Anaesthetics - Principles of Pharmacology Flashcards
Anaesthesia:
- Largest Single Hospital Specialty
- 78 Consultants in NHSG
- 125+ including trainees
There are multiple sub specialities which may include what?
Paediatrics
Cardiac
Maternity
Trauma
Crit care
Pre-hospital Care
Chronic Pain
Acute Pain
Vascular
Neuro
Hepato-Biliary
what does anaethesia literally mean?
without feeling/perception
what is general anaesthesia?
- produces insensibility in the whole body, usually causing unconsciousness
- Centrally acting drugs – hypnotics/analgesics
what is regional anaesthesia?
- producing insensibility in an area or region of the body
- Local anaesthetics applied to nerves supplying relevant area
Essence is that the anaesthetic agent is applied to the nerve anywhere from the spinal cord to the periphery and anaesthesia produced in a distal site served by that nerve, therefore effect is remote from the injection
what is local anaesthesia?
- producing insensibility in only the relevant part of the body
- Local anaesthetics applied directly to the tissues
Local anaesthetic is injected directly into the tissues to be anaesthetised. Unlike regional anaesthesia, anaesthesia is produced at the site of injection
what were the problems in ancient surgery? and what were their solutions?
in the early years how was anaesthesia given?
- General anaesthesia delivered as a monotherapy - Single agent e.g. chloroform, ether
- Toxic effects - cardiac depression, respiratory depression
To achieve the depths of anaesthesia needed to provide surgical operating conditions, e.g. muscle relaxation the patient had to be given very high concentrations of that agent. The muscle relaxation needed for abdominal surgery for instance was essentially a side effect of very deep anaesthesia. This made other, toxic, side effects such as cardiac dysrhythmia very common and frequently fatal
what is shown here?
Schimmelbusch Mask
very simple method for administering ether - Metal frame over which Gauze is placed
Ether or chloroform was dripped on and the mask with soaked gauze held over patients face and inhaled air – no added oxygen – was drawn through the gauze picking up the anaesthetic agent as it went
The anaesthetist would stand and slowly drip the agent onto the mask with only very crude control over the amount being inhaled by the patient
what durgs and techniques are now used in the modern era?
Huge progress in areas of equipment, monitoring and techniques available. Great advances in safety. Modern anaesthesia v safe
this is a modern anaesthetic machine
what are its functions?
- Regulation of fresh gases and mixing to deliver precise concentrations of gaseous agents
- Addition of precise concentrations of inhaled anaesthetic gases
- CO2 removal to allow recirculation of inhaled gases
- Mechanical ventilation, now microprocessor controlled contained within machine
- Most monitoring now normally integrated into anaesthetic machine
how safe is anaesthesia now a days?
very safe
- Overall operative mortality approx 1:25 (4%)
- Anaesthetic mortality 1:400,000 (0.00024%)
- ASA system: Mortality concentrated in ASA groups 3-5
what is the triad of anaesthesia?
- Hypnosis – unconsciousness. Necessary component of any general anaesthetic
- Analgesia – Pain relief, can also be taken in this context to mean “removal of perception of unpleasant stimulus” since not all unpleasant stimuli patients need protected from are necessarily painful. E.g. handling of gut. If patient is unconscious and therefore unaware of pain, analgesia is usually still required to suppress reflex autonomic responses to painful stimulus
- 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
Each drug, or type of drug may do more than one job
Balanced anaesthesia:
- Different drugs to do _______ jobs
- _______ doses separately & therefore more accurately to requirements
- Avoid ___________
- Enormous ________
This is the concept introduced on the previous slide of using different drugs to do different ____. Big advantage of balanced anaesthesia is that it allows a great degree of control over the individual components of the _____
different
Titrate
over-dosage
flexibility
jobs
triad
what is the problems that may be faced with balanced anaesthesia
- Polypharmacy - Inc chance of drug reactions/allergies
- Muscle relaxation - requirement for artificial ventilation, means of airway control
- Separation of relaxation & hypnosis - awareness:
The possibility of having patients awake yet paralysed and unable to communicate (Termed awareness) also now exists as it is possible to have a patient paralysed with muscle relaxant yet insufficiently anaesthetised. This is due to the separation of hypnosis from muscle relaxation making the latter possible without the former
whta are the 2 main groups of genereal anaesthetics?
IV
inhalation
General anaesthetic agents, inhaled and intravenous, provide _____________ as well as a small degree of ______ _______. They may to differing extents also provide some analgesia. But for all except Ketamine, analgesia is negligible
They are all potent drugs and the main thing which separates a general anaesthetic drug from a sedative drug really is potency
unconsciousness
muscle relaxation
How do general anaesthetic agents work?
