Anaesthesia

• Anaesthesia is a state in which there is a lack of sensation. Also applies to means used to produce this state
• general anaesthesia is A state of unconsciousness produced by anaesthetic agents, with absence of pain sensation over the entire body. Voluntary control and protective reflexes are lost or diminished but noxious stimuli are still responded to (like pain but processes differently)
• regional anaesthesia is lack of sensation caused by interrupting the sensory nerve conduction of any region of the body (may be used in conjunction with GA)
• sedation is A state of reduced irritability or excitement
• Anxiolysis is a state of reduced anxiety
• Analgesia is Insensibility to pain without loss of consciousness
• narcosis is A state of stupor or unconsciousness
• Hypnosis is An artificially induced state of passivity
• Reasons for anaesthesia:

  ·       Moral/ethical – to prevent suffering caused by stress and/or pain

  ·       Technical or practical – to prevent movement during surgery and reduce human risk

  ·       Legal requirement – anaesthesia is mandatory for any procedure with or without the use of instruments which causes interference with the sensitive tissues or bone structure of an animal (protection of animals (anaesthetic) act 1954)
• Exceptions to the need for anaesthesia are in emergency first aid, castrating males under a specified age or for specific procedures such as pig tail docking @ 7 days or younger
• The specified castration ages without anaesthesia are:

  Pigs: less than 7 days

  Calves: less than 2 months (7 days rubber ring)

  Lambs: Less than 3 months (7 days rubber ring)
• Aims of anaesthesia:

  1.      Safety for donor and recipient- minimum side effects and high therapeutic index; should not cause harm (but all have side effects)

  2.      Efficacy- act as described e.g. anaesthesia or muscle relaxation.

  3.      Predictability- should produce the same effect on every occasion and in every animal, in the same time course. (applies to adverse and desirable effects).

  4.      Convenience- ease of administration. More complex a technique = more likely to be forgotten or go wrong

  5.      Cost- should be affordable

  6.      Legality- particularly in food animals
• An ideal anaesthetic should provide narcosis, analgesia and muscle relaxation - The Anaesthetic Triad. This is usually done via balanced anaesthesia (using different agents to produce each element)
• Phases of anaesthesia:

  1.      Pre-operative: animals are examined and prepared for surgery. Drugs and fluids are given to control pre-existing conditions, anaesthetic equipment is prepared for use

  2.      Pre-anaesthetic medication: drugs used to sedate the animal, making them less anxious, relieve pain, reduces drug doses required for subsequent events

  3.      Induction of anaesthesia: animal rendered unconscious

  4.      Maintenance of anaesthesia: unconsciousness is maintained

  5.      Recovery: drug administration ceases, animal regains consciousness. Most deaths occur in this period.
• Factors influencing choice of anaesthetic technique:

  ·       Drugs and equipment available

  ·       Experience of the anaesthesiologist and surgeon

  ·       Assistance available

  ·       Patient factors (health, previous anaesthesia reactions)

  ·       Type of procedure

  ·       Duration of procedure
• Anaesthesia always carries some risk to the patient so the client must be fully informed of the risks,  pros, cons and importance in order to give informed consent
• Risk factors identified in small animal anaesthetic studies have been identified particularly with:

  -          Poor anaesthetic management

  -          Lack of observation or monitoring

  -          Some higher risk breeds, particularly brachycephalics

  -          Patients who are not intubated

  -          Sick animals

  -          Increased operative time

  -          Unfamiliarity with anaesthetic technique

  Most animals who die under anaesthetic die in recovery (particularly cats and rabbits). Dogs are more likely to undergo respiratory arrest during surgery.
• Particular anaesthetic risks in horses:

  -          Cardiac arrest in the first 30 mins (less common since first survey)

  -          Post op fractures (in recovery)

  -          Post op colic

  -          Upper airway obstruction in recovery

  -          Sudden collapse after standing

  -          Post op myelomalacia
• Pre-anaesthesia history questions:

  §  any previous anaesthetics, any unusual responses or adverse events?

  §  Age

  §  Is there any disease history and concurrent medication?

  §  How is the animal’s exercise tolerance? (this gives a good indication of any cardiac disease)

  §  Has there been any V+/D+ and have they eaten or drunk?

  §  Any coughing or respiratory abnormalities?

   

  The owner should sign a consent form. This should be INFORMED consent - i.e. that they know risks/benefits and can make an appropriate decision. Consent should include both the anaesthetic and surgical procedure.
• -          Horses, cats and rabbits are at higher risk under anaesthesia than dogs

  -          Cats- check for any hypertrophic cardiomyopathy?

  -          Extremes of size are associated with increased risk. Also must have suitable equipment

  -          Unfamiliarity with the species e.g. exotics increases risk
• Breed related pre-an Qs:

  -          Brachycephalics - airway problems

  -          CKCS – mitral regurgitation murmurs (endocarditis)

  -          Protuberant eyes - risk of globe proptosis

  -          Giant breeds - dilated cardiomyopathy +/- A-fib

  -          Dobermans- Von Willebrand’s disease (clotting disorder)

  -          Devon rex cats – more likely to have vitK dependant coagulopathy

  -          heavy horse breeds- increased risk of myopathy

  -          Dachunds, bassets – difficult venous access (leg morphology)
• Some Collies have genetic mutation MDR1 which can alter their response to some drugs
• Animal morphology needs to be considered pre-anaesthetic:

  -          Very thin – increased heat loss, altered drug distribution, may leady you to suspect other disease processes present.

