Anaesthesia Lecture Flashcards
Anaesthesia
is the reversible loss of response to noxious stimuli
Types of anaesthesia
LOCAL – numbs one small area of the body when
consciousness is maintained;
IV sedation – uses a mild sedative to relax you and pain
medicine to relieve pain. You may stay awake but may not remember the procedure;
Regional anaesthesia – blocks the pain in an area of the
body, e.g. epidural anaesthesia;
General anaesthesia – when anaesthesia is associated with loss of consciousness. It affects your whole body. You go to sleep and feel nothing. You have no memory of the procedure afterwards;
Local Anesthetic Mechanisms
Local anaesthetics act by stopping transmission of signals generated by nociceptors
Block Na+ channels: prevents axonal depolarisation and formation of action potential
Local Anesthesia Duration
Lidocaine is short acting with a rapid on-set
Mepivacaine is long acting with a rapid on-set
Duration of action can be extended by adding a vasoconstrictor like epinephrine
Topical
most are applied to mucous membranes but some
preparations will be absorbed through skin
0.5% Proxymetacaine and 0.4% Oxybuprocaine are
used for ocular anaesthetic
Lidocaine and benzocaine sprays are used to assist in
intubation (OTC in sore throat and dental pain)
Infiltration
Multiple intradermal or subcutaneous injections of local
anesthetic along proposed incision line
May contain epinephrine (1:200,000) to increase effect and duration
Regional blocks (nerve blocks)
Injection into the connective tissue surrounding a nerve
Can produce loss of sensation and/or paralysis in the region supplied by the nerve
Requires smaller volumes than field blocks, reducing the risk of toxicity
Epidurals
Administered alone or in combination with other analgesics
If combined, smaller doses can be used, decreasing risks of adverse effects
Can cause motor deficits at higher doses
General Anaesthesia causes
A variety of drugs are given to the patient that have different effects with the overall aim of ensuring
unconsciousness, amnesia and analgesia.
General anesthesia causes:
• loss of consciousness
• analgesia
• amnesia
• muscle relaxation (expressed in different extent)
• loss of homeostatic control of respiration and cardiovascular function
General Anaesthesia
Premedication:
–sedation-anxiolytics
»benzodiazepines (diazepam, temazepam, lorazepam)
–Analgesia
»opiates, NSAIDS
–suppress salivation & autonomic reflex (eg bradycardia)
»Cholinergic antagonists –hyoscine
–prophylactic antibiotics
–prophylactic anti-coagulants
–Stop oral hypoglycemics agents, MAOIs, warfarin,
Anaesthetics:
–intravenous – for induction and short procedure
–inhalational – for maintenance
Mechanical ventilation may be necessary
–neuromuscular blockade for tracheal intubation and to
facilitate abdominal surgery
Analgesia
Recovery
Anesthesia agents
- Inhalation anesthetics (volatile anesthetics) - gases : N2O, xenon - Fluids (vaporisers)
- Intravenous anesthetics - Barbiturans: thiopental
Others : propofol, etomidat - Pain killers - Opioids: fentanyl, sufentanil, alfentanil, remifentanil, morphine. NSAID: ketonal, paracetamol
- Relaxants - Depolarising : succinilcholine –
Non depolarising : atracurium, cisatracurium, vecuronium, rocuronium - Adiuvants -benzodiazepins: midasolam, diazepam
- Inhalational Anesthetics
Typically used to maintain unconsciousness
- Advantage of controlling the depth of anesthesia.
- Metabolism is very minimal.
- Excreted by exhalation
•Volatile liquids Desflurane Isoflurane Enflurane Halothane Methoxyflurane Sevoflurane halogenated
Diethyl ether (out of date)
•Gases: Nitrous oxide – old, weak, used as adjunct
Xenon – lately introduced
Mechanism of action of inhalational anaesthetics
• Not fully clear
• Cause generalized depression of CNS temporarily inhibit synaptic transmission a general loss of consciousness, lack of sensation, often muscle relaxation, loss of autonomic reflex
• Lipid solubility theory anaesthesia reflects change in neuronal cell lipid bilayer expands membrane and disrupts protein function
• Membrane receptors
Direct interaction with ion channel?
activation of inhibitory ion channels (eg GABA, Glycine)
inhibition of excitatory ion channels (eg 5HT3, NMDA)
General pharmacokinetics
Vapor pressure is expressed as a percentage of barometric pressure at sea level, i.e. 760 mmHg, to determine the maximum concentration, For example, halothane has a saturated vapor pressure of 244 mmHg at 20°C; therefore the maximum concentration of halothane that can be delivered at this temperature is 32% (244/760 × 100 = 32%).
The aim in using inhalation anesthetics is to achieve a partial pressure of anesthetic in the brain sufficient to depress CNS function and induce general anesthesia.
Thus, anesthetic depth is determined by the partial pressure of anesthetic in the brain.
To reach the brain, molecules of anesthetic gas or vapor must diffuse down a series of partial pressure gradients, from inspired air to alveolar air, from alveolar air to blood and from blood to brain:
Inspired air → Alveolar air → Blood → Brain
The Minimum Alveolar Concentration - MAC
• CNS partial pressure to monitor the level of anaesthetics;
• the minimum alveolar anesthetic concentration (% of the
inspired air) at which 50% of patients do not respond to a
surgical stimulus;
• MAC value is a measure of inhalational anesthetic potency; inversely related to potency, i.e. high MAC equals low potency; conceptually equivalent to an EC50; MAC values 1.1 to 1.2 used during surgery.
