Download General Anesthesia

The state of general
anesthesia is drug induced
absence of perception of
all sensations
PRINCIPLES OF THE ADMINISTRATION
OF GENERAL ANAESTHETICS
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Uptake and Distribution of inhalational
general anesthetics
Depth of Anesthesia - Tension of anaesthetic
agent in brain - controls - rates of induction
and recovery
Tension and partial pressure are
interchangeable terms
Tension in the arterial blood and brain are
same and are determined by:
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Concentration of the anaesthetic in the inspired air
depends on rate of ventilation
Transfer of the gas from the alveoli to the blood
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Decreased in disease (ex. emphysema)
Rate of transfer is determined by:
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Solubility(Blood gas partition coefficient.)
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Rate of blood flow(directly proportional to the C.O.
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Partial pressures of the agent in arterial and mixed
venous blood.
Loss of the agent from the arterial blood to all the
tissue of the body
ANAESTHESIA MACHINES
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Are devices by which the anesthesiologist is
able to deliver
Measured quantities of anaesthetic gases
and oxygen through accurate flowmeters and
with the use of special vaporizers it is
possible to add the vapor of volatile
anesthetic liquid to the gas stream. The
mixture of oxygen and anaesthetic agent is
then delivered to a breathing circuit for
administration by Inhalation.
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General Anesthetics are the most
dangerous drugs
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Therapeutic index ranges between 2-4
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2-3 times dose causes circulatory failure
Oral dose delivers the total dose
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When gas or vapor is inhaled only a small amount is
absorbed
Whereas the rest is exhaled out in next 1-2 seconds
The drug reaches the brain by leaving the blood
Anaesthetic blood levels of these cannot be
measures accurately
Concentrations in the lungs can be easily frequently
and accurately be measured
The partials pressures of the anaesthetic in the lung
and the brain are almost equal at equilibrium
Minimum Alveolar Concentration
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MAC is the measure of potency of
general anesthetics
It is the minimum alveolar concentration
(MAC) at one atmospheric pressure that
produces immobility in 50% patients or
animals exposed to noxious stimuli
ELIMINATION OF
GENERAL ANAESTHETICS
Free gases and vapors wash out of the lungs:
The arterial blood tension declines first.
Followed by that in the tissues where the anaesthetic
agent persists for a longer time.
Tissue having low blood flow (muscle) relieve the agent
much slowly.
OTHER ROUTES: These agents are also eliminated in
smaller quantities from skin, mucous membrane and the
kidneys
DEPTH OF GENERAL ANAESTHESIA
OCCURS IN STAGES
STAGE - 1
STAGE - 2
STAGE - 3
STAGE - 4
ANALGESIA
DELERIUM
SURGICAL ANESTHESIA
MEDULLARY
PARALYSES
APPROACHES FOR TESTING
DEPTH OF ANAESTHESIA
- Blinking of eyelids on striking the eyelashes.
- Swallowing
- Regularity and depth of respiration.
- Increase in respiratory rate and B.P.on incision
- Tightness of jaw muscles.
Above responses fade on deepening of the anesthesia.
Deep anesthesia Leeds to:
• Respiratory depression
• Apnea
• Lowering of B.P.
• Asystole
PREANAESTHETIC MEDICATION
Decrease anxiety with out drowsiness
Amnesia
Relieve preoperative pain
Decrease requirement for an inhalational agent.
Minimising undesirable effects of anesthetics.
(salivation, decrease in heart rate, coughing, vomiting)
Decrease volume and acidity of the gastric contents.
Decrease stress response in preoperative period
PREANAESTHETIC MEDICATION
2-3 drugs are used concomitantly:
Sedatives Hypnotics
Antianxiety drugs
Opoids
Antiemetics
H-2 antagonists
Gastrokinetic agents
Anticholinergics
DRUGS USED IN
PREANAESTHETIC MEDICATIONS
Benzodiazepines: Diazepam, Lorazepam, Midazolam.
Barbibiturates: Pentobarbitone, secobarbitone.
Antihistamines: Hydroxyzine, Diphenhydramine.
Phenothiazines: Promethazine.
Butyrophenones: Droperidol.
Opoids: Morphine, fentanyl, meperidine.
Anticholinergics: atropine, scopolamine, glycopyrrolate.
Antiemetics: Ondansetron
Drugs decreasing gastric acidity:
H-2 antagonists.
Antacids.
Gastrokinetic agents.
MECHANISM OF ACTIONS OF
GENERAL ANAESTHETICS
All drugs belong to diverse groups
Inert gases as xenon
Inorganic/Organic compds as Nitrous oxide and Chloroform.
Complex Organic Molecules
Halogenated Alkanes and ethers
The Mode of action is without any satisfactory explanation.
POSTULATIONS:
-Influence synaptic transmission.
-Axonal conduction is unaffected.
-Potentiate release of inhibitory neurotransmitters.
-Inhibit excitatory synapses.
MECHANISM OF ACTIONS OF
GENERAL ANAESTHETICS
Action of these agents is on Lipid bilayer and/or protein
lipid interface
OR
ION CHANNELS (Na/K/Ca)
OR
Ligand gated Channels
l-Glutamate.
NMDA
NAChR
GABA-A
MECHANISM OF ACTIONS OF
GENERAL ANAESTHETICS
At the molecular level, anesthetics probably exert their
effects by direct interactions with proteins rather than
by disturbing the matrix of the lipid bilayer as earlier
postulated. Anesthetics May bind to hydrophobic pockets
or clefts, producing small changes in protein
Conformation alerting receptor of channel function. It is
also possible that specialized areas Of the membrane
such as the boundary lipids surrounding membrane
Proteins are important sites of anaesthetic binding and
action.
MECHANISM OF ACTION:
The exact mechanism by which inhalational
anesthetics function is not known. There appears to
be a correlation between anesthetic potency and
lipid solubility (Meyer-Overton theory), suggesting
that these anesthetics likely affect the lipid matrix of
nerve cell membranes in the brain. Furthermore,
NMR and electron spin resonance studies indicate
that anesthetics cause a local disordering of the
lipid membrane matrix, possibly decreasing the
number of molecules that alternate simultaneously
between the gel and crystalline states, and thereby
altering membrane function.
NEUROLEPT ANALGESIA
- State of quiescence
- Reduced motor activity
- Reduced anxiety
- Indifference to the surroundings without loss of
consciousness
The patient responds to commands.
Drugs: a neurolept compd (Droperidol) plus an opoid
analgesic (fentanyl)
Neurolept analgesia may be converted into neurolept
anaesthesia by concominant administration of 65% nitrous
oxide
DISSOCIATIVE ANAESTHESIA
a state of sedation, immobility, amnesia
and marked analgesia (feeling of dissociation)
a single drug such as ketamine can result in this
state.
GENERAL ANAESTHETICS
CLASSIFICATION
Inhalation agents: NEWER AGENTS
Halothane
Enflurane
Volatile liquids:
Isoflurane
Desflurane
Sevoflurane
Gases:
Nitrous oxide
Intravenous agents:
Thiopentone
Benzodiazepines
Etomidate
Ketamine
Propofol
OLDER DRUGS
Chloroform
Ether
Ethylchloride
Trichlorethylene
Cyclopropane
Ethylene
CHARACTERSITICS OF AN
IDEAL ANAESTHETIC
1. Rapid and pleasant induction
2. Rapid changes in the depth of anesthesia
3. Adequate Muscle Relaxation
4. Wide margin of safety
5. Absence of toxic/adverse effects