1. PART 3: Breathing Circuit
The system that brings the fresh gas from the vaporizer to the patient and takes the expired gases from the patient to the scavenger.
May contain unidirectional valves, reservoir bag, pop off valve, CO2 canister, negative pressure relief valve, pressure manometer.
Non-rebreathing
System
Rebreathing System

2. Rebreathing System
Set up as a circle system  exhaled gases are recirculated and rebreathed by the patient
New fresh gas is also added in
Used for patients weighing:
Can be used as a closed or semi-closed system
Begins with the fresh gas inlet
Vaporizer outlet port connects to the fresh gas inlet

3. Rebreathing System: Closed
Pop-off valve is completely closed!
What’s a pop-off?!
Only provides enough oxygen to meet metabolic demands of the patient
Oxygen flow rates (L/min) must be kept very low!!
The volume of gas added to the circuit = the volume of gas used by the patient
*This is very tricky to achieve and is therefore not commonly used

4. Rebreathing System: Semi-Closed
Also known as partial rebreathing system
This is much more common!
Pop-off valve is open or partially closed
This system requires a scavenging system because more oxygen than the patient needs is being provided

5. Unidirectional Valves
One-way valves that allow the flow of fresh gas to enter the inhalation valve and exit the exhalation valve.
Valve is either a rigid disk or a flap that flutters as gas flows past it
Inspiratory/Inhalation valve opens as patient inhales
Fresh gas enters the inspiratory breathing hose, then the ET tube and then the patient’s lungs
Gas exchange occurs now = sleepy patient
O2 CO2

6. Unidirectional Valves
CO2 and anesthetic gases are then exhaled and travel up the expiratory breathing hose.
Gas flows through the unidirectional exhalation valve
This valve prevents the expired gases from traveling back to the patient before the CO2
is removed.

7. Breathing Circuits for Rebreathing System
1. Y Tubes – rubber or plastic
Come in large animal, small animal, and pediatric
Large animal tubes can only be used on large animal machines
2. Universal F-circuit
Come in adult (green-blue) and pediatric (red or green)
Pediatric = patients lbs
Adult = patients weight > 35 lbs
The inspiratory tube is located within the expiratory tube
Each connects to the machine separately at their respective flutter valves
 CO2 absorber

8. Universal F-circuit
Y tubing

9. CO2 Canister
Exhaled gases flow past the exhalation valve and into the CO2 canister
CO2 canisters usually contain calcium hydroxide granules that
selectively absorb CO2 from other gases breathed out.
Example: Soda Lime granules
“Exhausted” soda lime granules no longer absorb CO2 patient breathes in CO2

10. CO2 Absorber HOW DO I KNOW WHEN THE GRANULES ARE EXHAUSTED?
Color change to violet, off-white or pink depending on the brand.
CO2 saturated granules are hard and brittle, new ones can be chipped and crumbled with your fingers
Once color becomes abnormal, it is possible that it changes back to normal within hours!
VTI note: Change q 8 hours of use
next, gas flows into the rebreathing bag AND through the pop-off simultaneously 

12. Reservoir Bag- aka rebreathing bag
Expandable bag that fills as gases enter the circuit or as patient exhales
Deflates as patient inhales
Range in size from 0.25 L to 30 L
Volume of bag should be minimally 60 mL/kg

13. Reservoir Bag Calculation
If your patient weighs 15 lbs, how many mLs of air should the bag hold?
How many liters is this?
*Always round up to next highest bag
 0.5 L bag
*If that size doesn’t work for your patient, switch it out!

14. Reservoir Bag Functions
Helps in determining correct ET tube placement movement of bag with breaths = tube in trachea (yay!)
Allows assessment of respiratory rate and depth
Good for when you can’t see your patient
Why can’t you see your patient?!
Allows manual ventilation of the patient
Also known as “giving a breath” or “bagging”
*You must close the pop-off valve to do this!

15. Indications for Manual Ventilation
Reverse/Prevent atelectasis if present
Done by closing pop-off and gently squeezing bag
Performed once q 10 minutes = “sighing” patient
Removal of CO2 and anesthetic that builds up when RR and TV decrease
Anesthetics decrease tidal volume up to 50%
Forcing fresh gas in stimulates gas exchange in the alveoli
Assist or control ventilation – especially if in respiratory arrest or “deciding not to breathe”
IPPV

16. Is Your Reservoir Bag the Right Size?
Bags should consistently be ~ ¾ full upon exhalation
1. If bag is going from over-inflated to flat:
Bag is too small and doesn’t hold enough oxygen  patient won’t be able to fully inflate lungs on inhalation
AND
Bag will overinflate on exhalation increases pressure in the circuit
2. If bag is flat during respirations (under-inflated):
Bag is too big  will not see movements with each breath
and there’s wasted space in circuit

17. Pressure Manometer
Measures pressure of gas within the breathing circuit this includes the patient’s lungs!
Unit is:
Should read between 0-2 cm H2O during normal respirations
Pressure increases as system is closed
How do we close the circle and why?
Gauge should never exceed 20 cm H20 in small animals!
Watch manometer while “bagging” patient

18. Pop-off Valve - aka pressure relief valve, exhaust valve
Always kept open for semi-closed system
Allows exhaled gas to leave the breathing circuit and be scavenged
Prevents the build-up of excess gas or pressure within the circuit.
If the pressure were allowed to build up (forgot to open pop-off valve), the alveoli in the lungs could rupture!

19. Pop-off Valve
The degree that the pop off valve is opened changes the flow rate and how full the reservoir bag is. (semi-closed system)
If you see your reservoir bag unintentionally expanding very quickly- check pop-off!
Only close the pop-off valve when you are currently providing breaths for the patient
Must be opened between breaths
Rebreathing
System
Non-rebreathing
System

20. Occlusion Valve This one is safety regulated

21. Negative Pressure Release Valve aka Air Intake Valve
Special feature activated in emergencies
Indicated by a completely empty reservoir bag
1. Instances when there is excessive suction
Ex: When an active scavenging system is utilized, if negative pressure is detected in the circuit, this valve opens and allows room air in.
2. O2 flow rate is too low or tank runs out of O2
ALWAYS keep an eye on your tank pressure gauge
*Better for the patient to breathe room air than no air

22. Scavenging System A&A Pg 356
To reduce exposure to waste anesthetic gases
Can be active or passive scavenging system
Active Scavenging System
Connected to a vacuum that sucks the waste gases out of the machine…concerns?
Runs through a drop down line, through pipes in the ceiling, and outside –must be turned on!
Downsides?
Passive Scavenging System
Uses pressure in circuit to expel gases from the machine, out through a hole in the wall
Must be room adjacent to the outside

23. Scavenging System Activated charcoal cartridge
Cheaper than installing a scavenger
Is mobile
Easy to use with little maintenance
Example: F/air canister
Must be discarded after 12 hours of use or
a gain of:
Down side: does not absorb nitrous oxide

24. Oxygen Flow Rates Mask Induction: 2-3 L/min for < 10 kg
Chamber Induction: 5 L/min
*Your inhalant will most likely be at 5% too
Intubated patient:
Rebreathing system: 40 ml/kg/min
Note: this may be increased if you are inducing,
changing anesthetic depth, or recovering patient

25. Rebreathing O2 Flow Rate Calculation
Ex: If your patient is 25 kg, what should their O2 flow rate be while intubated?
Flow meter measures in L/min:
Ex: 16 lb patient
If you calculate < 0.8 L/min round up to 0.8 L/min