ANSWERS

1. Define local anaesthesia.
   Local anaesthesia is defined as the reversible blocking of nerve conduction resulting in analgesia of an area of the anatomy; local anaesthetics may be injected or applied topically.
2. Lignocaine is a commonly used local anaesthetic; list 6 advantageous properties of this drug.
   • Good surface analgesia.
   • Rapid onset of action.
   • Long acting (1-1 and a half hours).
   • Effectively spread throughout the body tissue.
   • Low toxicity.
   • Antiarrythmic.
3. Describe the signs of local anaesthetic overdose in the order in which they would be seen.
   • Sedative effect.
   • Twitching.
   • Convulsions.
   • Coma.
   • Death.
4. Give 2 ways in which local anaesthetic overdose may be avoided.
   • Use the minimum dose required to produce an effect.
   • Use regional techniques where possible rather than local infiltration.
5. Why is adrenaline commonly incorporated into local anaesthetic solutions such as Lignocaine?
   Local anaesthetics such as Lignocaine cause vasodilation and the subsequent increased blood supply limits analgesic action and increases systemic toxicity. Adrenaline is a vasoconstrictor and opposes this effect.
6. Why might the hair of a patient change colour at the site of Lignocaine administration, and what other more serious complication may arise?
   The vasoconstriction caused by adrenaline may lead to ischaemia (deficiency in the blood supply to a part of the body) which can cause the hair to change colour following subcutaneous or intradermal administration. A more serious complication is necrosis of an area with a limited blood supply such as the teats of the mammary glands.
7. List 6 preparations of Lignocaine used in veterinary practice.
   • Vial for injection.
   • Cartridges - used in dental syringes
   • Creams and ointments.
   • Lubricant gel.
   • Aerosol spray.
   • Ophthalmic preparations.
8. What is the active ingredient in Ophthaine, a popular ophthalmic preparation?
   Proparacaine.
9. List the 4 ways in which local anaesthetics are used to produce analgesia and briefly describe each technique.
   • Surface local anaesthesia - topical applications used in analgesia of the mucous membranes such as those of the cornea, urethra and larynx.
   • Local infiltration - repeated injections of local anaesthetic to a surgical site; a field block avoids interference by adrenaline of wound healing.
   • Regional anaesthesia - 3 types:

     i) Conduction block - sterile local anaesthetic is injected into nerves rather than nerve endings; the intercostal nerves may be blocked in this way to provide analgesia following a thoracotomy operation

     ii) Spinal analgesia - local anaesthetic is injected into the cerebrospinal fluid filled space below the arachnoid mater

     iii) Epidural analgesia - local anaesthetic is injected into the space between the dura mater and the periosteum lining the spinal canal.

   • Intravenous regional anaesthesia - a technique used for surgical procedures in limb extremities; an Esmarch bandage is applied to stop arterial blood flow, and local anaesthetic may be injected into any vein distal to the tourniquet.
10. List 4 surgical procedures in which epidural analgesia may be useful.
    • Exploratory laporotomy; especially in toxic patients.
    • Caesarean section.
    • Hind limb surgery.
    • Perineal surgery.
11. What is the main risk of lumbar epidural analgesia?
    Infection; the procedure must be carried out aseptically. Other risks to note include systemic toxicity, respiratory paralysis, spinal cord damage and hypotension.
12. Give examples of 2 surgical procedures in which intravenous regional analgesia may be useful.
    • Removal of an inter-digital foreign body.
    • Amputation of a digit.
13. List 4 local anaesthetic drugs in addition to Lignocaine and Proparacaine.
    • Amethocaine - suitable for skin and aural preparations.
    • Bupivacaine - four times stronger than Lignocaine, all preparations contain adrenaline.
    • Mepivacaine - suitable for conduction blocks in equine species.
    • Procaine - good analgesia but poor tissue penetration; unsuitable for regional nerve blocks.
14. List the 4 main indications for intravenous anaesthesia.
    • As the sole agent to facilitate examination or to perform a short surgical procedure.
    • As induction for inhalation anaesthesia.
    • To supplement inhalation anaesthesia.
    • Central nervous system suppression in conditions such as poisoning cases.
15. What are the advantages of intravenous anaesthesia?
    • Simplicity - only a needle and syringe are required for administration.
    • Rapid induction - relatively stress-free for a calm or sedated animal.
    • Rapid recovery.
