Mechanics of respiration Flashcards
During spontaneous inspiration how far does the diaphragm move down?
1 - 1.5cm
What forces explain passive expiration
Relaxation of inspiratory muscles that increased the volume of the chest cavity
Passive contraction of elastic lung tissue
Alveolar surface tnesion
Respiratory muscles are efficient or inefficient
Inefficient 90% of the energy is lost as heat
Describe the change in alveoalr pressure over the respiratory cycle
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.
Intrapleural pressure vs time?
-5cmH20 –> -8cmH20 at peak inspiration
What is transpulmonary pressure
alveolar pressure - pleural pressure
Varies from +5cmH2O at baseline pre inspiration –> 8cmH20 throughout inspiration, and gradually returning to +5 throughout expiration
Airflow over time in the lungs
Inspiraory flow at baseline peaks 20L/min
Expiratory flow 10-15L/min
What is the bucket handle and pump handle mechanisms?
◦ 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
What is required for the lung to expand in pressures
Negative pleural pressure overcome the elastic recoil of the lung causing expansion
Describe passive expiration
◦ 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.
What 4 variants can be used to descirbe inspiratory and expiratory processes
What happens in each fo these zones
- Diaphragm
- Chest wall
- Pleural cavity pressures
- Alveoli - including pressure, gas flows, relative volumes
Define compliance
- The change in lung volume per unit change in transmural pressure gradient (usually 100ml/cmH20)
What are the two methods of describing lung compliance?
Static and dynamic compliance
What is static compliance
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
What is static compliance composed of? normal values
- Chest wall compliance - 200ml/cm H20
- Lung tissue compliance - also usually cm H20
What is dynamic compliance
- 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
How is dynamic compliance different to static compliance
Flow or no flow
This implicates resistance when flow is invovled
Airway resisatnce is dependent on
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
How does dynamic compliance compare with static compliance
Dynamic compliance is always lower than static compliance as there is an extra form of resistance
What is dyanamic compliance measured between
PIP and PEEP
However there is no pause allowing for equilibrationn means slower lung units not ventilated increasing resistance
What is specific compliance?
Compliance normalised for a lung volume –> usually FRC allowing comparison between people
Factors affecting respiratory complaince
Lung
Chest wall
Lung comppliance is affected by 6
2 wet stuff
2 P’s
2 resistance/elastic
Surfactant
Blood volume
Position
Volume
Resistance/dynamic compliance
Elastic recoil
Describe the inluence of lung surfactant on complaicne
◦ Increases lung compliance, conversely a loss of surfactant reduces lung compliance
◦ Surface tension is reduced by surfactant
How is lung compliance affected by volume?
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
How is lung compliance influenced by elastic recoil? What conditions change this?
◦ Loss of connective tissue with aging
◦ Loss of connective tissue with
‣ Oedema
‣ Fibrosis or emphysema
‣ Fluid
Draw a diagram illustrating the relationship of volume to pressure to illustrate the compliance relationship of volume
How does posture affect compliance? Why?
◦ 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.
How does dynamic compliance influence lung compliance
◦ Increased airway resistance e.g. asthma
◦ Increased air flow - high respiratory rate
◦ Viscosity of gas
Give 5 factors increasing lung compliance
◦ Lung surfactant
◦ Lung volume - compliance is at its highest at FRC
◦ Posture - supine, upright
◦ Loss of lung connective tissue associated with age
◦ Empysema
Give 5 factors decreasing static compliance
◦ Loss of surfactant (ARDS)
◦ Decreased elasticity - fibrosis, oedema
◦ Decreased functional lung volume - pneumonectomy, pneumonia, atelectasis, small stature
◦ Alveolar decruitment
◦ Alveolar overdistension
What increases chest wall compliance? 5
◦ Ehler’s Danlos and other connective tissue disorders
◦ Cachexia
◦ Rib resection ◦ Flail segment rib fractures ◦ Open chest
What factors reduce chest wall compliance?3
◦ Structural - kyphosis, pacts excavatum, scoliosis, circumferential burns, surgical rib fixation
◦ Functional - muscle spasm, tetany
◦ Extrathoracic influences
‣ Obesity
‣ abdominal compartment syndrome
‣ Prone
What is hysteresis?
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
Why does hysteresis occur 4
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
What is lung resistance? units
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
Reynolds number
Reynolds number = gas density x flow x length / viscosity
Reynolds number ranges and what they mean
Reynolds number = gas density x flow x length / viscosity
High Reynolds numbers reflect increased turbulence —> increasing resistance to flow
<2000 vs 2000-4000vs >4000
Determinants of airway resistance
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
What effect does density have on gas flow?
