Anesthetic Breathing Systems and How They Work

Question 1

isoflurane in a bottle

Answer 1

in bottle, liquid isoflurane vaporizes naturally
at room temp there is a liquid and vapor layer

Question 1

what % of isoflurane is required to adequately and safely anesthetize the cat?

Answer 1

1.6%

Question 2

what are the 2 types of rebreathing systems?

Answer 2

1. rebreathing/circle: all or part of the exhaled gas can be combined with new (fresh) gas and rebreathed by the patient
2. non-rebreathing: all exhaled gas is eliminated by fresh gas flow

Question 2

what are the minimum basics with anesthetizing a patient?

Answer 2

1. provide at least basal metabolic oxygen
2. deliver inhalant anesthetic precisely
3. exclude expired carbon dioxide
4. manually ventilate the patient

Question 3

what components do anesthetic systems have? (think pressures)

Answer 3

1. high pressure components - oxygen source/tank
2. medium pressure components - working system pressure
3. low pressure components (a circle or non-rebreathing circuit)- always what is attached to the patient - breathing circuit pressure

Question 4

what is the oxygen source for anesthesia machines?

Answer 4

1. tank
2. house gas
   or both

Question 5

T/F: oxygen tanks in the US are either green or white

Answer 5

false, they are conventionally green

Question 6

T/F: volume of oxygen (L) is directly proportional to tank pressure (psi)

Answer 6

true

Question 6

what is the PSI and volume of a full oxygen tank?

Answer 6

PSI: 2000
H tank: 6,900 L
E tank: 650 L

Question 7

what is PISS?

Answer 7

pin index safety system: specific pin configuration on the yoke that prevents the attachment of the wrong gas to the yoke

Question 8

what are high pressure components (oxygen source) regulated to ? where ?

Answer 8

regulated where they attach to the anesthetic system to a constant working or medium pressure (approx 50psig)- psi gauge

Question 9

what are the 2 ways oxygen is delivered from the medium pressure component to the low pressure components (breathing circuit)

Answer 9

1. via the oxygen flow meter to the fresh gas inlet
2. via the oxygen flush valve to the fresh gas inlet

Question 10

what is the oxygen flow meter?

Answer 10

L/min or mL/min: low pressure delivery device, which then feeds thru the vaporizer allowing inhalant anesthetic vapor to enter the system

Question 11

what is the oxygen flush valve?

Answer 11

medium pressure delivery device that bypasses the flow meter and the vaporizer. delivers high flow, medium pressure and puts ONLY oxygen into the patient. even when you turn the vaporizer off, pt still getting anesthetized

Question 12

after oxygen has flown through from either the oxygen flow meter or the oxygen flush valve, are the rest of the circuit components high, low or medium pressure? how is this maintained?

Answer 12

the rest are low pressure components, remain low because it has an adjustable pressure relief valve. when open, excess gas will go out scavenge

Question 13

what pressure is the low pressure system at?

Answer 13

typically neutral (0-2cmH2O) because directly attached to patient’s airway
- pressure can be negative (-1 to -6cmH2O) when patients spontaneously ventilate
- or positive (8-10cmH2O for small animals, 20-30 cmH2O for larger animals) when we manually ventilate

Question 14

in order to generate positive pressure in the system does the pressure relief valve need to be open or closed? why?

Answer 14

needs to be OPEN so no gas can escape to the scavenge system

Question 15

what is another name for the pressure relief valve?

Answer 15

pop off valve

Question 16

when we manually ventilate a patient, should the pop off valve be on or off?

Answer 16

off, so that positive pressure can be generated in the system and no gas can escape

Question 17

T/F: the pop off valve should ALWAYS be open

Answer 17

true!

Question 18

when are the only times the pop off valve should be closed?

Answer 18

1. to temporarily pressurize the system to check for leaks in circuit components
2. to temporarily provide a positive pressure manual breath to the patient
3. if the patient is attached to a mechanical ventilator, which has its own pressure relief valve

Question 19

how are breaths getting to the patient?

Answer 19

either they are breathing the circuit gas spontaneously or positive pressure breaths are delivered by manually squeezing gas from the rebreathing bag OR the ventilator delivers gas mechanically

Question 20

T/F: the circle system is 1-way and unidirectional

Answer 20

true, forces exhaled CO2 to go thru a CO2 absorbant

Question 21

how can expired gas escape

Answer 21

via pressure relief valve to the scavenging system
OR
can remain in the circle to be rebreathed after it passes thru the CO2 absorbent

Question 21

what are CO2 absorbers?

Answer 21

bases that neutralize acids (CO2)

Question 22

what are the end products after CO2 has been thru an absorber?

Answer 22

water and carbonate absorbing CO2 and removing gas so inhalant and O2 gas come back

Question 23

how much O2 do patients need to meet basal metabolic demands? do they get this in a typical anesthesia machine?

Answer 23

4-6mL/kg/min of O2 for a "closed circle" but most use flow rates of 20-30mL/kg/min (a "semi closed circle")- easier to change concentrations in this way

Question 24

instead of a circle system, what could be used?

Answer 24

a non-rebreathing system: high and medium pressure components are the same exhaled air goes out to scavenge

Question 25

what is a nonrebreathing system?

Answer 25

"coaxial": 2 limbs in 1 piece - fresh gas enters small inner tube, gets inhaled - exhaled gas exits via the outer corrugated tube to the reservoir bag or via the pop off valve to the scavenge system exhaled gas is NOT rebreathed or recycled because high fresh gas flow clears the tubing of exhaled gas very quickly

Question 26

pros of non-rebreathing system

Answer 26

- very rapid changes in concentration of inhalant anesthetic at patient - simple: no requirement for CO2 absorbent or complex equipment

Question 27

what are minimum fresh gas flow rates for non-rebreathing systems?

Answer 27

200mL/kg/min: very high! need fresh gas rates at 10x higher. may need to be increased if patient has abnormal respiratory pattern

Question 28

cons of nonrebreathing systems

Answer 28

- high fresh gas flows limit patient size (typically less than 5-10kg) - system inefficient and produces excess waste gas