General Anesthetics.
- General anesthetics are the pharmacological agents which on administration produce loss of all modalities of sensations along with a reversible loss of consciousness.
- They are used to perform major surgeries.
- e.g. Anesthetic ether, Midazolam.
Anesthesia: -
- Anesthesia is a medical technique of producing a reversible loss of sensations and awareness.
- Anesthesia is of 3types
- General Anesthesia:-
- It is a total reversible loss of senses, reflex & consciousness.
- It is used for major surgeries.
- Local anesthesia:-
- It is a localized reversible loss of sensation without affecting the consciousness of a patient.
- It is used for minor surgical procedures like tooth extraction.
- Spinal anesthesia:-
- It is achieved by injecting a local anesthetic into spinal column at L2-L3- position (in subarachnoid space).
- It is useful for surgeries of lower limb & gynecological procedures.
Classification of General Anesthetics:
- General anesthetics can be classified as per their chemistry and pharmacology.
- The pharmacological classification of general anesthetics is as follows,
| Sr. No. | Class | Examples |
| A | Volatile or Inhalational Anesthetics | |
| 1) Volatile Liquids. | · Diethyl ether · Chloroform · Halothane. | |
| 2) Gases. | · Nitrous Oxide. · Cyclopropane. | |
| B | Non-Volatile or Intravenous Anesthetics | |
| 1) Barbiturates | · Thiopentone Sodium. · Methohexitone Sodium. | |
| 2) Benzodiazepines. | · Midazolam. · Diazepam. | |
| 3) Opioid Analgesics. | · Fentanyl. | |
| 4) Other | · Ketamine. | |
Ideal properties of general anesthetics:-
- Faster induction of anesthesia.
- Smooth recovery.
- Easy to administer.
- It should be non- inflammable & non- irritant.
- Should produce good analgesia & adequate muscle relaxation.
- Should have a wide margin of safety & there should be no side effects.
- Should have sufficient duration of action.
- Should not affect the liver, kidney, lungs & heart.
Stages of anesthesia:-
Stage 1:- Stage of analgesia.
Stage 2: Stage of delirium.
Stage 3: Stage of surgical anesthesia.
Stage 4: Stage of medullary paralysis.
Stage 1: Stage of analgesia.
- It starts from beginning of anesthetic inhalation to loss of consciousness.
- Pain is progressively reduced.
- The patient remains conscious, can hear & see & feels a dream-like state.
- Reflexes & respiration are normal.
- Minor surgical procedures such as incision of abscess, dental extraction can be carried out during this stage.
Stage of delirium:-
- This stage extends from loss of consciousness to the beginning of regular respiration.
- The patient is in the excited state.
- Breathing is rapid and jerky.
- Heart rate, blood pressure may rise.
- This stage is not suitable for any type of operative procedure.
- Preanaesthetic medication is used to control the stage.
Stage 3: Stage of surgical anesthesia
- It starts from the onset of regular respiration to end of spontaneous breathing.
- It is characterized by steady respiration, fixed eyeballs with lost corneal and light reflexes.
- This stage is suitable for major surgical operations as,
Muscle tone is decreased.
Breathing is regular.
All reflexes are lost.
Stage 4: Stage of Medullary Paralysis
- It starts from the cessation of breathing to the failure of circulation & death.
- The patient should not reach this stage for which doses of anesthetics should be adjusted.
- It is characterized by dropped respiration, heart rate, blood pressure.
- Only prompt and vigorous efforts can save the life of a patient.
Mechanisms of action of General Anesthetics:
- Inhalational anesthetics: potentiate GABA-A receptor linked chloride channel in thalamus & reticular activating system. Nitrous oxide inhibits excitatory glutamate receptor.
- Thiopentone, methohexitone & Midazolam are agonists of GABA-A receptor and act by opening the linked chloride ion channels to cause CNS depression.
- Ketamine inhibits excitatory glutamate receptor.
- Fentanyl is an agonist of Opioid receptors.
Preanesthetic Medication:
- The medication given prior to anesthesia to make induction and recovery smoother is known as pre-anesthetic medication.
- Objectives of pre-anesthetic medication:-
- To reduce the dose of anesthetics.
- To minimize adverse effects of anesthetics.
- Proper preparation of the patient for anesthesia.
- Relief from anxiety.
- To reduce salivary & respiratory secretion.
- To produce antiemetic effect extending to postoperative period.
- To potentiate analgesic effect.
- To produce adequate muscle relaxation prior to surgery.
- Different classes of drugs are used as pre-anesthetic medication depending on the need of patient which is as follows….AnalgesicsE.g. Morphine (10 mg i.m.) or pethidine (50 mg i.m.) are used to potentiate analgesic effect.
Antianxiety:
E.g. Diazepam (5-10 mg oral) or lorazepam (2 mg i.m.) are used to reduce the anxiety of patient.
Antisecretory:
E.g. Atropine (0.6 mg i.m./i.v.) is used to reduce salivary & tracheobronchial secretion.
H2 blocker:
Ranitidine (150 mg oral) or famotidine (20 mg oral) is given to prevent gastric acidosis & stress ulcers.
Antiemetic:-
Metoclopramide (10-20 mg i.m.) is used to reduce postoperative vomiting. Ondansetron (4-8 mg i.v. ) is highly effective.
Skeletal muscle relaxant. Succinylcholine or vecuronium or atracurium are used as musical relaxants.
