Prepare and monitor anaesthesia in animals

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Prepare and monitor anaesthesia in animals

• VAPORISERS

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• VOLATILE ANAESTHETICS

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Purpose of Vaporisers

• Deliver volatile anaesthetic vapour to the
  circuit
• Volatile anaesthetics come as liquids at room
  temperature but vaporise readily

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Volatile Anaesthetics

• Ether (no longer)
• Halothane (BP 50C)
• Methoxyflurane
• Enflurane (BP 56C)
• Isoflurane (BP 49C)
• Sevoflurane (BP 59C)
• Desflurane (BP 23C)

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Speed of Recovery

• Halothane
• Isoflurane
• Sevoflurane

Quicker recovery
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Ether

• No longer used
• Irritating vapour
• Explosive
• Some analgesia
• Sympathetic stimulation
• Less cardiac depression

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Chloroform

• Another older anaesthetic
• Sometimes together with ether

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Halothane

• Sweet smelling, clear liquid
• Decomposes in light
• Up to 32 possible (5 lethal)
• Induce on 1-3
• Maintain on 0.75-2
• Poor analgesia
• Modest muscle relaxation

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Halothane

• Reduces blood pressure
• Weakens heart contractions
• Predisposes heart to arrhythmias (if affected by
  adrenaline)
• Dilates blood vessels in skin
• Reduces respiration
• Reduces depth
• Reduces rate

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Isoflurane

• Similar to halothane, but
• More expensive
• More pungent odour
• Less tolerated for mask inductions
• Quicker induction recovery
• Therefore good for neonates (caesareans),
  geriatric patients long surgeries
• Needs sl. higher settings for maintenance
  (1.5-2.5)
• Does not promote adrenaline-induced arrhythmias
  (unlike halothane)
• More vasodilation
• More respiratory depression

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Isoflurane

• Volatile, colourless and non flammable
• Minimal effects on CVS
• Decreased RR
• Good muscle relaxation
• Some analgesia
• Can be used with nitrous
• Crosses placenta

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Isoflurane

• Anaesthetic choice for birds
• Best for patients with cardiac, renal or hepatic
  disease
• Entirely secreted through RS and is not
  metabolized by the liver or kidneys

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Rate of Absorption into Blood

• Depends on
• Concentration in air sacs
• Rate of ventilation

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Nitrous Oxide (N2O)

• Adjunct to anaesthetic
• Has anaesthetic properties- Nitrous oxide (N2O)
  is a dissociative that can cause analgesia,
  euphoria, dizziness, flanging of sound, and, in
  some cases, slight hallucinations and mild
  aphrodisiac effect.
• Reduces dose of halothane (caesarians) in 2 ways
• Anaesthetic sparing effect eg reduces halothane
  dose by 25
• 2nd gas effect rapid uptake from alveolus
  increases concentration of anaesthetic
• Some analgesia
• Little cardiopulmonary effect
• Disadvantages
• Pollution issues
• Little odour
• Cannot be scavenged by charcoal canister
• Long term may be toxic (bone marrow)
• Diffusion hypoxia (3 mins 100 O2 upon recovery)
• Greenhouse gas (1 molecule same effect as 300
  CO2 molecules)
• Expensive
• Gas spaces expanded (birds, bloat etc)
• Less used nowadays
• Also known as laughing gas used by human
  dentists because preserves consciousness whilst
  providing analgesia

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Nitrous Oxide

• Inert gas
• Weak analgesia
• Used in combination allowing the amount of the
  second agent to be reduced
• Sedation
• Minimal effects on CVS and RS
• Poor muscle relaxation
• Weak anaesthetic
• Rapid effect

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Nitrous Oxide

• Diffusion hypoxia
• During the first few moments following
  termination of N2O breathing, large volumes of
  N2O pour back into the lungs from the blood as
  the pressure gradient is reversed
• Alveolar volume expands and resident gases (e.g.
  O2) are diluted
• Causing arterial O2 partial pressure and
  saturation to fall
• Therefore patient should breath 100 O2 during
  the three (3) minutes after N2O is disconnected

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Nitrous Oxide problems

• At high concentrations can cause hypoxia
• Pure N2O will cause asphyxiation!
• Dont forget to have O2 flowing equal to N2O rate
  (i.e. 11)
• N2O O2 of 21 66 N2O (can be used)
• N2O O2 of 11 50 N2O (safer)
• Set both O2 to N2O 30 mL/kg/min
• Avoided in shunts or other severe ventilation
  perfusion mismatches
• Diffuses into gas filled spaces so it is
  contraindicated in several conditions.
• Pneumothorax
• Gastric dilation
• Ruminants
• OHS risk

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2 Common Types of Vaporizers