    • No explosion or pollution hazards (these are a risk of inhalation anaesthesia).
16. What are the disadvantages of intravenous anaesthesia?
    • Rapid induction means that the stages of anaesthesia are not as recognisable as in inhalation techniques and are therefore more difficult to monitor.
    • Risk of overdose.
    • Unsuitable for major surgery due to the swiftness of recovery.
17. Intravenous anaesthesia may be given as a single dose, incrementally (repeated top-up injections) or as a continuous infusion. Under which technique will recovery be quickest?
    Single dose administration. Incremental doses in particular will prolong the recovery time and may predispose the patient to excitement upon awakening.
18. Name the vein of choice for intravenous anaesthetic administration and list 2 other veins which would be suitable if this vein was inaccessible.
    The cephalic (radial) vein is preferred since the animal is restrained in such a way that the airway can easily be observed. Other suitable veins are the saphenous (recurrent tarsal vein) or the jugular.
19. How do injectable anaesthetics work to produce unconsciousness?
    The brain, with its rich blood supply, receives a high concentration of anaesthetic drug shortly after intravenous injection. Unconsciousness occurs once a critical concentration has been exceeded, while organs that are less well perfused take up the drug later (redistribution). Movement of the drug from the brain to the plasma takes place due to a diffusion gradient created by falling plasma levels.
20. Define barbiturates, and give 2 examples of drugs of this kind commonly used as veterinary anaesthetic agents.
    Barbiturates are a large group of sedative and hypnotic drugs derived from barbituric acid. Most are controlled drugs under Schedule 3 of the Misuse of Drugs Act 1971 and Misuse of Drugs Regulations 1985. 2 examples are:
    • Methohexitone sodium (Brietal).
    • Pentobarbitone sodium (Sagatal).
    • Thiopentone sodium (Intraval, Thiovet).
21. If thiopentone sodium is accidentally injected outside of a vein, severe skin sloughing may occur; why is this, and what should be done if extravasation occurs?
    Thiopentone sodium is very alkaline (pH 14) and irritant, particularly if used in concentrations of greater than 2.5%. Should extravasation occur, the area around the vein must be injected with sterile water immediately to dilute the concentration of the thiopentone.
22. Why is pentobarbitone sodium not recommended as a sole anaesthetic agent?
    Pentobarbitone sodium is a long-acting drug producing both prolonged induction and recovery. Hypothermia is common, and both respiratory and cardiovascular depression are marked; therefore oxygen must be available to animals given this agent.
    Note: Its therapeutic index is very low therefore a small overdose can have fatal consequences.
23. Give an example of a steroid anaesthetic drug and explain why it is unsafe to use in dogs.
    Alphaxolone (Saffan) is an example of a steroid anaesthetic. In many practices it is the intravenous agent of choice for cats since it produces rapid induction and recovery, and also has a better safety margin than barbiturates. Saffan is not recommended for use in dogs since the cremaphor EL (polyoxyethylated castor oil used to help dissolve the steroids within the solution) elicits histamine release with all the signs of anaphylaxis which can be fatal.
24. What is the main disadvantage of steroid anaesthetics other than that they cannot be safely used in dogs?
    Mild anaphylactic reactions can occur in cats including the swelling of the pinnae and paws. Very occasionally, fatal pulmonary oedema may occur.
25. Give an example of a dissociative anaesthetic drug in the cyclohexanone group and explain what is meant by dissociative.
    Ketamine is an example of a dissociative agent commonly used in veterinary practice. It is usually administered by intramuscular injection and produces a state of profound analgesia with superficial sleep. The eyes remain open, and palpebral, pedal, laryngeal and pharyngeal reflexes are present. The patient is often hypersensitive to sound.
26. What precautions must be taken when ketamine is used?
    • Constant careful monitoring - due to the unique state of anaesthesia produced, depth may be difficult to assess.
    • Ophthalmic ointment - the patient's eyes require protection and lubrication since they remain open and "awake" during the procedure and the cornea may become dry.
    • Warmth - ketamine produces hypothermia.
27. Why is ketamine alone unsuitable for dogs?
    Ketamine used as a sole agent in dogs has been found to increase muscle activity to a level of convulsions. Excessive muscle tone or movement in cats may be controlled by the administration of a barbiturate at one quarter of the induction dose.
28. List the drugs that may be given in combination with Ketamine to improve its anaesthetic effects.
    • ACP - sedation and prolonged anaesthetic effect.
