Abstract
Pulmonary function tests (PFTs) are used to quantify an individual patient’s respiratory physiology. A battery of tests and manoeuvres are performed to measure the performance of the different lung components.
What are the clinical uses of pulmonary function tests? What equipment is needed?
Pulmonary function tests (PFTs) are used to quantify an individual patient’s respiratory physiology. A battery of tests and manoeuvres are performed to measure the performance of the different lung components:
Large and small airways;
Alveoli;
Pulmonary vasculature;
Respiratory muscles.
The clinical uses of PFTs are:
Diagnosis of respiratory disease;
Grading the severity of respiratory disease and guiding its pharmacological management;
Estimation of surgical risk, in particular of thoracic surgery.
Spirometers are used for performing PFTs. There are many types of spirometer, classified as:
Volume-sensing; for example, the vitalograph, based on a bellows mechanism;
Flow-sensing; for example, the pneumotachograph, which is much more portable.
Which variables are measured using spirometry?
Spirometers are used to take many different lung measurements, broadly classified as:
Static lung volumes. The patient breathes in and out of a spirometer, first with tidal volume breaths and then with vital capacity breaths. As discussed in Chapter 12, all static lung volumes and capacities can be measured, with the exception of residual volume, functional residual capacity (FRC) and total lung capacity (TLC).
Dynamic spirometry. Lung measurements that depend on the rate (i.e. volume per unit time) at which air flows in and out of the lungs are called ‘dynamic’. Dynamic PFTs include:
– Forced expiratory volume in 1 second (FEV1);
– Forced vital capacity (FVC);
– Peak expiratory flow rate (PEFR);
– Expiratory flow–volume curve;
– Flow–volume loops.
Special tests such as diffusion capacity (which gives a measure of alveolar diffusion – see Chapter 10), gas dilution and N2 washout (used to calculate FRC – see Chapter 12).
How are FEV1, FVC and PEFR measured?
Forced spirometry is a simple bedside test. From full inspiration, the patient breathes out as hard and as rapidly as possible into the spirometer, to full expiration, resulting in the expiratory volume–time graph (Figure 13.1).
Figure 13.1 Normal forced expiratory volume–time graph.
Two parameters are measured: FEV1 and FVC. These are compared with their ‘predicted’ values, based on normal patients matched for age, gender, height and ethnic origin. One parameter is calculated: FEV1/FVC ratio – an FEV1/FVC ratio less than 0.7 is considered abnormal. Use of this ratio identifies a relative difference between FEV1 and FVC: a patient with low FVC will also have a low FEV1 simply as there is less gas to be expelled, rather than necessarily being due to an obstructive pathology.
PEFR can also be calculated from the forced spirometry trace: flow is volume per unit time, so the gradient of the spirometry curve represents flow. The ‘peak’ flow is therefore the initial gradient of the forced volume–time curve (Figure 13.1). However, PEFR is more commonly measured by a separate device: the peak flow meter.
Forced spirometry is particularly useful for the diagnosis of obstructive and restrictive lung diseases:
Obstructive airways diseases (asthma and chronic obstructive pulmonary disease, COPD) can be diagnosed by comparing forced spirometry measurements with predicted values (Figure 13.2a). Diagnostic criteria are:
– FEV1 < 80% predicted;
– FEV1/FVC ratio < 0.7.
– Mild disease, FEV1 50–79% predicted;
– Moderate disease, FEV1 30–49% predicted;
– Severe disease, FEV1 < 30% predicted.
PEFR can also be used for the diagnosis of obstructive airways disease (a diurnal variation of >20% is suggestive of asthma), but is more commonly used to compare a patient’s baseline respiratory function with that during an exacerbation.
Differentiation between asthma and COPD is based on the history and the reversibility of airway obstruction. Forced spirometry is performed before and 15 min after administration of a bronchodilator – an improvement in FEV1 of 400 mL is said to correspond to significant airway reversibility, suggesting asthma. Some patients, usually over the age of 40 years, have chronic airways obstruction that is only partially reversible, as well as features suggestive of both asthma and COPD. This is referred to as ‘asthma–COPD overlap syndrome’ (ACOS). ACOS may represent an intermediary condition in the spectrum from asthma to COPD.
Restrictive lung diseases (e.g. lung fibrosis, kyphoscoliosis, respiratory muscular weakness) are characterised by (Figure 13.2b):
– FVC1 < 80% predicted;
– FVC < 80% predicted;
– FEV1/FVC ratio > 0.7; that is, ‘normal’ or even ‘high’, the latter due to increased FEV1 from decreased pulmonary compliance.
