To Örebro University

For measurements of absolute lung volumes, special methods are required. Mainly there are two different ways to perform this, either by body plethysmography, or gas dilution/nitrogen washout. Body plethysmography measures all intrathoracic gas whilst gas dilution techniques measure all gas in free communication with the airway opening. Previous studies have shown that there are small differences in functional residual capacity measured by body plethysmography (FRC

pleth) in comparison with nitrogen washout (FRCN2) in healthy subjects; although in significant obstructive patients there is an elevated FRCpleth. However, other studies show that there will be an elevated FRC-value in nitrogen washout due to the release of tissue nitrogen. Clinical observations have shown mixed results, both the impression of an elevated FRC value in nitrogen washout but also a lowered FRC value compared to body plethysmography.

The aim of this study was to investigate if there is a systematic difference between FRC calculated with nitrogen washout and FRC calculated with body plethysmography, with or without correction for estimated tissue nitrogen.

Thirty-one patients underwent dynamic spirometry, body plethysmography and multiple-breath nitrogen washout (MBW). Seventeen patients had normal lung function; five were diagnosed with some grade of obstructive lung disease and seven with some grade of restrictive lung disease.

FRC

N2 provides slightly higher values (165-206mL) compared with FRCpleth, which should be considered regarding interpretation in clinical practice. After correction for tissue nitrogen, FRCpleth yields higher values (154-195mL). These results seem to be applicable in general and in healthy subject, but further studies should be conducted with the inclusion of subgroup analyzes.