L21 - Airflow through Tubes AND Expiratory Flow Limitations AND Flow-Volume Loops

Airflow through tubes

• Pressure falls with distance as flow proceeds through a tube

• Due to frictional forces, which are caused by the resistance to the flow.

• The greater the resistance to the flow, the greater the fall in pressure for any given flow

Airflow through tubes

Flow profile of Laminar flow

Parabolic flow profile → molecules move faster in the middle and slowest at the edges as this is where frictional forces are largest 

Airflow through tubes

Flow profile of Turbulent flow

Linear flow profile

Airflow through tubes

The amount pressure falls with distance depends on…

the profile of the flow itself

Airflow through tubes

Two extremes of flow patterns

• laminar flow

• turbulent flow

Airflow through tubes

Sustaining flow of laminar vs turbulent flow

Different flow profiles require different amounts of energy in order to sustain flow

• Laminar flow: Δ Pressure ∝ flow

• Turbulent flow: Δ Pressure ∝ flow²

A greater pressure difference (and therefore more energy) is required to sustain a turbulent flow rather than a laminar flow:

• Laminar flow: If I want to double the flow, I would just have to double the pressure difference

• Turbulent flow: If I want to double the flow I would need a 4x increase in pressure difference.

Airflow through tubes

What determines flow pattern (laminar or turbulent)

Reynolds number

• Larger Reynolds Number= more likely to have turbulence. 

  • Increasing radius and/or increasing velocity increases turbulence. 

Air we breathe will have a constant viscosity and density, so ignore those terms.

Airflow through tubes

Use Reynolds number to show type of flow in resting breathing

Large number = more turbulent

• Flow transitions from laminar air flow in the small airways towards turbulent flow in the larger airways 

• This is because the radius of the airways and speed of gas both increases.

There is always a pressure drop from the alveolus to mouth, this drop is greatest in the larger airways in order to maintain a constant turbulent airflow).

Airflow through tubes

What does Bernoulli’s Principle State

If we have flow going through a tube, at the point where the tube narrows the velocity must increase in order to maintain a constant flow

Bernoulli’s Principle: When velocity of a fluid increases, there must be a fall in pressure

Airflow through tubes

Bernoulli’s principle + lift in aeroplanes

• Aeroplane wings have a specific shape that generate a greater path length on the surface than underneath.

• Because airflow is moving at a fixed flow, air molecules will have to go faster on top of wing the wing than below.

• Therefore, according to Bernoulli’s principle, pressure underneath the wing must be greater than pressure above the wing if flow is constant

• This then generates the lift.

Airflow through tubes

Bernoulli’s Principle and Lungs

• Flow of gas (V̇) = u (speed, cm/sec) x A (area, cm²)

• Therefore, if flow is constant then the velocity (V) must increase as total surface area falls. 

• Smaller air ways have largest total surface area so the velocity of air increases from the lower airways to the upper airways.

• Using Bernoulli’s principle → by air speeding up as it moves through the airways during expiration (alveolus to trachea) there is a further fall in pressure. 

  • P1 greater than P2 anyway during expiration but Bernoulli's principle is adding to that.

• The greater we want to accelerate air through airways, on expiration, the greater the fall in pressure must be

Airflow through tubes

Pressure falls as air progresses from the alveoli, up the airways, and out the mouth.

Why (3)

• Pressure is lost as the resistance of airways is overcome (energy lost as heat)

• Velocity increases and radius increases as you move up the airways

  • Reynolds number increases

    • So turbulence increases, causing an even greater fall in pressure.

• Decreasing total cross sectional area

  • Velocity of gas must increase

    • Get an additional fall in pressure (Bernoulli effect)

Expiratory flow limitations

When can intrapleural pressure be positive

intrapleural pressure = P_pl

When you breathe out maximally

Expiratory flow limitations

What is peak flow determined by

Pressure inside the airways falls as air moves through them 

• Therefore, there may come a point where intrapleural pressure becomes greater than the pressure inside the airways at a certain point. 

• At this point the airways will collapse (unless it is supported by a tracheal ring) which will restrict airflow.

• So your peak flow is determined by the point where airways collapse.

