Anaesthesia Flashcards
Local anaesthetics are…
Drugs that reversibly block the conduction of impulses in the PNS, inhibiting excitation-conduction process in neutrons near the site of administration
LA blocks the…
Initiation and propagation of AP by blocking the Na+ channels. The threshold potential is thus not reached, inhibiting depolarisation, and with no impulse conduction the nerve is blocked from signalling a pain response.
In general, LA blocks conduction in…
Small-diameter nerve fibres more readily than large fibres, preferentially affecting the pain fibres as these are thinner and more easily penetrated by drugs (motor fibres are thicker=more resistant)
LA agents include
-aines
LAs are potentially
toxic. Consider age, weight, physical condition and liver function for dosage
Examples of LA amides
Bupivacaine, Levobupivacaine, Lidocaine (anaesthesia), Prilocaine, Ropivacaine
Examples of LA esters
Cocaine and tetracaine (skin)
List the AEs of LA
Allergic reactions and vasoconstriction (these are rare)
List the application techniques of LA
Topical/surface, Infiltration, Bier’s block (regional IV), Nerve block, Epidural, Spinal
Topical or surface application
LA is applied directly to the target area of skin, nose, throat or urethra.
Infiltration is
Performed by injecting the diluted agent into the skin and subcutaneous tissue. Adrenaline is often added to intensify anaesthesia and prevent bleeding as a vasoconstrictor. It is used for minor skin surgery and dental extraction.
Bier’s block is
A short procedure of the upper limb where the 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). It requires full results monitoring. It is not recommended in children due to tourniquet discomfort.
Nerve block is
LA injected to a single nerve or a group of nerves. This is usually performed under ultrasound guidance.
Epidural
An epidural is an injection of LA into the epidural space. It is commonly used at the thoracic or lumbar region. Insertion of an epidural catheter is completed in a strict sterile procedure. LA is delivered through the catheter. The local anaesthetic in the epidural will slowly be absorbed into the subarachnoid space where it blocks the nerves of the spinal cord. This LA method is commonly used for obstetric, urology and abdominal surgery.
Complications include blockade of the sympathetic nerve fibres and an epidural haematoma.
• Placement confirmation: Resistance test with nervous system (NS) sensation test. The test dose and ultrasound.
Spinal is
LA injected into the CSF in the subarachnoid space. This must be performed under strict sterile conditions to avoid infection (e.g. meningitis). LA is injected in close proximity to its site of action, smaller volumes are required, and onset is rapid. There will only be one injection; usually in the lumbar region. The choice of LA is based on the length of the procedure. Common side effects include headache, hypotension and infection.
LA toxicity occurs as a result of
A therapeutic error
Clinical manifestations of LA toxicity
CNS: agitation, seizures, coma
CV: bradycardia, hypotension, atrial and ventricular dysrhythmias
Resp: resp depression and apnoea
Toxicity management for LA
Stop injection, call for help, DRSABCD, manage arrhythmia, provide CV support to suppress seizure, lipid emulsion (mechanism of action unclear but it’s supposed to remove the LA from the target tissue; may have inotropic effect)
General anaesthesia (GA) is
A pharmacologically induced reversible state of unconsciousness which is maintained despite the presence of noxious stimuli.
GA strives to achieve the 4 As which are:
Awareness (a lack of awareness); amnesia (lack of memory of the event); analgesia; akinesia (lack of overt movement)
GA consists of 4 phases:
Induction:(inducing unconsciousness) is performed through either intravenous (IV) injection or via inhalation (or a combination of both)
Maintenance: where there is a level of anaesthesia and homeostasis is achieved and maintained through the procedure
Emergence: the transition from an unconscious state to a conscious state
Recovery: fully conscious
Pre-medications are 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.
Pre-medications e.g.
Benzodiazepines (anxiolytics/hypnotics/sedatives), analgesics (narcotic/opioids or non-opioids), anticholinergics (to reduce salivation and control bradycardia (atropine)), antiemetics (to reduce nausea and vomiting) and antibiotics (ordered preoperatively to reduce the incidence of wound infection).
Induction agents produce
Unconsciousness, which is pleasant, rapid (seconds), and maintains haemodynamics. Includes some resp and circulatory depression
Induction agents e.g.
Thiopentone, propofol, ketamine, midazolam
Thiopentone (thiopental)
Enhances or mimics the action of GABA in the CNS and depresses the action of excitatory neurotransmission. Thiopentone is IV administered and has both respiratory and cardiac effects.
GABA is
Gamma-aminobutyric acid is the most widely distributed inhibitory neurotransmitter in the CNS. GABA is 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.
Thiopentone action:
Marked respiratory depression, which is dose-dependent, and can be used in conjunction with an inhalation agent. It decreases cardiac output (CO) blood pressure (BP) as the plasma concentration rises. It has a prolonged elimination and is lipid-soluble (fat distribution). It has no analgesic effect.
Propofol
Commonly used and works by activating a specific site within the GABA receptor. It shortens channel opening times at the neuronal nicotinic acetylcholine receptor (nAChR) and Na+ channels in the cortex. This causes a rapid induction; suitable for maintenance and sedation; minimal cardiovascular effects with no analgesic properties. The recovery from propofol is more complete, with less “hangover” when compared to thiopental, likely due to its high plasma clearance. It can cause pain at the injection site.
Ketamine is
N-methyl-D-aspartate (NMDA) receptor antagonist and interacts with muscarinic acetylcholine receptors (mAChR), voltage-gated Ca2+ channels and opioid receptors. Ketamine’s MOA is complex, but the major effect is likely due to reducing neuronal excitability by blocking NMDA (glutamate) receptors.
Ketamine is used in
Induction and maintenance and is a potent analgesic. Referred to as a “dissociative” anaesthetic as it is possible for the patient to remain conscious but with insensitivity to pain and short-term amnesia. It can produce surgical anaesthesia suitable for brief procedures on its own.
Ketamine does what to the CV?
It’s a CV system stimulant, causes hypertension, tachycardia, ECG changes
What can be experienced during emergence after ketamine was used?
Hallucinations and nightmares. Nystagmus is common.
Why is ketamine used in paediatric populations?
Because it maintains airway reflexes (only if aged >12)
Ketamine is restricted in adults because?
Incidence of hallucinations and dysphoria is much higher than in kids
Ketamine is suitable for what kind of procedures?
Short, painful ones (e.g. face lacerations, fracture reduction)
Precautions for ketamine
NBM prior with strict vitals and cardiac monitoring and resuscitation equipment nearby. Keep pt in a quiet area, dim lighting and don’t stimulate prematurely. Keep parents near kids to make them feel safe
Ketamine side effects
Hyper-salivation, emesis, transient laryngospasm, recovery agitation
Midazolam (benzodiazepine)
Potentiates the inhibitory effects of GABA in the CNS, resulting in sedative, hypnotic, anterograde amnesic and muscle relaxant effects. It is fast-acting and the patient is typically unconsciousness within 80 seconds. It can cause dangerous cardio-respiratory effects.
Volatile agents administration via
Inhalation (via an anaesthetic machine) to pt with a mask or tube by mixing with a carrier gas (air, oxygen)
Volatile anaesthetic is
Liquids vaporised to gases via a vaporiser that is inhaled into the lungs and crosses the alveolar-capillary membrane, into circulation, then the brain
