Breathing systems Flashcards
What are the functions of a breathing system?
- Supply oxygen
- Supply anaesthetic agent
- remove CO2
What are the 2 types of breathing circuit?
- Re-breathing
- Non-rebreathing
Describe the main features of non-rebreathing circuits
- High flow rates used to flush out CO2
- Expensive
- More oxygen, nitrous oxide and inhalant used
- Cannot be used for large animals
Name the main types of non-rebreathing circuit
- Magill
- Lack (and mini Lack)
- Bain (and mini Bain)
- T piece
Describe the Magill breathing circuit
- Valve near patient
- Bag at opposite end
- Circuit factor 1
- Should no longer be used
- Used in large dogs over 10kg
What are the disadvantages of the Magill breathing circuit?
- Heavy end near patient due to valve, risk of pulling ET tube out of the patient’s mouth
- Inefficient for IPPV
- Risk of breathing own CO2
- Alveolar gas is vented and dead space gas is rebreathed
What are the advantages of the Magill breathing circuit?
Efficient for spontaneous breathing
Describe the Lack breathing circuit
- Bag and valve at machine end
- Parallel running tubes
- Green tube is fresh gas
- Circuit factor 1
- Only for animals >10kg spontaneously breathing
- Inefficient for IPPV
Describe the mini Lack breathing circuit
- Smooth bore tubing
- Lightweight valve
- Minimal resistance
- Circuit factor 1
- Animals 2-10kg
- Not suitable for long term IPPV unless fresh gas flow increased to 600ml/kg/min
Describe the Bain and mini Bain breathing circuits
- Smaller tube within another tube (green is inner, supplies fresh gas)
- Circuit factor 2.5
- High flow rates
- Good for IPPV
- Bain can be used on animals 7-20kg
Describe the T piece breathing circuit
- No valves
- Low resistance
- Used for animals <10kg
- High flow rates
- Circuit factor 2.5
- Good for IPPV
- Long green tube, smaller bore
- Shorter white tube with bag ad valve at end
- Valve at machine end
- Human pediatric versions often used with teddy on bag
Explain how fresh gas flow rate is calculated for non-rebreathing systems
- Assume minute volume is approx 0.25L/kg
- Flow rate (L/min) = circuit factor x body weight x 0.25
What factors may affect the fresh gas flow rate required?
- Pain
- Pyrexia
- Depth of anaesthesia
- Size of animal
In what proportion of fresh gas flow should nitrous oxide be given?
- Should be given as 2/3rds of total fresh gas
- 1/3rd oxygen
- I.e. 5L/min total FGF, give 3L nitrous oxide, 2L oxygen
What are the different types of rebreathing circuits?
- To and Fro
- Circle
Describe To and Fro circuits
- Outdated
- Soda lime between patient and bag
- Can lead to soda lime dust inhalation
- CO2 channeling over soda lime so is not absorbed
- Dead space increases with time
Describe the benefits of circle circuits
- Efficient
- Warms and humidifies gas
- Less pollutant
- Easy to use and perform IPPV
- Lower fresh gas flows, less oxygen, less inhalant required
Describe the layout of circle circuits
- Fresh gas flow enters from commo gas outlet of anaesthetic machine
- Flows through one way valve to patient through inspiratory tube
- Through Y-piece to patient
- From patient through expiratory tube, through one way valve on expiratory limb
- In and out of reservoir bag
- Through soda lime cannister and back to patient mixed with fresh gas
What is the main difference between rebreathing and non-rebreathing circuits?
Rebreathing circuits have absorbent (most commonly soda lime) in circuit
Describe absorbents in rebreathing circuits
- Usually calcium hydroxide (soda lime) and barium hydroxide lime
- Incorporate indicators to show activity
Give the chemical reaction that occurs in soda lime
2H2CO3 + 2NaOH + Ca(OH)2 -> CaCO3 + Na2CO3 + 4H2O + heat
H2O+CO2 -> H2CO3
Describe the valves used in rebreathing circuits
- Pressure generated by patient’s breathing, causes disc to move, gas passes in one direction only
- Transparent dome allows visualisation of movement
- Valves/discs can be made of deformable rubber or mica or plastic (light materials)
- Must be checked as can get stuck
What volume of air must be in the breathing bag on a circuit?
- Must be greater than the patient’s inspiratory capacity
- Estimated to be 30ml/kg
What volume should be absorber cannister be?
- At least double that of the tidal volume of the patient in order to optimise efficiency
- Soda lime contains 50-70% air around the granules
What are the 2 types of fresh gas flows that can be used with circle circuits?
- Closed
- Semi-closed/open
Describe the closed fresh gas flow system
- O2 consumption (m/min)= 10xbodyweight^0.75
- Only supplying enough oxygen for the patient to stay alive
- E.g. for a 20kg dog, require 95ml/min of O2, only supply this amount
- Requires FIO% monitor
- Risky, not used
Describe the semi-closed/open fresh gas flow system
- O2 flow rate exceeds patient’s O2 consumption
- Excess lost via pressure relief valve into scavenge
- Initially have high flow rate, high vaporiser setting to raise concentration of anaesthetic in circuit
- For maintenance, both are reduced
- General rule: 2-3L/min for first 10-15 min, then 500-1L/min during maintenance
- For large animals e.g. horses, 8-10L/min initially then reduce
What are the advantages of lower fresh gas flow?
- Less wastage
- Cheaper (less inhalant and carrier gas used)
- Less pollutant
- Gases warmed and humidified
Describe the steps required when changing inhalant concentration used during anaesthesia in an open circle system
- Open valve
- Increase fresh gas flow
- Set to desired concentration
- Expel old concentration using bag
- Close valve but leave semi-open
- Reduce FGF after 10-15mins to maintenance
In what ratio is nitrous oxide used in circle systems?
