RESUMO
BACKGROUND: The two most commonly used methods for the measurement of lung volumes are helium dilution and body plethysmography. Two methods have been developed which are both easier and less time consuming to perform. Mathematical modelling uses complex calculations from the flow-volume loop to derive total lung capacity (TLC), and the nitrogen balance technique uses nitrogen from the atmosphere to calculate lung volume in a similar way to helium dilution. This study was designed to compare the two new methods with the two standard methods. METHODS: Sixty one subjects were studied, 23 with normal lung function, 17 with restrictive airway disease, and 21 with obstructive ventilatory defects. Each subject underwent repeated measurements of TLC by each of the four methods in random order. Reproducible values were obtained for each method according to BTS/ARTP guidelines. Bland-Altman plots were constructed for comparisons between the methods and paired t tests were used to assess differences in means. RESULTS: Bland-Altman plots showed that the differences between body plethysmography and helium dilution fell into clinically acceptable ranges (agreement limits +/-0.9 l). The agreement between mathematical modelling or the nitrogen balance technique and helium dilution or body plethysmography was poor (+/-1.8-3.4 l), especially for subjects with airflow obstruction. CONCLUSIONS: Neither of the new methods agrees sufficiently with standard methods to be useful in a clinical setting.