Anaesthetics - principles + pharmacology Flashcards
- Understand the mechanism, action and pharmacological kinetics of: local anaesthetic agents, general anaesthetic agents, opiates and muscle relaxants.
- Describe the “triad of anaesthesia” and discuss how this relates to the concept of balanced anaesthesia.
- Discuss the physiological effects of general and regional anaesthesia and how these may interact with patients’ underlying illness.
. Pharmacokinetics - ADME, how the body deals with the drug
Purpose of general anaesthetic
Produces insensibility in the whole body, usually causing unconsciousness
Purpose of regional anaesthetic
The agent is applied to the nerve and insensibility is produced in a distal area, remote from the injection, e.g. spinal or epidural block
Purpose of local anaesthetic
Agent applied directly to tissue and insensibility is produced in only that part
Types of anaesthetic drugs (5)
Inhalational anaesthetics, i.e. general Intravenous anaesthetics, i.e. general Muscle relaxants Local anaesthetics Analgesics, e.g. opiates
What is the triad of anaesthesia
-an anaesthetic may consist of varying contributions from all 3 but doesn’t need to have all 3
Analgesia
Hypnosis
Relaxation
Describe hypnosis
Unconsciousness.
Necessary component of any general anaesthetic
Describe analgesia in terms of anaesthesia
Removal of perception of unpleasant stimulus
Describe relaxation in terms of anaesthesia
Refers to skeletal muscle relaxation necessary to provide immobility for certain procedures
Which components of the triad of anaesthesia do the following have
- GA
- LA
- Opiates
- Muscle relaxants
GA - all 3
LA - analgesia + relaxation
Opiates - analgesia + hypnosis
Muscle relaxants - relaxation
What is balanced anaesthesia
Means that you can control the individual components of the triad and allow different drugs to do different jobs, e.g. can use less GA by adding a muscle relaxant
Essentially allows doses of individual drugs to be minimised
Problems with muscle relaxants
Means that artificial ventilation is needed to maintain the airway
Patients can be awake but paralysed and unable to communicate due to the separation of hypnosis (unconsciousness) from muscle relaxation
What is this called
Awareness
Mechanism of GA agents
-what ion channels are opened
Suppress neuronal activity by hyperpolarising neurones (so they’re less likely to fire) - hyperpolarised by opening chloride channels or suppressing excitatory synaptic activity
How do inhalational GA agents hyperpolarise neurons
-what ion channels opened
Dissolves in the membrane and causes the Cl channel to open
How do IV GA agents hyperpolarise neurones
-what ion channels opened
Allosteric binding to GABA receptors which stimulates Cl channels to open
Action of GA
- what is lost
- what is spared
Removes cerebral function
- complex processes lost first, primitive functions later
- remove consciousness
Reflexes are spared (e.g. spinal reflexes) and other automatic functions
Cautions during GA use (2)
Need to maintain airway
Control breathing
Name some IV anaesthetic agents
Propofol
Thiopentone
How do drugs like propofol cause rapid onset unconsciousness and rapid recovery
Fat soluble so cross BBB very quickly and cause unconsciousness as soon as it hits the brain
Also leave the circulation very quickly
Pharmacokinetics of IV anaesthetics
- absorption
- distribution
- metabolism
- excretion
Aborsbed into blood so blood conc. of the agent high initially then falls as it moves into highly perfused tissues
Distributed to organs, muscle and fat
Metabolised by liver
Excreted by kidney
Does muscle or fat absorb IV anaesthetics quicker
Muscle, but the effect is large because of huge mass of skeletal muscle
Fat can store large amounts of the agent due to to high fat solubility of the drug
Inhaled anaesthetics are what kind of compounds + name on
Halogenated hydrocarbons, e.g. sevoflurane
How do inhalational anaesthetics work once inhaled
The gas moves down the conc. gradient into the blood (from the lungs) then to the brain to acheive a high enough partial pressure to produce unconsciousness
Continuously breathed during the procedure
Inhalational drugs have a MAC (minimum alveolar conc.) which is a measure of
What would a drug with a low MAC mean?
the concentration of the drug required in the alveoli in order to produce any anaesthetic effect
A low MAC means the drug is potent as less of it is needed
Is induction of inhalational anaesthetics fast or slow
Slow
Main role of inhalational anaesthetics
To prolong or continue anaesthesia, i.e. MAINTAIN ANAESTHESIA
(usually to maintain IV induction)
How are inhalational anaesthetics reversed
Machine is switched off and patient is given a gas mixture with no anaesthetic agent in it to reverse the conc. gradient and decrease the alveolar conc. therefore decreasing the blood conc. of the drug and subsequently the brain
Most common sequence of GA uses what agents
IV induction followed by inhalational maintenance
Physiological effects of GA
on the CVS
-central (3)
-peripheral (3)
Central effects
- reduce sympathetic outflow activity
- reduce contractility (negative inotropic effect)
- vasodilation
Peripheral
- reduce contractility of vessels (negative inotropic)
- vasodilation leading to reduced peripheral resistance
- venodilation –> decreased VR –> decreased CO
Physiological effects of GA on resp function (4)
Reduce hypoxic and hypercarbic drive
Reduce tidal volume so increase RR
Paralyse cilia
Decrease FRC (i.e. decrease lung volumes so interferes with V/Q matching)
GA agents decrease lung volumes so interfere with V/Q matching in the lungs and this effect persists post-op, so patients should be given what post-op
Oxygen (probably several days)
Action of muscle relaxants
Paralyse skeletal muscle in an indiscriminate way, therefore airway muscles are affected too even though you don’t want to
Indications for muscle relaxant use (3)
Ventilation & intubation
When immobility is essential
(e.g. microscopic surgery, neurosurgery)
Body cavity surgery
Disadvantages of muscle relaxants (3)
Loss of awareness - awake but paralysed
Incomplete reversal - airway obstruction/resp insufficiency persists in immediate post-op period (i.e. at the end of the anaesthetic) but unlikely to persist when they’re back on the ward
Apnoea - hence the need for airway + ventilation support during op
If the muscle relaxant is given systemically (orally) then also must provide which triad of anaesthesia
Hypnosis (unconsciousness)
Analgesia is most commonly used in what way in terms of anaesthetising?
In conjunction with unconsciousness (i.e. GA) as part of ‘balanced anaesthesia’
Does analgesia always have to be accompanied by GA ?
No, can use it as regional anaesthesia where it’s used itself, e.g. spinal or epidural analgesia
What’s the point of intra-operative analgesia for someone who’s unconscious?
+ benefit of opiate analgesics
Prevents arousal from the pain
Opiates have direct sedative effect, contributing to GA
Suppress reflex responses to pain, e.g. tachycardia, hypertension
Benefits of local + regional anaesthesia (4)
Retain awareness
Complete analgesia with no hypnotic effect
Can allow lower levels of GA to be used if GA is needed
Less physiological effects than GA, mainly effects on CVS
Mechanism of local anaesthetic
Block Na+ channels so prevent AP from propagating
Disadvantages of local anaesthetic
Lots of side effects and toxic if delivered wrongly
Local + regional anaesthesia mainly have effects on which organ system
CVS, resp function spared
