Anaesthetic Agents
·
Concentrations. Four most common methods of specifying
concentration are:
o Mg/ml: weight of drug per volume of dilutent
o % - number of grams of drug per 100ml. E.g. 50% = 50 grams per 100 ml or
500 mg/ml. 1% = 10 mg/ml. Multiplying by 10 gives mg/ml. Often used for
lignocaine (Xylocaine)
o 1:1000 is the same as 0.1% (1 mg/ml).
Used for adrenaline and isoprenaline
o mmol/l: used for electrolytes
·
Infusions:
o Use for drugs with very short T½ where fluctuations in dose of drug are
dangerous
o Common for adrenaline, dopamine, dobutamine & lignocaine
·
E.g. post arrest infusion of
lignocaine at 2 mg/min with 2% xylocaine. This is 20 mg/ml, so want 0.1 ml per
minute. Using a burette system can deliver 6 drops a minute (60 drops = 1 ml)
or dilute 10 fold and run at 60 drops per minute (easier)
·
Gases = compounds in the gaseous
state at a temperature above their critical temperature.
·
Vapours = compounds in gaseous
phase but can be returned to liquid by in pressure. Ambient temperature
must be below the critical temperature for the compound
·
The partial pressure in the brain
necessary to achieve anaesthesia will depend on its potency
·
Poorly soluble gases will have
higher partial pressure at a given concentration and will have a fast onset.
·
Lipid soluble drugs are more
potent
·
So more soluble drugs have slower
uptake in blood and require lower partial pressure to achieve their effect
·
Factors affecting uptake:
o Inspired partial pressure
o Alveolar ventilation
o Circulation
o Properties of the anaesthetic agent (e.g. blood solubility)
·
Nitrous Oxide (N2O):
o Pleasant, non-irritating, non-flammable (but supports combustion)
o Stored in blue cylinders
o Poorly soluble in tissues ® rapid uptake and elimination
o Good analgesic but low potency
o Always given with at least 30% O2 to avoid hypoxia
o Expands in enclosed air spaces: so avoided for middle ear operations and where air embolism may occur
o Give 100% O2 at end of N2O anaesthesia to avoid diffusion hypoxia – N2O
rapidly leaves pulmonary capillaries ® ¯alveolar O2 concentration
· Halothane:
o High potency (Minimum Alveolar Concentration, MAC = 0.7% - level that produces immobility in 50% of people, NO2 is 105% Þ not so good)
o No irritation and moderately rapid induction and recovery
o Poor relaxant and analgesic properties
o Requires accurate vaporisers
o High concentrations produce profound respiratory and cardiac depression
o Rare complication is hepatic necrosis
·
Isoflurane:
o Similar to halothane, but more irritant so difficult to use for
induction
o More potent vasodilator ® hypotension and tachycardia
o Also sevoflurane: not so irritant, quicker onset, but more
expensive. Use in kids if can‟t
cannulate
·
Can be given as bolus for
induction or infusion for maintenance
·
All have similar characteristics:
o All are lipid soluble
o Unionised fraction crosses the blood brain barrier
o Loss of consciousness usually occurs in one arm to brain circulation
time (approx. 30 secs)
o Rapidly redistributed to tissues with high blood flow so rapid fall in
peak plasma concentration
·
Thiopentone:
o Barbiturate derivative containing sulphur (® yellow)
o Highly alkaline in solution ® thrombophlebitis
o Blood level results from distribution (seconds), redistribution (e.g. from muscle to fat – minutes/hours) and metabolism (15%/hour by liver)
o CNS depression - ¯cerebral O2 consumption, potent anticonvulsant
o Respiratory depression: especially initially – may be apnoeic for short period
o Circulatory effects - ¯cardiac outputs, ¯vasomotor depression ® peripheral vasodilation ® ¯BP
o Poor analgesia and prolonged hangover
o Dose: 4 mg/kg – less for elderly, more for robust male
o Complications: acute intermittent porphyria and IgE mediated
