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Respiratory > Mechanics of respiration > Flashcards

Mechanics of respiration Flashcards

1
Q

During spontaneous inspiration how far does the diaphragm move down?

A

1 - 1.5cm

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2
Q

What forces explain passive expiration

A

Relaxation of inspiratory muscles that increased the volume of the chest cavity
Passive contraction of elastic lung tissue
Alveolar surface tnesion

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3
Q

Respiratory muscles are efficient or inefficient

A

Inefficient 90% of the energy is lost as heat

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4
Q

Describe the change in alveoalr pressure over the respiratory cycle

A

Inspiration varies from -1 to -2 –> expiration +1

Sine wave

◦ As the lung volume expands during inspiration, the alveolar pressure drops to below atmospheric.
◦ This produces a pressure gradient between the upper airway and the alveoli
◦ This pressure gradient produces airflow into the lungs
◦ At the end of inspiration, alveolar pressure equals atmospheric pressure, and there is no flow because there is no pressure gradient.
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5
Q

Intrapleural pressure vs time?

A

-5cmH20 –> -8cmH20 at peak inspiration

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6
Q

What is transpulmonary pressure

A

alveolar pressure - pleural pressure

Varies from +5cmH2O at baseline pre inspiration –> 8cmH20 throughout inspiration, and gradually returning to +5 throughout expiration

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7
Q

Airflow over time in the lungs

A

Inspiraory flow at baseline peaks 20L/min

Expiratory flow 10-15L/min

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8
Q

What is the bucket handle and pump handle mechanisms?

A

◦ Inspiratory skeletal muscle contract:
‣ “Bucket handle” movement: elevation of the ribs (mainly by the external intercostals)
‣ “Pump handle” movement: elevation of the sternum (by the sternomastoid muscle)
◦ This also increases intrathoracic volume and opposes the effect of atmospheric pressure

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9
Q

What is required for the lung to expand in pressures

A

Negative pleural pressure overcome the elastic recoil of the lung causing expansion

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10
Q

Describe passive expiration

A

◦ By relaxing, allows the chest wall and abdominal content to decrease the intrathoracic volume.
◦ Abdominal muscles, by maintaining tone, put pressure on the abdominal contents and push it into the chest cavity, making the diaphragm take on a dome shape.
◦ By intruding back into the chest cavity, the intrathoracic volume is decreased by the relaxation of the diaphragm.

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11
Q

What 4 variants can be used to descirbe inspiratory and expiratory processes

A

What happens in each fo these zones
- Diaphragm
- Chest wall
- Pleural cavity pressures
- Alveoli - including pressure, gas flows, relative volumes

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12
Q

Define compliance

A
  • The change in lung volume per unit change in transmural pressure gradient (usually 100ml/cmH20)
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13
Q

What are the two methods of describing lung compliance?

A

Static and dynamic compliance

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14
Q

What is static compliance

A

Static compliance change in lung volume per unit change in pressure in the absence of flow. This requires time allowed for mobile respiratory elements to relax

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15
Q

What is static compliance composed of? normal values

A
  • Chest wall compliance - 200ml/cm H20
  • Lung tissue compliance - also usually cm H20
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16
Q

What is dynamic compliance

A
  • Change in volume divided by change in pressure measured during normal breathing between points of apparent zero flow at the beginning and end of inspiration (PIP and PEEP measurements reached = start and stop); its components
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17
Q

How is dynamic compliance different to static compliance

A

Flow or no flow

This implicates resistance when flow is invovled

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18
Q

Airway resisatnce is dependent on

A

Frequency dependent
Pressure contribution of airway resisatnce
Preferential distribution of flow into lung units with shorter time constants –> this tendancy increases with shorter inspiratory times and increasing RR

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19
Q

How does dynamic compliance compare with static compliance

A

Dynamic compliance is always lower than static compliance as there is an extra form of resistance

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20
Q

What is dyanamic compliance measured between

A

PIP and PEEP

However there is no pause allowing for equilibrationn means slower lung units not ventilated increasing resistance

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21
Q

What is specific compliance?

A

Compliance normalised for a lung volume –> usually FRC allowing comparison between people

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22
Q

Factors affecting respiratory complaince

A

Lung
Chest wall

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23
Q

Lung comppliance is affected by 6

A

2 wet stuff
2 P’s
2 resistance/elastic

Surfactant
Blood volume
Position
Volume
Resistance/dynamic compliance
Elastic recoil

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24
Q

Describe the inluence of lung surfactant on complaicne

A

◦ Increases lung compliance, conversely a loss of surfactant reduces lung compliance
◦ Surface tension is reduced by surfactant

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25
Q

How is lung compliance affected by volume?

