Comparison of two new methods for the measurement of lung volumes with twostandard methods

Background-The two most commonly used methods for the measurement of lung v olumes are helium dilution and body plethysmography. Two methods have been developed which are both easier and less time consuming to perform. Mathema tical modelling uses complex calculations from the flow-volume loop to deri ve total lung capacity (TLC), and the nitrogen balance technique uses nitro gen 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 w ith restrictive airway disease, and 21 with obstructive ventilatory defects . Each subject underwent repeated measurements of TLC by each of the four m ethods in random order. Reproducible values were obtained for each method a ccording to BTS/ARTP guidelines. Bland-Altman plots were constructed for co mparisons between the methods and paired t tests were used to assess differ ences in means. Results-Bland-Altman plots showed that the differences between body plethys mography and helium dilution fell into clinically acceptable ranges (agreem ent Limits +/-0.91). The agreement between mathematical modelling or the ni trogen balance technique and helium dilution or body plethysmography was po or (+/-1.8-3.41), especially for subjects with airflow obstruction. Conclusions-Neither of the new methods agrees sufficiently with standard me thods to be useful in a clinical setting.