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Dancing Machine; skillful manipulation of the anesthetic mothership
Kristen Cooley BA, CVT, VTS (Anesthesia/Analgesia
University of Wisconsin School of Veterinary Medicine
Veterinary Anesthesia Support and Training
vastwise@gmail.com

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The design
Anesthetic machines are designed to deliver volatile anesthetic gases in the presence of oxygen

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Concentrating...
- Why is 100% oxygen necessary?
- Delivery of oxygen based on need
- Reduced tidal volumes - Respiratory depression

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Following the Yellow Brick Road...

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Oxygen
Compressed gas
Pressurization
Two most common sizes
E
700 Liters
H
7000 Liters
Full cylinders
2200 psi of gas

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Medical Gas Coding
Oxygen - green
Medical Air - Yellow
Vacuum - White
Nitrous Oxide - Blue
Carbon Dioxide - Gray
WAGD - Purple
Instrument Air - Red

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Following the Yellow Brick Road...

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Pressure Regulator
- Tank = HIGH PRESSURE - Pressure regulator = Safe, constant operating pressure
- Regardless of the pressure in the oxygen tank or central system

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Oxygen Flush Valve
- Vaporizer bypass
- 30-50 L/min!
- Safety 15 L/min
- Handy but dangerous
- Over pressurization is easy

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CAUTION!
Use with care or not at all when the circuit is connected to the
patient
NEVER use it with a non-rebreathing system
NEVER use when pop-off is closed NEVER use in small patients
Decreases anesthetic concentration in the system and in the patient!

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Flowmeter
- Adjust O2 flow rate
- L/min
- O2 enters the bottom
- Indicator ball support

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- A time constant represents the volume of the machine
in relation to the flow of gases - Machine volume (components of breathing system)
- Sodasorb canister ~ 1 L
- Rebreathing bag - depends on size
- Breathing hoses- depends on length and diameter
- Example volume 5 L

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Time Constant Math
- It takes approximately 3 time constants to see a 95% change in the concentration of gases within the system
3 TC
X 5 L machine at 1 L/min = 15 min (15 min to approach a steady state)
3 TC
X 5 L machine at 2 L/min = 7.5 min (7.5 min to approach a steady state)

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Example
- You want to go from A to B
- Your flow rate is 1 L/min
- Your volume is 5 L
- You know it will take 3 time constants to go from A to B
- 3TC X 5 L / 1L per min = 15 min
- You are at A and in 15 minutes you will be solidly at B.

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Example
- You are impatient. 15 minutes is too long.
- What can you do to speed things up?
A. Reduce your volume
B. Increase your flow
C. Change your time constant
D. Use the oxygen flush valve

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Example
- You want to go from A to B quickly.
- Your flow is now 2 L/min - Your volume is 5 L
- Your time constant is constant at 3...
- 3TC X 5L / 2L per min = 7.5 min!
- Now we are getting somewhere!

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Time Constants
- Factors affecting changes
- Hypoventilation
- Apnea
- Lack of breathing = lack of gas exchange
- This includes oxygen as well as anesthetic!

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Caution with high flows
- Monitor closely!
- Increases system flow = increased patient pressure
- Never close pop-off valve when you have higher flows
- High flows waste anesthetic gas, oxygen, and money

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Back on the road...

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Vaporizer
- Converts liquid anesthetic to gas anesthetic
- Precision
- Temperature controlled
- Back pressure compensated

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Vaporizers
- Agent specific
- Must be calibrated and cleaned regularly!
- Never fill vaporizer with an unspecified anesthetic!

