General anaesthetics (GAs) - 1

Drugs which produce reversible loss of all sensation and consciousness. The cardinal features of general anaesthesia are:

Loss of all sensation, Sleep (unconsciousness), Muscle relaxation and, Abolition of reflexes. Modern day combination of drugs are used

History

Horace Wells: A dentist demonstrated the use of laughing gas (N2O) for anaesthesia in 1844, at Massachusetts General Hospital Boston

Willium TG Morton: With the help of a surgeon Dr. Warren, in 1846 succeeded in demonstration of the general anaesthesia by ether (Ether Dome) at Boston.

Simpson (Scottish) 1946: Used Chloroform for the relief of child birth pain and he wrote Chloroform is decoy of Satan. In the year 1853 Queen Victoria gave birth to her 7th baby without pain under the influence of Chloroform

Synthetic unaesthetic cycloprorane was invented in 1929, at University of Wisconsin

Halothane invented in 1956 by the British Research Council

The i.v. anaesthetic thiopetone is invented in1935

MECHANISM OF ANAESTHESIA

Mayer and Overton (1901); Lipid theory

1994 Protein theory (specific membrane protein modulation)

STAGES OF ANAESTHESIA

I. Stage of Analgesia

II. Stage of Delirium

III. Surgical anaesthesia: From onset of regular respiration to cessation of spontaneous breathing. Divided into 4 planes which may be distinguished as: ­

Plane 1: Roving eye balls, 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, and dilated pupil

IV. Medullary paralysis

Pharmacokinetics of Inhalational Anaesthetics

MAC is an index of potency ,Partial pressure (PP)

1. PP of anaesthetic in the inspired gas

2. Pulmonary ventilation

3. Alveolar exchange

4. Solubility of anaesthetic in blood

5. Solubility of anaesthetic in tissue

6. Cerebral blood flow

Elimination

Techniques of inhalation of anaesthetics

1. Open drop method Liquid anaesthetic

2. through anaesthetic machines

(a) Open system

(b) Closed system

(c) Semi-closed system

Classification of General Anaethetics

Inhalational

Gas: Nitrous oxide

Liquids: Ether, Halothane, Enflurane, Isoflurane, Desflurane, Sevoflurane

Intravenous

Inducing agents; Thiopentone sod, Methohexitone sod, Propofol, Etomidate

Slower acting drugs: Benzodiazepines, Diazepam, Lorazepam, Midazolam

Dissociative anaesthesia

Ketamine, Neurolept analgesia, Fentanyl + droperidol

Category: Pharmacology Notes