Comparison of methods of measuring static lung volumes.

The measurement of static lung volumes is important for the accurate diagnosis of lung disorders, and when making volume-dependent measurements, such as airways resistance. There are a variety of methods available. The most accurate method is that of constant volume body plethysmography, which provides an estimate of total lung capacity regardless of the presence of airflow obstruction. Whilst this method may overestimate lung volumes in asthmatics, and is technically more demanding than gas dilution methods, this should be regarded as the principal method for estimating lung volumes. Gas dilution estimates of multi-breath helium or nitrogen dilution or single-breath estimates using the same gases all underestimate total lung capacity in the presence of airflow obstruction. Single-breath methods will underestimate volumes to a greater extent than multi-breath methods. Multi-breath helium dilution is currently regarded as the acceptable alternative to body plethysmography. Estimates of lung volumes from chest radiographs provide an estimate of lung volumes independent of airflow obstruction. They are probably prone to greater variability than body plethysmographic estimates, and it is regarded as unacceptable to expose patients to excess radiation. Other methods being developed include estimates from nuclear magnetic imaging and computed tomography.