50:50, but little benefit of using nitrous oxide, MAC is unachievably high
What are the advantages of the HUmphrey ADE circle (combination) system?
- Compact, robust, easy to clean
- Versatile (2-100kg)
- Positive End Expiratory Pressure Valve maintains positive pressure between breaths and stops collapse of alveoli in small patients
- Excellent for IPPV
- Metal is non-ferrous, can go close to MRI
Describe induction chambers
- Useful as removes need for stressful restraint of small animals
- Scavenge from top or lid
- Oxygen and volatile enter at bottom
- removable clear partition may be available to accommodate all sizes
What is IPPV?
Intermittent Positive Pressure Ventilation
Describe how IPPV is performed
- AMBU bag or bag on breathing system
- AMBU bag is self inflating
- Deliver room air or oxygenated air to patient where they are not doing this themselves by squeezing the bag
When is IPPV required?
When the patient cannot ventilate itself in order to prevent hypoxaemia (and consequent hypoxia)
Describe the anaesthetic machine safety check
- Switch off flow control valves
- Ensure cylinders are closed and fitted securely on hanger yolks
- Press emergency O2 button until no gas flows from common gas outlet
- Check flowmeters and pressure gauges read 0
- Switch on oxygen cylinder slowly and check pressure gauge, replace if low
- Switch on flowmeter and check smooth operation
- Check emergency O2 flush (switch off, then test second cylinder if present)
- Check N2O cylinder
- Switch O2 to 2L/min, N2O to 4L/min, then switch off )2 - oxygen failure device should alarm
- check vaporiser is connected, full of agent, smooth dial movement when gas off, no leaks
- Check emergency overpressure valve and air intake valve if fitted
How useful is the nitrous oxide pressure gauge at indicating volume remaining in cylinder?
Poor, as nitrous oxide is stored a liquid/vapour and so will always read as full until empty, as vapour is constantly replaced from the liquid reservoir
What is the importance of the flow control valves (flowmeters) on an anaesthetic machine regarding the part of the machine?
- Divides machine into 2 parts
- High pressure circuit upstream of flowmeters
- Low pressure circuit downstream of flow control valves
How is flow into the low pressure system of an anaesthetic machine regulated?
- Regulated by the operator
- Adjust flowmeter control valves
What is the function of the pin index on anaesthetic machines?
- Pin Index Safety System is a safeguard that prevents cylinder interchaging
- each gas or combination of gases has its own particular pin pattern to prevent wrong placement of gases onto yokes
Where is gas flow rate read from depending on the type of float in a flowmeter?
- Read from middle of ball
- Top of bobbin/float
How do hypoxia prevention devices on anaesthetic machines work?
- Interface oxygen and nitrous oxide systems to ensure minimum of 25% oxygen at common gas outlet
- But are not foolproof
What is the purpose of interlock devices on anaesthetic machines?
Prevent concurrent use of more than one vaporiser
What is a potential outcome of tipping of vaporisers?
Movement of liquid anaesthetic into the bypass chamber, leading to very high anaesthetic agent concentrations being released to the patient
What are the ISO colours for oxygen, nitrous oxide, medical air, suction and CO2 cannisters?
- Oxygen: white
- NO2: light blue
- Medical air: black and white
- Suction: yellow
- CO2: grey and green
What causes turbulent flow in breathing systems?
Bends and changes in direction of the gas and increases the resistance further
When should soda lime be exchanged?
When the indicator has changed colour in approx 2/3rd of the absorbent
What are important considerations when using circle circuits for small patients?
- Standard circle absorbers cannot be used in small patients because of dead space
- Have small tidal volumes and not enough pressure to open the valves
- Effective dead space of Y-piece is larger than it appears so smaller Y-pieces must be used
What affects the rate of change of anaesthetic concentration?
- Fresh gas flow rate
- High FGF rate will achieve equilibration must faster than if low FGF rate is used
Describe the Humphrey ADE-circle system
- Combines advantages Mapleson A (spontaneous breathing patients) , D and E (preferable for controlled ventilation and small patients) systems
- Possibility of adding absorbent canister and converting into circle
What are the advantages of passive scavenging systems?
- Inexpensive to set up
- Simple to operate
- Charcoal cannisters are mobile
What are the disadvantages of charcoal cannister scavenges?
- Continuing cost of replacemeent
- Can be refilled with fresh charcoal but is messy
- Does not remove nitrous oxide
- Heating cannister causes release of inhalational agents
Describe a simple active scavenge
- Reservoir made of plastic pipe 3” diameter, 2 feet long
- Capped at both ends, needle valve at bottom
- 2 holes drilled in top cap
- 1 hole for hose from outlet of anaesthetic machine to be inserted
- Other to allow passage of atmospheric air
- Valve adjusted so air is slowly sucked into pipe when anaesthetic machine is in use, preventing waste gas escape into the atmosphere
What is an advantage of a simple active scavenge?
Convenient in large hospitals/practices where many machines are in use in different locations
What is a disadvantage of a simple active scavenge?
Vacuum system and pipe work is major expense and may need continual adjustment
What is meant by ambulatory infusion?
- Animal is freely moving without need for tether to connect with catheter
- Normally only in large animals that can carry jackets with infusion pump and compound reservoir
What is meant by an atraumatic procedure?
Minimal tissue injury caused during the procedure
What is meant by biocompatibility?
Good toleration of implants by animal tissues after implantation
What is a biofilm?
A coating which develops on implanted materials derived from the animal’s own tissue fluids and cells
What is the friction coefficient?
The surface friction of a catheter, affects how easily a catheter can be inserted into a blood vessel
