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Anaesthetic FRCA Primary - eLFH Equipment > eLFH - Gas Supplies, Breathing Systems and Ventilators > Flashcards

eLFH - Gas Supplies, Breathing Systems and Ventilators Flashcards

1
Q

Vapour definition

A

A gas below its critical temperature

Thus compression to the liquid is possible

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1
Q

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

A

Move to next card

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2
Q

Fixed gas definition

A

A gas above its critical temperature

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3
Q

Entonox gas mixture

A

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

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4
Q

Classification of breathing systems

A

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

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5
Q

Diagram illustrating Mapleson A to F breathing systems

A
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6
Q

Mapleson systems commonly used in paediatric patients

A

Mapleson E and F

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7
Q

Mapleson systems used in resuscitation in critical areas

A

Mapleson C

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8
Q

Mapleson systems most commonly encountered in routine anaesthetic practice

A

Mapleson A and D

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9
Q

Examples of Mapleson A system

A

Magill system

Lack system

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10
Q

Lack system features

A

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

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11
Q

Coaxial system definition

A

Expiratory limb within the inspiratory limb

Or Inspiratory limb within the expiratory limb

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12
Q

Bain system features

A

Coaxial variant of Mapleson D system for compact design

Inspiratory limb within the expiratory limb

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13
Q

Three elements of breathing cycle

A

Inspiration

Expiration

Expiratory pause

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14
Q

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

A

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

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15
Q

Mapleson A system - breathing cycle in spontaneous respiration

A

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

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16
Q

Mapleson A system - breathing cycle in controlled ventilation

A

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

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17
Q

Mapleson A system - key features

A

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

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18
Q

Approximate alveolar ventilation value

A

~ 70 ml/kg/min

19
Q

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

A

Controlled ventilation

20
Q

Mapleson D system - breathing cycle in spontaneous respiration

A

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

21
Q

Mapleson D system - breathing cycle in controlled ventilation

A

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

22
Q

Mapleson D Bain system - key features

A

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

23
Q

Circle system definition

A

Most efficient system but also most complex

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

24
Q

Essential elements of Circle system

A

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)

25
Q

Advantages of the Circle system

A

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

26
Q

Disadvantages of Circle system

A

Multiple components to maintain and test

High potential for leaks

Potential for circuit to empty if used in closed configuration with inadequate FGF

27
Q

What is used in circle system to absorb CO2

A

Soda lime

28
Q

Constituents of soda lime

A

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

29
Q

CO2 absorption by soda lime chemical reaction

A
30
Q

CO2 absorption - beneficial features of the reaction

A

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

31
Q

Amount of CO2 absorbed by 1kg of soda lime

A

120 L

32
Q

Colour change of soda lime when exhausted

A

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

33
Q

Cardiff Aldasorber

A

Absorbs anaesthetic volatile agents onto activated charcoal

Form of scavenging system

34
Q

Main modes of ventilation

A

Pressure control

Volume control (constant flow)

35
Q

Cycling mode definition

A

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

36
Q

Cycling modes

A

Time cycled

Flow cycled

Pressure cycled

Volume cycled

37
Q

Volume control pressure and flow graphs

A
38
Q

Pressure limitation with volume control ventilation

A

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

39
Q

Patients where volume control would be preferred and why

A

Brain injury patients

Volume control allows optimisation of PaCO2

40
Q

Patients where volume control is not preferred and why

A

Paediatric patients with uncuffed tubes

Volume control is unable to compensate for leaks

41
Q

Pressure control pressure and flow graphs

A
42
Q

Why pressure control potentially reduced control over PaCO2

A

Set pressure

Decelerating flow - generates tidal volume dependent on compliance and airways resistance

Poor handling of compliance / resistance changes

43
Q

Advantage of pressure control

A

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

44
Q

Methods to optimise oxygenation in ventilated patient

A

Increasing PEEP

Increasing mean airway pressure

Increasing FiO2

Moving from supine to semi-recumbent position

45
Q

Why is semi-recumbent position better for ventilation than supine

A

When semi-recumbent, FRC is increased and atelectasis reduced

46
Q

When will increasing fresh gas flow help to reduce PaCO2

A

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

Anaesthetic FRCA Primary - eLFH Equipment (4 decks)

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