Principles of pharmacology Flashcards
What are the main types of anaesthesia?
General
Regional
Local
What is general anaesthesia?
General anaesthesia produced insensibility in the whole body, usually causing unconsciousness
This is achieved using centrally acting drugs (GA agents)
What is the difference between local and regional anaesthetics
Regional anaesthetics:
- produces insensibility in an area or region of the body
- achieved by Local anaesthetics applied to nerves supplying relevant area
Local anaesthetics:
- producing insensibility in only the relevant part of the body
- achieved by applying local anaethetic directly to tissues
On this lovely photo of an anaesthetic machine - identify what the labelled bits are
What is the triad of anaesthesia?
Analgesia - (pain relief)
Hypnosis - (sleepy sleepy time)
Muscle relaxation - (of skeletal muscle)
Drugs used in anaesthetics often have effects on multiple parts of the triad of anaesthesia.
What drugs produce hypnosis?
General anaesthetic agents
Opiates do a wee bit too
What drugs can produce analgesia?
Opiates
Local anaesthetics
Certain General anaesthetic agents (special K) do a wee bit too
What drugs produce muscle relaxaton?
Muscle relaxants
A wee bit - GA & LA
What is balanced anaesthesia?
Using a mixture of different anaesthetic agents to do different jobs on the triad of anaesthesia
This way the mixture and so the effects produced can be tailored to the patient and procedure.
It avoids over dosage and gives enormous flexibility
What problems are associated with balanced anaesthesia?
Polypharmacy:
- The use of lots of drugs in combo means theres higher risk of allergy or adverse reaction
Seperation of relaxation & hypnosis:
- ‘Awareness’ happens when a patient in surgery is conscious but paralysed
Muscle relaxtion:
- requirements for artificial ventilation
How can General anaesthetic agents be administered?
IV
Inhaled
What are the effects of general anaesthetic agents on the triad?
All produce Hypnosis
They provide a little bit of relaxation
For all exept Ket - analgesia is negligible
How do general anaesthetic agents work?
What is the difference between how IV and inhaled agents work?
Interfere with neuronal ion channels - causing them to be hyper-polarised
Inhaled agents - dissolve in membranes and have a direct physical effect
IV agents - allosteric binding to GABA receptors - which opens chloride channels
When GA is administered - in what order are cerebral functions lost?
Cerebral function lost from top down
The most complex cerebral functions are the first to go,
More primitive functions are lost later,
Primitive reflexes are relatively spared
How quickly do IV General anaesthetic agents work?
Very 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 basement membranes (incl. the Blood-brain barrier) heavy fast - thus allowing them to act on neural tissues quickly
How quickly do IV GA agents leave the circulation?
These are removed very quickly from the circulation but not because they are metabolised quickly…
The rapid fall of [GA agent] in blood is mainly because of it leaving the circulation and moving into other parts of the body
Whats this?
Target Controlled Infusion (TCI) pump system
Allows very accurate infusion to achieve specific blood or brain concentrations of agents using complex pharmacokinetic algorithms
What types of molecule are Inhaled agents?
How are they excreted?
Inhalational anaesthetic agents are all halogenated hydrocarbons and are taken up and almost exclusively excreted via the lungs
With inhaled anaesthetic agents - what is the significance of the MAC value?
MAC (minimum alveolar conc.) is the concept of the concentration of the drug required in the alveoli which is required to produce anaesthesia with any particular agent.
Therefore a low MAC value means an agent is potent. E.g. Halothane (MAC = 0.8%) is more potent than Desflurane (MAC = 6%) because it takes a lesser concentration of the agent to produce the same effect.
As a general - what types of GA agents are used for:
- Induction?
- Maintenance?
Induction = IV*
Maintenance = Inhaled
However, there is an increasing use of IV agents for maintenance - such as IV propofol, opiates (remifentanil)
*Inhaled for children
How does General anasethesia affect the Cardiovascular system?
Central effects:
-
Depression of cardiovascular centre
- this reduces sympathetic outflow which:
- has negative inotropic/chronotropic effect on the heart
- reduces vasoconstrictor tone –> vasodilation
- this reduces sympathetic outflow which:
Direct effects on myocardium and vascular smooth muscle:
- negatively inotropic (reduced strength of heart contraction)
- vasodilation (reduced PVR)
- venodilation (decreased venous return & CO)
All of this contributes to decreased MAP (MAP = CO x SVR)
WHat are the effects of general anaesthesia on the respiratory system?
All anaesthetic agents are respiratory depressants:
- reduced hypoxic and hypercarbic drive
- decreased tidal volume and increased rate
Paralyse cilia
Decrease FRC:
- lower lung volumes
- V/Q mismatch
What are the indications for usage of muscle relaxants?
Ventilation & intubation
When immobility is essential
Body cavity surgery (for access)
What are the problems with muscle relaxant drugs?
Awareness - if not enough hypnosis
Incomplete reversal of effects post op
Apnoea - dependence on airway & ventilatory support
Why is intraoperative analgesia needed? (ie if the person is asleep why do they need analgesia)
Prevention of arousal
Opiates contribute to hypnotic effect of GA
Suppression of reflex response to painful stimuli:
- tachycardia, hypertension etc
Opiates are an important part of almost every general anaesthetic. They are strong analgesics but also contribute (supplement) to hypnosis
What opiates are used in general anaesthesia?
What are their benefits?
Fentanyl:
- short acting & potent
- only really used for intra-operative analgesia
Morphine, oxycodone:
- used for intro-operative analgesia which we want to continue into the post-operative period
Remifentanil:
- V short acting, V potent
- Has to be given IV
How do local anaesthetic drugs work?
Local anaesthetic drugs, lignocaine, bupivacaine and ropivacaine produce analgesia with no hypnosis
Work by blocking Na+ channels and preventing axonal action potential from propagating
What are the effects of local/regional anaesthetics on the cardiovascular and respiratory systems?
Affects CVS physiology - proportional to size of anaesthetised area
Respiratory function relatively spared