A

Compliance is at its highest at just above the FRC at the range of normal tidal volume breathing
◦ Under-distension of the lung reduces compliance because
‣ Decreased FRC in pneumonecrtomy, pneumonia, atelectasis and small stature —> increased energy to open collapsed alveoli stuck together, whereas inflated alveoli are elastic
◦ Over distension reduced compliance
‣ Alveolar overdistension - over-distension reflects the elastic property of the lung but also reducing alveolar surface tension as the molecules are stretched apart increasing surface tension and reducing compliance

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26
Q

How is lung compliance influenced by elastic recoil? What conditions change this?

A

◦ Loss of connective tissue with aging
◦ Loss of connective tissue with
‣ Oedema
‣ Fibrosis or emphysema
‣ Fluid

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27
Q

Draw a diagram illustrating the relationship of volume to pressure to illustrate the compliance relationship of volume

A
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28
Q

How does posture affect compliance? Why?

A

◦ upright > supine > lateral > prone
‣ Chest wall resistance is lowest upright and highest prone.
‣ Lung resistance is lowest upright and highest supine, with low lung resistance prone and lateral.

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29
Q

How does dynamic compliance influence lung compliance

A

◦ Increased airway resistance e.g. asthma
◦ Increased air flow - high respiratory rate
◦ Viscosity of gas

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30
Q

Give 5 factors increasing lung compliance

A

◦ Lung surfactant
◦ Lung volume - compliance is at its highest at FRC
◦ Posture - supine, upright
◦ Loss of lung connective tissue associated with age
◦ Empysema

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31
Q

Give 5 factors decreasing static compliance

A

◦ Loss of surfactant (ARDS)
◦ Decreased elasticity - fibrosis, oedema
◦ Decreased functional lung volume - pneumonectomy, pneumonia, atelectasis, small stature
◦ Alveolar decruitment
◦ Alveolar overdistension

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32
Q

What increases chest wall compliance? 5

A

◦ Ehler’s Danlos and other connective tissue disorders
◦ Cachexia

◦ Rib resection
◦ Flail segment rib fractures
◦ Open chest
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33
Q

What factors reduce chest wall compliance?3

A

◦ Structural - kyphosis, pacts excavatum, scoliosis, circumferential burns, surgical rib fixation
◦ Functional - muscle spasm, tetany
◦ Extrathoracic influences
‣ Obesity
‣ abdominal compartment syndrome
‣ Prone

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34
Q

What is hysteresis?

A

energy applied to the lung in inspiration not recovered in expiration. Dissipating energy
◦ Derecruitment and recruitment - added mechanical energy to open is lost
◦ Alveolar surface tension - when lung is stretched surface tension increases reducing compliance
◦ Stress relaxation - imperfect elastic property
◦ Gas absorption during measurement

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35
Q

Why does hysteresis occur 4

A

energy applied to the lung in inspiration not recovered in expiration. Dissipating energy
◦ Derecruitment and recruitment - added mechanical energy to open is lost
◦ Alveolar surface tension - when lung is stretched surface tension increases reducing compliance
◦ Stress relaxation - imperfect elastic property
◦ Gas absorption during measurement

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36
Q

What is lung resistance? units

A

a combination of resistance to gas flow in the airways and resistance to deformation of tissue of both lung and chest wall. It is usually expressed as a change in pressure per unit flow in cmH20 per litre per second

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37
Q

Reynolds number

A

Reynolds number = gas density x flow x length / viscosity

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38
Q

Reynolds number ranges and what they mean

A

Reynolds number = gas density x flow x length / viscosity
High Reynolds numbers reflect increased turbulence —> increasing resistance to flow
<2000 vs 2000-4000vs >4000

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39
Q

Determinants of airway resistance

A

Laminar vs Non laminar flow
Reynolds number = gas density x flow x length / viscosity
High Reynolds numbers reflect increased turbulence —> increasing resistance to flow
<2000 vs 2000-4000vs >4000

Hagen Poiseulle

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40
Q

What effect does density have on gas flow?

A

Reduced density = reduced turbulence = increased laminar flow = reduced resistance

Reynolds number = gas density x flow x length / viscosity
High Reynolds numbers reflect increased turbulence —> increasing resistance to flow
<2000 vs 2000-4000vs >4000

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41
Q

What is density

A

Measure of the space between two partticles in a fluid (mass per unit of volume)

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42
Q

Viscocity

A

Resistance to flow

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43
Q

Viscocity vs resistance?

A

Promotes laminar flow reducing resistance

However is a factor in increasing resistance in the Hagen Poiseulle equation

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44
Q

Airway diamtre is influeced by 2 physiological factors and 4 pathological factors

A

Lung volume - resistance decreases with increased lung distension
Physiological variation with increasing cross sectional area and slower flow producing reduction in airway resistance in very small airways

Pathological
1. Smooth muscle tone
2. Swelling
3. Obstruction

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45
Q

How does airway diametre and resistance change along the airways?

A

steadily narrowing airways as gas proceeds distally howeever cross sectional area becomes exponentially greater slowing flow down and thus total airway resistance reduces
◦ airway resistance reduces as flow becomes laminar rather than turbulent with airways branching in parallel further lowering resistance
◦ Resistance is maximl at the 7th bronchial division

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46
Q

Where is airway resistance maximal in the ariways?