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Fresh gas outlet/inlet

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Fresh gas outlet/inlet
- Exits vaporizer and enters machine
- Non-rebreathing system connection

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Flutter Discs
- One-way valves
- Circle system
- Inspect daily and clean regularly

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Manometer
- Pressure of gases
- Manual ventilation
- Pressure in system/patient
- Not to exceed 15-20 cmH2O or 10-14 mmHg

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Pop-off of APL valve
- Prevents pressure build-up
by allowing gases to escape out to scavenge
The pop-off occlusion valve vents pressure at 2 cm H2O

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Pop-off or APL valve
- A closed pop-off allows pressure to build within the machine and patient
- This pressure build-up may impede venous return to the heart or cause barotrauma

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Carbon dioxide absorbent
- Absorbent granules
- Removes CO2

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CO2absorber
- Lowers fresh gas flows
- Reduces waste
- Re-breathing of gases
- Lowers overall cost

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The granules
- Calcium hydroxide or barium hydroxide
- Exothermic reaction
- Must be changed when exhausted
- pH color indicator
- White to lavender or blue - Short lived chemical reaction
- Fresh granules are easily chipped or crumbled
- Saturated granules are hard and brittle

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Scavenge
- Collects waste gas from breathing system
- Passive disposal
- Non-circulating ventilation systems
- Activated charcoal
- F/air canisters
- Enviro-Pure
- Active disposal
- Central vacuum system
- Personnel safety should be a priority!

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Anesthetic machine

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Breathing System
- After the common gas outlet
- Anesthetic tubing
- Rebreathing bag
- CO2 absorber
- Scavenge system

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The Breathing System
- Oxygen and anesthetic gas
- Remove CO2
- Controlled ventilation

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Breathing Systems
- Circle system
- Patients > 3 kg
- Inspiratory and expiratory valves, sodasorb canister
- O2 flow rate of 40 ml/kg/min
- Non-rebreathing system
- Patients < 3 kg
- O2 flow rates of 180-300 ml/kg/min
- Closed or partially closed system
- Refers to O2 flow rate (not position of pop-off valve)
- Also called low-flow anesthesia
- Flow rates 11-22 ml/kg/min or less than 40 ml/kg/min

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Circle Systems

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NRB
- Follow the flow of oxygen
Same as circle system up to fresh gas inlet...
Bypasses flutter valves
Bypasses carbon dioxide absorber
High gas flows

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Non-rebreathing systems

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Breathing Bag/Reservoir Bag
- Reservoir for gases
- Should hold a minimum of 60 ml/kg
- 6 X tidal volume
- (10-20 ml/kg/min)
- Calculate the range
- Pick a size within the range

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Breathing Bag
- Large breath
- Observe breathing
- Assist breaths

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Bag Size
- If bag is too large...
- Impairs monitoring
- Machine volume
- Slows changes
- If bag is too small...
- Bag collapse
- Unable to take adequate breath

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Breathing Hoses
- Transports anesthetic gas
- Diameter hoses;
- 22 mm for larger dogs
- 15 mm for small dogs and cats
- Different lengths

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Endotracheal Tubes
- One time use only in humans
- Cuffs tend to wear out
- Always check for leaks!
- Make sure cuff inflates
- Discard any leaky tubes
- Place inflated tube into a bowl of water and look for bubbles/leaks

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Mechanical Dead Space
- Bi-directional flow
- Extra pieces
- CO2 monitor adapter
- Swivel
- Elbow
- Excess pieces should be avoided in small patients
Hose length does not contribute to dead space

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Best Practice
- Always pressure test the machine before every case and every time a component of the breathing circuit is changed.

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Machine Troubleshooting
Common areas prone to leaks;
- Around CO2 absorbent canister
- Around valve caps
- Re-breathing bag
- Around any joint that gases pass through
If leaking when connected to patient, check ET tube first, then check machine

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If you suspect a leak...
- Go through and tighten all joints and connections and hope that you catch the leak
- If the leak remains...
Keep O2 on and spray dilute soapy liquid around all joints in the breathing circuit The gas sneaking out of the system will make bubbles!

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Wrapping it all up...
- The anesthesia machine is a dynamic piece of equipment
- Understanding how it works allows you to better serve your patients
- Mindfully choosing breathing systems can improve patient outcomes
- Regular and proper maintenance is important to the machine's longevity

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Thank you!