hypersensitivity
·
Propofol:
o White, oil in water emulsions, 20 ml ampoules, 1% solution = 10 mg/ml
o Structurally different to barbiturates
o Highly lipid soluble: so rapid onset – and short duration due to rapid
distribution, metabolism (T ½ = 35 – 50 minutes) and delayed return from poorly
perfused tissues (e.g. fat)
o Smooth induction, rapid recovery
o Poor analgesic
o Apnoea for about 30 seconds common
o More ¯BP but less nausea/vomiting than thiopentone
o Induction dose: 2 mg/kg in young and 1.5 mg in elderly
·
Midazolam:
o H2O soluble benzodiazepine
o Muscle relaxant, amnesic, anxiolytic, anticonvulsant
o Rapid onset and rapid elimination (T½ = 2 hours) cf. other
benzodiazepines
o Unlike other iv anaesthetics can be reversed, with flumazenil
o Used for minor procedures, ICU sedation
o Slower, more variable induction cf. thiopentone. Small dose may ® deep
anaesthesia
o Dose for sedation: 1 – 5 mg titrated
o Dose of induction: 10 – 15 mg (highly variable)
·
Ketamine:
o Related to angel dust
o Dissociative anaesthesia
o BP and respiration well supported, bronchodilator
o Dose for induction: IV 1 – 2 mg/kg, IM 5 – 12 mg/kg
o Smaller doses ® potent analgesia
o A sympathetic stimulant, can have bad dreams
·
Muscle relaxants are used to:
o Facilitate intubation and artificial ventilation
o To facilitate abdominal access during abdominal surgery
o To allow lighter levels of anaesthetic to be used ® rapid
recovery and less cardiovascular depression
·
Block neuromuscular transmission
from motor nerve to voluntary muscle (ACh transmission across synaptic cleft,
broken down by cholinesterase)
·
Non-depolarising blocking agents:
o Competitive antagonists of the nicotinic cholinergic motor end plate
receptors. E.g. rocuronium, pancuronium, atracurium, vecuronium
o Degree of block determined by train of four stimuli. Each successive
twitch is a lower amplitude – trails off. No interference with twitch response
until 75 – 80% receptors blocked. 90 – 95% block required for surgery
o Reversed by anticholinesterase drugs: neostigmine (onset in 2 minutes,
lasts 20 minutes), pyridostigmine, edrophonium. Inhibit acetylcholinesterase ® ACh, but
also effects muscarinic sites in parasympathetic nervous system. Side effects
include salivation, bronchospasm, gut motility and bradycardia.
These effects blocked by atropine – which blocks the parasympathetic system
o Analgesia ended with anticholinesterase (commonly neostigmine 2.5 mg)
and atropine 1.2 mg
·
Depolarising Neuromuscular
Blocking Agents:
o Faster onset and shorter duration (4 – 6 mins) than non-depolarising.
Used for short procedures (quick intubation), supplemented with
non-depolarising for long procedures or given by constant infusion
o Suxamethonium: reacts with ACh receptors to produce depolarisation (®
fasciculation), but because it is not so rapidly eliminated as ACh,
depolarisation persists ® inexcitability of membrane around endplate. Repolarisation happens when
suxamethonium is hydrolysed by pseudocholinesterase (unless atypical genetic
variant ® prolonged effect). If overdose or poor elimination ®
drug-receptor complex forms, unavailable for ACh ® dual or
phase II block. No antagonist to suxamethonium except fresh frozen plasma
(contains pseudocholinesterase)
o Train of four different to competitive antagonists: each twitch the same
height (although reduced over normal)
o Has other muscarinic side effects due to similarity to ACh: BP, intra-ocular
pressure, bronchial secretions, ¯heart rate, post operative muscle pain
·
Suxamethonium is contraindicated
in major burns, neurological injuries, hyperkalaemia, myasthenia and myotonic
diseases, ¯pseudocholinesterase and history of malignant hyperthermia
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