Flow Volume Loops

Question 1

What is a typical spirometer recording?

Answer 1

Quiet breathing
Maximum inspiration
Maximum expiration.
It measures the change in airflow (L/s), and the change in volume, both plotted against time.
Airflow and volume positive deflection is air coming in.
See picture.

Question 2

What is volume?

Answer 2

A measure of flow over time.
e.g. flow of 500mL/s over 5 seconds, the volume = 1000ml.

Question 3

What is velocity?

Answer 3

Flow = velocity x area.
As you move down the airways, the airways get smaller with each generation, but total cross-sectional area increases, so velocity decreases.
This is because flow must stay constant.
Inverse relationship between velocity and area.

Question 4

What is velocity in expiration?

Answer 4

There is a smaller total cross-sectional area in the larger airways.
The means that there is increased velocity in the larger airways.
This is because flow must remain constant, so if velocity increases, area decreases.

Question 5

What is Bernoulli’s principle?

Answer 5

Increasing gas velocity causes decreasing pressure.
So in the larger airways there is reduced pressure.

Question 6

What is Reynolds number?

Answer 6

Re = 2 x radius x density x velocity / viscosity.
There is increased velocity in larger airways, increased radius, so more turbulent flow.
Turbulent flow is proportional to the square root of pressure gradient, so pressure decreases more quickly in turbulent flow (in larger airways).

Question 7

What is the relationship between airway resistance and lung volume?

Answer 7

Airways are embedded in the lung.
As the lungs increase in volume, the airways are stretched and their radius increases.
Resistance = r^4
So increasing the volume increases airway radius, and decreases airway resistance.

Question 8

How does airway resistance differ?

Answer 8

There is the largest airway radius at total lung volume, therefore the lowest airway resistance.

Question 9

How does total resistance change down the airways?

Answer 9

Each individual airway increases in resistance as their radius decreases.
But there is a larger number of airways in parallel, so this decreases total resistance.

Question 10

What is laminar flow resistance?

Answer 10

With laminar flow, flow rate increases proportionately as the pressure driving the flow is increased.
Once flow becomes turbulent, the flow rate is no longer proportional to pressure. There is now less flow for any driving pressure.
See picture

Question 11

What is the pressure difference between the alveoli and the atmosphere?

Answer 11

The pressure gradient from the alveolar space to the atmosphere generates flow, but the gradient is not linear.
Pressure falls more rapidly towards the alveolar end, is a curve.
This is because of turbulent airflow in large airways, which causes the pressure to fall more quickly.

Question 12

How does the structure of the airways change?

Answer 12

In the upper airways, there are cartilage rings holding the airways open. So if there is a collapsing force they don’t collapse and still has airflow.
Further down the airways, there is no cartilage so they are deformable, and will collapse and have no airflow.

Question 13

What are transmural pressures?

Answer 13

The alveoli, is dependent on the elastic recoil of the lung, and pleural pressure.
The bigger the elastic recoil, the bigger alveolar transmural pressure generated.
In the airway, the difference between airway pressure and barometric pressure. This is determined by where you measure in the airways, and how quickly pressure falls.

Question 14

What is elastic recoil?

Answer 14

Dependent on tissue components and surface tension.
A greater elastic recoil means a bigger collapsing force on the lung, and a bigger transmural pressure can be generated.
As stretch the lungs, this generates elastic recoil which tries to collapse the lungs.

Question 15

What are the pressures in passive expiration?

Answer 15

Barometric pressure is always 0.
At the end of inspiration, the negative pleural pressure becomes more negative, as the lungs are inflated more, so there is more elastic recoil.
When the inspiratory muscles are relaxed, elastic recoil will reduce the lung volume.
The alveolar pressure increases, which forms a pressure gradient and air flows out.

Question 16

What is the pressure gradient in passive expiration?

Answer 16

There is a larger drop in pressure in the larger airways, than the small airways.
There is always a positive descending pressure inside of the airways which holds them open - so no collapsing force.

