General Anaesthesia

ANAESTHESIA

General Anaesthesia

·        Anaesthesia = absence of all sensation

·        The reversible induction of:

o   Hypnosis

o   Autonomic reflex suppression

o   Muscle relaxation

·        One drug doing all (e.g. inhalational anaesthetic such as isoflurane) would require sufficiently high doses to cause major side effects (e.g. respiratory depression, prolonged recovery)

·        Balanced anaesthesia: one drug for each component ® minimal homeostatic depression and quick recovery: 

o   Hypnotic: e.g. thiopentone or halothane

o   Opiod: abolish autonomic response to painful stimuli

o   Muscle Relaxant

·        Stages:

o   1 – confusion

o   2 – exaggerated reflexes, severe confusion

o   3 – Light, surgical and deep anaesthesia

o   4 – medullary paralysis

·        With increasing depth there is:

o   Loss of reflexes causing cough and swallowing ® aspiration

o   ¯Muscle tone ® airway obstruction

o   Progressive depression of ventilation

o   At deep level, medullary paralysis with apnoea and cardiovascular collapse

·        Major problem is awareness if anaesthetic too light: anaesthetist won‟t know due to paralysis.

·        Principal signs are stress responses: tachycardia, hypertension, mydriasis (large pupils), sweating, pallor. Additional hypnotic will remove consciousness of pain, additional opiod will suppress CNS reception of pain

·        Monitoring:

o   ECG: heart rate, rhythm and ischaemia 

o   Blood pressure: depressed by excessive depth, extensive local (e.g. spinal blocks), or blood loss, elevated by awareness, inadequate pain blockade or hypercarbia 

o   End tidal CO2: if ­ then inadequate ventilation, if ¯ then excessive ventilation

o   Pulse oximeter: O2 saturation 

o   Temperature: anaesthetic agents ® vasodilation ® loss of temperature, also cold iv fluids

o   Muscle relaxation: neuromuscular block monitor

·  Rapid Sequence Induction/Intubation: Intubation of someone not properly prepared for surgery (eg not fasted so risk of gastric aspiration/regurgitation). Drugs to paralyse (e.g. Suxamethonium – fast acting) and sedate (e.g. propofol). Press down on cricoid cartilage until intubated to stop aspiration (closes of oesophagus) – but not if actively vomiting (otherwise gastric or oesophageal rupture)