  -          Obese- impaired ventilation during anaesthesia
• Very young or very old animals pre-op:

  -          Neonate/paediatric- may have reduced thermoregulatory ability. Cardiac output is rate dependent, poorly developed renal and hepatic function.

  -          Geriatric- reduced cardiovascular and respiratory reserves, increased risk of coexisting disease. Increased chance of poor renal/hepatic function. reduced thoracic compliance. Often arthritic, therefore very important to consider positioning if in one position for a long time.
• ASA classifications:

  ASA I – Normal, healthy animal

  ASA II – mild systemic disease or impairment e.g. elderly but otherwise healthy

  ASA III – more severe systemic which is well compensated or controlled by treatment e.g. heart murmur but BAR and active

  ASA IV – severe systemic disease which is not compensated e.g. showing signs of pulmonary failure

  ASA V – moribund, unlikely to survive 24 hours

  E – added to any classification if the anaesthetic is an emergency
• Withholding food reduces risk of vomiting, regurgitation (and aspiration) and reduces intra-abdominal pressure.

  -          Dogs/cats: 4-6 hours

  -          Horses: 6-12 hours

  -          Cattle: Historically 18-24 hours but prolonged starvation increases the fluid content of the rumen, increasing aspiration risk. Also metabolically messy
• Preparation allows you to fully assess risk in individuals, and to stabilise and correct any abnormalities.

  In light of the clinical findings, the correct pre-med, induction agent, maintenance and recovery procedures can be chosen.

  The anaesthetic machine and equipment MUST be checked and prepared for use.

  Fluids, catheters, ET tubes and planned emergency drugs should all be to hand

  Preparation of the surgeon is equally important.
• the choice between GA or sedation is mostly dependent on the species. In small animals sedation is appropriate in some cases however GA allows full control of the airway so we can use inhaled drugs (avoiding hepatic metabolism) so they are easier to monitor
• patient positioning has the potential to affect

  §  Blood flow

  §  Intracranial pressure

  §  Muscle perfusion

  §  Joint pressure

  §  Ventilation

  Undervalued but extremely important especially in horses and arthritic animals
• Normal thermoregulation is diminished under anaesthesia, and heat is lost via conduction, convection, radiation and evaporation. This is particularly marked in small patients.
• Hypothermia can be prevented by increasing the temperature of the environment (OR, prep, recovery), insulating the animal  with aluminium foil or bubble wrap, use of blankets etc.

  -          You must protect them against burns though!
• Thermoregulation:

  Anaesthetic factors:

  Favour short-acting anaesthetics, ensure animal is not too deep, don’t hyperventilate, use rebreathing systems where appropriate.

  Surgical factors:

  Avoid unnecessary wetting and clipping of non-surgical areas. Minimise surgical time, minimal incision size. Keep visceral surfaces moist with warm fluids.
• 50-65% of anaesthetic deaths occur in recovery. The environment should be warm, comfortable and quiet but observed.

   

  Extubation should be carried out once a dog is swallowing, before a cat is swallowing (risk of laryngospasm) , prior to swallowing in the horse and once a ruminant is vigorously swallowing.

   

  Analgesia should be considered based on pain scores and observation (posture, movement, behaviour, facial expression)
• After surgery, there should be a rapid return to feeding as patients will be in a catabolic state. This may require specialist support such as a feeding tube in some cases

   

  Catheters should be checked, flushed and properly bandaged. Continuing fluid therapy may be required.
• Each gas cylinder has a pressure gauge, a regulator and a check valve. Piping before the machine has an oxygen failure alarm and a nitrous cut off for patient and operator safety. Gases pas through a flowmeter and then a vaporiser. A high pressure release valve and emergency air intake are passed by before reaching the patient. An oxygen flush also bypasses the flowmeters.
• the formula for remaining gas volume from pressure is (Current pressure/original pressure)x original volume = current volume
• Oxygen: black with white shulders / white

  Nitrous oxide: blue

  Medical air: grey with white and black
• Safety features:

  Pin index system

  Bodak seal - prevents leaks

  Check valve - unidirectional, prevents backflow

  Shrader probe - tube collars have unique diameter and non-interchangeable screw thread

  Pressure gauges

  Pressure regulator - reduces high cylinder pressure to a usable level

  Oxygen failure alarm - insufficient oxygen pressure causes a whistle, nitrous cut off.

  Flowmeters - prevent excessive gas volume. anti-hypoxic devices (can’t turn on nitrous w/o oxygen)

  Vaporiser splitting valve - fresh gas flow split, correct concentration anaesthetic vapour administered
• Vaporiser safety measures:

  Key filling

  Colour coded

  Ensure is kept vertical!

  Backbar- locks vaporiser to machine

  Safety interlock- prevents multiple vaporisers being turned on
• Oxygen flush: A high pressure (400kPa) pure oxygen flow which bypasses all other gas supplies  and the vaporiser to just deliver oxygen to the patient. Because it is high flow, care must be taken especially with small animals .