Principles and Pharmacology of Anaesthetics Flashcards
What does anaesthesia mean?
Without feeling/perception
Types of anaesthetic
General
Regional
Local
What does a general anaesthetic do?
Produces insensibility in the whole body, usually causing unconsciousness
Centrally acting drugs - hypnotics/analgesics
What does regional anaesthetics do?
Producing insensibility in an area or region of the body
Local anaesthetics applied to nerves supplying relevant area
What do local anaesthetics do?
Produce insensibility in only the relevant part of the body
Local anaesthetics applied directly to the tissues
Drugs used in anaesthetics
Inhalational anaesthetics Intravenous anaesthetics Muscle relaxants Local anaesthetics Analgesics
Techniques and equipment in anaesthetics
Tracheal intubation Ventilation Fluid therapy Regional anaesthesia Monitoring USS fibre optics BIS Sensors Modern anaesthetic machine
Triad of anaesthesia
Analgesia
Hypnosis
Relaxation
What types of drugs are used to form the triad of anaesthesia?
Opiates
Muscle relaxants
General anaesthetic agents
Local anaesthetics
What parts of the triad of anaesthetics do opiates effect?
ANALGESIA
hypnosis
What part of the triad of anaesthetics do muscle relaxants affect?
Relaxation
What part of the triad of analgesia does local anaesthetics effect?
Analgesia
Relaxation
What parts of the triad of analgesia does general anaesthetic agents effect?
ALL 3
Problems with anaesthetics
Polypharmacy
- including chance off drug reactions / allergies
Muscle relaxation
- requirement for artificial ventilation
- means of airway control
Separation of relaxation and hypnosis
- awareness
How do general anaesthetics work?
By suppressing neuronal activity in a dose dependent fashion
- interfere with neuronal ion channels
- hyperpolarise neurones = less likely to ‘fire’
- inhalational agents dissolve in membranes (direct physical effect)
- intravenous agents - allosteric binding (GABA receptors - open chloride channels)
What do intravenous and inhaled general anaesthetics do to the patient?
Provide unconsciousness
Small degree of muscle relaxation
What happens to the cerebral function in general anaesthesia?
Most complex processes interrupted first
LOC early - hearing later
more primitive functions lost later
What happens to reflexes in general anaesthesia?
Relatively spared
Primitive
Other autonomic functions spared also
small number of synapses
Features of unconsciousness in IV anaesthesia
rapid onset
1 arm - brain circulation time
Causes unconsciousness as soon as they reach the brain
Highly fat soluble drugs and cross membranes extremely quickly
Features of recovery in IV anaesthesia
Rapid recovery
Due to disappearance of drug from the circulation and moving to other parts of the body (compartments)
Very little due to termination of action of IV agent given as bolus
Blood concentration of anaesthetic dose
Blood level very high initially
Then falls straight away as the drugs move into highly perfused tissues
Muscle concentration of anaesthetic dose
Picks up the drug more slowly
The effect is large because of the relative mass of skeletal muscle in the body
Fatty tissue concentration of anaesthetic dose
Picks up drugs even more slowly than muscle
But can store large amounts due to the high fat solubility of these drugs
What does the Target Controlled Infusion (TCI) pump system allow?
Very accurate infusion to achieve specific blood or brain concentrations of agents
Using a TCI pump system, what is used to make calculations and assumptions about the patients physiology?
Age
Sex
Size
What is used as inhalational anaesthetics?
Halogenated hydrocarbons
How is the uptake and excretion of inhalational anaesthetics done?
Via lungs
concentration gradient - lungs > blood > brain
Cross alveolar BM easily
Arterial concentration equates closely to alveolar partial pressure
What does MAC stand for?
Minimal alveolar concentration
What is MAC a measure of?
Potency
Low number = high potency
Induction speed of inhalational anaesthetics
Slow
Maintenance of anaesthesia in inhalational anaesthetics
Prolong duration
Very flexible
Awakening in respect to inhalational anaesthetics
Stop inhalational admin
Washout - reversal of concentration gradient
Most common sequence of general anaesthesia
IV induction followed inhalational maintenance
Induction of general anaesthesia involves…
Inhalation
Intravenous
Maintenance of general anaesthesia involves…..