Expiratory flow limitations

What is transmural pressure

Transmural pressure (P_TM) = P_A – P_pl

• A positive P_TM indicates a distending pressure (P_in>P_out), it holds the airway open

Expiratory flow limitations

What determines P_pl

• Recoil of the chest wall and the lung pulling away from each other

• Or in a maximal expiration where the chest wall is pushing down to make pleural pressure positive

Expiratory flow limitations

What determines P_A

• Pleural pressure (green)

• Elastic recoil pressure of the lung (P_el) (red)

• Therefore P_A = P_el + P_pl

Expiratory flow limitations

Combination of P_TM and P_A equations

P_A = P_el + P_pl

Therefore, by substitution:

• P_TM = P_A - P_pl

• P_TM = (P_el + P_pl) – P_pl

• P_TM = P_el

Under static conditions the transmural pressure is the elastic recoil pressure.

Expiratory flow limitations

Loss of elasticity and transmural pressure

P_TM = P_el

If elasticity is lost (emphysema) you will able to generate a lower P_A → greater tendency for the airways to collapse

Expiratory flow limitations

• At FRC P_A = 0.

• Therefore: P_TM = 0 - P_pl (e.g. if P_pl = -0.5kPa, P_TM = 0 - -5 = +0.5kPa)

• Greater P_TM indicates a greater more negative P_pl which indicates a greater lung volume → indicates a greater elastic recoil → indicates a greater distending pressure

• More intrapleural pressure when pulling the lungs away from where they want to be and is retracting

  • This retraction back to its equilibrium volume is generating a greater elastic recoil → greater distending pressure

  • Larger lung volume creates a greater flow

Expiratory flow limitations

P_pl at forced expiration

• P_TM is +ve in quiet breathing as P_pl is always negative (P_TM = P_A - P_pl) so there is always a distending pressure throughout a breath when breathing quietly so airways are held open. 

• However: during eg exercise, expiratory muscles become active (crush down volume) and P_pl can become +ve. (likely to be in large airways as pressure falls greatest in larger airways)

• If P_pl can become +ve and > P_A, then small airways (which lack smooth muscle or cartilage) may collapse and limit airflow during expiration

• This is determined, in part, by ΔP along the airway during expiration.

Expiratory flow limitations

What determines whether an airway collapses?

• Difference between P_A and P_pl.

• Remember:

  • P_A = P_el+ P_pl 

  • P_A - P_pl = P_el

• So, elastic recoil of the lung is a major determinant of P_TM so determines whether airways collapse during expiration

  • Flow limitation (determined by height, age and sex)

Expiratory flow limitations

Can you train yourself to have a higher peak flow

Elastic recoil is a major determinant of  P_TM

Elastic recoil determines peak flow (thus airway collapse)

You can’t train yourself to have a higher peak flow

Flow Volume Loop

What is on the x and y axis

explain what does the +ve and -ve y axis mean

• Y axis: Flow (L/sec)

  • -ve Y axis: inspiratory flow rate

  • +ve Y axis: expiratory flow rate

• X axis: Lung volumes: TLC → FRC → RV

• At both Residual volume and TLC, flow = 0

Flow Volume Loop

in Quiet breathing

• Small change in volume

• Horizontal diameter of small circle = Tidal Volume

• In quiet breathing both inspiratory flow rate expiratory flow is ~equal → get circle.

Flow Volume Loop

PEF to RV

What is the Maximum Envelope

Occurs during forced expiration from TLC to RV.

• Horizontal diameter of peak expiratory flow loop is vital capacity.

• Maximum envelope

  • Deep breath in + exhale → cannot increase flow from the maximum envelope (line which goes from PEF to RV during forced expiration)

Flow Volume Loop

• PEF is much greater/less than PIF

• PIF occurs…….

• PEF occurs……..

• Expiratory flow decline is normally linear/exponential/plateaus

• PEF is much greater than PIF

• PIF occurs mid-inspiration

• PEF occurs close to TLC

• Expiratory flow decline is normally linear

Flow Volume Loop

• FEF - Fractional Expiratory Flow

• FIF - Fractional Inspiratory Flow

• FEF and FIF gives information of flow in smaller airways

  • Respiratory disease often progress from smaller airways

  • If someone presents with a reduced PEF → often a progressed disorder

• PEF measures flow at larger airways

Why is Maximum Expiratory flow near TLC? (4 factors)

• Elastic recoil

• Surface tension

• Pleural pressure

• Lung volume

These create a greater transmural pressure, less chance of developing a choke point which would reduce peak expiratory flow.

