eLFH - Gas Supplies, Breathing Systems and Ventilators Flashcards
Vapour definition
A gas below its critical temperature
Thus compression to the liquid is possible
For gas supplies - refer to flashcard decks in Anaesthetic FRCA Primary - eLFH Physics
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Fixed gas definition
A gas above its critical temperature
Entonox gas mixture
Oxygen and Nitrous oxide 50:50 mix by VOLUME (not by weight)
Classification of breathing systems
Mapleson A to F
Mapleson F was added later
Classifies breathing systems according to location of reservoir bag, APL (adjustable pressure limiting) valve and fresh gas inflow
Diagram illustrating Mapleson A to F breathing systems
Mapleson systems commonly used in paediatric patients
Mapleson E and F
Mapleson systems used in resuscitation in critical areas
Mapleson C
Mapleson systems most commonly encountered in routine anaesthetic practice
Mapleson A and D
Examples of Mapleson A system
Magill system
Lack system
Lack system features
Mapleson A system but with weight of the APL valve removed from the facemask towards the anaesthetic side of the circuit
Available as parallel and coaxial variants
Coaxial system definition
Expiratory limb within the inspiratory limb
Or Inspiratory limb within the expiratory limb
Bain system features
Coaxial variant of Mapleson D system for compact design
Inspiratory limb within the expiratory limb
Three elements of breathing cycle
Inspiration
Expiration
Expiratory pause
With which form of ventilation is Mapleson A system most efficient and why
Spontaneous respiration
Rebreathing of dead space gas (which is essentially fresh gas with humidification) allows a fresh gas flow of ~ 70% of minute ventilation to be used
Mapleson A system - breathing cycle in spontaneous respiration
During expiration, fresh gas flow initially fills the reservoir bag while dead space gas enters the system
During expiratory pause with bag now filled, pressure in system rises and opens APL valve - alveolar gas now leaves the system through valve and continued FGF will purge residual alveolar gas +/- dead space gas
At beginning of next inspiration, dead space gas inhaled first contributing to 30% of tidal volume, so only remaining 70% needs to be met by FGF
Mapleson A system - breathing cycle in controlled ventilation
Expiration:
FGF initially fills bag while dead space gas enters the system
Expiratory pause:
Alveolar gas reaches reservoir bag before APL valve opens
Very high FGF required to vent this gas mixture prior to inspiration
Inspiration:
Manual squeeze of reservoir bag generates high positive pressure for inspiration.
Therefore APL valve opens and FGF + dead space gas are wasted resulting in inefficiency
Mapleson A system - key features
Tubing 110 - 180 cm long
Fresh gas runs in the outer tube of coaxial variants
Efficient for spontaneous respiration, inefficient for controlled ventilation
Requires FGF equal to alveolar ventilation
Dead space too great to use in children < 25 - 30 kg
Approximate alveolar ventilation value
~ 70 ml/kg/min
With which form of ventilation is Mapleson D Bain system most efficient
Controlled ventilation
Mapleson D system - breathing cycle in spontaneous respiration
Expiration:
In spontaneous expiration, dead space, alveolar and some fresh gas pass to the bag
Expiratory pause:
High FGF required to purge alveolar gas
Inspiration:
Initially inhales fresh gas
If FGF is insufficient, extra flow requirement will draw from reservoir bag contents - results in rebreathing
Mapleson D system - breathing cycle in controlled ventilation
Expiration:
Dead space, alveolar and some fresh gas pass through tubing to the bag, as it does with spont respiration too
Expiratory pause:
Fresh gas fills the distal part of the tube
Inspiration:
Squeezing bag produces positive pressure which opens APL valve - Alveolar gas vented out through valve and fresh gas driven into lungs
With sufficient FGF, the bag acts as a driving gas and is not re-breathed
Mapleson D Bain system - key features
Tubing 180 cm but increasing length doesn’t affect its performance
Fresh gas runs in inner tube of coaxial Bain system
Efficient for controlled ventilation, inefficient for spontaneous respiration
Required FGF equal to alveolar ventilation in controlled ventilation
Circle system definition
Most efficient system but also most complex
Doesn’t fit into Mapleson classification as it is a closed system
Essential elements of Circle system
Reservoir bag
APL valve (never between patient and inspiratory valve)
CO2 absorber cylinder
Inspiratory and expiratory tubing both with unidirectional valve
Source of fresh gas and anaesthetic vapour (never entering between patient and expiratory valve)
Advantages of the Circle system
Minimal equipment dead space
Conservation of heat and humidity
Ability to remove CO2 and recycle gas
Closed circuit configuration minimises fresh gas consumption
Reduced pollution
Disadvantages of Circle system
Multiple components to maintain and test
High potential for leaks
Potential for circuit to empty if used in closed configuration with inadequate FGF
What is used in circle system to absorb CO2
Soda lime
Constituents of soda lime
Calcium hydroxide (75%)
Water (20%)
Sodium hydroxide (4%)
Potassium hydroxide (1%)
Indicator dye
CO2 absorption by soda lime chemical reaction
CO2 absorption - beneficial features of the reaction
Exothermic
(warms gases)
Produces 1 mole water for each mole CO2 removed
(humidifies gases)
Although conventionally fresh gas enters the circuit after the soda lime
Amount of CO2 absorbed by 1kg of soda lime
120 L
Colour change of soda lime when exhausted
Depends on the particular pH sensitive colour dye used by the brand
Commonly dyes change granules from white to purple, or from pink to white
Cardiff Aldasorber
Absorbs anaesthetic volatile agents onto activated charcoal
Form of scavenging system
Main modes of ventilation
Pressure control
Volume control (constant flow)
Cycling mode definition
The parameter used to determine when the ventilator should cycle from inspiration to expiration
Cycling modes
Time cycled
Flow cycled
Pressure cycled
Volume cycled
Volume control pressure and flow graphs
Pressure limitation with volume control ventilation
Can set pressure limitation as safeguard against barotrauma if compliance suddenly decreases
Older ventilators would truncate breath is pressure limitation reached leading to hypoventilation
Modern limiters employ decelerating flows so most of remaining volume can be delivered within pressure limit
Patients where volume control would be preferred and why
Brain injury patients
Volume control allows optimisation of PaCO2
Patients where volume control is not preferred and why
Paediatric patients with uncuffed tubes
Volume control is unable to compensate for leaks
Pressure control pressure and flow graphs
Why pressure control potentially reduced control over PaCO2
Set pressure
Decelerating flow - generates tidal volume dependent on compliance and airways resistance
Poor handling of compliance / resistance changes
Advantage of pressure control
Less likely to worsen or cause lung injury
More able to compensate for leaks in breathing system - e.g. paediatrics with uncuffed tubes
Decelerating flow pattern improves gas exchange and homogeneity of ventilation especially in patients with ventilatory distribution issues
(i.e. lung units with grossly varying time constants)
Higher mean airway pressure than volume control for given tidal volume - improves oxygenation
Methods to optimise oxygenation in ventilated patient
Increasing PEEP
Increasing mean airway pressure
Increasing FiO2
Moving from supine to semi-recumbent position
Why is semi-recumbent position better for ventilation than supine
When semi-recumbent, FRC is increased and atelectasis reduced
When will increasing fresh gas flow help to reduce PaCO2
When using semi closed circuit to reduce rebreathing - i.e. Mapleson circuit
