Pulmonary Function Tests

Question 1

Normal PFT values depend on what 4 things?

Answer 1

Age
Gender
Height
Possibly ethnicity (We do not differentiate in the UK)

Question 2

What are the 2 most simple and common used PFTs

Answer 2

Spirometry

Peak expiratory flow rate (PEFR)

Question 3

What do Peak Flow Meters measure?

Answer 3

Measure the highest velocity of airflow than can be achieved during maximal expiration from Total Lung Capacity

(Litres per second, Volume-Time graph)

Question 4

What 2 things does Vital Capacity depend on?

What are 3 causes of it being reduced?

Answer 4

Maximal Inspiration and Expiration

• Lungs not filled normally in inspiration
• Lungs not emptied normally in expiration
• Or both

Question 5

Compare a Restrictive and Obstructive deficit

Suggest conditions where they are seen

Answer 5

Restrictive;

• Reduced maximal filling of lungs (e.g increased stiffness)
• Diffuse Pulmonary Fibrosis (Due to Interstitial Lung Disease)
• Myasthenia Gravis

Obstructive;

• Reduced airflow during expiration (e.g airway narrowing)
• Asthma
• COPD

Question 6

How do we distinguish between an Obstructive and Restrictive defect?

Answer 6

Measuring FEV1/ FVC ratio

FEV1= Forced Expiratory Volume in 1 second

Question 7

What is the FEV1/ FVC ratio in normal individuals?

Answer 7

FEV1/ FVC> 0.7 (More than 70%)

Question 8

Explain the FEV1/ FVC ratio in an Obstructive defect

Answer 8

• FVC nearly normal in early disease (may decrease as disease progresses if there is air trapping)
• FEV1 is reduced DISPROPORTIONALLY
• Thus, FEV1/ FVC ratio is <0.7

Question 9

Explain the FEV1/ FVC ratio in a Restrictive defect

Answer 9

• Reduced TLC, therefore reduced FVC
• FEV1 reduced proportionally
• Thus, FEV1/ FVC ratio is normal/ raised

Question 10

How can FEV1/ FVC ratio be used to rule out Obstructive defects?

Answer 10

If FEV1/ FVC ratio is >/= 0.7

Question 11

Compare Obstructive and Restrictive defect, with regards to;

• FEV1/ FVC ratio
• FVC

Answer 11

Obstructive;

• FEV1/ FVC <0.7
• Nearly normal FVC

Restrictive;

• FEV1/ FVC >/=0.7
• Reduced FVC

Question 12

When is PEFR at its highest?

Answer 12

At start of expiration (Lungs are expanded and airways stretched open)

(As expiration continues, small airways are narrowed by compression of the lungs)

Question 13

Why can FEV1 not be read off of a Flow Volume loop graph?

Answer 13

No time axis (Y-Flow, X-Volume)

Question 14

What affects Peak Flow most in normal individuals?

Answer 14

Large airway resistance, (as opposed to small airways)

Question 15

On a Flow-Volume Loop, what are 2 signs of Obstructive Defect?

Answer 15

• Reduced PEFR (More so in severe obstruction)

- ‘Scalloping’ (Steeper curve during expiration)

Question 16

How does a Restrictive defect appear on a Flow-Volume Loop?

Answer 16

• PEFR not significantly reduced
• Narrow loop (Due to reduced TLC-> FVC)

(“Wizard hat” flow-volume loop)

Question 17

Why does Myasthenia Gravis cause a Restrictive Defect?

Answer 17

Weak chest elastic recoil due to weaker muscles

Question 18

What methods can be used to measure;

• Residual volume
• Dead Space

Answer 18

Residual Volume: Helium dilution test

Dead Space: Nitrogen washout/ Fowler’s method

Question 19

What is the Diffusion conductance?

Answer 19

Resistance to diffusion across alveolar membrane

Question 20

Describe how measuring Diffusion Capacity help to distinguish between causes of Restrictive Defect

Answer 20

Problems intrinsic to lung parenchyma (Fibrosis);
- Abnormal DLCO and Restrictive pattern

Extrinsic disease (Myasthenia Gravis);
- Normal DLCO and Restrictive pattern

Question 21

Suggest a cause of abnormal DLCO (Diffusing capacity of CO) with NO OTHER DEFECTS/ ABNORMALITIES

Answer 21

Pulmonary arterial hypertension

Thickened arterioles-> Reduced diffusion