Why is Maximum Expiratory flow near TLC?

Role of Elastic recoil

• Elastic recoil is determined by intrinsic properties of the lung and by the lung volume → the greater the lung volume, the greater the elastic recoil pressure. 

• The greater the elastic recoil pressure the greater the P_A that can be generated (P_A = P_el + P_pl), and therefore the greater the difference between P_A and P_pl.

• So at TLC (the largest lung volume) there is the largest elastic recoil so on expiration you can produce the largest P_A and so the greatest difference between P_A and P_pl

Why is Maximum Expiratory flow near TLC?

Surface tension

• Surface tension → contributes to elastic recoil. 

• Surfactant reduces surface tension forces.

• At TLC, the surfactant molecules are the most spread out, so surfactant is least effect, so the surface tension forces are greatest and then so is the elastic recoil of the lung → so the greatest P_A can be generated at TLC, with the greatest difference between P_A and P_pl

Why is Maximum Expiratory flow near TLC?

Pleural pressure

• Most +ve on expiration from TLC

• The chest wall is maximally above its equilibrium volume at TLC (NOT at FRC) and so it has its greatest collapsing tendency → so the greatest P_A can be generated at TLC with the greatest difference between P_A and P_pl

• Expiratory muscles are at maximum tension development length

Why is Maximum Expiratory flow near TLC?

Lung volume

• Greater lung volume results in a greater elastic recoil which induces greater tethering to the chest wall.

• Greater tethering forces means a greater diameter of the airways and so there is a lower resistance. 

• Therefore, resistance is at its least at TLC → greatest flow can be generated.

Airways being held open

In normal passive expiration

• You always have a -ve intrapleural pressure. 

• During expiration we generate a slightly positive P_A 

  • Because of recoil compared to barometric pressure

• Then get a fall in pressure as we move through the airways towards the mouth 

• But because P_pl is –ve the airways will always be held open.

Airways being held open

In forced expiration starting at TLC

• We take a maximum deep breath in and then force air out. 

• This generates a high +ve intrapleural

  • As chest wall is collapsing strongly with expiratory muscle activity

  • Have a larger elastic recoil pressure

• These will generate a high P_A (P_A = P_pl + P_el) e.g. 90

• Therefore, because you have generated a large elastic recoil pressure, max Transmural pressure will be high → 30cm H₂O (P_A-P_pl)

• As we breathe out the pressure in the airways will fall but will stay greater than Ppl until we reach the trachea (where may be lower) so the airways are held open.

Airways being held open

In forced expiration starting at mid-lung volume

• 4 factors are not working together to create a large P_TM

• Lower elastic recoil and generates a lower +ve intrapleural pressure e.g., 30 cm H₂O

  • Generates a lower P_A (P_A = P_el+ P_pl)

• Therefore max Transmural pressure is lower (P_TM = P_el) = 20 cm H₂O 

• Results in intrapleural pressure equalling airway pressure at some point in the smaller air ways (no cartilage) = choke point

Choke point

Intrapleural pressure equalling airway pressure at some point in the smaller air ways (no cartilage)

• Equal pressure point (EPP) develops in smaller, peripheral airways if expiration has begun between TLC and FRC

• Continue to breathe out (forced expiration)

  • Airway pressure behind choke point builds up and so the choke is relieved. 

• But as choke is relieved the airway pressure falls again and choke re-develops. (presented as wheezing)

• This limits your flow.

Can you ‘train’ expiratory muscles to create a greater force and hence greater P_A - and hence greater flows at volumes less than TLC?

• No! Trying to exhale more forcibly raises P_pl, which raises P_A by the same amount (P_A = P_el +P_pl) and so the max transmural pressure remains the same (20cm water) – so you still get choke point. 

• Whether a choke point occurs (which limits flow) is determined by the size of transmural pressure.

  • You can not change your elastic recoil pressure so you can not change your flow rates for a set lung volume.

