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Comparative Study
. 2012:7:119-26.
doi: 10.2147/COPD.S26419. Epub 2012 Feb 22.

Total lung capacity by plethysmography and high-resolution computed tomography in COPD

Jamie L Garfield  1 , Nathaniel Marchetti, John P Gaughan, Robert M Steiner, Gerard J Criner
Affiliations
Comparative Study

Total lung capacity by plethysmography and high-resolution computed tomography in COPD

Jamie L Garfield et al. Int J Chron Obstruct Pulmon Dis. 2012.

Abstract

Aim: To characterize and compare total lung capacity (TLC) measured by plethysmography with high-resolution computed tomography (HRCT), and to identify variables that predict the difference between the two modalities.

Methods: Fifty-nine consecutive patients referred for the evaluation of COPD were retrospectively reviewed. Patients underwent full pulmonary function testing and HRCT within 3 months. TLC was obtained by plethysmography as per American Thoracic Society/European Respiratory Society standards and by HRCT using custom software on 0.75 and 5 mm thick contiguous slices performed at full inspiration (TLC).

Results: TLC measured by plethysmography correlated with TLC measured by inspiratory HRCT (r = 0.92, P < 0.01). TLC measured by plethysmography was larger than that determined by inspiratory HRCT in most patients (mean of 6.46 ± 1.28 L and 5.34 ± 1.20 L respectively, P < 0.05). TLC measured by both plethysmography and HRCT correlated significantly with indices of airflow obstruction (forced expiratory volume in 1 second/forced vital capacity [FVC] and FVC%), static lung volumes (residual volume, percent predicted [RV%], total lung capacity, percent predicted [TLC%], functional residual capacity, percent predicted [FRC%], and inspiratory capacity, percent predicted), and percent emphysema. TLC by plethysmography and HRCT both demonstrated significant inverse correlations with diffusion impairment. The absolute difference between TLC measured by plethysmography and HRCT increased as RV%, TLC%, and FRC% increased. Gas trapping (RV% and FRC%) independently predicted the difference in TLC between plethysmography and HRCT.

Conclusion: In COPD, TLC by plethysmography can be up to 2 L greater than inspiratory HRCT. Gas trapping independently predicts patients for whom TLC by plethysmography differs significantly from HRCT.

Keywords: gas trapping; high-resolution computed tomography; lung capacity; lung volume measurement errors; plethysmography.

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Figures

Figure 1
Figure 1
Diagram of the lungs of a 61-year-old man with COPD. The transverse view (top left), coronal view (top right), and sagittal view (bottom left) are at the same anatomical level, as well as three-dimensional rendering of tracheobronchial tree (bottom right).
Figure 2
Figure 2
Correlation between inspiratory lung volume by HRCT and TLC derived from PFT results. Notes: r = 0.92, P < 0.01. Abbreviations: HRCT, high-resolution computed tomography; TLC, total lung capacity.
Figure 3
Figure 3
This plot of the difference between the methods against their mean illustrates the relationship between the measurement error and the true value. Total lung capacity measured by plethysmography is 1.12 L greater than total lung capacity measured by HRCT. Greater variation from the mean is seen at higher lung volumes, suggesting measurement error is greater at higher lung volumes. Abbreviations: HRCT, high-resolution computed tomography; pleth, plethysmography.
Figure 4
Figure 4
Relationship between TLC% predicted and difference between lung volumes determined by plethysmography (pleth) and HRCT. Notes: r = 0.39, P < 0.01. Abbreviations: HRCT, high-resolution computed tomography; TLC%, total lung capacity, percent.
Figure 5
Figure 5
Relationship between RV% predicted and difference between lung volumes determined by plethysmography (pleth) and HRCT. Notes: r = 0.29, P < 0.01. Abbreviations: HRCT, high-resolution computed tomography; RV%, residual volume, percent.
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