Lung Function Testing

Question 1

What is the purpose of lung function testing?

Answer 1

• evaluation of breathless patient
• screening for COPD/occupational lung disease
• lung cancer (fitness for treatment)
• pre-operative assessment
• measure disease progression/treatment response
• monitoring drug treatment toxic to lungs
• pulmonary complications of systemic disease

Question 2

Explain how spirometry is carried out

Answer 2

• patient takes big breath in as far as they can and then blow it out as hard as they can for as long as possible then take another breath all the way in
• forced expiratory manoeuvre from total lung capacity followed by full inspiration
• best of 3

Question 3

What are the disadvantages of spirometry?

Answer 3

• needs an appropriately trained technician
• effort and technique dependent
• if patient is frail they might not be able to do it
• same if patient is in pain/too unwell

Question 4

What can be directly measured in spirometry?

Answer 4

• vital capacity (total amount exhaled after maximal inspiration)
• inspirational capacity (total volume of air that the lungs can accomodate)
• inspiratory reserve volume (amount of air that can be inhaled after maximal expiration)
• tidal volume (the volume of air that moves in and out of the lungs with each respiratory cycle)
• expiratory reserve volume (volume of air that can be expired after normal expiration)

Question 5

How much air is released in expiration in a second normally?

Answer 5

3L = 75% FEV1/FVC ratio

Question 6

Describe the features of obstructive lung disease

Answer 6

• asthma or COPD
• FEV1/FVC <70%
• severity of COPD stratified by %predicted FEV1
• mild >80%
• moderate 50-80%
• severe 30-50%
• very severe <30%

Question 7

Describe reversibility testing in asthma

Answer 7

• nebulised or inhaled salbutamol given
• spirometry taken before and 15 minutes after
• if 15% and 400ml reversibility in in FEV1 = asthma

Question 8

What testing can be done for asthma?

Answer 8

• peak flow (look for diurnal variation, response to inhaled corticosteroids, occupational asthma)
• reversibility testing with spirometry
• bronchial provocation

Question 9

Describe the features of restrictive lung disease

Answer 9

• FEV1 and FVC reduced
• FEV1/FVC <70%

Question 10

What are the causes of restrictive lung disease?

Answer 10

• interstitial lung disease (stiff lungs)
• kyphoscoliosis/chest wall abnormality
• previous pneumonectomy
• neuromuscular disease
• obesity
• poor effort/technique

Question 11

How can you measure gas exchange in the lungs?

Answer 11

• transfer factor (TLCO, KCO, DLCO)
• single breath of very small concentration of CO
• CO has very high affinity for Hb
• measure concentration in expired gas to derive uptake in lungs

Question 12

What factors affect transfer factor uptake in the lungs?

Answer 12

• alveolar surface area
• pulmonary capillary blood volume
• Hb concentration
• ventilation perfusion mis-match

Question 13

What conditions cause a reduction in transfer factor uptake?

Answer 13

• emphysema
• interstitial lung disease
• pulmonary vascular disease
• anaemia

Question 14

What are the 2 ways of measuring lung volume?

Answer 14

• helium dilution: inspire known quantity of inert gas
• body plethysmography: respiratory manoevures in a sealed box causes changes in air pressure which can be measured

Question 15

What is oximetry?

Answer 15

• non-invasive measurement of saturation of Hb by O2
• depends on oxyhaemoglobin and deoxyhaemoglobin absorbing infrared light differently
• depends on adequate perfusion (eg. Compromised in shock/cardiac failure)
• at a pressure of 12-13 O2kPa 99% RBC bound to O2 so level of O2 will have no effect on carrying capacity

Question 16

List the main causes of hypoxaemia

Answer 16

• hypoventilation (eg. Drugs, neuromuscular disease)
• ventilation / perfusion mis-match (eg. COPD, pneumonia)
• shunt (eg. Congenital heart disease)
• low inspired O2 (eg. Altitude, flight)

Question 17

Describe the mechanism of ventilation perfusion mis-match

Answer 17

• some areas of the lung are perfused but not well ventilated
• results in mixing of blood from poorly ventilated and well ventilated parts of the lung cause hypoxaemia
• in extreme cases can form a shunt which blood bypasses the lungs entirely and cannot be corrected with O2 administration

Question 18

What is the alveolar arterial oxygen gradient?

Answer 18

• the difference between calculated alveolar pO2 and arterial pO2 (measured in ABG)
• difference should be <2-4 kPa
• if more = V/Q mis-match