• Non-precision in-circuit vaporizers
• Stephens machine
• Vaporizer output unknown dial notches are only
  relative approximations
• Precision out-of-circuit vaporizers
• Tec series (Fluotec III, Stinger)
• Vaporizer output agrees with dial measurement
• E.g. 2 set on dial and O2 flow 1L/min 20ml/min
  of vapor

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• VAPORISERS

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Precision Non-Precision
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Filling port
Emptying port
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Vapour Potency

• Vapour concentration above the liquid in the
  bottle is 30
• Lethal dose is only 3-4 (if sustained)

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Rate of vaporisation

• The slower the O2 passes through, the more vapor
  it can pick up
• The hotter the temperature, the more vapor that
  will be produced from the liquid form of the
  anesthetic

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Precision Vaporisers
                                                                           
Tec 2
Ohio 100
Tec 3
Drager 19
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Precision Vaporisers
Tec 2
Ohio 100
Tec 3
Drager 19
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Precision Vaporisers

• Metal containers with complex internal wicks
• Usually placed out of circuit (VOC)
• More expensive

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Precision Vaporisers
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Precision Vaporisers
Gas outet
Splitting valve
Gas inlet
Baffles
Temp sensitive valve
Wick
Volatile anaesthetic
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Precision Vaporisers

• Deliver close to the exact percentage represented
  by the figure on the dial
• Variations in quality- only the more advanced
  (and expensive) are fully compensated
• Compensate to a variable extent for ambient
  temperature, flow rates and back pressure
• Less expensive models often have charts that
  enable calculations to be made at low flow rates

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Fluotec 2

• Very inaccurate at low flows, but good for flow
  rates greater than 5 l/min

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Fluotec 3

• Accuracy tends to fall off at high dial setting
  and flow rates

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Ohio 100

• Output increases at high flow rates and vice
  versa.

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Dräger 19

• Very accurate at all dial settings over a wide
  range of flow rates.

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Precision Vaporisers
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Precision Vaporisers

• Not self-regulating
• ie they need trained staff to actually monitor
  the patient!
• But still probably safer in the long run

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Precision Vaporisers

• Vaporiser output depends on oxygen flow and
  vaporiser setting
• Known concentration is delivered
• Changes in ventilation have no effect on output
  of vaporiser
• Positive pressure ventilation will not change the
  inspired anaesthetic concentration
• Increasing fresh O into circuit brings inspired
  anaesthetic concentration closer to value on
  vaporiser setting
• Can be used with small patients (modified
  circuit) and large patients
• Do not leak

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Simple Vaporisers
Boyles Bottle
Ohio Bottle
StephensUniversal
Goldman
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Simple Vaporisers

• Often glass containers
• Resulting Anaesthetic varies with temperature
  gas flow
• i.e. not temp. or flow compensated
• To be safe, they must be inefficient in picking
  up anaesthetic gas (to reduce the chance of
  picking up a lethal dose)
• Very low resistance
• Dangerously high concentrations of anaesthetic
  can develop when performing intermittent positive
  pressure ventilation use with care with IPPV

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Stephens machine

• Uses a simple vaporiser

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Stephens machine
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Boyles bottle - Stephens Machine
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Simple Vaporisers

• Placed in circuit
• Advantages
• Cheaper to run
• Use less gas (lower flow rates)
• Some claim that they are self regulating
• As animal goes deep, they breath less and more
  shallowly
• As animal goes light, they breath more, and more
  deeply

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Simple Vaporisers

• Vaporiser output depends on patient ventilation
• Vaporiser output is not known
• Increased ventilation increases the amount and
  concentration of anaesthetic
• Positive pressure ventilation greatly increases
  the inspired concentration
• Increasing fresh O flow into the circuit dilutes
  the inspired anaesthetic concentration
• Works best on 10-30 kg dogs in mild temperatures
• Generally leak- anaesthetic vaporises and
  contaminates environment (especially when
  refilling bottle)

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Filling Vaporisers

• Coded filler tubes

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O2 Flush button
O2 Flush bypass pipe
O2 Flowmeter
Lock screw
Fill port
Fill level window
Drain port
Lock screw
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Steps in Filling Vaporisers

• Choose well ventilated area
• Put on mask
• Attach filling device to bottle
• Insert key into filling port
• Fill to FULL mark (watch carefully)
• Drain key into bottle
• Remove key and close port
• Cap bottle
• Seal key for storage

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Multiple vaporizers

• Vaporisers can be put in series
• But in a specific order to prevent vapour damage
  downsrteam

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Masks for anaesthetic gases
Small type
Large type pregnant women
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Spills of Volatile Anaesthetic

• Small spill
• Mop up with absorbent material and seal in
  container (e.g. polyethylene bag or jar)
• Large spill
• Evacuate area
• Wear absorbent mask mop up
• Ventilation

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Ether
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The End