Principles of Spirometry

Question 1

what is spirometry?

Answer 1

Spirometry is used to analyse lung volume changes as a function of time.
Flow can also be measured (as flow is the rate of change of volume)
Spirometry is quantitative measure of lung function.

Question 2

what is the diagnostic value of spirometry?

Answer 2

Spirometry results are key to diagnosing and understanding the effects of respiratory diseases.
For example, our lungs have a huge reserve capacity.
Therefore, we can have disease progression going on but we will not notice it for quite a while as for the majority of the time we are breathing tidally, so we will not notice a decrease in lung capacity.
Spirometry can uncover respiratory diseases earlier as it stresses the system (sees the maximum capability of the lungs) to uncover a disease.
We have predictive lung volumes (static & dynamic) values which are very accurate for predicting what someone’s normal lung volume should be.
They can be adjusted based on age gender and heigh to get accurate prediction.
Therefore, any major deviation from these lung volumes gives a good indication of sometimes abnormal going on.
Static volumes –> volume only
Dynamic volumes –> time to exhale a volume

Question 3

what are the 2 ways that lung volume can be measured in?

Answer 3

directly or indirectly

Question 4

how do you measure lung volumes directly?

Answer 4

• Piston spirometer and a water bell spirometer can be used to directly measure lung volumes.
• They are connected to a pen and some paper which record the lung volumes on a trace:
• We can also see how volume changes over time - get flow rates.
• To get accurate measurement of volume changes using these devices:
• The system needs to as frictionless as possible
• Use of sealed mouth piece and nose clips –> ensures we are measuring everything that is coming in and out of the lungs.

Question 5

what are the 2 ways that you can measure lung volumes indirectly?

Answer 5

Peak flow meter and pneumotachograph

Question 6

what is a pneumotachograph?

Answer 6

Pneumotachograph
A pneumotachograph measures flow and volume indirectly
If it is connected to a computer, we can get a trace to see how flow and volume changes over time.
Using both peak flow metres or pneumotachographs required the patient to take a maximal breath in up to TLC. They will then need to blow out as hard and fast as you can all the way down to residual volume.

Question 7

what is a peak flow meter?

Answer 7

Peak flow meters measure the maximum (peak) flow that is generated in a breath.
You get a single number from peak flow meters which has limited value.
However, peak flow meters can be mass produced so can be given to lots of patients.
Also, multiple uses can show how lung function changes over time.
Electronic peak flow metres provide more accurate and more information:
The give you your peak flow and also the volume of air you have blown.
NB: Peak flow metres do not measure how flow and volume change over time.

Question 8

what 3 variables do you get from measuring lung volumes?

Answer 8

By measuring lung volumes, you can obtain data for 3 variables: Flow, volume and time.
You can then plot all of these against each other.