    • Atropine - to prevent salivation.
    • Barbiturates - to prevent excessive muscle tone or movement.
    • Diazepam - for a smooth, slow induction in debilitated patients.
    • Midazolam.
    • Medetomidine (Domitor) - Prolonged effect; Atipamezole (Antisedan) may be used to antagonise the effects.
    • Xylazine - for sedation and muscle relaxation.
      Ketamine with Diazepam and Xylazine have anti-convulsant effects rendering them safe combinations to use in dogs.
29. Give an example of an anaesthetic drug of the phenol group, and state the breed of dog in which this is the intravenous anaesthetic of choice.
    Propofol (Rapinovet) is an example of an anaesthetic drug of the phenol group. It may be used in both cats and dogs, and is especially useful in greyhounds who suffer a remarkably long recovery from thiopentone sodium (Intraval, Thiovet). This is due to a lack of body fat for redistribution of thiopentone.
30. Give 4 advantages of propofol.
    • Produces rapid unconsciousness.
    • Produces rapid recovery.
    • Incremental injections (oxygen to be given) do not prolong recovery.
    • Rapid hepatic metabolism; non-cumulative.
31. Give 4 disadvantages of propofol.
    • Respiratory depression.
    • Hypotension.
    • Expensive, and contains no bacteriostat, so unopened vials must be discarded.
    • Special care required if administered to animals with liver disease (due to rapid hepatic metabolism).
32. Pentobarbitone is an old-fashioned intravenous anaesthetic agent largely superseded by safer, more recent drugs. It does have a place in veterinary practice however; what are its uses?
    Pentobarbitone is still sometimes used as an anaesthetic agent (usually in laboratory animals). Both induction and recovery are prolonged due to the delay in the drug crossing the blood-brain barrier. Nowadays, it is most commonly used as an anticonvulsant drug or for euthanasia.
33. What is the main advantage of inhalation anaesthesia.
    Inhalation anaesthesia is a relatively safe technique since the animal is usually intubated (unless a mask is used) thus protecting and preserving the airway. Oxygen may be administered and intermittent positive pressure ventilation may be performed in the event of apnoea.
34. List the 4 gaseous agents used in inhalation anaesthesia and briefly describe their uses.
    • Carbon dioxide (grey cylinder) - used to produce normocapnia in controlled breathing when blood-gas analysis unavailable, also used to increase the depth of anaesthesia when volatile agents are employed and to stimulate the onset of breathing after a period of intermittent positive pressure ventilation.
    • Cyclopropane (orange cylinder) - no longer used in veterinary practice since it is highly inflammable.
    • Nitrous oxide (blue cylinder) - a sweet-smelling potent analgesic with anaesthetic sparing effects used as a carrier gas; its inclusion also preserves cardiopulmonary performance and accelerates the induction of anaesthesia.
    • Oxygen (black cylinder with white neck) - used as a carrier gas in inhalation anaesthesia; also used in cases where normal delivery of atmospheric oxygen is threatened and during recovery from anaesthesia until a patient is able to maintain haemoglobin concentration with room air.
35. Nitrous oxide is often used as a carrier gas due to its analgesic and anaesthetic sparing effects; it does however possess certain disadvantages most notably diffusion hypoxia and gas filled viscus. Describe these terms.
    • Diffusion hypoxia = when delivery of nitrous oxide ends, the direction of diffusion reverses (from blood into the alveolar space); this means that alveolar oxygen may be diluted, and therefore the animal requires pure oxygen (100%) rather than room air (20%) for at least 3 minutes after termination of nitrous oxide delivery to prevent hypoxia.
    • Gas filled viscus = nitrous oxide is relatively insoluble in blood and therefore accumulates in gas-filled organs such as the dilated stomach of a dog with gastric dilation; the pressure is increased within such spaces and hence compromises the patient.
36. What is meant by the "open method" and "semi-open method" of anaesthesia? Why are these techniques not commonly used today?
    Open method = volatile anaesthetic is placed onto absorbent material which is held near to the animal's nose and inhaled.
    Semi-open method = volatile anaesthetic is placed onto absorbent material held within a mask through which the animal inhales.
    Both techniques possess the following disadvantages:
    • Pollution since no scavenge system is employed.
    • Depth of anaesthesia is difficult to control.
    • Hypoxia and hypercapnia are common.
    • Resuscitation is difficult.
    • Patient resistance during induction.