Inhalational
Intravenous
- Propofol
- opiate (remifentanil)
CVS in general anaesthesia
Central = depresses cardiovascular centre
- reduced sympathetic outflow
- negative inotropic/chronotropic effect on heart
- reduced vasoconstrictor tone -> vasodilation
Direct
- negatively inotrophic
- vasodilation (increased peripheral resistance)
- venodilation - decreased venous return and CO
Respiratory physiology in general anaesthesia
All anaesthetic agents are respiratory DEPRESSANTS - reduce hypoxic and hypercarbic drive - decreased TV and increased rate Paralyse cilia Decreased FRC (may be prolonged effect) - lower lung volumes - VQ mismatch
What do muscle relaxants to?
Relax (i.e. paralyse) skeletal muscle
Indications for muscle relaxants
Ventilation and intubation When immobility is essential - microscopic surgery - neurosurgery Body cavity surgery (access)
Problems of muscle relaxants
Awareness
- due to separation of unconsciousness from hypnosis in the triad of anaesthesia which muscle relaxants allow
Incomplete reversal
- airway obstruction
- ventilatory insufficiency in immediate post op period
Apnoea - dependence of airway and ventilatory support
What is analgesia usually used with in general anaesthesia?
Unconsciousness
+ / - muscle relaxation
Why might using analgesia may mean you dont need to use muscle relaxants?
Because regional techniques usually provide a reasonable muscle relaxation by blocking motor nerves so spinal or epidural analgesia might not require additional muscle relaxation
Why use intraoperative analgesia?
Prevention of arousal
- pain wakes you up
Opiates contribute to hypnotic effect of GA
- have a direct sedative effect
Suppression of reflex responses to painful stimuli
- e.g. tachycardia, HTN, gross movements
What does regional anaesthesia not have?
No direct hypnotic / sedative effects
What is the result of a local / regional anaesthesia?
Produce analgesia
No hypnosis
Retain awareness / consciousness
How do local / regional anaesthetics work?
Blocking Na+ channels and preventing axonal action potential from propagating
Effects every tissue so toxic if delivered wrongly (IV)
Effects of local / regional anaesthetics
Lack of global effects of GA
Derangement of CVS physiology
- proportional to size of anaesthesia area
Relative sparing of resp function
Benefit of local / regional anaesthetics
Avoidance if reliance on opioid analgesics
What is the colour of the needle and the nerve in an USS guided regional anaesthesia?
Needle = yellow Nerve = dark coloured
Local anaesthetic drugs
Lignocaine
Bupivacaine
Ropivacaine
Limiting factor of local anaesthetics
Toxicity
How do local anaesthetics result in toxicity?
Produced by high plasma levels of LA
- IV injection
- absorption > rate of metabolism = high plasma levels - therefore vasoconstrictors reduce blood flow, reduce absorption
Therefore if high doses are injected into high absorption (well perfused) areas, then absorption will be high and exceed the rates of removal
Toxicity of local anaesthetics depends on…
Dose used
Rate of absorption (site dependent)
Patient weight
Drug
Most toxic to least toxic local anaesthetic drugs
Bupivacaine
Lignocaine
Prilocaine
Presentation of local anaesthetic toxicity
Circumoral and lingual numbness and tingling Light headedness Tinnitus Visual disturbances Muscular twitching drowsiness Cardiovascular depression convulsions coma cardiorespiratory arrest
How can we provide analgesia without paralysis?
LA differential blockage
- due to differential penetration into different nerve types
- myelinated, thick fibres are relatively spared
Motor fibres = relatively spared
Pain fibres = blocked easily
CVS physiology in neuraxial block
Similar to CVS effects of GA but all effects of RA are due to sympathectomy due to t LA blockage of mixed spinal nerves
What does sympathectomy result in?
Veno and vaso dilation
Respiratory physiology in neuraxial block
Inspiratory function (relatively) spared - unless high block Expiratory function relatively impaired - cough dependent on abdo muscle function Decreased FRC Increased V/Q mismatch
Blocks with increasing physiological impact
Local anaesthesia Field blocks - hernia repair Plexus blocks Limb blocks - e.g. femoral and sciatic nerve Central neural (neuraxial) block - epidural - spinal