A

steadily narrowing airways as gas proceeds distally howeever cross sectional area becomes exponentially greater slowing flow down and thus total airway resistance reduces
◦ airway resistance reduces as flow becomes laminar rather than turbulent with airways branching in parallel further lowering resistance
◦ Resistance is maximl at the 7th bronchial division

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47
Q

Pathological factors affecting airway diamtre 3

A

◦ Smooth muscle tone -
‣ bronchospasm,
‣ irritants e.g. histamine,
‣ PSNS agonists
◦ Decreased smooth muscle tone -
‣ bronchodilators,
‣ SNS agonists
◦ Decreased internal cross-section -
‣ oedema,
‣ hypertrophy of smooth muscle ro mucosa,
‣ secretions
‣ Foreign body
‣ Tumour
◦ Mechanical obstruction or compression -
‣ extrnisic by tumour
‣ dynamic compression due to gas trapping ro forceful expiratory effort
‣ artificial airways and kinking
‣ infection

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48
Q

What changes smooth msucle tone in the airways

A

◦ Smooth muscle tone -
‣ bronchospasm,
‣ irritants e.g. histamine,
‣ PSNS agonists
◦ Decreased smooth muscle tone -
‣ bronchodilators,
‣ SNS agonists
◦ Decreased internal cross-section -
‣ oedema,
‣ hypertrophy of smooth muscle ro mucosa,
‣ secretions
‣ Foreign body
‣ Tumour
◦ Mechanical obstruction or compression -
‣ extrnisic by tumour
‣ dynamic compression due to gas trapping ro forceful expiratory effort
‣ artificial airways and kinking
‣ infection

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49
Q

What causes a reduction in internal cross section fo airways aside from smooth muscle

A

◦ Smooth muscle tone -
‣ bronchospasm,
‣ irritants e.g. histamine,
‣ PSNS agonists
◦ Decreased smooth muscle tone -
‣ bronchodilators,
‣ SNS agonists
◦ Decreased internal cross-section -
‣ oedema,
‣ hypertrophy of smooth muscle ro mucosa,
‣ secretions
‣ Foreign body
‣ Tumour
◦ Mechanical obstruction or compression -
‣ extrnisic by tumour
‣ dynamic compression due to gas trapping ro forceful expiratory effort
‣ artificial airways and kinking
‣ infection

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50
Q

What causes mechanical obstruciton to the airways

A

◦ Smooth muscle tone -
‣ bronchospasm,
‣ irritants e.g. histamine,
‣ PSNS agonists
◦ Decreased smooth muscle tone -
‣ bronchodilators,
‣ SNS agonists
◦ Decreased internal cross-section -
‣ oedema,
‣ hypertrophy of smooth muscle ro mucosa,
‣ secretions
‣ Foreign body
‣ Tumour
◦ Mechanical obstruction or compression -
‣ extrnisic by tumour
‣ dynamic compression due to gas trapping ro forceful expiratory effort
‣ artificial airways and kinking
‣ infection

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51
Q

How does flow affect resistance

A
  • RR - increased flow rate
  • Inspiratory and expiratory work - forced inspiration or expiration
  • Inspiratory flow pattern generated by mechanical ventilator - rise time/I time/inspiratory flow set rate
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52
Q

Via the Hagen Poiseulle equation what factors other than radius can affect the resistance of the lungs?

A

length

  • Lung volume –> stretch –> elongates bronchi
  • Artifiical airways increse or decrease airway length
    ◦ Tracheostomy vs ETT
  • Age and size of person
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53
Q

Tissue resistance to lung deformation is made up fo

A

Tissue resistance from lung parenchyma (~70%)
Tissue resistance from chest wall (~30% )

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54
Q

How quantitatively is resistance measured?

A

Indirectly via pressure adn flow measurements
- Spiromettre
- Body plethysmography - a subject breathing in a closed chamber generates pressure changes in the chamber which can be recorded, flow is measured simultaneously. Respiratory resistance can then be calculated
- Flow and volume waveforms on variable orifice flow metres, screen pneumotachyogrpahy or ultrasonic flow metres

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55
Q

Qualitative measure of airway resistance

A

End tidal
Wheeze
Prolonged expiratory phase
Patient respiratory effort

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56
Q

How does an inspiratory hold measure resisatnce?

A
  • Inspiratory hold in mechanically ventilated patint -flow delivered constant (square waveform) and pressure difference between peak pressure and early plateau used to calculate resistance
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57
Q

What is an airway interrupter resistance measurement device? What does it measure?

A
  • Airway interrupter reisstance measurement - normal breathing subject has their airway transiently occluded during respiratory for 100 millisecond period and flow immediately before and pressure immediately after are used to calculate resistance
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58
Q

What is respiratory elastance

A
  • Elastance is defined as the reciprocal of compliance, or change in pressure divided by change in volume
    ◦ Elastance (E) = ∆P/∆V
    ◦ In cmH20/L
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59
Q

What is the intrinsic nature of the lung - to expand or contract?