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
What is density
Measure of the space between two partticles in a fluid (mass per unit of volume)
Viscocity
Resistance to flow
Viscocity vs resistance?
Promotes laminar flow reducing resistance
However is a factor in increasing resistance in the Hagen Poiseulle equation
Airway diamtre is influeced by 2 physiological factors and 4 pathological factors
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
How does airway diametre and resistance change along the airways?
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
Where is airway resistance maximal in the ariways?
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
Pathological factors affecting airway diamtre 3
◦ 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
What changes smooth msucle tone in the airways
◦ 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
What causes a reduction in internal cross section fo airways aside from smooth muscle
◦ 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
What causes mechanical obstruciton to the airways
◦ 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
How does flow affect resistance
- 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
Via the Hagen Poiseulle equation what factors other than radius can affect the resistance of the lungs?
length
- Lung volume –> stretch –> elongates bronchi
- Artifiical airways increse or decrease airway length
◦ Tracheostomy vs ETT - Age and size of person
Tissue resistance to lung deformation is made up fo
Tissue resistance from lung parenchyma (~70%)
Tissue resistance from chest wall (~30% )
How quantitatively is resistance measured?
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
Qualitative measure of airway resistance
End tidal
Wheeze
Prolonged expiratory phase
Patient respiratory effort
How does an inspiratory hold measure resisatnce?
- Inspiratory hold in mechanically ventilated patint -flow delivered constant (square waveform) and pressure difference between peak pressure and early plateau used to calculate resistance
What is an airway interrupter resistance measurement device? What does it measure?
- 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
What is respiratory elastance
- Elastance is defined as the reciprocal of compliance, or change in pressure divided by change in volume
◦ Elastance (E) = ∆P/∆V
◦ In cmH20/L
What is the intrinsic nature of the lung - to expand or contract?
Tends to collapse, exerting a positive pressure
At some minimum volume the lung pressure would be 0
What pressure does the lung exert at FRC?
5cmH20
At normal volumes such as in the range of normal tidal breathing what is the lung compliance?
200ml/cmh20
What is transmural pressure
Alveolar pressure - intrapleural pressure
Draw a curve representing the passive recoil of the lung against pressure and volume
Draw a pressure volume curve for the lung
What does the chest wall try to do at baseline volumes
Expand, and exerts a negative pressure at virtually all volumes
At residual volume the chest wall pressure is
-20cmH20
At FRC the chest wall pressure is
-5cmH20
At a volume of 70-80% of vital capacity what is the chest wall pressure
0 cmH20
outline the pressure volume curve distribution the chest wall elastic recoil
Draw a transmural pressure vs %VC graph depicting the relaxation pressure curve of the respiratory system
Draw a relaxation pressure curve of the respiratory system
Draw a curve depicting the relationship between chest wall and lung elastance in the respiratory system
Work done in equation
Force x distance
What is the units for work
joules
1 N per 1m displacement
In respiratory physiology what is work?
pressure x volume
What is the normal work of breathign
0.35J/L
What si the power of breathing?
2.4J/min
what is the oxygen cost of ventilation
0.25-0.5ml per 1000ml
1-2% of BMR
What type of work is done in respiration?
Elastic work
Resistive work
Describe in graphical terms the elastic work done
Elastic recoil of the lung work is described as? What causes it to increase?
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
What is work done to overcome the elastic recoil of the chest?
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
Resistive work dissipates as?
heat
resistive work is composed of 4
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
Work done to overcome tissue or viscous resistance is what % of total work?
What type of work is this 4
10-20%
1. Chest wall resisatnce to movement
2. Lung resistance
3. Compression of mediastinal structures
4. Displacement of abdominal organs
Why would tissue resisatnce work increase?
Raised intrabaodminal pressure
Mediastinal masses
Pleural disease
Work done to overcome airway resistance includes
- Resisatcne to inspiration and expiration in the airways
- Resistance of airway devices
How important is airway resistance to work done?
When is it increased 2
Minimal contribution
Increased where flow is increased or airway diamtre increased
Draw a diagram outlining how work changes with airway resisatnce?
Demonstrate how work is increased by reducing lung compliance
Increased work to overcome the increased elastic recoil of the lungs
* FRC decreases
Demonstrate graphically how chest wall resistance affects work?
