Lung Function Tests

Question 1

What causes variations in lung function?

Answer 1

Age, ethnicity, sex and height.
Tall, young adults will have peak lung function.

Question 2

How do you measure peak expiratory flow (PEF)?

Answer 2

The patient takes a full breath and exhales as rapidly as possible into a Wright’s Peak Flow Meter or a spirometer.

Question 3

What are some advantages and disadvantages of measuring PEF?

Answer 3

Portable and suitable for home monitoring.
Not very precise.

Question 4

Describe the shape of a graph of flow rate against lung volume.

Answer 4

Flow rate rises rapidly to reach the peak value relatively early in exhalation.
Flow rate declines slowly until exhalation is complete.

Question 5

What are typical values for PEF?

Answer 5

620 (+/-60) litres/min.
A reduced PEF value suggests airflow obstruction (at any level from the small airways up to the larynx).

Question 6

How do you measure dynamic lung volumes?

Answer 6

During a forced expiration, the forced expiratory volume in one second (FEV1) and the total volume exhaled (FVC) are measured by a spirometer.

FEV1, FVC and FEV1/FVC give an indication of lung volume and the calibre of the airways.

Question 7

How can dynamic lung volumes be extrapolated from a graph?

Answer 7

Expired volume is plotted against time.
FEV1 can be read off a graph.
FVC = the total volume exhaled.

Question 8

What are some advantages and disadvantages of measuring dynamic lung volumes?

Answer 8

More reproducible than PEF measurements.
Less portable and effort dependent.

Question 9

What are typical values for dynamic lung volumes?

Answer 9

FEV1 = 4.5 (+/-0.5) litres.
FVC = 5.6 (+/-0.6) litres.
FEV1/FVC = 80 (+/-5)%.

Question 10

How do certain lung pathologies affect dynamic lung volumes?

Answer 10

Reduction of FEV1 and FEV1/FVC - obstructive lung lesion.
Reduction of FEV1 - restrictive ventilatory problem (due to intrinsic lung disease or problems with the chest wall).

Question 11

What is the inert gas dilution technique?

Answer 11

Measures static lung volumes.
The subject breathes in a closed circuit from a bag containing air with helium, until the gas in their lungs is fully mixed with the bag.
A sample of gas is taken, and [helium] is measured.

Question 12

How can static lung volumes be calculated by the inert gas dilution technique?

Answer 12

FRC is known before inert gas dilution.
Using the original bag volume and [helium], as well as the final [helium], lung volume can be calculated.

From FRC, RV and other lung volumes can be calculated by spirometry.

Question 13

What are the different classes of spirometry measurements?

Answer 13

Static - only considers volume exhaled.
Dynamic - the time for a certain volume to be exhaled is considered.

Question 14

What are typical values for static lung volumes?

Answer 14

FRC = 3.7 (+/-0.7) litres.
RV = 1.9 (+/-0.4) litres.
VC = 5.6 (+/-0.6) litres.
TLC = 7.3 (+/-0.8) litres.
RV/TLC = 25 (+/-0.4)%.

Question 15

What is obstructive lung disease?

Answer 15

Gas is trapped in the chest, causing SOB and difficulty exhaling.
The rate at which air is exhaled is slower.

Question 16

What is restrictive lung disease?

Answer 16

Small or stiff lungs cause a restriction in lung expansion.
The absolute rate of airflow and TV is reduced.

Question 17

What lung volumes are affected by obstructive lung disease?

Answer 17

Increased TLC, RV and FRC.
Decreased FEV1 and FEV1/FVC.

Question 18

What lung volumes are affected by restrictive lung disease?

Answer 18

Reduced FEV1, TLC, RV, FRC and VC.

Question 19

How can static lung volumes be calculated by carbon monoxide transfer?

Answer 19

The subject exhaled to RV and then inhales a known amount of CO and helium.
A breath hold allows CO to be absorbed.
A sample of exhaled gas is collected.
[CO] is measured.

Question 20

What calculations are made in CO transfer?

Answer 20

The rate of assumed transfer in the blood is calculated using initial [CO] and final [CO].
Exhaled [helium] is used to calculate the volume into which the CO was distributed.

Question 21

How is CO transfer (Tco) expressed?

Answer 21

As a rate, in mmol/min/kPa.

Question 22

What is the typical value of Tco?

Answer 22

11.6 (+/-1.7) mmol/min/kPa.

Question 23

What factors directly influence Tco?

Answer 23

Pulmonary blood flow and volume.
Hb concentration.
Background levels of COHb in the blood.

Question 24

When does Tco increase?

Answer 24

Where pulmonary blood flow and volume increases.
Shunts, pregnancies, alveolar haemorrhages.

Question 25

When does Tco decrease?

Answer 25

Many types of lung disease.
Anaemia.
Smoking.