A

Tends to collapse, exerting a positive pressure
At some minimum volume the lung pressure would be 0

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60
Q

What pressure does the lung exert at FRC?

A

5cmH20

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61
Q

At normal volumes such as in the range of normal tidal breathing what is the lung compliance?

A

200ml/cmh20

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62
Q

What is transmural pressure

A

Alveolar pressure - intrapleural pressure

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63
Q

Draw a curve representing the passive recoil of the lung against pressure and volume

A
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64
Q

Draw a pressure volume curve for the lung

A
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65
Q

What does the chest wall try to do at baseline volumes

A

Expand, and exerts a negative pressure at virtually all volumes

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66
Q

At residual volume the chest wall pressure is

A

-20cmH20

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67
Q

At FRC the chest wall pressure is

A

-5cmH20

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68
Q

At a volume of 70-80% of vital capacity what is the chest wall pressure

A

0 cmH20

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69
Q

outline the pressure volume curve distribution the chest wall elastic recoil

A
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70
Q

Draw a transmural pressure vs %VC graph depicting the relaxation pressure curve of the respiratory system

A
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71
Q

Draw a relaxation pressure curve of the respiratory system

A
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72
Q

Draw a curve depicting the relationship between chest wall and lung elastance in the respiratory system

A
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73
Q

Work done in equation

A

Force x distance

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74
Q

What is the units for work

A

joules
1 N per 1m displacement

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75
Q

In respiratory physiology what is work?

A

pressure x volume

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76
Q

What is the normal work of breathign

A

0.35J/L

77
Q

What si the power of breathing?

A

2.4J/min

78
Q

what is the oxygen cost of ventilation

A

0.25-0.5ml per 1000ml
1-2% of BMR

79
Q

What type of work is done in respiration?

A

Elastic work
Resistive work

80
Q

Describe in graphical terms the elastic work done

A
81
Q

Elastic recoil of the lung work is described as? What causes it to increase?

A

The work done to oppose the elastic recoil of the lung and bring it up from FRC to tidal volume

This work increases with increasing inspiratory volume

82
Q

What is work done to overcome the elastic recoil of the chest?

A

Subtracted from the work done to overcome the elastic recoil of the lung - as elastic recoil of the chest does work to inflate the lung

With smaller volumes work is actually done to reduce chet wall volume down to FRC

With larger tidal volumes energy for expiration increases beyond stored potential and work must be done to overcome this

The overlap represents work done to inflate the lung by the chest wall

83
Q

Resistive work dissipates as?

A

heat

84
Q

resistive work is composed of 4

A

Work done to overcome
- tissue/viscous resistance 10-20% of work
- work done to overcome airway resistance
- Work done to compress intrathoracic gas
- Work done to overcome inertia

85
Q

Work done to overcome tissue or viscous resistance is what % of total work?

What type of work is this 4

A

10-20%
1. Chest wall resisatnce to movement
2. Lung resistance
3. Compression of mediastinal structures
4. Displacement of abdominal organs

86
Q

Why would tissue resisatnce work increase?

A

Raised intrabaodminal pressure
Mediastinal masses
Pleural disease

87
Q

Work done to overcome airway resistance includes

A
  1. Resisatcne to inspiration and expiration in the airways
  2. Resistance of airway devices
88
Q

How important is airway resistance to work done?

When is it increased 2

A

Minimal contribution

Increased where flow is increased or airway diamtre increased

89
Q

Draw a diagram outlining how work changes with airway resisatnce?

A
90
Q

Demonstrate how work is increased by reducing lung compliance

A

Increased work to overcome the increased elastic recoil of the lungs
* FRC decreases

91
Q

Demonstrate graphically how chest wall resistance affects work?

A
  • FRC reduced - equilibrium of chest wall and lung elastic pressures at lower volume
92
Q

Draw a diagram depicting work of breathing against optimal respiratory rate

A
93
Q

Explain graphically how work of breathing and RR are linked - explain how increased elastic work of breathing and increased resistive work of breathing have different effects

A
  • If respiratory frequency too low tidal volumes must be higher - so work is wasted on defeating elastic structures of the chest
  • As frequency increases tidal volumes are small and work is wasted on ventilating the dead space - this is seen in the upward deflection of elastic work- optimal RR for elastic work is for essentially 50/50 dead space and alveolar ventilation with a higher RR
  • However for other components of work increasing frequency linearly increases work
    Increased elastic work - small tidal volumes favoured to deform the lung and chest wall less, so higher RR favoured, e.g. ARDS
    Resistive work increases - asthma - optimal RR slower as minimising work done against airway resistance is favoured (in asthma tachypnoea as increased elastic work of breathing with gas trapping)
94
Q

What is a time constant?