- FRC reduced - equilibrium of chest wall and lung elastic pressures at lower volume
Draw a diagram depicting work of breathing against optimal respiratory rate
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
- 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)
What is a time constant?
- 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 “
What is compliance in an equation
Change in volume for a chnage in pressure
What is resistance
Change in pressure/change in flow
When inspiratory flow is constant what can be said about time constants
Product of resistance and compliance
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 time constant
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 time constant
What is this
the time required for inflation up to 63% of the final volume, or deflation by 63%
A time constant
For a normal set of lungs the time constant is
0.1 - 0.2 seconds
So over 0.6 seconds 95% fo the total lung volume should be emptied
Time constant for a COPD intubated patient
2.5 seconds
Time constant in ARDS
600-700ms
If you have constant flow what happens to poor compliance units
Shortened or normal time constant - fill rapidly but incompletely
High resistance units have what type of time constant
Long time constants
Fill slowly
How is a high resistance different to a poor compliance unit with respect to time constants
Hihg resistance long time constants and fill slowly
Poor compliance units rapid time constants but fill incompletely
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?
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
What is pendeluft?
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)
Using a pressure over time curve demonstrate how 2 lungs one with higher resistance and the other normal fill
Using a pressure time curve show how a lung of poor compliance fills differently to one of normal compliance
Using a pressure versus time curve show how a lung of lower compliance and higher resistance fills differently
Why is dynamic compliance frequency dependent?
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
How much fluid does the pleural cavity have in it
2 - 20mls
What is pleural fluid
Ultrafiltrate from parietal pleural capillaries
Why is pleural pressure negative
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
Where is the pressure most negative in the lung?
Apex -10cmH20
What is the intrapleural pressure at the base of an uprgiht lung? What about the middle? What about the top?
◦ -10 cmH2O at the apex of lung
◦ -5 cmH2O at some average midzone
◦ -3cmH2O at the base
What contributes to a vertical pressure gradient in the lung 3
◦ 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
Why does a vertical pressure gradient in the lung matter?
◦ 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.
Why is the base better ventilated than the apex?
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.
How does positioning influence the vertical pressure gradient
◦ 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
Describe how an upright posture affects respiratory compliance
Best respiratory compliance
Respiratory resistance lowest
- Chest wall resistance optimal, lowest of all positions
- Lung resisatnce low
FRC highest
What happens to compliance and resistance with supine positioning
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
What happens to compiance and resisatnce with lateral lying
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
How does going prone affect respiratory mechanics
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)
Describe what happens when you go from supine to an upright position
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)
Supine to prone has what effect on lung mechanics
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
Show a work of breathing diagram of pressure vs volume ad demonstrate elastic work, resistive expiratory work, resistive inspiratory work
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
Show using a diagram how going from upright to supine affects FRC
Define compliance and give its units
Compliance is the change in volume for a given a change in pressure
Compliance is measured in ml.cmH2O-1.
Define elastance
elastance, which is the force at which the lung recoils for a given distension
What equation is used to define compliance
Compliance of the respiratory system is a function of both lung and chest wall compliance:
1/CT =1/CL+1/CW
.
What is the base compliance of the respiratory system?
100 ml/cmH20
What is the compliance of the chestw all
200ml/cm H20
What is the compliance of the lung tissue itself
200ml/cm H20
What pressure gradient is used for lung compliance calculations
Alveolar - intrapleural pressure gradient
What pressure gradient is used for chest wall compliance
intrapleural-ambient pressure gradient
What is the total compliance pressure gradient
alveolar - ambient pressure gradient
How is alveolar pressure calculated
Plateau pressure
How do you measure intrapleural pressure
oesophageal pressure using a blloon with an open glottis approximates intrapleural pressure
If you were to draw a static compliance curve what would it look like? What is it made up of
Elastic recoil of the lung
Surface tension of alveoli
Draw a dynamic compliance curve
What is the specific compliance equation
Specific compliance is the compliance per unit volume of lung, expressed as:
CS=CTot/FRC
Which of static and dynamic compliance curves is there hysteresis?
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.
Draw a static pressure volume relationship in the supine patient and explain the important lung volumes
Define compliance
• The change in lung volume per unit change in transmural pressure gradient (usually 100ml/cmH20)
What is the normal value for Chest wall compliance
200ml/cm H20
What is the normal value for lung tissue compliance
200ml / cmH20
What is static total lung compliance
1/compliance = 1/lung tissue compliance + 1/chest wall compliance
Define 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
Is static compliance measureable? Why?