37. Give 2 examples of closed anaesthetic circuits.
    • The circle system.
    • Water's canister (also known as the to and fro system).
      These circuits are described in Revision Module 6 - The Safe Use of Anaesthetic Equipment.
38. List 4 advantages of closed anaesthetic circuits (also known as re-breathing systems).
    • Efficient removal of carbon dioxide (particularly with the circle system).
    • Economic use of oxygen and volatile agent.
    • Low explosion and pollution hazards.
    • Heat and moisture conservation.
39. List 4 disadvantages of closed anaesthetic circuits.
    • Cumbersome; may prove obstructive in head or oral surgery.
    • Resistance and dead space from the presence of soda lime (and valves in the circle system, making it unsuitable for use in animals under 10 kg).
    • Level of anaesthesia slow to change.
    • Nitrous oxide cannot be safely used and de-nitrogenation must be regularly performed.
40. Give 4 examples of semi-closed anaesthetic circuits.
    • Ayres.
    • Bain.
    • Lack.
    • Magill.
      These circuits are described in Revision Module 6 - The Safe Use of Anaesthetic Equipment.
      
      
41. List 4 advantages of semi-closed anaesthetic circuits.
    • Low resistance.
    • Level of anaesthesia rapidly changed.
    • Nitrous oxide can be safely used.
    • Denitrogenation not required.
42. List 4 disadvantages of semi-closed anaesthetic circuits.
    • High gas flow requirements.
    • High volatile agent consumption
    • Loss of expired moisture and heat.
    • Different flow rates are required for different circuits.
43. Give 4 examples of volatile anaesthetic agents and briefly describe their properties.
    • Enflurane - Administered via an "Enfluratec" vaporiser; sweet smelling, reasonably potent anaesthetic producing fair muscle relaxation and a rapid, smooth recovery; expensive.
    • Halothane - Administered via a "Fluotec" vaporiser; sweet smelling, potent anaesthetic producing good muscle relaxation; the most popular volatile agent in practice at present.
    • Isofluorane - Administered via a "Fluotec" vaporiser; pungent anaesthetic producing good muscle relaxation and analgesia, useful for cardiac conditions as causes less cardiac depression than Halothane; may produce some transient excitable effects after painful surgery, expensive.
    • Metofane - Administered via a "Pentec" vaporiser; fruity smelling anaesthetic producing good muscle relaxation; not suitable for induction.
44. What is the purpose of soda lime?
    Soda lime crystals are contained within a canister incorporated into a closed anaesthetic circuit. They contain:
    • 90% Calcium hydroxide.
    • 5% Sodium hydroxide.
    • 1% Potassium hydroxide.
    • Silicates to prevent powdering.
      The hydroxides combine with carbon dioxide in the presence of moisture to form carbonates, thus allowing the re-breathing of expired air. Soda lime crystals change colour when spent (pink to white, or white to mauve) and it is important to renew the used crystals when indicated to avoid hypercapnia and hypoxia.
45. What is a fluabsorber?
    A fluabsorber is a canister containing activated charcoal for the purpose of removal of waste anaesthetic gases. It is attached to the scavenge valve via a pipe and is useful in portable anaesthetic machines where it may not be possible to employ a more efficient type of scavenge system. However, activated charcoal cannot remove nitrous oxide and should not therefore be used in conjunction with this gas. Fluabsorbers have a limited life and should be weighed regularly to ensure that spent canisters are replaced at once.
46. What is neuromuscular blockade, and list examples of neuromuscular blocking agents describing their properties.
    Neuromuscular blockade is the employment of muscle relaxants as a supplement to general anaesthesia. Absolute relaxation is achieved by the action of these agents upon the nicotinic receptors at the neuromuscular junction. Muscle relaxants are not anaesthetics, and do not possess analgesic properties. They paralyse the respiratory muscles and should therefore only be used when facilities for positive pressure ventilation are available. They are useful in cases such as intra-abdominal surgery and open chest surgery, but care must be taken since the depth of anaesthesia is difficult to monitor since reflex response is suppressed and spontaneous respiration abolished. There are 2 types of muscle relaxant; depolarising and non-depolarising. These are listed below:
    • Depolarising = Succinylcholine; a short acting relaxant with a rapid onset time used to facilitate endotracheal intubation in cats.
    • Non-depolarising = Alcuronium, Atracurium, Gallamine, Pancuronium and Vecuronium; these relaxants are intermediate to long acting and are used in surgical cases.