A
  • Time constant (τ) is the time required for inflation up to 63% of the final volume, or deflation by 63%◦ A time which represents the speed with which a particular system can respond to change, typically equal to the time taken for a specified parameter to vary by a factor of 1− 1/ e “
95
Q

What is compliance in an equation

A

Change in volume for a chnage in pressure

96
Q

What is resistance

A

Change in pressure/change in flow

97
Q

When inspiratory flow is constant what can be said about time constants

A

Product of resistance and compliance

98
Q

What does this refer to?

◦ A time which represents the speed with which a particular system can respond to change, typically equal to the time taken for a specified parameter to vary by a factor of 1− 1/ e "
A

A time constant

99
Q

What does this refer to?

◦ If an exponential decay curve tangent of the function was taken at the initial rate the amount of time for decay to reach 0% of starting amount is 1 time constant
A

A time constant

100
Q

What is this

the time required for inflation up to 63% of the final volume, or deflation by 63%

A

A time constant

101
Q

For a normal set of lungs the time constant is

A

0.1 - 0.2 seconds

So over 0.6 seconds 95% fo the total lung volume should be emptied

102
Q

Time constant for a COPD intubated patient

A

2.5 seconds

103
Q

Time constant in ARDS

A

600-700ms

104
Q

If you have constant flow what happens to poor compliance units

A

Shortened or normal time constant - fill rapidly but incompletely

105
Q

High resistance units have what type of time constant

A

Long time constants
Fill slowly

106
Q

How is a high resistance different to a poor compliance unit with respect to time constants

A

Hihg resistance long time constants and fill slowly

Poor compliance units rapid time constants but fill incompletely

107
Q

When gas flow ceases how does air move in the lung with respect to poor compliance lung units and high resistance lung units? What is this called?

A

gas may flow from lung units with poor compliance into lung units with high resistance

Gas flow from lung units with different time constants is called Pendeluft

108
Q

What is pendeluft?

A

Gas flow from lung units with different time constants is called Pendeluft

	‣ Can be demonstraed with inspiratory hold - flow stops, and airway pressure falls abruptly as resistance no longer contributes; then as one holds for longer there is a gradual downward shift in plateau pressure (this is why inspiratory hold should be for 2 seconds at keast - the other reason is relaxation fo the lung and chest walll tissues)
109
Q

Using a pressure over time curve demonstrate how 2 lungs one with higher resistance and the other normal fill

A
110
Q

Using a pressure time curve show how a lung of poor compliance fills differently to one of normal compliance

A
111
Q

Using a pressure versus time curve show how a lung of lower compliance and higher resistance fills differently

A
112
Q

Why is dynamic compliance frequency dependent?

A

In part because Faster RR = more resistance due to higher flows and more turbulence

The other factor being the faster the RR the shorter the inspiratory time and the greater proportion of flow goes to areas with fast time constants - this reduces compoiance

113
Q

How much fluid does the pleural cavity have in it

A

2 - 20mls

114
Q

What is pleural fluid

A

Ultrafiltrate from parietal pleural capillaries

115
Q

Why is pleural pressure negative

A

Recoil of the chest wall - wanting to expand, ribcage wants to spring out to 70% of TLC

Recoil of the lungs which want to collapse to a smaller volume than FRC putting further negative pressure in the space

Negative pressure exeerted by the lymphatic system draining fluid

116
Q

Where is the pressure most negative in the lung?

A

Apex -10cmH20

117
Q

What is the intrapleural pressure at the base of an uprgiht lung? What about the middle? What about the top?

A

◦ -10 cmH2O at the apex of lung
◦ -5 cmH2O at some average midzone
◦ -3cmH2O at the base

118
Q

What contributes to a vertical pressure gradient in the lung 3

A

◦ Gravity (i.e. weight of the lung) - the lung actually doesn’t weight much though - only accounts for half of the presure gradient
◦ Pressure from mediastinal contents
◦ Pressure from abdominal contents

119
Q

Why does a vertical pressure gradient in the lung matter?

A

◦ Apical alveoli are more distended than basal alveoli
◦ Bases of the lungs are therefore more compliant
◦ Even though pleural pressure changes evenly throughout the cavity, the bases will be ventilated better because of this.

120
Q

Why is the base better ventilated than the apex?

A

Vertical pressure gradient

◦ Apical alveoli are more distended than basal alveoli
◦ Bases of the lungs are therefore more compliant
◦ Even though pleural pressure changes evenly throughout the cavity, the bases will be ventilated better because of this.
121
Q

How does positioning influence the vertical pressure gradient

A

◦ In the upright position, the vertical pleural pressure gradient is greatest (8cm H20)
◦ In the supine or head-down position, this gradient is halved - i.e. pleural pressure more evenly distributed
◦ In the prone position, the gradient is the smallest (2cmH20)
◦ In the lateral position, the weight of the mediastinal content makes the pleural pressure of the dependent cavity less negative

122
Q

Describe how an upright posture affects respiratory compliance

A

Best respiratory compliance

Respiratory resistance lowest
- Chest wall resistance optimal, lowest of all positions
- Lung resisatnce low