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
What is dynamic compliance? Define
• 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
Airway resistance is dependent on
◦ 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
What is dynamic compliance measured between?
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
WHat is specific compliance?
Compliance normalised by a lung volume usually FRC
Cs = C (total)/ FRC
Define hysteresis
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
Why does Hysteresis exist?
- Surfactant
- Relaxation of lung tissue - loss of energy with stretch as it is imperfectly elastic so as stretch occurs consumption of energy
- Recruitment and derecruitment of alveoli
- Gas absorption
- Differences in inspiratory and expiratory air flow influences dynamic compliance
What are 5 factors increasing lung compliance
• Increased
◦ Lung surfactant
◦ Lung volume - compliance is at its highest at FRC
◦ Posture - supine, upright
◦ Loss of lung connective tissue associated with age
◦ Empysema
What are 5 factors reducing lung compliance
◦ Loss of surfactant (ARDS)
◦ Decreased elasticity - fibrosis, oedema
◦ Decreased functional lung volume - pneumonectomy, pneumonia, atelectasis, small stature
◦ Alveolar decruitment
◦ Alveolar overdistension
What are 7 factors influencing lung compliance
- lung volume - affected by PEEP, hyperinflationon
- Lung elastic recoil - age, disease states
- Chest wall compliance
- Pulmonary blood volume - congested = less compliant
- Dynamic lung compliance affected by RR and bronchoconstriction
- Lung surfactant
- Posture
Chest wall compliance is increased by?
• Increased
◦ Ehler’s Danlos and other connective tissue disorders
◦ Rib resuection
◦ Cachexia
◦ Flail segment rib fractures
◦ Open chest
Chest wall compliance is decreased by?
◦ Structural - kyphosis, pacts excavatum, scoliosis, circumferential burns, surgical rib fixation
◦ Functional - muscle spasm, tetany
◦ Extrathoracic influences
‣ Obesity
‣ abdominal compartment syndrome
‣ Prone
WHat are the methods of measuring static compliance
- Super syringe method
- Constant flow
- Multiple occlusion method
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?
• 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
How is the constant flow measure of compliance done? What type of compliance does it measure?
What flaws does it have?
• 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
What is the multiple occlusion method of lung compliance? What are some advantages?
• 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
What re the limitations of all methods of static compliance
• 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
What change in pressure is observed when measuring lung compliance
Intrapleural - alveolar
How is intrapleural pressure measured?
Oesophageal balloon - as oesophageal pressure is a close surrogate
The measurements of compliance performed on a ventilator represent what?
Respiratory system compliance
How do you measure purely lung compliance without chest wall influence?
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.
How is chest wall compliance measured
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
What is the equation for dynamic compliance
Dynamic Compliance= Vt/(PIP-PEEP)
Static Compliance= Vt/(PPlat-PEEP)
Specific Compliance =(Vt/(PPlat-PEEP))/FRC
WHat is the static compliance equation
Dynamic Compliance= Vt/(PIP-PEEP)
Static Compliance= Vt/(PPlat-PEEP)
Specific Compliance =(Vt/(PPlat-PEEP))/FRC
WHat is the specific compliance equation
Dynamic Compliance= Vt/(PIP-PEEP)
Static Compliance= Vt/(PPlat-PEEP)
Specific Compliance =(Vt/(PPlat-PEEP))/FRC
What is dynamic airways compression
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
How is dynamic airway compression exacerbated?
Increased resistance
Increased compliance - decreased driving pressure
Expiration from low lung volumes
How is a flow volume loop for emphysema different
Total lung volume is higher with the chest overexpanded
Maximal flow is reduced
Total volume expired less
Descending lumb may be concave
Inspiration involves which intercostals?
External
Diaphragm contributes what % to TV
Diaphragm is responsible for 70% of tidal volume
When external intercostals contract what movement do they cause
Ribs to move upwards and forwards
Bucket handle - Increases the Transverse
Pump handle - AP diametre
How far does the diaphragm descend in tidal breathing
1cm
How far does the diaphragm move in a vital capacity rbeath
10cm
What is the cost of tidal ventilation
3mls O2 per minute
1%
What is the pressure gradient in expiration in tidal breathing
1cm H20
What is responsible for elastic recoil of the lungs
70% from surface tension fo the thin film of fluid lining the alveolus
30% Stretched elastic fibres in the lung parecnhyma
