Anaesthetics - Principles and Pharmacology

Question 1

General anaesthesia

Answer 1

Implies loss of consciousness and global lack of awareness.

Question 2

Regional anaesthesia

Answer 2

• 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.

Question 3

Local anaesthesia

Answer 3

Local anaesthetic is injected directly into the tissues to be anaesthetised.

Question 4

Triad of anaesthesia

Answer 4

Analgesia, hypnosis, muscle relaxation

Question 5

Hypnosis

Answer 5

Unconsciousness

Question 6

Analgesia

Answer 6

• 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.

Question 7

Muscle relaxation

Answer 7

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.

Question 8

Problems of seperation of components of anaesthetic

Answer 8

• Polypharmacy
  
  • Increased chance of drug reactions / allergies
• Muscle relaxation
  
  • Requirement for artificial ventilation
  • Separation of relaxation & hypnosis
    
    • awareness

Question 9

General anaesthetic role in triad

Answer 9

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.

Question 10

General anaesthetic agents mechanism

Answer 10

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.

Question 11

Describe why a general anaesthetic results in the loss of cerebral function from the to down.

Answer 11

• The most complex neuronal activity is the most susceptible to inhibition in this way
• Reflexes relatively spared
• Primitive
• Small number of synapses

Question 12

Describe why general anaesthetic agents work so quickly.

Answer 12

• 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.

Question 13

Target Controlled Infusion (TCI) pump system.

Answer 13

Allows very accurate infusion to achieve specific blood or brain concentrations of agents using complex pharmacokinetic algorithms.

Question 14

Inhalation anaesthetics

Answer 14

• Halogenated hydrocarbons
• Uptake and excretion via lungs
  
  • concentration gradient - lungs > blood > brain
  • cross alveolar BM easily
  • arterial concentration equates closely to alveolar partial pressure

Question 15

MAC = minimum alveolar concentration

Answer 15

• Measure of potency
• Low number = high potency

Question 16

General anaesthetic inhalation agents - induction, maintainance and awakening

Answer 16

• Induction
  
  • slow
• Maintenance of anaesthesia
  
  • prolonged duration
  • very flexible
• Awakening
  
  • stop inhalational admin
  • washout
  • reversal of concentration gradient

Question 17

Role of general anaeasthetic inhalation agents

Answer 17

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

Question 18

Central effect of general anaesthetic on cardiovasclular system

Answer 18

• depress cardiovascular centre
• reduce sympathetic outflow
  
  • negative inotropic effect on heart
  • reduced vasoconstrictor tone → vasodilation

Question 19

Direct effect of general anaesthetic on cardiovasclular system

Answer 19

• negatively inotropic
• vasodilation → decreased peripheral resistance
• venodilation → decreased venous return, decreased cardiac output

Question 20

Effect of general anaesthetic on respiratory system

Answer 20

• 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

Question 21

Muscle relaxents role in anaesthetic triad

Answer 21

They relax (i.e. paralyse) skeletal muscle. They do this indiscriminately and unfortunately respiratory and airway muscles are affected as much as any other.

Question 22

Muscle relaxants indications

Answer 22

• Ventilation & Intubation
• when immobility is essential
  
  • microscopic surgery, neurosurgery
• body cavity surgery (access)

Question 23

Muscle relaxants problems

Answer 23

• awareness
• incomplete reversal → airway obstruction, ventilatory insufficiency in immediate post op period
• apnoea = dependence on airway & ventilatory support

Question 24

Analgesia role in triad

Answer 24

• 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

Question 25

What is the reason for intraoperative analgesia?

Answer 25

• Prevention of arousal
• Opiates contribute to hypnotic effect of GA
• Suppression of reflex responses to painful stimuli e.g. tachycardia , hypertension

Question 26

Mechanism of local anaesthetics

Answer 26

• 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!!)

Question 27

Local anaesthetic drugs used

Answer 27

Lignocaine, bupivacaine and ropivacaine

Question 28

Benefits of local and regional analgesia

Answer 28

• Retain awareness / consciousness
• Lack of global effects of GA
• Derangement of CVS physiology - proportional to size of anaesthetised area
• Relative sparing of respiratory function

Question 29

Local anaethetic toxicity depends on:

Answer 29

• dose used
• rate of absorption (site dependant)
• patient weight
• drug ( bupivacaine > lignocaine > prilocaine )

Question 30

Signs and symptoms of local anaesthetic toxicity

Answer 30

• Circumoral and lingual numbness and tingling
• Light-headedness
• Tinnitus, visual disturbances
• Muscular twitching
• Drowsiness
• Cardiovascular depression
• Convulsions
• Coma
• Cardiorespiratory arrest

Question 31

Effect of neuroaxial block on respiratory system

Answer 31

• 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