FRC highest

123
Q

What happens to compliance and resistance with supine positioning

A

Upright posture
* Respiratory compliance - best of all positions
* Respiratory resistance
◦ Chest wall resistance - lowest of all positions
◦ Lung resistance - low
* FRC - highest of all positions

Supine
* Respiratory compliance - slightly decreased
* Respiratory resistance - worst in supine position as a combination
◦ Chest wall resistance - low
◦ Lung resistance - highest
* FRC - decreased

124
Q

What happens to compiance and resisatnce with lateral lying

A

Upright posture
* Respiratory compliance - best of all positions
* Respiratory resistance
◦ Chest wall resistance - lowest of all positions
◦ Lung resistance - low
* FRC - highest of all positions

Supine
* Respiratory compliance - slightly decreased
* Respiratory resistance - worst in supine position as a combination
◦ Chest wall resistance - low
◦ Lung resistance - highest
* FRC - decreased

Lateral
* Respiratory compliance - lowest of all positions
* Chest wall resistance - high
* Lung resistance - low similar to upright
* FRC - slightly decreased

125
Q

How does going prone affect respiratory mechanics

A

Prone
* Respiratory compliance - low compliance, may be slightly better than supine depending on abdominal supports
◦ decreases the pleural pressure gradient making compliance of units more uniform and reducing ventilator associated lung injury from alveolar overdistension (apex) and cyclic atelectasis (bases)
* Chest wall resistance - highest resistance
* Lung resistance - lowest resistance
* FRC - slightly decreased (better than supine)

126
Q

Describe what happens when you go from supine to an upright position

A

Thus, on going supine from an upright position:
* Compliance will decrease
* Chest wall resistance will increase slightly
* Lung resistance will increase
* FRC will decrease by ~ 30% (2.9 –> 2.1L on average)

127
Q

Supine to prone has what effect on lung mechanics

A

Upon turning the patient from supine to prone:
* Compliance will increase
* Chest wall resistance will increase
* Lung resistance will decrease
* Thus, total tissue resistance will remain unchanged
* FRC will increase
* The vertical pleural pressure gradient will decrease

128
Q

Show a work of breathing diagram of pressure vs volume ad demonstrate elastic work, resistive expiratory work, resistive inspiratory work

A
129
Q

Show a work of breathing diagram of pressure vs volume ad demonstrate elastic work, resistive expiratory work, resistive inspiratory work and the effect of PEEP or dynamic hyperinflation

A
130
Q

Show using a diagram how going from upright to supine affects FRC

A
131
Q

Define compliance and give its units

A

Compliance is the change in volume for a given a change in pressure
Compliance is measured in ml.cmH2O-1.

132
Q

Define elastance

A

elastance, which is the force at which the lung recoils for a given distension

133
Q

What equation is used to define compliance

A

Compliance of the respiratory system is a function of both lung and chest wall compliance:
1/CT =1/CL+1/CW
.

134
Q

What is the base compliance of the respiratory system?

A

100 ml/cmH20

135
Q

What is the compliance of the chestw all

A

200ml/cm H20

136
Q

What is the compliance of the lung tissue itself

A

200ml/cm H20

137
Q

What pressure gradient is used for lung compliance calculations

A

Alveolar - intrapleural pressure gradient

138
Q

What pressure gradient is used for chest wall compliance

A

intrapleural-ambient pressure gradient

139
Q

What is the total compliance pressure gradient

A

alveolar - ambient pressure gradient

140
Q

How is alveolar pressure calculated

A

Plateau pressure

141
Q

How do you measure intrapleural pressure

A

oesophageal pressure using a blloon with an open glottis approximates intrapleural pressure

142
Q

If you were to draw a static compliance curve what would it look like? What is it made up of

A

Elastic recoil of the lung
Surface tension of alveoli

143
Q

Draw a dynamic compliance curve

A
144
Q

What is the specific compliance equation

A

Specific compliance is the compliance per unit volume of lung, expressed as:
CS=CTot/FRC

145
Q

Which of static and dynamic compliance curves is there hysteresis?

A

There is hysteresis in both static and dynamic curves:
In dynamic compliance curves:
Airways resistance is a function of flow rate. Flow rate (therefore resistance) is maximal at the beginning of inspiration and end-expiration.

In static compliance curves:
There is no resistive component. Hysteresis is due to viscous resistance of surfactant and the lung.

146
Q

Draw a static pressure volume relationship in the supine patient and explain the important lung volumes

A
147
Q

Define compliance

A

• The change in lung volume per unit change in transmural pressure gradient (usually 100ml/cmH20)

148
Q

What is the normal value for Chest wall compliance

A

200ml/cm H20

149
Q

What is the normal value for lung tissue compliance

A

200ml / cmH20

150
Q

What is static total lung compliance

A

1/compliance = 1/lung tissue compliance + 1/chest wall compliance

151
Q

Define static compliance

A

change in lung volume per unit change in pressure in the absence of flow. This requires time allowed for mobile respiratory elements to relax

152
Q

Is static compliance measureable? Why?

