Common breathing systems Flashcards
what is normal peak inspiratory flow rate?
30 - 40 L/minute
Is it practical to supply a 30 - 40 L/m in flow rate to a patient to account for the possible peak inspiratory flow rate. What is done to avoid this?
NO.
Addition of a reservoir bag –> this prevents the creation of a negative pressure in the breathing system during peak inspiration and allows lower fresh gas flows
How is prevention of CO2 rebreathing accomplished?
- High enough FGF to flush exhaled CO@ from the system (dependent on circuit characteristics)
- A 1-way system that passes exhaled gases through a canister that removes CO2 chemically (circle systems)
What should APL be set at for spontaneous ventilation and manual ventilation
Spontaneous ± 1 -2 cmH2O
Manual ventilation ± 15 - 20 cmH2O
At what pressure will the valve always open even if completely closed
60 cm H2O - safety
What is the defining feature all Mapelson circuits?
These circuits rely on FGF to wash out CO2
What is the Mapelson A circuit?
What is its main advantage and disadvantage
Also known as the Magill system
FGF –>RB –> BT –> APL –> Patient
Advantage: More suited to spontaneous ventilation as low FGF required to prevent CO2 rebreathing. APL must be closed during controlled ventilation which means that higher FGF required to washout CO@ from the patient end of the circuit.
Disadvantage: Cumbersome to connect scavenging system to the exhaust valve near the patient’s airway
What system was designed to overcome the cumbersome position of the APL valve in the Mapelson A circuit?
The Lack System is a co-axial Mapelson A system. The expiratory limb of the breathing system extends within the inspiratory limb with two advantages:
- The APL valve can be connected distal o the patient
- Counter current heat exchange system is established for warming inhaled air.
What is a Mapelson D circuit
RB–>APL–>BT–>FGF–>patient
More suited to controlled ventilation than spontaneous breathing.
How is efficiency of a non-circle system breathing circuit determined
The minimum fresh gas flow required to prevent rebreathing of CO2.
Mapelson A - most efficient for spontaneous Ventilation (FGF = 1 x VE)
Mapelson D - most efficient for controlled ventilation
(FGF = 1.5 x VE)
What is Mapelson E and a Mapelson F circuit
Otherwise known as the Ayre’s T-piece, this is a lightweight open ended circuit with FGF near the patient.
BT –> FGF –> patient
Mapelson F or the Jackson-Rees modification of the Ayre’s T-piece adds a valveless open ended reservoir bag to the Ayre’s T-Piece
Open ended RB –> BT –> FGF –> PT
What is the Humphrey ADE circuit?
This is a system that provides the ability to interchange between Mapelson A, D and E arrangements
Describe the Mapelson B circuit
RB –> BT –> FGF –> APL –> PT
Rarely used
Describe the Mapelson C circuit and its use and why alternatives are more effective
RB –> FGF –> APL –> PTUsed for resuscitation BUT a self-inflating bag is better because it does not rely on FGF for ventilation
For a 70 kg person, define the minimum fresh gas flow required to clear CO2 adequately when breathing SPONTANEOUSLY for Mapelson A, C, D.
A - 5L/minute (VE)
C - >14 L/minute (>2 x VE)
D - 14 L/min (2 x VE)
What are the two additional components required in a circle system
Two unidirectional valves
CO2 absorber (soda lime)
What does the increased complexity of the circle system cause
More connections which increases the possibility for errors or disconnection
What is the theoretical O2 flow rate required in a perfect circle system that is fully equilibrated with anaesthetic gases. how is this different in practice
Matching the patient’s O2 consumption: 250 ml/minute
This is different in practice as equilibration is rarely reachedand the circle system may have a number of small leaks –> a higher flow of 1 L/min O2 is used
When during an anaesthetic are high fresh gas flows required
- At the start –> rapid uptake phase by patient
- If rapid changes of vapour concentration are required
- At the end of a case to washout the anaesthetic agents
