PIL B Flashcards
Objectives of anaesthesia
Prevent pain during the procedure (analgesia). Prevent distress during restraint (loss of consciousness). Provide immobility (muscle relaxation).
How is fear minimised during the anaesthetic process?
Careful handling, use of premedication; avoid face mask induction; careful selection of the method of anaesthesia
Requirements for successful anaesthesia
Careful selection of the method of anaesthesia.
Good pre/intra/post-operative care.
Selection of the method of anaesthesia
Animal species. Duration of procedure. Depth of anaesthesia. Purpose of the investigation. Equipment available. Experience of the staff.
Inhalational anaesthesics
Stressful induction, requires specialist equipment, risk to operators.
Good control during procedure, good recovery (often faster than injectable), can combine with local anaesthetic.
Apparatus: Anaesthetic, chamber, anaesthetic machine, vaporiser, anaesthetic circuit, endotracheal tubes.
Aim to fill chamber in less than a minute (using flow rate and volume of chamber).
Following induction of anaesthesia animal can be removed from chamber and brief (<30secs) procedures carried out. Usually more convenient to maintain anaesthesia by placing the animal on a face mask.
Examples of inhalational anaesthetics (6)
METHOXYFLURANE: non-irritant / non-flammable, use in neonatal, potent analgesia, nephrotoxic if prolonged use, slow induction.
HALOTHANE: easily vaporised, potent, mild hypotension, induces liver microsomal enzymes, malignant hyperthermia.
ISOFLURANE: very rapid induction and recovery, very little biotransformation. Excretion via lungs. Advantage = undergoes almost no bio-transformation and is almost completely eliminated in exhaled air -> little effect on liver microsomal enzymes -> minimal interference in drug metabolism or toxicology studies.
SEVOFLURANE – rapid induction & recovery, less pungent, depth of anaesthesia altered easily.
NITROUS OXIDE: minimal CVS and Respiratory depression, very low anaesthetic potency, decreases concentration of other agents, use 50/50, 60/40 ratio to oxygen, flush with oxygen at end of anaesthesia.
ETHER: inflammable and irritant, not really used
Injectable anaesthetics
I/V, I/M or I/P; delayed absorption, can combine different ones, some are reversible, weigh the animal, bolus injection, no specialist equipment required, requires careful dose calculation, lack of fine control of depth. Can anaesthetize multiple animals at the same time. Important to be able to deliver O2 as many can cause resp dep. Greater individual variability in response to a given dose.
Important to check that the dose used is appropriate for the particular strain, age and sex of animal used, by anaesthetising only one or two animals. The dose can then be adjusted before anaesthetising larger numbers.
Examples of injectable anaesthetics
BARBITURATES:- Pentobarbitone - severe CVS / Respiratory depression, poor analgesia, long recovery, hypnosis, I/v, I/p. Thiopentone- poor analgesia, short acting, I/v, induction agent.
DISSOCIATIVE:- Ketamine - cataleptic state, I/m, I/p, I/v, most often combined with diazepam or meditomidine, minimal CVS effects, raises intracranial pressure, atropine counteracts salivation (sheep, primates, cats, dogs, rabbits, rodents) .
STEROID:- Alfaxan (Alfaxalone) subcutaneous, intramuscular, intravenous. Many species, including primates. Continuous infusion. (saffan).
NEUROLEPTANALGESIC:- Hypnorm (fentanyl/fluanisone) - profound analgesia, moderate respiratory depression, poor relaxation. Add midazolam for surgical anaesthesia. Reverse with buprenorphine or naloxone.
Propofol:- rapid induction, short acting, i/v, little effect on renal/hepatic function.
Chloral Hydrate:- Rats, 1-2hrs light anaesthesia, minimal CVS effects, very poor analgesia, post anaesthetic ileus (with 10% conc).
Tribromoethanol:- Rats and Mice, I/p, irritant.
Chloralose:- Long acting, 8-10hrs, light anaesthesia, minimal CVS/Respiratory effects, poor analgesia.
Urethane:- Long acting, 6-10hrs, minimal CVS/Respiratory effects, sufficient analgesia, NON-RECOVERY, CARCINOGENIC.
Producing all the components of anaesthesia with a single injectable agent…
…usually requires a high dose to be given, and this can cause severe depression of vital body systems. Two or more agents, given in combination, at relatively low dose rated can produce anaesthesia without such severe side effects.
Injectable anaesthetic choice for rodents
Fentanyl / Fluanisone + Midazoloam. (reverse with Buprenorphine).
Ketamine + Midazolam or Diazepam Ketamine + Medetomidine. (reverse with Atipamizole).
All the above giving about 30 to 35 mins anaesthesia, and a 2-3 hour recovery.
Pre-op care
Clinical examination, acclimatize, measure food and water intake, weigh animal, no need to fast, premedicants.
Assessxent of depth of anaesthesia: withdrawal reflex- tail pinch response lost at medium planes, pedal withdrawal response lost at medium-deep planes; pattern and depth of breathing, CVS; eye position- in rodents, position of eye remains fixed, blink reflex may still be present at surgical planes of anaesthesia
Intra-op care
Body temp: insulate, provide additional heat (heating pad), avoid excess skin preps, minimal hair clipping, small rodent loses 5-10 degrees in 15min of anaesthesia.
Respiratory support: monitor resp, IMP respiratory monitor (<50% = concerning), maintain an airway (position/intubation), provide O2.
CVS support: mucous membrane colour, pulse oximeter (measures adequacy of oxygenation), CRT (certified respiratory therapist), pulse quality, ECG, avoid blood loss, infuse fluids (maintenance requirement is 40-80ml/kg/day).
Blood volume: ~70ml/kg body weight – rat (200g) = 14 ml; mouse (30g) = 2.1ml.
Protect eyes: blink reflex lost -> ophthalmic ointment can be placed in eyes, or lids can be taped closed with micropore tape during longer (>15min) periods of anaesthesia & whenever using injectable (recovery 30-60min).
Post-op care
Warmth (30-35oC until regained consciousness, then 25-30 for small rodents, 25 for larger animals, or 35 for neonatal animals) and comfort, respiratory support (place animal on chest or side), fluid therapy, analgesia, avoid sawdust or wood shavings bedding (use towel bed). Pain assessment (cont.): vocalisation, stature, behaviour, reduced food intake, avoidance, variance w/ species Pain relief: analgesics.
Post-op analgesics
Opioids (moderate-severe pain): Short duration, controlled drugs; SEs (rare): respiratory depression, raised intracranial pressure, hypotension, constipation; Eg Buprenorphine Butorphanol Morphine Pethidine Fentanyl Nalbuphine Pentazocine.
NSAIDs (mild-moderate pain: Long duration, injectable and oral preparations; SEs: GIT, platelet interference, renal, cranial injury; Eg Carprofen, Meloxicam, Flunixin.
Local anaesthetics: Inhibition of nerve transmission, can be combined with sedative or volatile agent, spinal/regional; Eg topical cream EMLA (eg for venipuncture), lidocaine (block nerve trunks ->larger areas)