A

Of note this is physiologically impossible to properly measure as if you are measuring compliance and close the respiratory circuit there will be a pressure drop as the gas redistributes between lung units with different time constants which is an active process fo flow, then as you wait further the measured volume will decrease with gas exchange

153
Q

What is dynamic compliance? Define

A

• Change in volume divided by change in pressure measured during normal breathing between points of apparent zero flow at the beginning and end of inspiration (PIP and PEEP measurements reached = start and stop); its components

154
Q

Airway resistance is dependent on

A

◦ Airway resistance - therefore frequency dependent which is due to
‣ Pressure contribution from airway resistance
‣ Preferential distribution of flow into lung units with shorter time constants, a tendency which increases with shorter inspiratory times and increasing respiratory rates

155
Q

What is dynamic compliance measured between?

A

Dynamic compliance is measured at PIP without allowing for equilibration pause at the time of measurement which means that the compliance will be lower as slower lung units are poorly or not ventilated and the faster the rate the more pronounced this is

156
Q

WHat is specific compliance?

A

Compliance normalised by a lung volume usually FRC

Cs = C (total)/ FRC

157
Q

Define hysteresis

A

difference between inspiratory and expiratory compliance and the lung volume at any given pressure during inhalation is less than the lung volume at the same pressure during exhalation
Definition: lung volume at any given pressure during inhalation is less than lung volume at any given pressure during exhalation

158
Q

Why does Hysteresis exist?

A
  1. Surfactant
  2. Relaxation of lung tissue - loss of energy with stretch as it is imperfectly elastic so as stretch occurs consumption of energy
  3. Recruitment and derecruitment of alveoli
  4. Gas absorption
  5. Differences in inspiratory and expiratory air flow influences dynamic compliance
159
Q

What are 5 factors increasing lung compliance

A

• Increased
◦ Lung surfactant
◦ Lung volume - compliance is at its highest at FRC
◦ Posture - supine, upright
◦ Loss of lung connective tissue associated with age
◦ Empysema

160
Q

What are 5 factors reducing lung compliance

A

◦ Loss of surfactant (ARDS)
◦ Decreased elasticity - fibrosis, oedema
◦ Decreased functional lung volume - pneumonectomy, pneumonia, atelectasis, small stature
◦ Alveolar decruitment
◦ Alveolar overdistension

161
Q

What are 7 factors influencing lung compliance

A
  1. lung volume - affected by PEEP, hyperinflationon
  2. Lung elastic recoil - age, disease states
  3. Chest wall compliance
  4. Pulmonary blood volume - congested = less compliant
  5. Dynamic lung compliance affected by RR and bronchoconstriction
  6. Lung surfactant
  7. Posture
162
Q

Chest wall compliance is increased by?

A

• Increased
◦ Ehler’s Danlos and other connective tissue disorders
◦ Rib resuection
◦ Cachexia
◦ Flail segment rib fractures
◦ Open chest

163
Q

Chest wall compliance is decreased by?

A

◦ Structural - kyphosis, pacts excavatum, scoliosis, circumferential burns, surgical rib fixation
◦ Functional - muscle spasm, tetany
◦ Extrathoracic influences
‣ Obesity
‣ abdominal compartment syndrome
‣ Prone

164
Q

WHat are the methods of measuring static compliance

A
  1. Super syringe method
  2. Constant flow
  3. Multiple occlusion method
165
Q

WHat is the super syringe method of lung compliance measurement
- What type of compliance does it measure?
- How is it performed? After each step what is done?
- How does this compare to other methods of lung compliance measurement
- Disadvantages?

A

• Static compliance measured by inflating the lung in volume increments of usually 100mls
• 2-3 seconds allowed for gas pressure to equilibrate between units with different time constants
• Gold standard for static compliance
• Disadvantage –>
◦ Time it takes to perform
◦ The need to disconnect the patients from the ventilator - which will result in some lost PEEP and recruitment lost so compliance may be measured worse than it is on the ventilator
◦ Compressibility of gas not taken into account which changes volume slightly with increased pressure
◦ Temperature and humidity not taken into account

166
Q

How is the constant flow measure of compliance done? What type of compliance does it measure?
What flaws does it have?

A

• Low inspiratory flow (as low as 1.7L/min) is administered over 10-15 seconds
• Low expiratory flow is then controlled to observe expiratory pressure change
• Because the flow is low airway resistance is said to contribute minimally
• This overestimates expiratory compliance and underestimates inspiratory compliance
• The advantage is you do not need to disconnect them from the ventialtor

167
Q

What is the multiple occlusion method of lung compliance? What are some advantages?