Question 17

What is the pressure in a forced expiration?

Answer 17

Lungs stretched further so more elastic recoil.
It generates a positive pleural pressure (Ppl).
A greater alveolar pressure is generated from pleural pressure and elastic recoil.

Question 18

What is equal pressure point?

Answer 18

The point where the alveolar pressure is equal to pleural pressure, determined by:
The generation of driving pressure by elastic recoil pressure.
The fall in pressure along the airway - due to velocity and turbulent flow.

Question 19

What is the effect of the equal pressure point?

Answer 19

Beyond the EPP, the pleural pressure is greater than alveolar pressure, so there is a collapsing force.
If the EPP happens closer to large airways, the cartilage ring holds them open.
If the EPP happens closer to small airways, they collapse and there is no airflow.

Question 20

What happens in forced expiration from total lung capacity?

Answer 20

At TLC, have maximum elastic recoil, will generate the highest alveolar pressure.
The alveolar pressure then drops from the alveoli to the atmosphere, so the EPP will be near the top of the airways.
The airways remain open due to cartilage rings.
See picture.

Question 21

What happens in forced expiration from below TLC?

Answer 21

The alveolar pressure is smaller, so the EPP is in the smaller airways, and the airways collapse.
This stops flow, but as it is during expiration, the pressure in alveoli increases until it opens the airways and air flows out again.
Airflow leaving then lowers the pressure in the alveoli and they collapse.
So airways rapidly open and close which produces smooth airflow.

Question 22

What is the effect of obstructive lung disease on EPP?

Answer 22

There is reduced elastic recoil, so cannot generate a high alveolar pressure.
The EPP happens lower down in the airways, so there is flow limitation and peak expiratory flow is reduced.
e.g. COPD

Question 23

What is peak expiratory flow?

Answer 23

PEF is greatest at TLC.
The surfactant is least effective (largest surface tension) at TLC due to being spread out more so cannot disrupt surface tension.
This means there is greatest elastic recoil at TLC, so can generate greatest positive pleural pressure, and the airway resistance is least.

Question 24

What are flow volume loops?

Answer 24

Volume is on the x axis
Flow is on the y axis.
TLC is on the left, residual volume on the right.
Expiration is a positive deflection (opposite of spirometry) inspiration is a negative deflection.
See picture.

Question 25

What is normal tidal breathing on the flow volume loop?

Answer 25

Symmetrical change in inspiration and expiration, occurs around functional residual capacity (FRC). Blue line on picture.

Question 26

What is a forced expiration on the flow volume loop?

Answer 26

Take a maximal breath in from residual volume to total lung capacity, this creates a symmetrical inspiratory loop, with peak inspiratory flow around halfway. Then at TLC, forced expiration, with the peak flow as soon as begin - peak expiratory flow. This is the red line on picture, all other breaths are inside this.

Question 27

What is a deep breath in, but not to TLC, on a flow volume loop?

Answer 27

Deep breath in, not to TLC, then a maximal effort expiration, there is peak flow, but a smaller peak. Green line on picture.

Question 28

What is a deep breath in to TLC without maximal expiration on flow volume loop?

Answer 28

Smaller peak flow that does not go to residual volume. Purple line on picture.

Question 29

What is the effect of an obstructive disease on a flow volume loop?

Answer 29

There is more airway resistance, so peak expiratory flow is reduced. The vital capacity is the same. e.g COPD, asthma. See picture

Question 30

What is the effect of a restrictive disease on flow volume loop?

Answer 30

Cannot hold as much air, so residual volume is increased, and FRC is increased. This means that vital capacity is reduced. This also reduces peak expiratory flow. e.g. fibrosis See picture.

Question 31

What are the differences between restrictive and obstructive disease?

Answer 31

Restrictive has a reduction in both vital capacity and peak expiratory flow. Obstructive has a reduction in peak expiratory flow, but normal vital capacity.

Question 32

What is a flow volume loop from a real recording?

Answer 32

See picture.