Alveolar Volume Determined by Single-Breath Helium Dilution Correlates with the High-Resolution Computed Tomography-Derived Nonemphysematous Lung Volume
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abstract
<i>Background:</i> The alveolar volume (V<sub>A</sub>), determined by single-breath helium dilution, is a measure for the total lung capacity (TLC) that is very sensitive to ventilatory disturbances. In chronic obstructive pulmonary disease (COPD), the emphysematous lung parts are less accessible to test gas; therefore, the V<sub>A</sub> is smaller than TLC measured by multiple-breath helium dilution (TLC<sub>He</sub>). <i>Objectives:</i> The aim of this study was to investigate whether the V<sub>A</sub> represents the nonemphysematous lung parts. <i>Methods:</i> We measured V<sub>A</sub> as part of the diffusing capacity for carbon monoxide (DL<sub>CO</sub>), TLC<sub>He</sub> and spirometry in 50 patients with COPD. High-resolution computed tomography (HRCT) scans of all subjects were analyzed with the density mask method, where parts with an attenuation of less than –950 Hounsfield units were considered as emphysematous. <i>Results:</i> A strong correlation was observed between the V<sub>A</sub> (mean 5.2 liters) and nonemphysematous HRCT lung volume (mean 5.2 liters, r<sup>2</sup> = 0.9) and between the TLC<sub>He</sub> (mean 6.6 liters) and total HRCT lung volume (mean 6.4 liters, r<sup>2</sup> = 0.9). Bland-Altman plots showed considerable disagreement between the V<sub>A</sub> and the nonemphysematous HRCT lung volume. A weak correlation between the forced expiratory volume in 1 s (mean 46% predicted) and DL<sub>CO</sub> (mean 46% predicted) versus the HRCT emphysema ratio (nonemphysematous/total HRCT lung volume) was observed (r<sup>2</sup> = 0.3 and 0.3, respectively). <i>Conclusion:</i> We concluded that the V<sub>A</sub> correlates with the nonemphysematous HRCT lung volume, although the two measurements are not equivalent, possibly due to technical factors.
