1

How can respiratory function be assessed?

History – symptoms, exercise tolerance and performance status (PS).

Radiological imaging – chest radiograph (CXR), computed tomography (CT) scan, ventilation/perfusion (V/Q) scan and positron emission tomography (PET) scan.

Oxygen saturations (O₂ sats) – at rest and on exertion.

Arterial blood gas (ABG) sampling.

Spirometry.

Flow-volume loops.

Peak expiratory flow rate (PEFR).

Static lung volume determination.

Gas transfer diffusion testing.

Shuttle test.

Six-minute walk test (6MWT).

Cardiopulmonary exercise test (CPEX).

2

What are the principles of lung function testing?

Lung function tests are important to:

a)

establish baseline lung function and quantify any respiratory impairment;

b)

investigate patients with respiratory symptoms and signs suggestive of pulmonary disease;

c)

monitor the progression of disease and response to treatment;

d)

evaluate patients prior to cardiothoracic surgery to assess operative risk.

Patients are advised not to smoke prior to the test, not to eat a large meal within 4 hours of the test and not to wear tight-fitting clothes.

Normal or predicted values are taken from large population studies of healthy subjects and are defined by age, height, gender and ethnicity.

Lung function tests are performed three times and the average taken to ensure accuracy and reproducibility.

Lung function tests comprise:

a)

dynamic studies – spirometry, flow-volume loops, peak expiratory flow rate;

b)

static lung volume determination;

c)

gas transfer diffusion testing.

3

What is spirometry?

Spirometry is the most commonly performed pulmonary function test and measures the volume (amount) and flow rate (speed) of air entering and leaving the lungs during respiration.

It assesses the mechanics of the lungs and their ability to move large volumes of air rapidly through the airways.

It is an inexpensive and simple ‘bedside’ test that can be carried out as screening for underlying respiratory pathology.

There are various types and sizes of spirometer, many of which are now handheld and suitable for use in the outpatient clinic.

The patient is asked to take in the deepest breath possible, place their mouth over the machine and breathe out as hard and fast as possible, for as long as possible, ideally for at least 6 seconds.

As it is highly dependent on patient effort and compliance, it is typically repeated several times and the average values utilised.

Results are given in L or L/sec and as a percentage of predicted value, with >80% of predicted considered normal.

The components of basic spirometry include (Figure 1):

a)

forced vital capacity (FVC);

b)

forced expiratory volume in 1 second (FEV₁);

c)

FEV₁/FVC ratio.

Spirometry can also be used to assess reversibility, with an increase of >12% and 0.2L in either FVC or FEV₁ deemed to represent a significant improvement, when performed 10-15 minutes after inhalation of a therapeutic dose of a bronchodilator.

4

What is forced vital capacity?

Forced vital capacity (FVC) is the total amount of air that can be forcefully exhaled from the lungs in a single breath from maximal inspiration to maximal expiration.

It represents the difference between total lung capacity (TLC) and residual volume (RV).

Observed values above 80% of the predicted value are considered normal.

A reduction in FVC is suggestive of a restrictive pulmonary defect (see below).

5

What is forced expiratory volume in 1 second?

Forced expiratory volume in 1 second (FEV₁) is the maximal volume of air exhaled in the first second of a forced expiration from a position of full inspiration.

Observed values above 80% of the predicted value are considered normal.

A reduction in FEV₁ without a corresponding decrease in FVC is suggestive of an obstructive pulmonary defect (see below).

A reduction in FEV₁ associated with a corresponding decrease in FVC is suggestive of a restrictive pulmonary defect (see below).

6

What is the FEV₁/FVC ratio?

The FEV₁/FVC ratio is a calculation of the rate of flow of air out of the lungs as a function of the total volume exhaled.

It should be approximately 75-80% in healthy patients.

An increase in this ratio suggests restrictive lung pathology, whilst a decrease suggests obstructive pathology.

7

What is forced expiratory flow?

Forced expiratory flow (FEF) represents the flow of air coming out of the lungs during the middle interval of a forced expiration.

The usual intervals are 25%, 50% and 75% of FVC.

It can identify early chronic obstructive disease but is less commonly used in clinical practice than FEV₁.

8

What are the spirometry findings in a patient with obstructive lung disease (Figure 2)?

Reduced FEV₁.

Stable FVC.

Reduced FEV₁/FVC ratio.

Obstructive lung diseases are characterised by reduced airflow secondary to bronchospasm, airway inflammation, increased bronchial secretions or a reduction in the parenchymal support of the airways, due to loss of lung elastic recoil.

Examples of obstructive lung disease include:

a)

asthma;

b)

chronic obstructive pulmonary disease (COPD)/emphysema;

c)

bronchiectasis/cystic fibrosis.

The severity of the obstructive pathology can be categorised by the reduction of FEV₁ (Table 1).

9

What are the spirometry findings in a patient with restrictive lung disease (Figure 3)?

Reduced FEV₁.

Reduced FVC.

Normal or increased (>80%) FEV₁/FVC ratio, especially as there is usually a more pronounced decline in FVC than in FEV₁.

Examples of restrictive lung disease include:

a)

intrinsic lung disease:

i)

idiopathic pulmonary fibrosis;

ii)

sarcoidosis;

b)

extrinsic thoracic pathology:

i)

obesity;

ii)

asbestosis;

iii)

chest wall deformity;

iv)

neuromuscular disorders.

The severity of the restrictive pathology can be categorised by the reduction of FVC (Table 2).

Assessment of TLC and RV by static lung volume determination can confirm the restrictive defect suggested by spirometry.

10

What is a respiratory flow-volume loop (Figure 4)?

A flow-volume loop graphically demonstrates the rate of airflow (Y-axis) against the total volume of air inspired or expired (X-axis).

The graph comprises a positive expiratory limb and a negative inspiratory limb.

The maximum flow rate during expiration (peak expiratory flow rate [PEFR]) can also be measured.

The morphology of the flow-volume curves can be used to diagnose obstructive and restrictive airway lesions.

11

What are the characteristic morphologies of normal and pathological respiratory flow-volume loops (Figure 5)?

Normal:

a)

flow-volume loop: rapid rise to the peak expiratory flow rate followed by a near linear fall in the expiratory curve, with a relatively symmetrical, saddle-shaped inspiratory curve.

Emphysema:

a)

flow-volume loop: concave appearance in the descending portion of the expiratory flow limb;

b)

mechanism: collapse of the distal airways due to a loss of elastic recoil and radial support resulting in reduced expiratory flow.

Variable intrathoracic upper airway obstruction:

a)

flow-volume loop: plateau across the expiratory flow limb with a normal inspiratory flow pattern;

b)

mechanism: reduction of airflow during forced expiration caused by narrowing or collapse of the airway, secondary to extraluminal pressure exceeding intraluminal pressure during expiration;

c)

aetiology: tracheomalacia, polychondritis, and tumours of the distal trachea or main bronchus.

Variable extrathoracic upper airway obstruction:

a)

flow-volume loop: reduced inspiratory flow during forced inspiration with a normal expiratory flow pattern;

b)

mechanism: airway narrowing, secondary to extraluminal pressure exceeding intraluminal pressure during inspiration;

c)

aetiology: unilateral and bilateral vocal cord dysfunction, laryngeal oedema and obstructive sleep apnoea.