (I.V) injectable anaesthesia
agents

The barbiturate in clinical use are all sodium
salt of barbituric acid or their oxy or thioderavitives, whose chemical structure had
been modified to increase their lipid solubility
and hence potency.
Pentobarbiturate



The important barbiturate drugs in veterinary
anaesthesia are:
Pentobarbiturate sodium (Segatal® 6%
Nembutal ® 10 %; Narkoderm®).
It is an oxybarbiturate drug, available as
sterile alkaline solution of 6.10 & 20 mg/ml.
Pentobarbiturate
Pharmacology
A- Effects on CNS relatively crosses the BBB slowly
 its peak effect delayed for few minutes (5-10 mins.)
 It is mainly a potent depressor to the motor areas of
the brain (cerebral cortex & probably the
hypothalamus)
 hence it is used to control convulsive seizures.
Pentobarbiturate
B- Respiratory system:
 The drug depresses the respiratory centers
at a dose dependent proportion.
 The sensitivity of the respiratory center for
CO2 is reduced.
Pentobarbiturate
C- cardiovascular system:
 The drug causes a fall in blood pressure
following direct depression of the vasomotor
center.
 It has a little direct toxic effect on the
myocardium.
Pentobarbiturate
D- Fat in the body:


The drug metabolized by the liver
products eliminated by rental excretion.
Pentobarbiturate
Uses & Doses:
 It is hazardous in most species, particularly in large
animals, to obtain surgical levels of anaesthesia with
pentobarbinate alone.
 it is used, particularly in large animals to prolong the
duration of narcosis following the administration of
another anaesthetic agent e.g., chloral hydrate.
 in small animals (dogs & cats) it is used a sole
anaesthetic agent.
Pentobarbiturate


Small animals: (Dogs & Cats) 20-25 mg/kg
B.wt.
Half to Tow-third of the calculated dose is
given I.V quickly to avoid excitement during
induction, then the remaining is given very
slowly until effect of general anaesthesia is
obtained.
Pentobarbiturate


Sheep: 20-25 mg/Kg B.wt.
It is a useful agent with duration of
anaesthesia ranging 5-10 up to 25 mins
when a single bolus dose is given. The short
duration of action in these animals are due
mainly to the rapid metabolism of the drug by
the liver.
Thiopentone sodium
Thiopentone sodium: Pentothal®, Interval®.
 It is a thiobarbiturate derivatives, with a
shorter duration of action than
pentobarbitone due partly to its higher fat
solubility & because it is metabolized in the
liver quite rapidly during early stages of
anaesthesia.
Thiopentone sodium



The drug prepared commercially as a
yellowish-white powder.
It is highly soluble in distilled water with an
alkaline pH (pH=11).
This it is extremely irritant to the body
tissues if given extravascularly.
Thiopentone sodium


Because, theopentone is a highly fatty
soluble drugs, it crosses the BBB with very
great speed
makes the drug very useful for the
production of anaesthesia with rapid
induction, short period of narcosis & rapid
recovery as the drug redistribute from the
plasma to other body tissues such as body
muscles and fat.
Thiopentone sodium
Pharmacology:
1- CNS:
Medullary depression.
Thiopentone sodium


2- Respiratory Syetem: Markedly depressed
following rapid administration.
Usually there is increase in depth of
respiration followed by a short period of
apnoea.
Thiopentone sodium
3- Cardiovascular System:
 Depresses the myocardium
 sever hypotension may occur if large doses
are given due to peripheral vasodilatation.
Thiopentone sodium
4- Fate in the body.
 After rapid I.V administration Thiopentone exerts its
effect on the CNS almost immediately.
 Then the drug level in the plasma fail rapidly due to
its redistribution into body muscles & fat.
 This is why rapid recovery from Thiopentone occurs.
 The drug is rapped metabolized in the liver & exerted
by the kidneys.
Thiopentone sodium
Uses & Dosage:
 1-Used as a sole anaesthesia agent with
premedication for short surgical procedures
in small & large animals.
 For larger procedures, additional increments
of Thiopentone I.V can be given to maintain
anaesthesia for a longer duration of time.
Thiopentone sodium
2- As an induction agent prior to the
anaesthesia by inhalation agents.
Thiopentone sodium



Large animals (Horses & Cattle): 1 g/90 Kg
B.wt. given to rapid I.V injection (10%
solution).
Sheep & Goat: 20-25 mg/Kg B.wt. (2.5-5%
solution).
Dogs & Cats: 25-35 mg/Kg 1% sol. Half of
the dose is given rapidly & the remaining is
given slowly to-effect.
Methohexitone Sodium (Braven
®).
It is a rapidly acting oxybarbiturates with
important differences from other barbiturates
a- Potency 2 to 3 times greater than that of
Thiopentone.
b- Shorter duration of effect.
c- More rapid recovery to full alerness even
after prolonged anaesthesia

Methohexitone Sodium (Braven
®).
Note:
Methohexitone sod. is administrated to small
animals as a 1% solution & large animals
concentrated solutions of up to 6% used.

Methohexitone Sodium (Braven
®).
Doses
 Cats: I.V doses of 5mg/Kg at 0.5% solution.
 Cattle: because recovery after
methohexitone Na is more rapid than that
after thiopentone, it might be better for use in
cattle. In adult cattle 25 mg/kg by rabid I.V
2.5-5% sol.
Methohexitone Sodium (Braven
®).



