eLFH - Gas Supplies, Breathing Systems and Ventilators

Question 1

Vapour definition

Answer 1

A gas below its critical temperature

Thus compression to the liquid is possible

Question 1

For gas supplies - refer to flashcard decks in Anaesthetic FRCA Primary - eLFH Physics

Answer 1

Move to next card

Question 2

Fixed gas definition

Answer 2

A gas above its critical temperature

Question 3

Entonox gas mixture

Answer 3

Oxygen and Nitrous oxide 50:50 mix by VOLUME (not by weight)

Question 4

Classification of breathing systems

Answer 4

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

Question 5

Diagram illustrating Mapleson A to F breathing systems

Answer 5

Question 6

Mapleson systems commonly used in paediatric patients

Answer 6

Mapleson E and F

Question 7

Mapleson systems used in resuscitation in critical areas

Answer 7

Mapleson C

Question 8

Mapleson systems most commonly encountered in routine anaesthetic practice

Answer 8

Mapleson A and D

Question 9

Examples of Mapleson A system

Answer 9

Magill system

Lack system

Question 10

Lack system features

Answer 10

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

Question 11

Coaxial system definition

Answer 11

Expiratory limb within the inspiratory limb

Or Inspiratory limb within the expiratory limb

Question 12

Bain system features

Answer 12

Coaxial variant of Mapleson D system for compact design

Inspiratory limb within the expiratory limb

Question 13

Three elements of breathing cycle

Answer 13

Inspiration

Expiration

Expiratory pause

Question 14

With which form of ventilation is Mapleson A system most efficient and why

Answer 14

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

Question 15

Mapleson A system - breathing cycle in spontaneous respiration

Answer 15

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

Question 16

Mapleson A system - breathing cycle in controlled ventilation

Answer 16

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

Question 17

Mapleson A system - key features

Answer 17

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

Question 18

Approximate alveolar ventilation value

Answer 18

~ 70 ml/kg/min

Question 19

With which form of ventilation is Mapleson D Bain system most efficient

Answer 19

Controlled ventilation

Question 20

Mapleson D system - breathing cycle in spontaneous respiration

Answer 20

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

Question 21

Mapleson D system - breathing cycle in controlled ventilation

Answer 21

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

Question 22

Mapleson D Bain system - key features

Answer 22

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

Question 23

Circle system definition

Answer 23

Most efficient system but also most complex

Doesn’t fit into Mapleson classification as it is a closed system

Question 24

Essential elements of Circle system

Answer 24

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)

Question 25

Advantages of the Circle system

Answer 25

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

Question 26

Disadvantages of Circle system

Answer 26

Multiple components to maintain and test High potential for leaks Potential for circuit to empty if used in closed configuration with inadequate FGF

Question 27

What is used in circle system to absorb CO2

Answer 27

Soda lime

Question 28

Constituents of soda lime

Answer 28

Calcium hydroxide (75%) Water (20%) Sodium hydroxide (4%) Potassium hydroxide (1%) Indicator dye

Question 29

CO2 absorption by soda lime chemical reaction

Answer 29

Question 30

CO2 absorption - beneficial features of the reaction

Answer 30

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

Question 31

Amount of CO2 absorbed by 1kg of soda lime

Answer 31

120 L

Question 32

Colour change of soda lime when exhausted

Answer 32

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

Question 33

Cardiff Aldasorber

Answer 33

Absorbs anaesthetic volatile agents onto activated charcoal Form of scavenging system

Question 34

Main modes of ventilation

Answer 34

Pressure control Volume control (constant flow)

Question 35

Cycling mode definition

Answer 35

The parameter used to determine when the ventilator should cycle from inspiration to expiration

Question 36

Cycling modes

Answer 36

Time cycled Flow cycled Pressure cycled Volume cycled

Question 37

Volume control pressure and flow graphs

Answer 37

Question 38

Pressure limitation with volume control ventilation

Answer 38

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

Question 39

Patients where volume control would be preferred and why

Answer 39

Brain injury patients Volume control allows optimisation of PaCO2

Question 40

Patients where volume control is not preferred and why

Answer 40

Paediatric patients with uncuffed tubes Volume control is unable to compensate for leaks

Question 41

Pressure control pressure and flow graphs

Answer 41

Question 42

Why pressure control potentially reduced control over PaCO2

Answer 42

Set pressure Decelerating flow - generates tidal volume dependent on compliance and airways resistance Poor handling of compliance / resistance changes

Question 43

Advantage of pressure control

Answer 43

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

Question 44

Methods to optimise oxygenation in ventilated patient

Answer 44

Increasing PEEP Increasing mean airway pressure Increasing FiO2 Moving from supine to semi-recumbent position

Question 45

Why is semi-recumbent position better for ventilation than supine

Answer 45

When semi-recumbent, FRC is increased and atelectasis reduced

Question 46

When will increasing fresh gas flow help to reduce PaCO2

Answer 46

When using semi closed circuit to reduce rebreathing - i.e. Mapleson circuit