Overview of General Anesthetics.pdf (Pharmacology)

This is a concise lecture on general anesthetics for undergraduates medical, nursing, pharmacy and other paramedic students.

1. General Anaesthetics
Dr. DB Dhami MD
Associate Professor
Department of Clinical Pharmacology and Therapeutic
Karnali Academy of Health Science, Jumla, Nepal

2. Learning Objectives
At the end of the lectures, students should be able to:
•Define General anaesthesia(GA)
•Enlist the ideal properties of GA
•States stages of GA
•Classify GA
•Describe mechanism of action, uses, advantages and disadvantages of
general anaesthetics agents.
•List Side effects of GA
•Describe preanesthetic medication

3. General Anesthesia (GA)
•Definition:
-General anesthesia is a state of reversible unconsciousness, loss of sensation,
and muscle relaxation induced by drugs.
-General anaesthetics: Drug which produce reversible loss of all sensation and
consciousness
-The feature of GA:
-Reversible loss of consciousness
-Reversible loss of sensation
-Analgesia and amnesia
-Muscle relaxation and abolition of reflexes

4. Properties of an ideal general anesthetic
•Rapid and smooth induction
•Effective analgesia and muscular
relaxation
•Non-irritating to mucous
membranes
•Non-toxic to heart, liver, kidney,
and brain
•Non-inflammable
•Prompt recovery upon cessation
•Cost-effective and easy to store

5. Alveoli →Blood→Brain
•Minimal alveolar concentration (MAC) is the lowest concentration of the
anaesthetics in pulmonary alveoli needed to produce immobility in
response to a painful stimulus (surgical incision).
•It indicates the potency of inhalational general anaesthetics (N2O>100%,
halothane 0.75%)
•Factors affecting the Partial pressure of anesthetic attained in the brain are:
•Partial pressure of anaesthetics in the inspired gas
•Pulmonary ventilationand Alveolar exchange
•Solubility of anaesthetics in blood and tissues
•Cerebral blood flow

6. Mechanism of action of GA
•Most anaesthetics depress reticular
formation by enhancing the activity
of inhibitory transmitters and
blocking the activity of excitatory
neurotransmitters.
•Inhibit the presynaptic voltage-gated sodium
channels in glutamatergic synapse, which inhibits
the excitation of the neuron by blocking the
release of presynaptic neurotransmitters

7. Stages of General Anesthesia
Stage I or stage of analgesia
Stage II or stage of delirium or excitement
Stage III or stage of surgical anaesthesia
•Plane 1: Roving eye balls; eyes become fixed at the end of this plane.
•Plane 2: Loss of corneal and laryngeal reflexes.
•Plane 3: Loss of light reflex and pupil starts dilating; respiration
abdominal and marked muscle relaxation.
•Plane 4: Dilated pupil, shallow abdominal respiration, intercostal paralysis.
Stage IV or stage of medullary paralysis

8. Classification of General Anaesthetics
1. Inhalation anaesthetics
-Gases:Nitrous oxide
-Volatile liquids: Ether, Halothane, Enflurane, Trichloroethylene,
Isoflurane, Sevoflurane
2. Intravenous anaesthetics:
-Fast inducers: Thiopentone sodium, Propofol
-Slow inducers: Diazepam, Lorazepam, Midazolam
-Dissociative anaesthesia: Ketamine

9. •It is a colorless, relatively odorless gas, “laughing gas”.
•It produces light anaesthesia without significant
depression of respiration
•Mechanism of action: enhance GABA activity and
inhibit glutamate activity.
•Advantages
-Rapid onset; Profound analgesia, Lack of irritation to mucous
membrane of respiratory tract, Non-inflammable
•Disadvantages:
-Low potency, Hypoxia, Poor muscle relaxation
•Status in anaesthesia: adjuvant
Inhalation anaesthetics: Gases: Nitrous Oxide

10. •Volatile liquid, characteristic odour.
•Advantages:
-It is a safest inhalation anaesthetics, It produces good analgesia and
muscular relaxation.
-It has no hepato-toxicity, renal toxicity and does not affect blood pressure,
pulse and cardiac rhythm during light to moderate anaesthesia.
-It is cheap and quite stable under proper storage conditions.
•Disadvantages
-It is explosive with Oxygen or air.
-Its vapors are irritating to mucous membranes of the respiratory tract.
-Induction is slow and stormy, Recovery is also slow.
-Although ether is a safe anaesthetics, it is not commonly used.
Inhalation anaesthetics: Volatile liquid:
Ether (diethyl ether)

11. •It is a volatile, non-inflammable, very potent and relatively nontoxic. It is the most widely
used inhalation anaesthetics.
•Advantages: Induction is smooth, rapid and pleasant. It is non-irritant to mucous
membranes of the respiratory tract and no bronchoconstriction. Recovery is also smooth.
•Disadvantages:
-It sensitizes the heart to sympathetic amines.
-It is hepato-toxic , may cause malignant hyperthermia
-It causes poor analgesia and poor muscle relaxation.
-It is expensive.
•Its main uses are:
-To maintain anaesthesia during major surgery,
-To supplement to anaesthetics action of nitrous oxide oxygen mixture.
Inhalation anaesthetics volatile liquid: Halothane

