Critical evaluation of computerised x ray planimetry for the measurement of lung volumes.

BACKGROUND Computerised x-ray planimetry has been advocated as an alternative to body plethysmography and helium dilution for measuring static lung volumes. The accuracy and reproducibility of this method has been assessed in comparison with these standard methods. METHODS Plethysmographic and planimetric measurements of total lung capacity (TLC) and functional residual capacity (FRC) were made in 10 normal subjects and in 12 patients with chronic obstructive pulmonary disease (COPD), with additional helium dilution measurements in the latter 12 patients. RESULTS Mean lung volumes (TLC and FRC) for groups of subjects measured by planimetry and by plethysmography were similar in both groups and larger than the helium dilution measurement in patients with COPD. Intraindividual agreement between planimetry and plethysmography was poor, however, with a wide confidence interval (-2.2 to +2.31). The planimeter did not measure reliably changes in volume from TLC to FRC in individuals. CONCLUSIONS Mean lung volumes measured by planimetry in a group of patients probably reflect a regression to the mean of the computer algorithm rather than accurate TLC estimation. The technique is not yet robust enough to replace the established techniques of helium dilution or plethysmography.