Anaesthetics - Principles and Pharmacology Flashcards
General anaesthesia
Implies loss of consciousness and global lack of awareness.
Regional anaesthesia
- Nerve and plexus blocks including central neuraxial block - spinal and epidural
- Essence is that the anaesthetic agent is applied to the nerve and anaesthesia produced in a distal site, remote from the injection.
Local anaesthesia
Local anaesthetic is injected directly into the tissues to be anaesthetised.
Triad of anaesthesia
Analgesia, hypnosis, muscle relaxation
Hypnosis
Unconsciousness
Analgesia
- Pain relief, can also be taken in this context to mean “removal of perception of unpleasant stimulus”.
- If patient is unconscious and therefore unaware of pain, analgesia usually still required to suppress reflex autonomic responses to painful stimulus.
Muscle relaxation
Refers to skeletal muscle relaxation necessary to provide immobility for certain procedures, allow access to body cavities and to permit artificial ventilation amongst other things.
Problems of seperation of components of anaesthetic
- Polypharmacy
- Increased chance of drug reactions / allergies
- Muscle relaxation
- Requirement for artificial ventilation
- Separation of relaxation & hypnosis
- awareness
General anaesthetic role in triad
General anaesthetic, inhaled and intravenous, agents provide unconsciousness as well as a small degree of muscle relaxation. They may to differing extents also provide some analgesia but for all except Ketamine this is negligible.
General anaesthetic agents mechanism
All general anaesthetic agents work by suppressing neuronal activity in a dose dependant fashion. This is largely done by opening chloride channels which hyperpolarise the neurones or suppressing excitatory synaptic activity.
Describe why a general anaesthetic results in the loss of cerebral function from the to down.
- The most complex neuronal activity is the most susceptible to inhibition in this way
- Reflexes relatively spared
- Primitive
- Small number of synapses
Describe why general anaesthetic agents work so quickly.
- IV agents, e.g. thiopentone or propofol all work extremely rapidly and cause unconsciousness basically as soon as they reach the brain.
- They are highly fat soluble drugs and cross membranes extremely quickly
- They also leave the circulation very quickly and consequently from the brain.
- This rapid fall in blood concentration is due mainly to the drug leaving the circulation and moving to other parts of the body (compartments).
- Metabolism of the drug actually contributes very little to the termination of action of an intravenous anaesthetic agent given as a bolus.
Target Controlled Infusion (TCI) pump system.
Allows very accurate infusion to achieve specific blood or brain concentrations of agents using complex pharmacokinetic algorithms.
Inhalation anaesthetics
- Halogenated hydrocarbons
- Uptake and excretion via lungs
- concentration gradient - lungs > blood > brain
- cross alveolar BM easily
- arterial concentration equates closely to alveolar partial pressure
MAC = minimum alveolar concentration
- Measure of potency
- Low number = high potency
General anaesthetic inhalation agents - induction, maintainance and awakening
- Induction
- slow
- Maintenance of anaesthesia
- prolonged duration
- very flexible
- Awakening
- stop inhalational admin
- washout
- reversal of concentration gradient
Role of general anaeasthetic inhalation agents
Main role of inhalational agents is in the extension or continuation of anaesthesia. The patient breaths a gas mixture containing the inhalational anaesthetic for the duration of the procedure and will remain unconscious for as long as the anaesthetic is administered
Central effect of general anaesthetic on cardiovasclular system
- depress cardiovascular centre
- reduce sympathetic outflow
- negative inotropic effect on heart
- reduced vasoconstrictor tone → vasodilation
Direct effect of general anaesthetic on cardiovasclular system
- negatively inotropic
- vasodilation → decreased peripheral resistance
- venodilation → decreased venous return, decreased cardiac output
Effect of general anaesthetic on respiratory system
- All anaesthetic agents are respiratory depressants
- Reduce hypoxic and hypercarbic drive
- Decreased tidal volume & increase rate
- Paralyse cilia
- Decrease FRC
- Lower lung volumes
- VQ mismatch
Muscle relaxents role in anaesthetic triad
They relax (i.e. paralyse) skeletal muscle. They do this indiscriminately and unfortunately respiratory and airway muscles are affected as much as any other.
Muscle relaxants indications
- Ventilation & Intubation
- when immobility is essential
- microscopic surgery, neurosurgery
- body cavity surgery (access)
Muscle relaxants problems
- awareness
- incomplete reversal → airway obstruction, ventilatory insufficiency in immediate post op period
- apnoea = dependence on airway & ventilatory support
Analgesia role in triad
- Most commonly analgesia is used in conjunction with unconsciousness as part of a balanced general anaesthetic technique with or without muscle relaxation
- Regional techniques usually provide reasonable muscle relaxation by blocking motor nerves so spinal or epidural analgesia may not require additional muscle relaxation
What is the reason for intraoperative analgesia?
- Prevention of arousal
- Opiates contribute to hypnotic effect of GA
- Suppression of reflex responses to painful stimuli e.g. tachycardia , hypertension
Mechanism of local anaesthetics
- Produce analgesia with no hypnosis.
- Work by blocking Na+ channels and preventing axonal action potential from propagating.
- Pharmacologically filthy with effects on every tissue so toxic if delivered wrongly (intravenously!!)
Local anaesthetic drugs used
Lignocaine, bupivacaine and ropivacaine
Benefits of local and regional analgesia
- Retain awareness / consciousness
- Lack of global effects of GA
- Derangement of CVS physiology - proportional to size of anaesthetised area
- Relative sparing of respiratory function
Local anaethetic toxicity depends on:
- dose used
- rate of absorption (site dependant)
- patient weight
- drug ( bupivacaine > lignocaine > prilocaine )
Signs and symptoms of local anaesthetic toxicity
- Circumoral and lingual numbness and tingling
- Light-headedness
- Tinnitus, visual disturbances
- Muscular twitching
- Drowsiness
- Cardiovascular depression
- Convulsions
- Coma
- Cardiorespiratory arrest
Effect of neuroaxial block on respiratory system
- Inspiratory function (relatively) spared unless high block
- Expiratory function relatively impaired
- Cough dependent on abdominal muscle function
- Decrease FRC - airway closure c.f. GA
- Increased V/Q mismatch