A

• During normal ventilator function, breath occlusions are repeated at different volumes with normal breaths in between
• No need to discontinue normal ventilation and that process can be automated

168
Q

What re the limitations of all methods of static compliance

A

• all methods require patietn to be sedated and paralysed
• possible escape of gas into pulmonary circleation which decreases lung volume during measurement
• Changes in gas pressure associated with increased humidity and temperature are ignored

169
Q

What change in pressure is observed when measuring lung compliance

A

Intrapleural - alveolar

170
Q

How is intrapleural pressure measured?

A

Oesophageal balloon - as oesophageal pressure is a close surrogate

171
Q

The measurements of compliance performed on a ventilator represent what?

A

Respiratory system compliance

172
Q

How do you measure purely lung compliance without chest wall influence?

A

In order to calculate “Lung Compliance”;
=Change in lung Volume/change in pressure (Intrapleural - Alveolar)
As transpulmonary pressure = alveolar pressure - intrapleural pressure

A person must have an oesophageal balloon inserted to measure “intrapleural pressure” (oesophageal pressure is a close surrogate)
Then breathe into a closed circuit spirometer to measure the change in volume, stopping periodically with an open glottis to equilibrate with the atmosphere/spirometer. Therefore chest wall muscles used to maintain constant volume

This enables the change in pressure (Intrapleural - Alveolar) to be calculated
And the change in Volume measured from the spirometer.

Chest wall compliance is done similarly, but the change in pressure is measured as Intrapleural - Atmospheric pressure difference, which is then used in the above formula.

173
Q

How is chest wall compliance measured

A

In order to calculate “Lung Compliance”;
=Change in lung Volume/change in pressure (Intrapleural - Alveolar)
Recoil pressure of the chest wall = intrapleural pressure - atmospheric pressure

A person must have an oesophageal balloon inserted to measure “intrapleural pressure” (oesophageal pressure is a close surrogate)
Then breathe into a closed circuit spirometer to measure the change in volume, stopping periodically with an closed glottis to equilibrate with the atmosphere/spirometer - it is closed as opposed to open in lung compliance measurement to allow chest wall muscles to be relaxaed

This enables the change in pressure (Intrapleural - Alveolar) to be calculated
And the change in Volume measured from the spirometer.

Chest wall compliance is done similarly, but the change in pressure is measured as Intrapleural - Atmospheric pressure difference, which is then used in the above formula.

It remains negative at all values up to 80% of TLC

174
Q

What is the equation for dynamic compliance

A

Dynamic Compliance= Vt/(PIP-PEEP)
Static Compliance= Vt/(PPlat-PEEP)
Specific Compliance =(Vt/(PPlat-PEEP))/FRC

175
Q

WHat is the static compliance equation

A

Dynamic Compliance= Vt/(PIP-PEEP)
Static Compliance= Vt/(PPlat-PEEP)
Specific Compliance =(Vt/(PPlat-PEEP))/FRC

176
Q

WHat is the specific compliance equation

A

Dynamic Compliance= Vt/(PIP-PEEP)
Static Compliance= Vt/(PPlat-PEEP)
Specific Compliance =(Vt/(PPlat-PEEP))/FRC

177
Q

What is dynamic airways compression

A

The limiting factor to maximum expiratory airflow

During forced expiration there is increased intrapleural rpessure conveyed to alveoli causing gas flow due to increased pressure difference to mouth pressure (atmospheric) when the mouth is open. Greater effort = greater intrapleural pressure = increased pressur egradient. However Maximumal flow is depedent on lung volume

Airways are exposed to the same intrapleural pressure increase and initially the airways are held open by airway pressure inside > intrapleural pressure. Due to resistance the airway pressure decreases as you progress from the alveoli until these pressures may be equal and then collapse may occur. The driving pressure thus becomes difference between alveolar and intrapleural pressure thus is the same regardless of effort.

Maximum flow decreases as lung volume decreases

178
Q

How is dynamic airway compression exacerbated?

A

Increased resistance
Increased compliance - decreased driving pressure
Expiration from low lung volumes

179
Q

How is a flow volume loop for emphysema different

A

Total lung volume is higher with the chest overexpanded
Maximal flow is reduced
Total volume expired less
Descending lumb may be concave

180
Q

Inspiration involves which intercostals?

A

External

181
Q

Diaphragm contributes what % to TV

A

Diaphragm is responsible for 70% of tidal volume

182
Q

When external intercostals contract what movement do they cause

A

Ribs to move upwards and forwards
Bucket handle - Increases the Transverse
Pump handle - AP diametre

183
Q

How far does the diaphragm descend in tidal breathing

A

1cm

184
Q

How far does the diaphragm move in a vital capacity rbeath

A

10cm

185
Q

What is the cost of tidal ventilation

A

3mls O2 per minute
1%

186
Q

What is the pressure gradient in expiration in tidal breathing

A

1cm H20

187
Q

What is responsible for elastic recoil of the lungs

A

70% from surface tension fo the thin film of fluid lining the alveolus
30% Stretched elastic fibres in the lung parecnhyma

188
Q
A

Respiratory (10 decks)

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