12. •Halothane: A volatile, potent, non-inflammable inhalation anesthetic.
•Advantages:
•Smooth, rapid induction
•Non-irritant to respiratory tract
•Smooth recovery
•Disadvantages:
•Sensitizes heart to sympathetic amines
•Hepatotoxic; risk of malignant hyperthermia
•Poor analgesia and muscle relaxation
•Expensive
•Uses: Maintains anesthesia during major surgery. Supplements nitrous oxide-oxygen
anesthesia
Inhalation anaesthetics volatile liquid: Halothane

14. 1.Inducing agent: Thiopentone sodium, Propofol
2.Dissociative agent: Ketamine
3.Opioid analgesic: Fentanyl (used in short procedures)
4.Benzodiazepines: Diazepam, Lorazepam, Midazolam
Advantages:
-Extremely rapid induction due to high blood concentration
Disadvantages:
-No channel for quick elimination like lungs
Intravenous Anaesthesia

15. Inducing agent: Thiopentone sodium
•Barbiturates, ultra short acting 10mint, highly lipid soluble
•Mechanism of action: facilitate GABA-mediated inhibition at GABA-A
receptors
•Advantages:
-Quick on set of action, Smooth induction, rapid and pleasant
•Disadvantages:
-Neither the good analgesic nor muscle relaxant
-cannot used alone due to Respiratory and circulatory depression,
-Severe hypotension
•Uses:
-Induction of GA, To control convulsion in status asthmaticus
-Medicolegal uses : truth serum (subtherapeutic dose of
thiopentone sodium produces altered consciousness when the capacity
of concort stories and lies lost.

16. Inducing agent: Propofol
•Propofol is oily liquid and most preferred anaesthetics
•Mechanism of action: enhancing the activity of GABA in the brain.
•Advantages:
-Quick induction, rapid recovery-preferred in day cases, antiemetic
property, safe in pregnancy
•Disadvantages:
-Respiratory depression, Vasodilation –fall BP
-Negative effects on heart , Pain at injection site
-Histamine release
•Uses:
-Induction and maintenance of GA
-Total IV anaesthesia continuous infusion

17. Dissociative anaesthesia: Ketamine
•Mechanism of action: Blocks the NMDA receptors
•Advantages:
-Good analgesia, amnesia, no respiratory depression, no
hypotension, convenient to use (epidural, IM, IV and rectal)
-Suitable in children, in asthmatic patients(bronchodilation)
•Disadvantages:
-↑ HR, CO and BP, ↑ intracranial pressure, hallucination,
delirium, involuntary movements, nystagmus
•Uses: short surgical and diagnostic procedures, topical
use for joint pain.

18. Common side effects of GA
1.Nausea and vomiting
2.Sore throat
3.Confusion and memory loss
4.Headache:
5.Muscle aches
6.Shivering
7.Allergic reactions
8.Changes in blood pressure or heart rate, respiratory depression, and damage
to teeth or other structures in the mouth.

19. Preanesthetic medication
•Preanesthetic medication refers to drugs given before anesthesia to prepare the
patient for surgery, typically to reduce anxiety, induce relaxation, manage pain, and
stabilize vital signs.
•Preanesthetic medication aims to:
1.Reduce anxiety
2.Counteract the secretory effects of general anesthetics
3.Facilitate smooth and rapid induction
4.Induce amnesia for pre-and post-operative events
5.Enhance analgesia during anesthesia
6.Provide antiemetic effects through the post-operative period
7.Decrease gastric acidity and volume to reduce aspiration risk
8.Manage post-operative pain

20. Drugs used for Preanaesthetic medication
1.Anti-anxiety drugs: Diazepam(5-10mg oral),and Lorazepam(2mg i.m.)reduce
preoperative anxiety.
2.Sedatives-hypnotics: Promethazine ((25mg i.m.) induces sedation and reduces nausea.
3.Opioid analgesics: Morphine(8-12mg i.m.)and Pethidine (50-100mg i.m.) provide
pain relief.
4.Anticholinergics:Atropine,(0.5mg i.m.)Hyoscine,0.5mg i.m.)and Glycopyrrolate (0.1-
0.3mg i.m.)decrease secretions and prevent bradycardia.
5.Antiemetics: Metoclopramide,(10mg i.m.)Domperidone (10mg oral), , and
Ondansetron (4-8mg i.v) prevent and treat nausea and vomiting.
6.Drugs reducing acid secretion: Ranitidine,(150mg oral)Famotidine (20-40mg oral),
and PPIs (e.g., Omeprazole (20mg)with Domperidone (10mg)) decrease gastric acid
production, reducing the risk of aspiration.

21. Anesthetic protocol
1.Premedication
2.Induction of anaesthesia (propofol)
3.Maintenance of anaesthetics(N2O + isoflurane/halothane)
4.Skeletal muscle relaxation
5.Analgesia: as premedication, during and after the operation
6.Uses of other drugs
•To reverse neurotransmitter blockade
•To reverse the residual effects of opioids

22. Any Question?
Thank you

23. Learning Objectives
At the end of the lectures, students should be able to:
•Define General anaesthesia(GA)
•Enlist the ideal properties of General anaesthesia
•States stages of general anaesthesia
•Classify General anaesthesia.
•Describe mechanism of action, uses, advantages and disadvantages of
General anaesthetics agents
•List Side effects of General anaesthesia
•Describe preanesthetic medication