Getting Gas to the Patient: Breathing Systems

Go to Part 2 PP for diagrams

Define Tidal Volume:

Volume of gas exhaled in 1 breath

• 10-20 mL/kg

Define Respiratory Minute Volume (minute ventilation):

Volume of gas exhaled in 1 minute

• Tidal volume x respiratory rate

  • ≈ 200 ml/kg

What is a rebreathing system?? What are the two different types of rebreathing?

Rebreathing:

• Inhalation of previously exhaled gas

Two types:

• Rebreathing of exhaled gas from which CO₂ has been removed (by an absorbent) is not detrimental

• Rebreathing of unchanged exhaled gas leads to build up of CO₂ (hypercapnia)

  • Moderate to severe hypercapnia is detrimental causing catecholamine release, cardiac arrhythmias and acidaemia

Define equipment dead-space:

Volume of breathing system that MAY CONTAIN EXHALED GAS and COULD BE REBREATHED during the subsequent breath

What are the two different arrangements of tubing you could see?

PARALLEL

• Side-by-side

COAXIAL

• 1 tube inside of the other

What is the adjustable pressure limiting (APL) valve in your breathing system?

Your “pop-off” or “expiratory valve”

• Provides a means of escape for excess gas, preventing pressure buildup

• Connects to scavenging system for disposal of waste gases

• “OPEN”: Slight increase in pressure during expiration lifts disc and opens valve 

  • Turn left to open

• “CLOSED”: Tension in spring opposes lifting of disc and valve remains closed 

  • Turn right to close

** Valve should always be FULLY open during spontaneous ventilation

• Only adjust during intermittent positive pressure (IPPV)

NON-rebreathing systems…

What is their function? What is required for them to perform this function?

Non-rebreathing systems expel CO2 exhaled from the patient, ensuring they don’t inhale it again

Removal of exhaled CO₂ depends on an adequate fresh gas flow (FGF) 

• Each system has a recommended FGF

  • Using oxygen as carrier gas, then FGF = O₂ flow

  • Oxygen + nitrous oxide (1 part O₂ to 2 parts N₂O)

  • Oxygen + medical air (30% O₂ & 70% air)

What are the 3 main kinds of non-rebreathing systems?

1) Lack (incl. minilack for patients smaller than 10kg)

2) T-piece

3) Bain

What is the biggest advantage for a non-rebreathing system?

Patient inspires fresh gas

• Patient breathes gas of known composition 

• Anaesthetic depth can be changed rapidly

What are some disadvantages to non-rebreathing systems?

• High fresh gas flow (FGF) 

  • Increased cost 

  • Increased potential for environmental pollution

• Fresh gas is cold & dry (breathing in REBREATHED air is already warm)

  • Hypothermia

  • Respiratory dessication

    • Not good for a patient with respiratory disease to be constantly breathing in cold air

How would you calculate FGF?

Using mL/kg/min

For example: Calculate FGF required for a 10 kg dog using a Lack

• Lack 150-200ml/kg/min

• T-piece 400-600ml/kg/min

• Bain 200-600ml/kg/min

So then:

FGF = 200 mL/kg/min x 10kg = 2L/min

• Doesn’t take into account respiratory rate

• May need to adjust flow based on how rapid patient is breathing (go towards the higher end of the range)

How is calculating the FGF not always super accurate?

Normal ventilatory pattern:

• Inspiration, expiration & then expiratory pause 

Expiratory pause is crucial:

• Fresh gas flushes expired gas out of system

• If too short there is insufficient time for expired gas to be removed & rebreathing occurs

• So increase FGF in patients with rapid respiratory rates 

What is one disadvantage to using a Lack system?

NOT suitable for sustained IPPV

• Reservoir bag on INSPIRATORY limb

• Rebreathing and hypercapnia can develop during prolonged IPPV

  • Can be reduced (not eliminated) by increasing FGF

  • Inefficient use of gas

Describe the Ayre’s T-piece system… what are two problems this system has?

Low resistance (no APL valve)

• Problems:

• No reservoir bag so difficult to observe ventilation

• IPPV possible but exposes lungs to high pressure

How has the T-piece system been modified?

