What are the overarching functions of the breathing system? (2)
- deliver oxygen and volatile anesthetic to the respiratory system
- remove CO2 from the respiratory system
What is the open drop method?
patient is enclosed in a chamber to which anesthetic is added; typically done with agents of low vapor pressure, such as methoxyflurane
Why does the open drop system work (albeit in a very crude way)?
- very soluble takes a long time to enter the system and induce the patient; but because it’s so soluble, it also takes a long time to LEAVE the system and the patient stays under longer after being removed from the chamber
- this wouldn't work with something less soluble, like isoflurane
What are the components of an anesthetic machine? (4)
- oxygen source
- flowmeter to regulate the flow rate of oxygen into the vaporizer
- vaporizer to convert the volatile liquid into a vapor
- breathing circuit to deliver oxygen and volatile anesthetic to and remove exhale CO2 from the subject
How can you tell if a tank is oxygen or nitrous oxide?
- oxygen: green in USA, white elsewhere
- NO: blue
Oxygen within a cylinder is present as ____________;The pressure drop within a gas cylinder is proportional to...
only a gas; the volume remaining within the cylinder (Boyle's Law) [oxygen tanks need a regulator to ensure that the pressure of gas release remains constant, even as the pressure in the cylinder drops]
Nitrous oxide within a cylinder is present as...
The relevance of this is...
- liquid, with a gas phase above the liquid
- pressure of this gas phase remains constant until all liquid is vaporized (unlike oxygen, which is present only as gas, so pressure drops as gas leaves and oxygen tanks need a regulator)
When is liquid oxygen used?
suitable for very large scale demands of oxygen (specialty hospitals, ICU with O2 cage)... not used for smaller scale operation b/c liquid evaporate from within the tank when not used fast enough
What is the function of the flowmeter?
regulate the flow rate of oxygen or nitrous oxide that enter the vaporizer
How does a flowmeter work?
- gas enters at bottom and exits through top of tube, which tapers toward the bottom (larger diameter at top than bottom)
- metal ball floats higher with higher flows
What is the function and use of the anesthetic vaporizer?
- agent specific; temperature and flow compensated
- adjusted to deliver a specific gas concentration
The breathing circuit is connected to the ___________ and is the direct connection of the ___________ to the ___________.
fresh gas outlet; anesthetic machine; patient
What are the 2 types of breathing circuits used in vet med?
- circle rebreathing
With the circle rebreathing system, exhaled gas enters the system via the ___________; ___________ assure unidirectional flow; CO2 is _________ removed and the __(2)__ are recirculated.
Y-piece and hoses; one-way valves; chemically; exhaled O2 and inhalant anesthetic
With the circle rebreathing system, fresh gas from the ___(2)___ are __________ added to the system to mix with exhaled gas; the __________ of gas _______________ determines how much of the exhaled gas is ___________ and how much is ___________.
flowmeter and vaporizer; continually; flow rate; leaving the flowmeter and passing through the vaporizer; rebreathed; removed through the pop-off valve
What oxygen flow rate do you use for a circle system for induction and maintenance?
- Induction Phase (first 5 min): 1-3L/min
- Maintenance Phase: 300mL/min- 1L/min (can use as low as 3-5mL/kg/min, which is more economical but you cannot rapidly change anesthetic conc at this rate)
The _________ the oxygen flow rate, the faster you can change the anesthetic concentration in a circle system.
The Y-piece connects the ____________ to the ____________.
2 breathing hoses; ET tube or mask
What are the functions of the one-way valves in a circle system? (2)
- maintain unidirectional flow of gas within the circuit
- minimize dead space
When should you change the soda lime?
when 2/3 of the canister has turned from white to purple
When and why are non-rebreathing circuits used?
used in very tiny patients (under 3kg) because there is less resistance to breathing
What is the purpose of the pop-off valve?
- removes excess gas from the system; gas flows out of pop-off valve into waste gas scavenging system
- only close valve when giving a breath
What is the function of the reservoir (rebreathing bag)?
- allows inhaled and exhaled gas to move into and out of the system
- can be used to deliver an assisted breath when pop-off is closed
Describe the function of the CO2 absorber canister.
- contains calcium hydroxide, which reacts with water and CO2 to produce calcium carbonate, causing pH to drop--> pH sensitive color change indicates when it's time to change
- this is an exothermic reaction
How is the pressure gauge used?
to determine "how hard" to squeeze the bag when giving a breath (typically use 10-20cm H2O pressure)
What are advantages and disadvantages of non-rebreathing systems?
- Advantage: less resistance to breathing, dead space is minimized
- Disadvantage: high fresh gas flow rates are wasteful and expensive; patient is breathing cold, dry gas, making it harder to keep their temperature up
Non-rebreathing systems have no __________; therefore, it relies on a ___________ so that...
CO2 absorption; heavy flow of fresh oxygen; exhaled breath is diluted out with fresh gas and exhaled CO2 is pushed away from the patient
With a non-rebreathing circuit, gas flows directly from the ....
vaporizer and common gas outlet to the ET tube
Describe a passive waste gas scavenging system.
waste gas flows passively from pop off valve through charcoal canister or out of room
Describe an active waste gas scavenging system.
waste gas is actively removed from pop-off valve via vacuum