  • Lungs cannot be trained to create a greater pressure as it is made up of smooth muscle (not skeletal muscle)

Airways being held open

Forced Expiration in Emphysema

• Patients with emphysema, have a reduced elastic recoil as lung tissue is damaged.

• Lose the ability to generate a high P_A.

  • Max transmural pressure is lower e.g. 10cmH₂O 

• Therefore, it is more likely they will get choke point forming in more peripheral (lower) airways limits ability to generate a high flow. 

elastic recoil and cross sectional area

Major determinant of small airways cross sectional area and thus airway resistance

Effect of tethering force

• Greater lung volume = greater elastic recoil = greater tethering to the chest wall

  • Lung volume is a major determinant of small airway (without smooth muscle) cross sectional diameter

  • Greater tethering forces means smaller airways are held open more → greater total cross-sectional area → ↓ resistance.

What determines the tethering effect

Integrity / elasticity of the lung tissue

Weak tethering effect

Consequence

• Weakened in e.g. emphysema

• Therefore, emphysema patients have a lower total cross-sectional area of their smaller airways

  • Higher airway resistance → increases the rate of fall in P_A during forced expiration

  • Further decreases the maximal expiratory flow rate

Differences/Similarities in Flow volume loop in Emphysema compared to normal (5)

• Reduced PEF

• Expiratory flow decreases rapidly after PEF

• ‘Expiratory Coving‘

• Normal inspiratory profile

• Tidal breathing moved to higher lung volume

Differences/Similarities in Flow/volume loop in Emphysema compared to normal

PEF

Reduced: 

• Elastic recoil reduced → greater tendency for airways to collapse as reduced peak alveolar pressure

• Results in a reduced driving pressure during expiration → reduced PEF

Differences/Similarities in Flow/volume loop in Emphysema compared to normal

Expiratory flow after PEF

Expiratory flow decreases rapidly after PEF

• Due to narrowing of airways (loss of tethering force so can’t get same flow due to higher resistance)

Differences/Similarities in Flow/volume loop in Emphysema compared to normal

‘Expiratory coving‘

Indicative of emphysema

PEF to RV is no longer a linear line

Differences/Similarities in Flow/volume loop in Emphysema compared to normal

Inspiratory profile

Normal

• P_pl is always negative so P_TM  is always +ve during inspiration so loss of elastic recoil has no effect

• No issue with airway collapse during inspiration

Differences/Similarities in Flow/volume loop in Emphysema compared to normal

Tidal breathing

Moved to higher lung volumes. 

• Every breath flow limited. Patient may develop pursed lip breathing and wheezing

Differences/Similarities in Flow/volume loop in Emphysema compared to normal

Tidal breathing moved to higher lung volumes

• Presentations (3)

• Barrel chest

• Pursed lip breathing

• Wheezing

Differences/Similarities in Flow/volume loop in Emphysema compared to normal

Tidal breathing moved to higher lung volumes

• Barrel chest

Loss of lung elasticity so is pulled up towards equilibrium volume of the chest wall

Differences/Similarities in Flow/volume loop in Emphysema compared to normal

Tidal breathing moved to higher lung volumes

• Pursed lip breathing

• Retains upper airways by splinting the airway open

• Instead of air pressure falling rapidly, it retains airway pressure

Differences/Similarities in Flow/volume loop in Emphysema compared to normal

Tidal breathing moved to higher lung volumes

• Wheezing

Due to opening and closing of choke point

Emphysema

Productive cough

Cough = maximal expiration against the closed glottis (space between vocal cords), squeeze and then an explosive airway flow to expel mucus

• Patients with emphysema have a reduced likelihood of producing a productive cough due to their low peak expiratory flow rate.

• So mucus is not removed from the airways and so infection occurs is highly likely

Flow volume loop in restrictive disease

less lung volume

the maximum flow rate is reduced, as is the total volume expired.

The flow is abnormally high in the latter part of expiration because of increased recoil.

Flow volume loop in obstructive disease

Flow rate is very low in relation to lung volume

Scooped-out appearance is often seen following the point of maximal flow

Flow volume loops provide a graphical illustration of a patient's spirometry efforts.