Anaesthesia and Pain Flashcards
What is Anaesthesiology?
Branch of medicine concerned with the administration of medications or anaesthetics agents to relieve pain and support physiological functions during a surgical procedure
What is Analgesia?
Suppression of physiological responses to stimuli (usually pain) through hypnosis (suppression of consciousness) and narcosis (dulling of consciousness and paralysis - no movement)
What is Anaesthesia?
Practice of administering medications by injection or by inhalation that blocks the feeling pain and other sensations or that produces a deep state of unconsciousness that eliminates all pain and other sensations - allows medical and surgical procedures without distress or discomfort
What is Local Anaesthesia (LA)? How does it work?
Administrations of an agent to induce the absence of pain in a part of the body
• Reversibly block the conduction of impulses in the PNS - inhibits excitation-conduction process = blocks the initiation and propagation of action potential = blocks Na+ channels = threshold potential not reached = inhibits depolarisation = no impulse conduction = nerve blocks from signalling a pain response
• Blocks conduction in small-diameter nerve fibres more readily than large fibres
• Pain fibres are affected preferentially because they are thinner and more easily penetrated by these drugs (Motor axons - larger in diameter = relatively resistant)
• Consciousness is not depressed
• Can be injected around a nerve or nerve trunk (e.g. nerve block) to produce a regional anaesthesia (RA) to a large region of the body
Selectivity of LA agents are they only effect neurons located near the site of administration and they suppress pain without generalised depression of the nervous system
What are the types of LA? Name some. What are the adverse effects?
Amides (-AINES): • Bupivacaine • Levobupivacaine • Lidocaine/ lignocaine (anaesthesia) • Prilocaine • Ropivacaine
Esters:
• Cocaine
• Tetracaine (skin)
Adverse Effects: If administered correctly, adverse effects are rare and include allergic reactions; adverse effects from added vasoconstrictors also occur.
What are the techniques of LA? (6) Give a brief summary of each.
Topical/ surface - applied directly to the target area of skin, nose, throat or urethra
Infiltration - injecting the diluted agent into the skin and subcutaneous tissue (Adrenaline is often added to intensify anaesthesia and prevent bleeding as a vasoconstrictor) - minor skin surgery and dental extraction
Bier’s Block (regional IV) - Upper limb where circulation is blocked by tourniquet (pressure above patient systolic BP) and local anaesthetics are injected into venous vessels distal to the occlusion - Early release of the tourniquet may cause toxicity (inflate for at least 20 mins) - requires full results monitoring *Not recommended in children due to tourniquet discomfort
Nerve Block - LA is injected to a single nerve or a group of nerves - usually performed under US guidance
Epidural - • injection of LA into the epidural space
•Commonly used at the thoracic or lumbar region
• Insertion of an epidural catheter is completed in a strict sterile procedure and LA delivered through the catheter
• Slowly be absorbed into the subarachnoid space - blocks the nerves of the spinal cord
• Commonly used for obstetric, urology and abdominal surgery
Complications: blockade of sympathetic nerve fibres and an epidural haematoma
• No CSF
Placement confirmation: Resistance test with nervous system (NS) sensation test. The test dose and ultrasound
Spinal - • injected into the CSF in the subarachnoid space
• Injected in close proximity to its site of action, smaller volumes are required, and onset is rapid
• Performed under strict sterile conditions to avoid infection (e.g. meningitis)
• Only 1 injection; usually in the lumbar region
• Choice of LA is based on the length of the procedure
Common side effects include headache, hypotension and infection
What is needed to be taken into account to avoid LA toxicity? How can it occur? What are the clinical manifestations? What is the management of LA toxicity?
LAs are potentially toxic drugs: patient’s age, weight, physical condition and liver function must be taken into account in determining the drug dosage
* Generally occurs as a result of a therapeutic error: too high a dose of ingested or topically administered local anaesthetic * Inadvertent venous or arterial injection
Clinical Manifestations:
• CNS: agitation, seizures and coma
• CV: bradycardia, hypotension, atrial and ventricular dysrhythmias
• Respiratory: respiratory depression and apnoea
Toxicity management for LA: stopping injection; call for help; DRSABCD; manage any arrhythmias and provide CV support to suppress seizure
• Administer lipid rescue (20%) - mechanism of action of lipid emulsion for the treatment of LA toxicity is unclear. A “lipid sink” is where the lipid binds the LA to remove it from the target tissue - may have inotropic effect
What is general anaesthetic (GA)
Induces reversible state of unconsciousness - maintained despite presence of noxious stimuli
• May be used alone or in combination with LA or RA
**An epidural and GA may be used for major abdominal surgery - allows for the continued use of the epidural for postoperative pain management
What are the 4 A’s that GA strives to achieve?
