Getting Gas to the Patient (Breathing Systems - Part 1 &2)

What are the three classifications and components of breathing systems.

• Non-rebreathing systems

• Lack

• T-piece

• Bain

• Rebreathing systems

• Circle

• Hybrid systems

• Humphrey ADE

What are the functions of breathing systems?

• Deliver oxygen to patient

• Deliver anaesthetic gas and/or vapour to patient

• Remove exhaled carbon dioxide

• Provide a means to ventilate patient

Reminder**

• What is tidal volume?

• What is respiratory minute volume?

• Tidal volume

- Volume of gas exhaled in 1 breath (AKA size of the breath)

• 10-20 ml/kg for most smaller patients

• Respiratory minute volume (minute ventilation)

• Volume of gas exhaled in 1 minute

• Tidal volume x respiratory rate

  • = 200 ml/kg

What is rebreathing?

• What are the two types?

• Rebreathing

• Inhalation of previously exhaled gas

• Two types:

- Rebreathing of exhaled gas from which CO₂ has been removed (by an absorbent) is not detrimental (usually with an absorbant)

- Rebreathing of unchanged exhaled gas leads to build up of CO2 (hypercapnia), since CO2 not removed

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

What is equipment dead-space?

• Equipment dead-space

- Volume of breathing system that may contain exhaled gas that could be rebreathed during the subsequent breath

What are the components of the breathing system?

• Tubing

• Reservoir bag

• Adjustable pressure limiting (APL) valve

• Carbon dioxide absorbent (e.g. soda lime)

• Unidirectional valves

What is the purpose of the tubing in a breathing system?

• What can alter the resistance of gas flow through these tubes?

• Tubing

- Conveys gases to and from patient

- Usually corrugated to resist kinking

• Causes turbulence of gas flow which increases resistance

- Smooth internal bore reduces resistance

How are the tubes typically arranged in the breathing system?

What is the purpose of the reservoir bag in the breathing system?

• Reservoir bag

- Various sizes

• Typically 0.5-6L in small animals

• 20-30L in horses!

- Functions include:

• Reservoir

• Visual aid

• Means of assisting ventilation

  • Can be squeezed to push gas into the patient

What is the purpose of the APL valve?

• Adjustable pressure limiting (APL) valve

- "Pop-off", "spill" or "expiratory valve"

• Provides a means of escape for excess gas preventing pressure build up - important that this valve is open

• Connects to scavenging system for disposal of waste gases

What does “open” and “closed” refer to in the APL valve?

• "OPEN"

- ANTI-CLOCKWISE to OPEN

- "Lefty loosey"

• "CLOSED"

- CLOCKWISE to CLOSE

- "Righty tighty"

When should the APL valve be open?

• Valve should always be fully open during spontaneous ventilation

• Only adjusted during intermittent positive pressure ventilation (IPPV), i.e. when we want to manually inflate the patient's lungs

• If you don’t notice: pressure can damage patient lungs and cause a pneumothorax

  • Reservoir Bag Over-Inflation: The breathing bag fills up completely and becomes rigid and hard to squeeze because fresh gas keeps flowing in with nowhere to escape.

What are the main features of non-rebreathing systems?

• Features

• Advantages

• Disadvantages

• Features: A non-rebreathing system is an anesthetic breathing circuit that delivers fresh oxygen and gas directly to a patient while venting all exhaled carbon dioxide out of the system rather than recycling it.

- 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)

• Advantages:

- Patient inspires fresh gas

• High Fresh Gas Flow: It relies on a high rate of incoming fresh gas flow to physically flush out exhaled CO₂ and waste gases before the next breath.

• Patient breathes gas of known composition

• Anaesthetic depth can be changed rapidly

  • Minimal Resistance: Because there are no heavy valves or chemicals for the patient to push air through, the breathing resistance is extremely low.

• Disadvantages

- High fresh gas flow (FGF)

• Increased cost

• Increased potential for environmental pollution

- Fresh gas is cold & dry

• Hypothermia

• Respiratory dessication

How can you calculate the fresh gas flow (Method 1)?

One ways to calculate FGF:

- Use respiratory minute volume or minute ventilation (MV)

• MV = resp. rate x tidal volume (10-20ml/kg)

• FGF = MV x “circuit factor”

- Circuit factors:

• Lack: 0.8-1

• Ayre's T-piece 2.5-3.5

• Bain: 1-3.5

How can you calculate the fresh gas flow (Method 2).

Second way to calculate FGF: (More common method)

• Recommended method:

- Lack: 150-200ml/kg/min

- T-piece: 400-600ml/kg/min

- Bain: 200-600ml/kg/min

Why is a calculated FGF not always sufficient?

• 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

• But by how much? Capnography helps!

What is the lack system?

The Lack anaesthetic system is a parallel or coaxial (tube-within-a-tube) breathing circuit used to deliver oxygen and volatile anaesthetic gases to a patient while removing exhaled carbon dioxide

• It is a modified version of the Magill circuit, falling under the Mapleson A classification of semi-open breathing systems.

• Fresh gas flows through an inner tube to the patient, while exhaled gases travel back through the wider outer corrugated tube toward an adjustable pressure limiting (APL) exhaust valve.

• The exhaust valve sits back at the anaesthetic machine end rather than right next to the patient's face.

Parallel Lack

• More bulky but probably safer

  • More widely used

Coaxial Lack

• Difficult to detect damage if inner tube is damaged but is less bulky by being a tube within a tube

• Damage/disconnection of central tube leads to marked rebreathing of COz

Is a lack system useful for sustained IPPV?

NO

• Alveolar Gas Push: When you squeeze the reservoir bag during IPPV with low fresh gas flows, the fresh gas cannot adequately clear the expiratory limb, forcing carbon dioxide-rich gas back into the patient's lungs.