Horses: used in horses premedicated with
acepromazine (0.04 mg/kg)
the I.V does of methohoxitone being used
for induction of anaesthesia is 0.5g 90kg.
B.wt. (6% solution).
Accidental extravascular injection does not
cause tissue nerosis as occur with
thiopentone & recovery is more rapid &
complete than after thiopentone.
Non- barbiturate intravenous
anaesthesia agents:
1- Chloral hydrate:
 A white translucent crystals substance which has a
pungent smell.
 It is water soluble.
 The drug is primarily a hypnotic but when given in
large quantities, it will produce narcosis and
anaesthesia with low analgesic action.
 The drug is irritant (pH=11)
 when given a perivascularly it will cause sever
tissue damage.
Non- barbiturate intravenous
anaesthesia agents:
Effects on Respiratory systems:
 In hypnotic doses the drug produce
depression, due to failure of respiratory
center.
 C.V.S: Myocardial contractility is reduced &
arterial blood pressure falls.
Non- barbiturate intravenous
anaesthesia agents:


It is administered in large animal in solution
of 10% concentration.
Doses for Horses and Cattle: 5g./50kg B.wt.
one third is run in over 4 minutes and as
soon as the animal becomes ataxic & then
on the ground the remainder is given to effect.
2- Chloral hydrate Magnesium Sulphate
Mixtures:
 A dose of 5 gm of each (i.e. Chloral hydrate
and magnesium Sulphate)/50 kg. B.wt. in
10% solution claimed to hastened to onset of
anaesthesia muscle relaxation & increases
the depth of anaesthesia and reduce the
toxicity produced by chloral hydrate when
given alone.
Non- barbiturate intravenous
anaesthesia agents:
3- Equithesin ® Army preparation:
 It is a mixture from the following
components:
1. Chloral hydrate 28gm.
2. Mag. Sulphate 14 gm.
3. Pentobarbitone Na 6.5 gm.
4. Distilled water up to 1000 c.c.
Non- barbiturate intravenous
anaesthesia agents:
Note:
 This solution must be prepared within one
hour before administration to prevent
precipitation occurs as a result of reaction
between chloral hydrate and pentobarbonate.
Non- barbiturate intravenous
anaesthesia agents:

Advantages:
1.
The mixture has low toxicity.
Low quantity required for anaesthesia.
Excitement during induction is less.
2.
3.
Non- barbiturate intravenous
anaesthesia agents:
Disadvantages:
the solution is very irritant if given
perivascularily.

Non- barbiturate intravenous
anaesthesia agents:
Doses:
 Horses and Cattle 20-30 ml/50kg. B.wt I.V.
slowly.
Non- barbiturate intravenous
anaesthesia agents:
4- Dissocitative agents (Cyclohexylamine
derivatives)
A group of chemically related compounds of
which 3 are of clinical importance, namely:
Phenecyclidine, Tiletamine & Ketamine. But
the last agent i.e. Katamine is the most
widely for use in men animals.
Non- barbiturate intravenous
anaesthesia agents:


These agents are described as having a
cataleptic, analgesics & anaesthetic actions,
but without hypnotic properties.
Catalepsy is defined as a characteristic
akmetic state with loss of orthostatic reflexes
but without loss of consciousness in which
the extremities appear to paralyzed
Non- barbiturate intravenous
anaesthesia agents:



Ketamine : is the only dissociative agent
Widely accepted for chemical use
The drug metabolized by the liver
In cat a large part is excreted unchanged by
the kidney and the reminder metabolized by
the liver
Non- barbiturate intravenous
anaesthesia agents:
Effects on the body:
CNS
 Ketamine alters the reactivity of CNS to various
sensory impulses without blocking sensory input at
spinal or brain stem levels.
 Its anaesthetic action requires the presence of a
functional cerebral cortex.
 The drug increases the intracranial pressure due to
increased cerebral blood flow.
Non- barbiturate intravenous
anaesthesia agents:
CVS
It causes a transient hypotension & Brady
cardia followed by an increase in HR and BP.
Non- barbiturate intravenous
anaesthesia agents:
Respiratory system:
 It doesn't dependent ,respiratory depression
 Salivation simulated by Ketamine, but can be
controlled with atropine.
Non- barbiturate intravenous
anaesthesia agents:
Uses and Dosage:

Horse: used in combination with xylazin which
produce a smooth & slow induction & provide 5-15
mins of anaesthesia &10-20 mins. of recumbency.

Xylazine 1.1 mg/kg I.V after 2-5 mins. and
katamine 2.2 mg/kg I.V.

Either mixed in the same syring or given separately.
Non- barbiturate intravenous
anaesthesia agents:

Sheep: 0.05 mg/kg xylazine and 4mg/kg
katamine. The two drugs either mixed or
given separately and gives 50-90 minutes of
analgesia & restraint.
Non- barbiturate intravenous
anaesthesia agents:
Small animals:
 Dogs: Atropine sulphate 0.6 mg I.M or S.C.
Xylazine 2mg/kg I.M.
Katamine10-15 mg/kg I.M 10 to 15 min. better.
 Cats: Atropine sulphate 0.3 mg/kg S.C.
Xylazine 1.1mg/kg I.M.
Katamine 15-20 mg/kg I.M 10 to 20 min.
better.
Disoprofol



Is a very new anaesthetic agent.
Its active ingredient is 2.6-disoprophylphenol.
It is a rapidly acting agent which produce
anaesthesia for short duration & without
excitatory side-effects.
Disoprofol


This agent has shown to be compatible with
a wide range of drugs used for premedication,
inhalation anaesthesia & neuromuscular
agents.
2mg/kg of Disoprofol I.V is an effective agent
for smooth induction of anaesthesia in
horses after xylazine premedication.
anaesthesia