Awareness (a lack of awareness)
Amnesia (lack of memory of the event)
Analgesia
Akinesia (lack of overt movement)
What are the 4 phases of GA?
- Induction (induce unconsciousness): performed through either IV or inhalation (or combination of both)
- Maintenance: level of anaesthesia and homeostasis is achieved and maintained through the procedure
- Emergence: transition from an unconscious state to a conscious state
- Recovery
What are pre-medications used for?
administered to reduce patient anxiety, relieve pain, produce sedation and amnesia (to aid a smoother induction of anaesthesia). They also reduce salivary and bronchial secretions:
• Benzodiazepines (anxiolytics/hypnotics/sedatives)
• Analgesics (narcotic/opioids or non-opioids)
• Anticholinergics (reduce salivation and control bradycardia (atropine))
• Antiemetics (reduce nausea and vomiting)
• Antibiotics (ordered preoperatively to reduce the incidence of wound infection)
What are the 2 types of inducing agents?
Intravenous: produce unconsciousness - pleasant, rapid and maintains haemodynamic - Induce transition from awareness to an anaesthetised state within seconds (include some respiratory and circulatory depression)
- Thiopentone (thiopental)
- Propofol
- Ketamine
- Midazolam/ benzodiazepine
Inhalation: Volatile Agents
How does Thiopentone work?
Enhances or mimics the action of GABA in the CNS and depresses the action of excitatory neurotransmission (IV administered); Respiratory and cardiac effects:
○ Respiratory depression (dose-dependent) can be used in conjunction with an inhalation agent.
○ Decreases CO and BP as plasma concentration rises
○ Prolonged elimination and is lipid-soluble (fat distribution)
○ No analgesic effect
**Gamma-aminobutyric acid (GABA): most widely distributed inhibitory neurotransmitter in the CNS; naturally synthesised in presynaptic neurons, stored in vesicles. Upon neuronal activation, GABA is released from vesicles into the synapse and acts on postsynaptic GABA receptors
How does propofol work?
commonly used and works by activating a specific site within the GABA receptor
○ Shortens channel opening times at the neuronal nicotinic acetylcholine receptor (nAChR) and Na+ channels in the cortex = rapid induction = suitable for maintenance and sedation
○ Minimal cardiovascular effects with no analgesic properties
○ Recovery more complete, with less “hangover” (cf to thiopental) - likely due to high plasma clearance
Can cause pain at the injection site
How does Ketamine work?
N-methyl-D-aspartate (NMDA) receptor antagonist and interacts with mAChR, voltage-gated Ca2+ channels and opioid receptors
○ Complex MOA; major effect likely due to reducing neuronal excitability by blocking NMDA (glutamate) receptors
○ Used in induction and maintenance and is a potent analgesic
○ “dissociative” anaesthetic - patient remain conscious but with insensitivity to pain and short-term amnesia
○ Can produce surgical anaesthesia suitable for brief procedures on its own
○ CV system stimulant (HTN, tachycardia, ECG changes) and hallucinations and nightmares may be experienced during emergence ○ Used in paediatric population as airway reflexes are maintained ○ High incident of hallucinations and dysphoria which restricts its use in adults, but a lower incidence in children makes it suitable for minor paediatric surgery. Nystagmus is common ○ Indications for use in children is that they must be aged >12 months as there is an increased risk of airway complications in children aged <12 months, especially <3 months ○ Suitable for short, painful procedures (e.g. face lacerations, fracture reduction) ○ Pt has to remain nil by mouth prior with strict vitals and cardiac monitoring and resuscitation equipment nearby ○ Nurse in a quiet area with minimal noise, dim lighting and do not stimulate prematurely. Parents should be next to the child so they feel safe. Side effects include hypersalivation, transient laryngospasm, emesis and recovery agitation
