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)
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