16th February 2025

Spirometry

During a spirometry test, the patient inhales maximally, then exhales into the spirometer as hard as possible until no breath remains. The main values of interest are as follows, with predicted values calculated from the patient’s height and demographics.

Peak expiratory flow rate (PEFR)

Also measured using a peak flow meter, the PEFR is the maximum rate of expiratory flow the patient can achieve, in L/min, typically attained at the beginning of expiration. A reduction in PEFR is suggestive of airway obstruction, such as COPD or asthma. 

In asthma, variability in PEFR during the day is typically observed, with the lowest values usually being recorded early in the morning or after exercise. PEFR is usually reduced but stable in COPD. There is also a rare phenotype of asthma where fixed PEFR is observed, as in COPD.

PEFR is frequently used for monitoring acute asthma in the inpatient setting. BTS guidelines recommend the patient is safe for discharge when PEFR is >75% of best or predicted, with a diurnal variation of less than 25%.

FVC, FEV1 and FEV1/FVC ratio

The forced vital capacity (FVC) is the total volume of air expelled from full inspiration to full expiration. Above 80% of predicted is considered normal. 

The forced expiratory volume in one second (FEV1) is the volume of air expired in the first second of forced expiration. Typically, the FEV1 is interpreted using the FEV1/FVC ratio, which is more useful as it is independent of other factors. A ratio of >70% is considered normal, with a ratio of <70% suggesting airway obstruction.

Obstructive pattern

An obstructive pattern of PFTs is where the FEV1/FVC ratio is less than 70%, with normal or low FVC.

This usually indicates COPD or asthma, which can be differentiated by a reversibility test, where the test is repeated following administration of a bronchodilator such as salbutamol. If the FEV1 or FVC increases by more than 12%, the obstruction is said to be reversible or bronchodilator responsive, which is suggestive of asthma, although a small degree of reversibility can also be seen in COPD. 

Alternatively, diurnal variability in PEFR can be used to differentiate the two, as mentioned above.

Restrictive pattern

A restrictive pattern of PFTs is where the FVC is reduced, with a preserved FEV1/FVC ratio (>70%). This occurs in three main aetiologies:

Reduced lung compliance/volume, such as lung fibrosis or pneumonectomy.

Muscle weakness, such as myasthenia gravis or myopathy.

Reduced chest wall compliance – see following section.

FVC monitoring

Monitoring of FVC is also used for assessing decline in respiratory function in acute neuromuscular disease, such as in Guillain-Barré syndrome and myasthenic crisis. An FVC of less than 15-20 ml/kg body weight suggests endotracheal intubation may be necessary.