Determination of functional residual capacity with 133-xenon radiospirometry. Comparison with body plethysmography and helium spirometry. Effect of body position.

This study was undertaken to estimate the accuracy of 133-xenon radiospirometry for determination of FRC in healthy subjects. Forty healthy volunteers, both smokers and non-smokers, were examined. The FRC of each subject was concurrently determined with radiospirometric, He-dilution in closed circuit, and body plethysmographic methods. The radiospirometric and He-dilution measurements were done in supine and in sitting positions, the body plethysmography on sitting subjects, only. The mean FRC measured by radiospirometry (FRCRS) was 0.72 1 larger than that measured by helium spirometry (FRCHe) in sitting position (P < 0.01). In supine position the FRCRS was 0.65 1 larger than the FRCHe (P < 0.01). The body plethysmography gave FRC (TGV) 0.35 1 larger than the FRCHe sitting (P < 0.01). The FRCHe and the FRCRS in the sitting position were 0.48 and 0.55 1 larger than in the supine position (P < 0.01), respectively. Trapped air correlated significantly (P < 0.01) with the difference FRCRS-FRCHe, when sitting. The results indicate that the FRC determined radiospirometrically is significantly larger than the FRC determined with He-spirometry. The difference is systematic, suggesting that it is caused by 133-xenon dissolved in blood and accumulated in tissues of the thoracic cage and by dissimilar representation of trapped air in FRCRS and FRCHe. With eventual correction of the systematic error, the FRC obtained as a by-product of radiospirometry may be used, e.g. for clinical purposes.