Lung function made easy: assessing lung size.

Spirometry is available in most GP surgeries and provides an invaluable tool for assessing respiratory function in chronic obstructive pulmonary disease (COPD) and asthma. Spirometry alone may not provide the clinician with an accurate assessment of lung disease as it misses two important measurements of lung volume. By measuring Residual Volume (RV) and Total Lung Capacity (TLC) it is possible to determine true restrictive or hyperinflated disease processes. Helium dilution, body plethysmography and nitrogen washout are three different methods which may be used to measure lung volume. These tests are normally only provided in the acute setting. Comparing values of RV and TLC to predicted values makes it possible to grade the severity of disease far more accurately than spirometry. Four case studies of asthma, obesity, COPD and pulmonary fibrosis clearly demonstrate anomalies that may arise when interpreting lung disease from spirometry compared to the interpretation made with additional lung volume data.