• Rebreathing Risks: Unless extremely high fresh gas flow rates (far exceeding normal recommendations) are used to continuously flush the circuit, the patient will re-breathe expired carbon dioxide, causing hypercapnia.

What patients is a parallel lack and minilack system useful for?

Parallel Lack

KEY FACTS:

• Patients 10 kg & over

• FGF 150-200 ml/kg/min

• Not suitable for prolonged IPPV

MiniLack system

Parallel Lack adapted for smaller patients

• Patients 10 kg & less

• FGF 200 ml/kg/min

• Not suitable for prolonged IPPV

What are the main features of (Basic) Ayre’s T-piece system?

• Features

• Advantages

• Disadvantages

An Ayre's T-piece is a lightweight, valve-less anesthetic breathing system (classified as a Mapleson E or Jackson-Rees Mapleson F circuit) used to deliver oxygen and anesthetic gases

How does the Jackson-Rees Modification of the T-piece function?

Addition of an open ended bag

• JACKSON-REES MODIFICATION:

  • Classed as Mapleson F

  • Allows observation of respiration

  • Allows more control during IPPV

  • BUT Difficult to scavenge

  • Attaching a standard scavenging tube directly to an open bag tail can easily cause the soft bag to twist, occlude, or collapse. This creates dangerous, inadvertent positive pressure or airway obstruction in small pediatric patients.

How does the Mapleson D Modification of the T-piece work?

Adaptation to facilitate scavenging

• Includes closed reservoir bag & APL valve

KEY FACTS:

• Patients up to 10 kg

• FGF of 400-600 ml/kg/min

• Suitable for IPPV

What are the main features of the Bain system?

• Features

• Advantages

• Disadvantages

• Mapleson D

• Modification of T-piece

- Parallel or coaxial (most common)

KEY FACTS:

• Patients 10 kg & above

• FGF 200-600 ml/kg/min

• Suitable for IPPV

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

• Slower respiratory rate

• Longer expiratory pause

What is the difference between a coaxial bain and a coaxial lack?

COAXIAL:

• Damage/disconnection of central tube leads to marked rebreathing

• BUT: we can test integrity of inner tube in coaxial Bain, cannot do so easily with the coaxial lack

Always check coaxial Bain prior to use !

What are the main features of rebreathing systems?

• Features

• Advantages

• Disadvantages

FEATURES:

• Exhaled gases are rebreathed after removal of CO, by an absorbent

• Relatively low FGF can be used

• Patient inspires a mixture of fresh gas & exhaled gas

ADVANTAGES:

• Lower fresh gas flow

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

• Less environmental contamination

• Gases are warmed and humidified

DISADVANTAGES:

• Greater resistance to breathing

• Soda lime canister (CO2 absorber)

• Unidirectional valves

• Unsuitable for small patients

• 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 traditional absorbant for carbon dioxide?

• Traditional absorbent is SODA LIME

- 80% calcium hydroxide

- 4% sodium hydroxide

- 14-20% added water

- Indicator dye

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

What aspect of the soda lime changes allowing you to see exhaustion via CO2 absorption (Indicating fresh soda lime is needed)?

• pH of soda lime changes, allowing use of indicator dyes to reveal exhaustion (pink to white or white to purple)

• Exothermic reaction

• More water is produced than is consumed

How does FGF determine the extent of rebreathing in these anesthetic systems?

• High FGF not needed to expel exhaled COz

• 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)

What are the FGF requirements for full (complete) rebreathing?

• What are some problems with rebreathing?

• 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

Problems during use:

• Flowmeters may be inaccurate

• Vaporisers may be inaccurate

• Marked dilutional effect

• Unsure of exactly what they breath, discrepancy between vaporizer and what they breath, difficult to rapidly increase or decrease what they breath due to the volume of the system

• Turning up FGF will increase effects

• 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

Advantages:

• Very economical (& sustainable)

BUT

• Difficult to use!

What are the FGF requirements for partial rebreathing?

• What are some problems with rebreathing?

• Oxygen supplied is greater than that required for metabolic O, 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

Compare a partial rebreathing system to a full rebreathing system.

Compared to full rebreathing mode:

• Flowmeters should be accurate

• Vaporisers should be accurate

• Less dilution effect (though it still occurs)

- Easier to adjust anaesthetic depth

- N20 can be used (relatively) safely

What is important to remember for the lung contents of a patient at the start of anesthesia (Using a rebreathing system)?

At the start of anaesthesia:

• Patient's lungs and breathing system are full of room air (79% N2)

• No circulates diluting inspired concentration of anaesthetic & oxygen

• Need to DENITROGENATE post-induction

How is denitrogenation performed?

What are the two rebreathing systems?

• To & Fro System

• Circle Circuit

What are the main components of a circle rebreathing system?

COMPONENTS:

• Soda lime canister

• Reservoir bag

• Unidirectional valves

• Flow through soda lime is unidirectional - gas needs to go in one direction

• Patient size - depends!

• Human adult circle > 15 kg

• Veterinary specific circles ≥ 10 kg

What governs which system you should choose for anesthesia?

Describe how the Humphrey ADE system works for different sized patients.

• Pros and Cons

HYBRID SYSTEM:

• For patients 10 kg and over:

- Include soda lime canister

1. Use as a circle

• For patients < 10 kg:

- Remove soda lime & use in Non-RB mode

2. With lever up for spontaneous breathing (= MiniLack)

3. With lever down for IPPV ( = Bain/T-piece)

PROS & CONS:

• Easy to change from spontaneous breathing to IPPV (? advantage)

• Suitable for wide range of patient sizes

• Economical to run

• Expensive to purchase