Adaptation to facilitate scavenging

• Includes closed reservoir bag & APL valve

Called Mapleson D T-piece

• For patients UP TO 10kg

• FGF of 400-600 mL/kg/min

• Now suitable for IPPV

Describe a Bain system…

Either parallel or coaxial (most common)

• Damage/disconnection of central tube leads to marked rebreathing

• BUT: we can test integrity of inner tube in coaxial Bain

Use for patients 10kg and above

• FGF 200-600 mL/kg/min

• Suitable for IPPV

Essentially:

• Modification of T-piece (FGF 400-600 ml/kg/min) but used in larger patients 

  • Slower respiratory rate

  • Longer expiratory pause

REBREATHING systems…

How are these different from your non-rebreathing systems?

• Exhaled gases are rebreathed after removal of CO₂ by an absorbent

• Relatively low FGF can be used

• Patient inspires a mixture of fresh gas & exhaled gas

  • Less predictable in terms of how much anesthetic / fresh O2 patient is receiving

What are the advantages of using a rebreathing system?

• Lower gas flow

  • More economical - less O₂, N₂O and anaesthetic agent used

  • Less environmental contamination

• Gases are warmed and humidified

What are the DISadvantages of using a rebreathing system?

• Greater resistance to breathing

  • Soda lime canister (CO₂ absorber) 

  • Unidirectional valves

• Unsuitable for small patients due to resistance

• Patient inspires a mixture of fresh gas & exhaled gas

  • Composition of inspired mixture unknown

    • Anaesthetic vapour

    • Oxygen (build-up of nitrous oxide)

• Slower to alter anaesthetic depth

What is the absorbent that takes care of exhaled CO2 in a rebreathing system?

SODA LIME

• 80% calcium hydroxide

• 4% sodium hydroxide

• 14-20% added water

• Indicator dye

  • pH of soda lime changes, reveals exhaustion

  • More water produced than consumed

** Newer CO₂ absorbents that lack strong bases (e.g. NaOH) are available that don’t interact with sevoflurane to produce compound A (damaging)

What is the relationship between rebreathing systems and fresh gas flow?

• High FGF not needed to expel exhaled CO₂

• Relatively low FGF can be used

• FGF used will determine extent of rebreathing

  • Full (complete) rebreathing

  • Partial rebreathing

  • No (minimal) rebreathing

    • > 200 ml/kg/min (minute volume)

Describe full (complete) rebreathing in regards to FGF…

Oxygen supplied is just sufficient to meet the patient’s metabolic oxygen requirement

• Oxygen requirement ≈ 5-10 ml/kg/min 

• FGF 10 ml/kg/min generally recommended as a safe minimum 

No gas exits via the APL valve 

• Alternative name is “closed” system but do NOT actually close APL valve

Maximal rebreathing occurs

What could be some problems with using a full rebreathing technique?

1) Flowmeters may be inaccurate

2) Vaporisers may be inaccurate

• Both may not be calibrated for such a low flow for a 10kg dog

3) Marked dilutional effect

• Inspired concentrations of inhalant & oxygen may differ significantly from those set

• Slow to adjust anaesthetic depth

• Cannot use N₂O unless inspired concentration of O₂ is monitored

EX: for full rebreathing a 20kg dog requires 200 ml/min of oxygen (= FGF 200 ml/min)

Describe partial rebreathing in regards to FGF…

MOST PEOPLE USE THIS

• Oxygen supplied is greater than that required for metabolic O₂ consumption but less than the minute ventilation

  • > 10 ml/kg/min but < 200 ml/kg/min

  • For convenience a FGF of 1 or 2 L/min is often used 

• Excess gas spills via APL valve 

  • Alternative name is “semi-closed” system

• Partial rebreathing occurs

How does the partial rebreathing technique compare to the full rebreathing technique?

• Flowmeters should be accurate

• Vaporisers should be accurate

• Less dilution effect (though it still occurs)

  • Easier to adjust anaesthetic depth

  • N₂O can be used (relatively) safely

What are the two different types of rebreathing SYSTEMS?

1) To and fro system

2) Circle system (see image)

• Usually for larger patients ( > or equal to 10kg)

• These systems can be used in full or partial rebreathing modes depending on the FGF

Compare non-rebreathing systems to rebreathing systems…

What system should you choose for a patient < 10kg? What about 10-15 kg? >15 kg?

<10kg → T-piece or MiniLack 

• For IPPV → T-piece

10-15kg → Bain, Lack, (Circle)

• For IPPV → Bain or Circle

>15kg → Bain, Lack or Circle

• For IPPV → Bain or Circle

NB: the larger the dog the less economic non-RB systems become