General Anaesthesia

General Anaesthesia

There are intravenous and inhalational agents used in general anaesthesia to achieve loss of conciousness. The most widely used intravenous agents are,

·        Thiopentone sodium

·        Propofol

·        Ketamin

The widely used inhalational agents are,

·        Halothane

·        Isoflorane

·        Sevoflorane

Thiopentone Sodium

·        Thiopentone sodium is a barbiturate.

·        Dose – 4mg/kg

·        Produce anaesthesia in less than 30 seconds.

·        Consciousness is regained in 5-10 minutes.

·        Other effects- precipitate laryngeal and bronchospasms

                     Depress respiration, apnoea may occur

                     Depress the myocardium

                     Produce peripheral vasodilatation

·        Metabolized in the liver and excreted via the kidneys.

·        Contraindications – porphyria

                             Airway obstruction

                             Bronchial spasms

Propofol

·        A non barbiturate

·        Dose – 2-2.5 mg/kg

·        For induction and maintenance (TIVA)

·        Rapid recovery

·        Little hang over effect

·        Less incidences of post operative nausea and vomiting

·        Laryngeal and pharyngeal reflexes are depressed to a greater degree, and tracheal intubation can be achieved with Propofol alone.

·        Other effects – Apnoea

Arterial vasodilatation

Pain on injection

·        CI – children

       Pregnancy

       Electroconvulsive therapy

Ketamin

·        An non barbiturate

·        Produce dissociative anaesthesia

·        Dose – 1-2 mg/kg

·        Unconscious for 10-15 min

·        Potent analgesic

·        Muscle tone is maintained; therefore airway obstruction does not occur. Therefore, useful in difficult airway.

·        Pharyngeal and laryngeal reflexes are preserved.

·        Produce bronchodilatation – useful in asthmatics.

·        Other effects – increase heart rate

Increase arterial pressure (sympathomemetics activity)

Increased salivation – may require atropine

·        CI – Ischaemic heart disease

Hypertension

         Head injury

        Open eye injury

Other Intravenous general anaesthetic agents –

·        Etomidate – most cardiovascular stable induction agent

                      Less respiratory depression

                      CI – epilepsy

                             Adrenocortical insufficiency

·        Benzodiazepines – induction, sedation, anxiolytic, anticonvulsant, muscle relaxation

o   Diazepam

o   Midazolam

o   Flumazenil

Inhalational Agents

Halothane 

·        For induction and maintenance

·        0.5-2%

·        A bronchodilator

·        Lowers cardiac output and blood pressure

·        Sensitizes the myocardium to catecholamines.

·        Uterine relaxation at concentrations >2%

·        SE – hepatotoxicity

Isoflurane

·        Less myocardial depression

·        Arterial dilatation causes hypotension

Sevoflurane

·        Used in children

·        It is metabolized to flurane, which is hepatotoxic. Therefore, not used in patients with renal diseases.

·        Soda lime also degrades it into another hepatotoxic agent: Compound A.