All general anaesthetic agents 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
This is largely done by opening chloride channels which hyperpolarise the neurons, suppressing excitatory synaptic activity. The common mechanism then is that neurons become (reversibly!) hyperpolarized and therefore less able or likely to reach their threshold potential and fire, sending signals (as propogated action potentials) to other neurons
in general anaesthesia, what processes are lost first and last?
- Cerebral function “lost from top down”
- Most complex processes interrupted first
- LOC early - hearing later
- More primitive functions lost later
- Reflexes relatively spared - Primitive - Small number of synapses
Not surprisingly the most complex processes which rely on the greatest and most ________ neuronal activity (i.e. consciousness) are the most susceptible to inhibition in this way and therefore cerebral function is lost “from the top down” with relative sparing of the simpler more primitive functions including ________ (i.e. spinal reflexes) and other automatic functions
complex
reflexes
intravenous anaesthesia:
This rapid fall in blood concentration is due mainly to what?
the drug leaving the circulation and moving to other parts of the body
Muscle picks up the drug more slowly but the effect is large because of the relative high mass of skeletal muscle in the body. Fatty tissue picks up drug even more slowly but given lengthy enough exposure can store large amounts due to the high fat solubility of these drugs.
whta is a Target Controlled Infusion (TCI) pump system?
Allows very accurate infusion to achieve specific blood or brain concentrations of agents using complex pharmacokinetic algorithms
Big problem with Total Intravenous anaesthesia (TIVA) is that we currently can’t measure the drug concentration in real time
Therefore we use computers to calculate what is in essence, a real time guess
The calculations are based on mathematical models which mimic the movement of anaesthetic drug into different tissues as described on the previous slide
another strength of inhaled agents is our ability to measure them
how is it done?
measure CO2 in expelled gas and know when end of experation is and measure the concentration of anaesthetic in same gas flow
This represents Alveolar gas and gives info regarding alveolar gas concentrations
give apox value of arterial concentration
no direct way of measuring IV anaesthetic conecetraiton
what ist he sequence of GA?
The most common sequence of general anaesthesia is Intravenous induction followed by inhalational maintenance
More modern agents with more sophisticated infusion techniques (computer controlled infusions) allow use of intravenous maintenance which has claimed advantages of better recovery
All general anaesthetic agents IV or inhalational have adverse effects on the cardiovascular system and are almost universally depressant (Ketamine is the exception)
physiology of GA: how does GA affect the CVS centrally?
Central effects arise due to the depressant effects of the agent on the CNS and, more specifically, the cardiovascular centres and nuclei in the brainstem
depress cardiovascular centre:
- reduce sympathetic outflow
- negative inotropic/chronotropic effect on heart
- reduced vasoconstrictor tone → vasodilation
physiology of GA: how does GA affect the CVS directly?
direct effects of anaesthetic agents on vascular smooth muscle and myocardium which compound the effects of the reduced sympathetic activity
- negatively inotropic
- vasodilation → decreased peripheral resistance
- venodilation = decreased venous return, decreased cardiac output
what are muscle relaxants?
only effect one limb of the triad of anaesthesia
interfering wIth neuromuscular junction in skeletal muscle, dont effect cardiac or smooth muscle, only the somatic skeletal msucle effected
If muscle relaxation is provided by systemic muscle relaxant drugs then unconsciousness must be provided. Being paralysed and awake is extremely unpleasant.
These drugs do exactly what the name says. They relax (i.e. paralyse) skeletal muscle. They do this indiscriminately and unfortunately respiratory and airway muscles are affected as much as any other
what can be used to cause analgesia?
what are opiates used for and what is their role?
main role is anagesia and they supplement hypnosis
what do local anaesthetics do?
lots of analgaesia and little muscle relaxant as blocks both motor and sensory nerves
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 (i.e. intravenously!!)
Ultrasound Guided Regional Anaesthesia:
US to guide needle placement for regional anaesthesia
Ultrasound allows safer and more effective delivery of the local anaesthetic (LA) drug with less likelihood of the LA going intravenously or direct nerve or vascular injury which can happen with blind, anatomical landmark based, needle positioning
slide shows a needle approaching the brachial plexus in the interscalene groove. Yellow line highlights needle which is coming in from the right
Nerves are the dark bundles three in a row one above another
USS with a needle in plane approach to the femoral nerve – again, yellow line highlights needle, approaching from the left this time. Nerve is bright oval with two dark dots just below needle tip. Dark area about needle tip is injected local anaesthetic. Big dark circle to right is the femoral artery. Approach would probably be to inject half of the local anaesthetic here and reposition needle below the nerve to surround it with drug and ensure effectiveness of the block. The Artery is clearly seen and can be avoided. Safety feature is the ability to see the injectate in the tissues therefore we know the injection is not intravascular
